Illumination device and device and method for generating multispectral images

Abstract

An illumination device (13) for generating multispectral images is provided. The illumination device (13) comprises a light source arrangement (21, 23A-D, 33A-F) which is designed to illuminate an object (5) with illumination light having a broad spectral range and a hue corresponding to a specific point in a color space, and which makes it possible to independently vary the radiant powers arriving at the object (5) in at least two narrow spectral ranges (I, II). Furthermore, the illumination device (13) comprises a control device (25) for controlling the light source arrangement such that the radiant powers in the at least two narrow spectral ranges (I, II) are varied sequentially over time, wherein the sequential changes each include a change in the radiant power in at least one of the at least two narrow spectral ranges (I, II).The control device (25) is designed to control the light source arrangement in such a way that the change in the radiant power in at least one of the at least two narrow spectral ranges (I, II) in the radiant power arriving at the object (5) only causes color tone changes which do not exceed the predetermined maximum permissible deviation from the specific point of the color space. Furthermore, a device for generating multispectral images with such an illumination device (13) and a method for generating multispectral images are provided.

Description

The present invention relates to an illumination device for use in generating multi-spectral images. In addition, the invention relates to an apparatus and a method for generating multi-spectral images.In many fields of medicine, tissue should be made distinguishable as much as possible without stress to the patient. Examples of these are the differentiation of dental fillings compared to healthy teeth, the differentiation of tumor tissue compared to nontumor-largest tissue during resection operations, the differentiation of active cortical tissue compared to nonactive cortical tissue, etc.There are different approaches to distinguishing tissue. One approach is, for example, to bring about a distinguishability of tissue by means of fluorescence. During the recording of fluorescence images, filters are used which allow essentially only the fluorescence light to pass to the image sensor. As a result, on the one hand, the color impression of the object is changed and, on the other hand, the intensity of the light emanating from the object is significantly reduced. A surgeon must therefore generally switch back and forth between the fluorescence mode and the white light mode of a surgical microscope and interrupt his work during the recording of fluorescence images. In addition, taking fluorescent images frequently requires the injection of a fluorescent dye.An alternative to making tissue distinguishable with the aid of fluorescence is the recording of so-called multi-spectral images or hyper-spectral images. An imaging system which can record white light images and hyperspectral images is described, for example, in WO 2018 / 049215 A1. A common color image detects light reflected and / or scattered by the object in three color channels, namely a red, a green and a blue color channel. The individual color channels can be recorded using a monochromatic image sensor in a time-sequential manner by recording an image for the red color channel, an image for the green color channel and an image for the blue color channel, which are then combined to form a color image. Alternatively, all three color channels may be recorded simultaneously. For this purpose, the image sensor must be designed as a color sensor, which is sometimes also referred to as a Bayer sensor. In a color sensor, the image sensor is preceded by a matrix which arranges either a red, a green or a blue spectral filter before each pixel of the image sensor. This matrix is referred to as a Bayer matrix.In the case of multi-spectral images and hyperspectral images, the number of color channels is increased and can also comprise color channels in the infrared or ultraviolet spectral range. The time-sequential recording method is often used to record the multi-spectral images or hyperspectral images, wherein the object is illuminated with white light and spectral filters are successively introduced into the beam path between the observation object and the camera in order to record the individual color channels in a time-sequential manner. Such a procedure is described, for example, in CA 2 942 069 A1. In this way, a monochrome image is obtained for each color channel, which represents the intensity distribution of the light reflected and / or scattered by the object under observation in the respective color channel. For each image point, the spectral distribution of the reflected and / or scattered light can then be determined from the images recorded in the respective color channels, wherein the more color channels are used, the better resolution the spectral distribution can be determined. As an alternative to introducing spectral filters into the beam path between the observation object and the camera, it is also possible to illuminate the observation object in each case only with the corresponding narrow wavelength ranges in order to record the individual color channels of a hyperspectral image or of a multispectral image. Such a procedure is likewise described in CA 2 942 069 A1.CA 2 942 069 A1 also describes a method with which a surgeon is to be enabled to continue working during the recording of hyperspectral images. For this purpose, hyperspectral images are recorded between two white light images. However, this requires a high frequency of acquisition if a flicker-free white light image is to be presented to a surgeon. A flicker-free image requires about a frame rate of 30 images per second. If, for example, n color channels are to be contained in the hyperspectral image, this requires the recording of n images in different wavelength ranges in addition to the recording of a white light image within a thirtieth second, which is why the images in the individual color channels must be recorded with short exposure times, in particular if a large number of color channels is to be used.DE 10 2011 053 250 A1 discloses a method for recording images of an observation object in which time-sequential illumination with N different wavelengths takes place upon illumination of the observation object with white light.DE 10 2017 108 036 A1 describes a method for controlling a lighting device having at least two electrical light sources. The light sources have at least a first and a second spectral light distribution, which are superimposed on one another in order to generate a superimposed light distribution. The superposition light distribution is adapted on the basis of incoming light measured at the illuminated object such that a setpoint value of the light arriving at the illuminated object is achieved.Compared with this prior art, it is a first object of the present invention to provide an illumination device which makes it possible to record multi-spectral images without a surgeon having to interrupt his work during the recording of the multi-spectral images, it being possible to avoid the above-described problems of short exposure times.In addition, it is a second object of the present invention to provide a device and a method for generating multi-spectral images without a surgeon having to interrupt his work during the recording of the multi-spectral images, wherein the above-described problem of short exposure times can be avoided.The first object is achieved by an illumination device according to claim 1, the second object by a device for generating multi-spectral images representing spectral properties of an object according to claim 9 and by a method for generating multi-spectral images representing spectral properties of an object according to claim 12.A lighting device according to the invention comprises a light source arrangement and a control device. Insofar as a control device or a control is mentioned within the scope of the present description, a control device or a control is also intended to be included here as a control device or a control which adjusts a controlled variable to a reference variable by means of a feedback loop. The light source arrangement is configured to illuminate an object with illumination light having a broad hue and a hue corresponding to a particular point of a color space. The term light is intended to encompass not only wavelengths in the visible spectral range, but also wavelengths in the ultraviolet (UV) and infrared (IR) spectral ranges. The color space may be any color space, for example an RGB color space such as the sRGB color space, a CIE color space such as the CIE2000 color space or the CIExy color space introduced by the International Lighting Commission for characterizing the perception of color contrasts between hues. The hue can also be, in particular, a white hue, for example a hue as can be characterized on the basis of a color temperature. Instead of a white hue, the hue may be a dark hue.The light source arrangement is also designed in such a way that it makes it possible to change the radiation powers arriving at the object independently of one another in at least two narrow spectral ranges. The control device is configured to control the light source arrangement such that the radiation powers in the at least two narrow spectral ranges can be changed in a time-sequential manner, wherein the time-sequential changes each contain a change in the radiation power in at least one of the at least two narrow spectral ranges. The narrow wavelength ranges can partially overlap in this caseTypically, the broad spectral range has a spectral width of at least 200 nm, in particular at least 300 nm, and further in particular a width of at least 400 nm. In special cases, it may even have a width of at least 700 nm. If the broad spectral range is intended to comprise the visible light, it has a width of at least 300 nm (for example at least from 400 to 700 nm) and in particular at least 370 nm (for example at least from 380 to 750 nm). If it is also intended to include the infrared spectral range and parts of the ultraviolet spectral range, for example, the broad spectral range can also be more than 400 nm or even more than 700 nm wide (for example from 360 to 800 nm or even from 360 nm to 1100 nm). By contrast, a spectral range whose spectral width is at most 35% of the broad spectral range is to be considered as a narrow spectral range. In particular, spectral width of the narrow spectral range can also be only a maximum of 20% of the broad spectral range, only a maximum of 15% of the broad spectral range or even only a maximum of 10% of the broad spectral range. A narrow spectral range has a center-of-gravity wavelength which characterizes the spectrum of the narrow spectral range and represents the weighted arithmetic mean of the wavelengths of the narrow spectral range. The width of the narrow spectral range is considered to be the full width at half maximum which is given by the amount of the spectral distance between those wavelengths at which the intensity value has dropped from a maximum intensity of the narrow spectral range to half the maximum intensity value. A centroid wavelength and a full width at half maximum can also be present in the broad spectral range. A broad spectral range can, however, also be composed of a number of narrow spectral ranges, which can then also have a plurality of local maxima in the intensity or the transmission. A spectral range also need not be symmetrical about a central wavelength. Typically, in particular, the broad spectral range is unsymmetrical if it is generated by an incandescent lamp whose illumination is based on the temperature of an incandescent element.According to the invention, a maximum permissible deviation from the specific point is predefined in the control device for the color space. In addition, the control device is configured to control the light source arrangement in such a way that the change in the radiant power in at least one of the at least two narrow spectral ranges in the radiant power arriving at the object causes only hue changes which do not exceed the predefined maximum permissible deviation from the specific point of the color space.The maximum permissible deviation from the specific point can be predefined, for example, by a fixed maximum value. The permissible deviation from the specific point may, however, also depend on the direction in the color space, for example in order to take account of different sensitivities of the eye to different colors. In addition, the maximum permissible deviation can also depend on the location of the specific point in the color space. Because of the lower sensitivity of the human eye in the red and in the violet spectral range compared to the green spectral range, wavelengths in the region of the transition between violet and ultraviolet as well as in the region of the transition between red and infrared can be changed more strongly than wavelengths in the green spectral range without any disruptive change in the color perception taking place.A maximum permissible deviation, which is dependent both on the direction in the color space and on the specific point in the color space, can be specified, for example, by means of the so-called MacAd ellipse. FIG. 11 schematically shows the CIExy color space 94 with MacAd ellipses drawn in around specific color tones (center points of the ellipses). In addition, the figure shows the so-called Planckian curve 99 which represents the colors of a black thermal radiator as a function of its temperature, the temperature of the black thermal radiator increasing along the Planckian curve 99 from right to left in the figure. The points 96, 98 and 100 drawn in FIG. 11 thus represent white light with different color temperatures. In the figure, the lower corner of the color space 94 represents blue light having a wavelength of 380 nm, the right corner of the color space 94 represents red light having a wavelength of 700 nm, and the rounded tip of the color space 94 represents green light having a wavelength of 520 nm. As can be seen from Figure 11, the size and ellipticity of a MacAd ellipse depend on the location in the color space. However, it should be noted that the MacAd ellipses drawn in are shown enlarged by approximately ten times for the sake of better visibility. The actual MacAd ellipses are thus considerably smaller than the ellipses drawn in FIG. 11.Color tones that lie within a MacAd ellipse cannot be distinguished by the human eye, so that when the radiation powers arriving at the object are changed in a time-sequential manner, the color tone changes of the light arriving at the object cannot be perceived by the human eye. A viewer of the object illuminated with the illumination device according to the invention thus does not perceive a color tone change in the illumination light, so that his color impression is maintained during the time-sequential change of the radiation powers arriving at the object in the at least two narrow spectral ranges. However, when the images are recorded by an image sensor, an evaluation device can evaluate the hue changes lying within a MacAd ellipse and thus extract the signal due to the respective narrow wavelength range. This allows acquisition of multi-spectral images during the operation of a surgeon without thereby impairing the surgeon's perception of the object under observation. It is therefore not necessary to change between a mode for recording multi-spectral images and a mode for recording white light images. Since the color tone changes are not perceived during the recording of the multi-spectral images, no restriction of the exposure time is necessary for recording the multi-spectral images either.Hue changes are particularly evident when compared to each other. For example, if the hue changes rapidly in a time-sequential manner, i.e., at a high frequency, the rapid sequence of hue changes may be perceived as irritating color flickering when the successive hues exceed a small predetermined maximum allowable deviation, i.e., do not lie within a MacAd ellipse, for example. On the other hand, if the time-sequential change of color tones is slow, a color tone change is less objectionable. In such cases, a larger maximum permissible deviation can be specified. For example, in such cases it may be sufficient if, as a result of the change in the radiant power in at least one of the at least two narrow spectral ranges, only hue changes are caused in the radiant power arriving at the object, which changes lie within an ellipse whose half axes each correspond to a multiple of a MacAd ellipse in the CIExy color space, for example twice a MacAd ellipse, four times a MacAd ellipse or even six times a MacAd ellipse. In addition, the acceptable hue changes may depend on the application using the light source (intraoperative generation of multi-spectral images, preoperative generation of multi-spectral images, postoperative generation of multi-spectral images, etc.). The control device can therefore be configured, for example, to control the light source arrangement in such a way that the change in the radiation power in at least one of the at least two narrow spectral ranges in the radiation power arriving at the object causes only hue changes which lie within an ellipse whose half axes each correspond at most to a predefined multiple of a MacAd ellipse in the CIExy color space, for example twice a MacAd ellipse, four times a MacAd ellipse or even six times a MacAd ellipse. In particular, however, the control device can also be configured to control the light source arrangement in such a way that the change in the radiation power in at least one of the at least two narrow spectral ranges in the radiation power arriving at the object causes only hue changes which lie within a MacAd ellipse. In addition, the control device can comprise an adjustment device, with the aid of which the maximum permissible deviation from the specific point in the color space can be adjusted. This setting device can be part of a control panel, be integrated into a graphical user interface (GUI), be part of a voice input module for inputting voice commands, etc.Instead of the specification of a maximum permissible deviation on the basis of a MacAd ellipse, other specifications which are not based on the MacAd ellipse can also be used. Other color spaces can also be used. For example, in the sRGB color space, a sphere having a predetermined radius may be laid around a point representing a color tone. If the radus is small enough, only hues that cannot be distinguished by the human eye are present within the sphere. The corresponding radius can then represent the maximum permissible deviation in the sRGB color space. Such a determination of the maximum permissible deviation is also possible in principle in any other color space. Since color spaces can be transformed into one another, it is also possible to transfer an ellipse or a sphere given in one color space into another color space. For example, the ellipse can be transferred from the CIExy color space into another color space, such as the sRGB color space, wherein the simple geometric shape of an ellipse is then not necessarily retained. A further possibility for presetting a maximum permissible deviation is, in particular in the case of white color tones, presetting a maximum permissible Duv value. The Duv value indicates the position of the color tone with respect to a white point 96, 98, 100 on the Planckian curve trace 99 (see FIG. 11 ). A positive Duv value represents a shift in hue toward a yellow-greenish hue, a negative Duv value represents a shift toward a purple hue. The smaller the Duv value, the closer the hue is to the corresponding white point 96, 98, 100, and the more difficult it is to recognize the difference between the hue and the corresponding white point 96, 98, 100. If a small Duv value is set as the maximum allowable deviation, the difference between the hue and the white point is difficult to perceive, and if a Duv value small enough is set, the difference cannot be perceived at all.In a first embodiment of the light source arrangement, the latter can comprise a broadband light source emitting over the entire broad spectral range and, for at least one of the narrow spectral ranges, at least one spectral filter which can be introduced into the beam path by the control device. In addition or as an alternative to the at least one spectral filter that can be introduced into the beam path by the control device, the light source arrangement can comprise at least one light source that can be adjusted by the control device in its radiation power and emits in one of the at least two narrow spectral ranges. The light source arrangement can comprise at least two light sources whose radiation power can be adjusted by the control device, for example two, three, four or more light sources, wherein the light sources emit in each case in a different narrow spectral range. In some variants, the light source arrangement can comprise at least six and in particular at least twelve light sources whose radiation power can be adjusted, which emit in different narrow spectral ranges.As broadband light sources, for example, at least one incandescent lamp such as, for example, can be used. Halogen incandescent lamps or at least one luminescence emitter, for example. For example, gas discharge lamps, light-emitting diodes (LEDs), in particular white light LEDs, organic light-emitting diodes (OLEDs), etc. may be used. The broadband light source can also contain a plurality of light sources which only form the broadband light source in cooperation. The spectral filters can each have a band with a bandwidth of not more than 50 nm, in particular not more than 30 nm, further in particular not more than 20 nm, and the center points of the bands can be at least 35 nm, in particular at least 45 nm and further in particular at least 60 nm apart from one another in the spectrum. The band of the respective spectral filter can in this case either result in an increased or decreased transmission of the filter compared to the broad spectral range.The light sources whose radiation power can be adjusted can be present, as described, in addition to a broadband light source.Alternatively, the narrow spectral ranges of the light sources adjustable in their radiation power can, however, also collectively form at least a partial range of the broad spectral range or even the entire broad spectral range. In the latter case, no additional broadband light source is necessary. The narrow-band light sources can be LEDs, OLEDs, lasers, etc., which emit in different narrow spectral ranges. The width of the narrow spectral ranges is typically not greater than 75 nm, in particular not greater than 50 nm and further in particular not greater than 25 nm, in the case of LEDs and OLEDs. In lasers, it can be even smaller, for example not greater than 10 nm, in particular not greater than 5 nm, further in particular not greater than 3 nm and even further in particular not greater than 1 nm or even not greater than 0.5 nm.If two or more narrowband light sources are to be combined to form a broader spectral range, there are fundamentally several possibilities for carrying out this combination. For example, the narrowband light sources can be arranged spatially so close to one another that the individual spectral components cannot be perceived spatially resolved, as is the case, for example, with RGB displays. In this case, the narrowband light sources can be arranged in a linear arrangement, in particular when the broader spectral range is intended to comprise only a few narrowband spectral ranges and therefore only a few narrowband light sources are required. Alternatively, the narrow-band light sources can be arranged in a matrix-like arrangement in a two-dimensional arrangement. A matrix-like arrangement can, for example, repeat the linear arrangement of narrow-band light sources perpendicular to the direction of extension of the linear arrangement, wherein the arrangement of the light sources in the individual linear arrangement can also be permuted. In addition, a matrix-like arrangement offers the possibility of repeating an already two-dimensional basic arrangement on narrow-band light sources in one or two dimensions. Thus, for each narrow spectral range, as many narrow-band light sources as repetitions are present in the matrix-like arrangement. This can entail advantages with regard to a homogeneous illumination of the observation object.The combination of the narrowband spectral ranges by the spatial proximity of the narrowband light sources is advantageous in that no further optical elements are necessary for the combination, provided that the narrowband light sources are small enough, as is the case, for example, with LEDs, OLEDs, semiconductor lasers, etc. In the case of larger narrowband light sources, it is possible to arrange the narrowband light sources as primary light sources at a greater distance from one another and to couple their light into optical fibers, the outputs of which can then be situated close to one another and form secondary light sources.In addition, it is possible to combine narrowband light sources emitting in different narrow spectral ranges with one another via spectral filters. For example, interference filters can be equipped with one pass band or a plurality of pass bands in which they allow certain wavelengths to pass, whereas they reflect the remaining wavelengths. The introduction of such a filter into the beam path makes it possible to combine light which passes through the filter on account of its transmission properties with light which is reflected by the filter. In this way, it is possible to superimpose different wavelengths along the same optical path.It is likewise possible to provide light in different spectral ranges with mutually orthogonal polarizations and then, for example, to use a polarization beam splitter which allows light with the one polarization to pass and reflects light with the polarization orthogonal thereto. This makes it possible to combine, in particular, narrow spectral ranges which lie very close to one another and which would place very high requirements on the spectral filter in a combination via a spectral filter, to form a somewhat broader spectral range. Such a somewhat broader spectral range can then optionally also be combined with further spectral ranges by means of spectral filters, as has been described above. As an alternative to polarization beam splitters which pass light with the one polarization and reflect light with the polarization orthogonal thereto, there is also a crystalline form of polarization beam splitters in which birefringence is exploited. In such polarization beam splitters, orthogonal polarizations have different refractive indices. This results in different exit angles from such a polarization beam splitter and thus in different light paths after exiting the polarization beam splitter.If the light source arrangement comprises more than two spectral filters that can be introduced into the beam path or more than two light sources that can be adjusted in terms of their radiation power, the control device can be configured in a variant of the invention to control the light source arrangement in such a way that a change in the radiation power in at least one of the at least two narrow spectral ranges is compensated for by a change in the radiation power in at least one other of the at least two narrow spectral ranges in such a way that only hue changes are caused by the change in the radiation powers in the narrow spectral ranges in the radiation power arriving at the object, which changes do not exceed the predefined maximum permissible deviation from the specific point of the color space. In a simple example, the radiation powers can be increased or decreased simultaneously at least in a first narrow spectral range and in a second narrow spectral range. With a suitable distance between the first narrow spectral range and the second narrow spectral range, the resulting hue change can be less than if only one of the two narrow spectral ranges were increased or decreased, so that spectral changes that can be easily evaluated can be brought about for an evaluation unit, which in total do not exceed the predefined maximum permissible deviation from the specific point of the color space. If, for example, in the case of illumination with a white color tone, two narrow spectral ranges lying in the red spectral range, two narrow spectral ranges lying in the green spectral range and two narrow spectral ranges lying in the blue spectral range are present and one of the narrow spectral ranges lying in the red spectral range, one of the narrow spectral ranges lying in the green spectral range and one of the narrow spectral ranges lying in the blue spectral range is increased or decreased in its radiant power, it can be achieved that the predefined maximum permissible deviation from the white color tone will not exceed even in the case of a marked increase or decrease in the respective radiant powers and the original white color tone is thus retained within the perception limits of the human eye.In another example, in the case of illumination with a white hue, the light source arrangement can be controlled in such a way that the radiation power is increased at least in a narrow spectral range and at the same time the radiation power is decreased in at least a second narrow spectral range, wherein the centroid wavelengths of the two spectral subareas, the radiation power of which is increased and decreased, are close to one another. This can achieve the effect that the effect of increasing the radiation power in the one narrow spectral range on the hue is compensated by the reduction in the other narrow spectral range to such an extent that the associated hue change is below the perception threshold of the human eye, i.e. the predefined maximum permissible deviation from the white hue is not exceeded. Thus, for example, the radiation power can be increased in a first blue spectral range and reduced in a second blue spectral range without the predefined maximum permissible deviation from the white hue being exceeded. The same applies to changes in the radiation powers in the green or red spectral range.In the light source arrangement according to the invention, the control device can also be designed for controlling the light source arrangement such that, when the radiation power is changed in at least one of the at least two narrow spectral ranges, only brightness changes in the radiation power arriving at the object occur which amount to a maximum of 10%, in particular a maximum of 5%, further in particular a maximum of 3% and even further in particular a maximum of 1% or even a maximum of 0.1%. In this case, the accepted maximum change in brightness, for the accepted maximum change in hue, may already depend on the frequency at which the changes take place and / or on the application in which the light source is used (intraoperative generation of multi-spectral images, preoperative generation of multi-spectral images, postoperative generation of multi-spectral images, etc.). Any brightness change caused by the change of the radiation power in at least one of the at least two narrow spectral ranges can be effected by limiting the change of the radiation power in at least one spectral range or in that a brightness change caused by the change of the radiation power in at least one of the at least two narrow spectral ranges is compensated for by changing the radiation power of the light source arrangement outside the at least one spectral range. In particular, if a plurality of spectral ranges outside the at least one spectral range is used for compensating, the brightness fluctuations can be kept low. The previously described increase of the radiation power at least in a narrow spectral range with a simultaneous reduction of the radiation power in at least a second narrow spectral range can also be used to keep brightness fluctuations low.According to the invention, a device for generating multi-spectral images representing spectral properties of an object is also provided. This device includes at least one image sensor for capturing images of an object and an illumination device according to the invention. In addition, the device contains an evaluation unit which generates the multi-spectral images of the object on the basis of the changes in the radiation power reflected and / or scattered by the object and arriving at the image sensor resulting from the time-sequential change in the radiation powers arriving at the object. In this case, the use of the light source arrangement according to the invention enables recording of multi-spectral images during the work of a surgeon without thereby impairing the surgeon's perception of the object under observation. It is therefore not necessary to change between a mode for recording multi-spectral images and a mode for recording white light images. Since the changes in color tone during the recording of the multi-spectral images are not perceived as disturbing, no restriction of the exposure time is necessary for the recording of the multi-spectral images either. The evaluation unit can be designed in particular such that it can carry out the evaluation in real time, i.e. with only a delay of a few frames, for example with a delay of at most five frames, preferably of at most two and ideally of less than one frame. This can be used, for example, to superimpose the multi-spectral images or parts thereof on the white light images in real time.The image sensor used in the device may be a monochromatic image sensor. However, this is not a need. If radiation power is supplied to different regions of the image sensor via one of at least two different spectral channels, the evaluation unit is designed to evaluate the differences in the radiation powers supplied to the respective regions when determining the spectral properties of the object. Image sensors in which different spectral channels are assigned to different regions of the image sensor, as is the case, for example, with color sensors, can thus also be used for recording the multi-spectral images. In a color sensor, three different spectral channels, namely a red, a green and a blue color channel, would be present. Each of these channels would accordingly allow only the respective spectral component of the light reflected and / or scattered by the object to pass to the image sensor. Depending on the spectral distribution of the illumination light, the portions of the light that has arrived at the image sensor via the respective spectral channel change, so that the spectral distribution in the light reflected and / or scattered by the object can be deduced via the ratios of the portions in the respective spectral channels. In this case, the spectral channels of the image sensor generally do not correspond to the spectral bands of the light source arrangement.The evaluation unit of the device according to the invention can also be designed to generate at least one digital image with the predefined color tone, for example with a predefined white tone, during the time-sequential change of the radiation power arriving at the object. In this case, the evaluation unit is designed to further reduce, in the at least one digital image having the predefined hue, a hue deviation lying within the predefined maximum permissible deviation from the specific point of the color space by digital hue adaptation. In this way, a digital image with the predefined color tone, for example a white light image with a predefined white tone, can be generated without flickering in the color tone even if a relatively large maximum permissible deviation is predefined.Furthermore, according to the invention, a method for generating multi-spectral images representing spectral properties of an object is provided. The method comprises the steps of:illuminating an object with light having a broad spectral range and a hue corresponding to a particular point of a color space.time-sequential changing of the radiation power arriving at the object, wherein the time-sequential changing of the radiation power arriving at the object includes a change of the radiation power in at least one of at least two narrow spectral ranges. The narrow wavelength ranges can partially overlap in this case.capturing at least one image with the respective radiation power arriving at the object after each time-sequential change of the radiation power arriving at the object in the at least two narrow spectral ranges.generating a multi-spectral image from the recorded images, wherein the spectral properties of the object are determined from time-sequential changes in the radiation power reflected and / or scattered by the object, wherein the time-sequential changes in the radiation power reflected and / or scattered by the object are caused by the time-sequential change in the radiation power arriving at the object.The change of the radiation power in at least one of the at least two narrow spectral ranges is effected in such a way that the change of the radiation power in at least one of the at least two narrow spectral ranges in the radiation power arriving at the object causes only hue changes which do not exceed a predefined maximum permissible deviation from the specific point of the color space. For example, the maximum permissible deviation may be defined on the basis of the MacAd ellipse, as has been described with reference to the lighting device according to the invention, such that, for example, only hue changes are caused which lie within a MacAd ellipse, or that only hue changes are caused which lie within an ellipse whose half axes correspond to at most a predefined multiple of a MacAd ellipse in the CIExy color space, for example twice a MacAd ellipse, four times a MacAd ellipse or even six times a MacAd ellipse. However, other types of specification of a maximum permissible deviation than the specification based on a MacAd ellipse can also be used, as has been described with reference to the light source arrangement according to the invention.The method according to the invention enables recording of multi-spectral images during the work of a surgeon without thereby interfering with the surgeon's perception of the object under observation. It is therefore not necessary to change between a mode for recording multi-spectral images and a mode for recording white light images. Since the color tone changes are not perceived or are hardly perceived during the recording of the multi-spectral images, no restriction of the exposure time is necessary for recording the multi-spectral images either.In one configuration of the method, a change in the radiation power in at least one of the at least two narrow spectral ranges can be compensated for by a change in the radiation power in at least one other of the at least two narrow spectral ranges in such a way that the change in the radiation powers in the narrow spectral ranges in the radiation power arriving at the object causes only color hue changes which do not exceed the predefined maximum permissible deviation from the specific point in the color space. This has been explained with reference to the lighting device according to the invention and is therefore not repeated again at this point.In the method according to the invention, the recording of the at least one image can comprise recording image signals in at least two different spectral channels. When generating the multi-spectral images, differences between the image signals in the at least two different spectral channels are then evaluated. This embodiment of the method makes it possible to use image sensors in which different spectral channels are assigned to different regions of the image sensor, such as color sensors, for example, for recording the multi-spectral images. In a color sensor, for example, three different spectral channels would be present, namely a red, a green and a blue channel. Each of these channels would accordingly allow only the respective spectral component of the light reflected and / or scattered by the object to pass to the image sensor. Depending on the spectral distribution of the illumination light, the portions of the light that has arrived at the image sensor via the respective spectral channel change, so that the spectral distribution in the light reflected and / or scattered by the object can be deduced via the ratios of the portions in the respective spectral channels. In this case, the spectral channels of the image sensor generally do not correspond to the spectral bands of the light source arrangement.Within the scope of the method according to the invention, the object can be illuminated in such a way that, when the radiation power is changed in at least one of the at least two narrow spectral ranges, only brightness changes in the radiation power arriving at the object occur which amount to a maximum of 10%, in particular a maximum of 5%, further in particular a maximum of 3% and even further in particular a maximum of 1% or even a maximum of 0.1%, as has also already been described with reference to the light source arrangement according to the invention.If images in the predefined color tone, for example a predefined white tone, are generated for an observer not only purely optically, but also digitally, at least one digital image having the predefined color tone can be generated during the time-sequential change of the radiation power arriving at the object. In this digital image having the predetermined hue, a hue deviation within the predetermined maximum allowable deviation from the specific point of the color space can be further reduced by digital hue matching. In this way, a digital image with the predefined hue, for example a white light image with a predefined white tone, can be generated without flickering in the hue even if a relatively large maximum permissible deviation is predefined.Further features, properties and advantages will become apparent from the following description of exemplary embodiments of the invention with reference to the appended figures. FIG. 1 shows a surgical microscope having an illumination device which enables the generation of multi-spectral images representing spectral properties of an object with the surgical microscope. FIG. 2 shows a circuit diagram for connecting LEDs with narrow spectral ranges to white light illumination in the surgical microscope from FIG. 1. FIG. 3 shows an alternative embodiment of the illumination device. FIG. 4 shows a further alternative embodiment of the illumination device. FIG. 5 shows a filter wheel as can be used in the lighting device shown in FIG. 4. FIG. 6 shows a circuit diagram for changing the radiation power in narrow spectral ranges of the illumination light. FIG. 7 shows a further schematic diagram for changing the radiation power in narrow spectral ranges of the illumination light. FIG. 8 shows yet another schematic diagram for changing the radiation power in narrow spectral ranges of the illumination light. FIG. 9 shows the spectral channels of a color sensor. FIG. 10 shows typical transmission curves of red filters, green filters and blue filters, as are commonly used in a color sensor. FIG. 11 schematically shows the CIExy color space with MacAd ellipses drawn in and the Planckian curve.An exemplary embodiment of a device for generating multi-spectral images representing spectral properties of an object is described below with reference to FIG. 1. In the exemplary embodiment, the device is designed as a surgical microscope or integrated into a surgical microscope. The surgical microscope can be a digital surgical microscope in which intermediate images generated by an optical system are recorded by means of image sensors and displayed on a display, or an analog surgical microscope in which the intermediate images are optically viewed by means of eyepieces and to which at least one camera is also connected, to which a part of the beam path of the optical system is coupled out for recording an image by means of an image sensor, as is schematically shown in the exemplary embodiment shown in FIG. 1.In the device for generating multi-spectral images of the exemplary embodiment, a monochrome image sensor is used for recording the multi-spectral images. However, it is also possible to record the multi-spectral images using a color sensor, as will be explained later. Color sensors differ from monochrome image sensors in that they are preceded by a so-called Bayer matrix, in which a spectral filter is arranged before each pixel of the image sensor, which filter allows either red, green or blue light to pass. Each pixel is thus supplied only with information in the red color channel or in the green color channel or in the blue color channel. The color information of the other pixels is then interpolated from the available color information. For example, the color information of the green color channel for those pixels to which only red or only blue light is supplied on the basis of the surrounding pixels to which green light is supplied is interpolated on the basis of the surrounding pixels.The surgical microscope 1 of the exemplary embodiment comprises two stereoscopic partial beam paths, a main objective 3 which images a divergent beam bundle 7 originating from a focal plane F in the object 5 into two parallel partial beam bundles, one for each stereoscopic partial beam path of the surgical microscope 1. This magnification changer can be designed, for example, as a Galilei changer, in which different lens combinations can be introduced alternately into the beam path, each lens combination providing a different magnification factor. Alternatively, the magnification changer can be designed as a zoom system with which a continuous change of the magnification factor can be realized. The magnification changer can be a "large" magnification changer, through which both stereoscopic partial beam paths pass. Alternatively, it is possible that the magnification changer has its own Galilean changer or a separate zoom system for each stereoscopic partial beam path.In the present exemplary embodiment, beam splitters 9, 11 are also arranged in the parallel partial beam paths. Each of these beam splitters 9, 11 can be designed as a partially transparent mirror or as a prism. The first beam splitter 9 is arranged in the beam path such that the illumination beam path B emanating from an illumination device 13 is deflected in the direction of the observation object 5. This beam splitter can be a "large beam splitter" which is passed by both stereoscopic partial beam paths of the observation beam path B, as is the case in the exemplary embodiment illustrated in FIG. 1. However, a small beam splitter can also be present in each stereoscopic partial beam path, which is then passed in each case only by one of the two stereoscopic partial beam paths. This is the case in particular when the illumination is to take place parallel to the stereoscopic partial beam paths of the observation beam path A (so-called coaxial illumination). Alternatively, the illumination beam path B can also be directed between the two stereoscopic partial beam paths of the observation beam path A in the direction of the observation object 5. In this case, instead of a beam splitter, a reflecting element, for example a mirror or a reflecting prism, can be used. In addition, it is possible to guide the illumination through an edge region of the main objective 3 to the object ( 5) at an angle to the observation beam path A (so-called oblique illumination). In this case, too, instead of a beam splitter, a mirror or another reflective element without transmission properties can be used. The angle of oblique illumination can be, for example, between 2° and 10° and in particular approximately 6°.The second beam splitter 11 couples light out of the observation beam path in the direction of an image sensor 15, which is symbolized by a camera in the present exemplary embodiment. Typically, the light coupled out in the direction of the image sensor 15 is coupled out from one of the stereoscopic partial beam paths. If a stereoscopic image is to be recorded, two image sensors are present, to each of which light from a different one of the two stereoscopic partial beam paths is fed, so that a stereoscopic partial image can be recorded with each image sensor. In the present exemplary embodiment, the image sensor 15 is a monochrome image sensor.In addition, the surgical microscope 1 in the exemplary embodiment comprises a binocular tube. This comprises a tube objective for each stereoscopic partial beam path, with which the parallel stereoscopic partial beam paths are each focused on an intermediate image plane. The aerial images thus formed in the intermediate image plane can be viewed by means of eyepieces 19A, 19B. Alternatively, image sensors can also be arranged in the respective intermediate image plane, onto which the parallel stereoscopic partial beam paths are focused and which record the stereoscopic partial images for display on one or more monitors. These image sensors are typically color sensors. In this case, the image sensors can also be used to record the multi-spectral images, so that the second beam splitter 11 and the image sensor symbolized by the camera 15 in FIG. 1 can be dispensed with.If the recorded stereoscopic partial images are displayed on a single monitor, this can be effected in particular in such a way that the left and right stereoscopic partial images are displayed in a temporal alternation, which is viewed by a user, for example, by means of shutter glasses or polarization glasses. A further possibility for displaying the stereoscopic partial images consists in a head mounted display (HMD), in which each eye is presented with a stereoscopic partial image by means of a display assigned to the respective eye. In addition, it is possible to divide the stereoscopic partial beam paths into two beam paths by means of further beam splitters, one of which is supplied to an image sensor and one to a tube objective, in order to make it possible for purely visual observation and at the same time display to take place on a monitor or in an HMD.In the present exemplary embodiment, the lighting device 13 comprises a halogen incandescent lamp 21 as a white light source, which emits in a broad spectral range and has a specific white tone in a color space, for example in the CIExy color space. Instead of a halogen incandescent lamp 21, however, other broadband light sources can also be used, for example white light LEDs or gas discharge lamps such as xenon lamps. In addition, the broadband light source does not necessarily have to emit white light. The broadband light source can also emit chromatic light, that is to say light which has a specific chromatic hue in the color space, for example light having a green, blue or red hue. In addition, the broadband light source can also emit light in the infrared and / or ultraviolet spectral range.In addition to the halogen bulb 21, the lighting device 13 includes a number of LEDs 23A to 23D emitting in different narrow spectral ranges. The width of the narrow spectral ranges is typically not greater than 75 nm, in particular not greater than 50 nm and further in particular not greater than 25 nm. The halogen incandescent lamp 21 forms, together with the LEDs 23A- 23D, a light source arrangement which is designed to illuminate the observation object 5 with light in a ready spectral range, in the present exemplary embodiment with white light, and is also capable of illuminating the observation object 5 also in different narrow spectral ranges. By suitably controlling the radiation powers emitted by the respective LEDs, it is possible to independently change the radiation power arriving at the observation object 5 in the narrow spectral ranges.The surgical microscope 1 is also assigned a control device 25, which is shown in FIG. 1 as a computer, but which can also be integrated as a dedicated control device, for example, into the surgical microscope 1 or into the illumination apparatus 13. The control means 25 acts on the lighting means 13 to control the radiation powers emitted from the respective LEDs 23A-23D of the light source arrangement. The control of the radiation power emitted by an LED can be effected, for example, by means of pulse width modulation of the current which is supplied to the respective LED. The control of the radiation powers emitted by the respective LEDs 23A- 23D of the light source arrangement is effected in such a way that the radiation powers emitted by the LEDs 23A- 23D in the narrow spectral ranges are changed in a time-sequential manner. The control device 25 acts on the image sensor 15 in addition to acting on the light source arrangement in order to synchronize the recording of images with the time-sequential change of the radiation power emitted by the LEDs 23A-23D. The aim is to make it possible to time-allocate the images recorded by the image sensor 15 to the LED 23A- 23D which is active at the respective recording time. This aim can be achieved, for example, by associating each recorded image with an identifier which is characteristic of the respective active LED. Alternatively, it is possible to store the times at which the respective LEDs were active and the times at which the images were recorded, so that an assignment of the images to the LEDs is possible subsequently on the basis of the stored times. This requires that the switching of the LEDs 23A-23D on the one hand and the image recording on the other hand take place with the same time signal or that the time signals are synchronized if different time signals are used for the switching of the LEDs and the image recording.A timing diagram for turning on the LEDs 23A-23D for white light illumination is shown in FIG. 2. FIG. 2 shows the switching on of the individual LEDs 23A- 23D to the white light of the halogen incandescent lamp 21 for different points in time t 0 to t 3. In addition to the halogen incandescent lamp 21, the LEDs 23A- 23D are switched on at the different points in time t 0 to t 3. The respectively connected LEDs 23A- 23D lead to an increased radiation power in the corresponding narrow spectral range at the observation object 5, which in turn leads to an increased luminance in the recorded monochrome image. In this case, the luminance represents a measure of the brightness recorded by the pixels of the image sensor 15. The luminance does not contain any color information and is determined by the luminance which results at the location of the image sensor from the radiation power reflected and / or scattered by the observation object 5. For each image point of a monochrome image recorded at a time t0, t1, t2, t3, the luminance value of the image point is a superposition of a luminance component resulting from the broadband spectral range of the halogen incandescent lamp 21 and the luminance component resulting from the narrowband spectral range of the LED active at the recording time. By subtracting the luminance component due to the incandescent halogen lamp 21 from the luminance value of the respective image point as an image point, an evaluation unit 27 can therefore extract the luminance component of the luminance value due to the narrow-band spectral range of the active LED for each image point. As a result, monochrome images are obtained in which the luminance values of the individual pixels only follow the narrowband spectral range of the LED that is active at the recording time. These images can be combined by the evaluation unit 27 into a multi-spectral image, for example by means of a suitable false color representation. In order to determine the luminance component due to the halogen incandescent lamp 21, an image without LEDs 23A-23D connected in can be recorded at the beginning of the recordings or at regular intervals.In the present exemplary embodiment, the evaluation unit 27 is implemented as the control unit 25 by a computer, in particular by the same computer as the control unit 25. However, it does not need to be realized by the same computer as the control unit 25 and in particular also not at all as a computer. Rather, it can be a computer separate from the control unit 25 or a dedicated unit which can be integrated into the surgical microscope, for example.If the difference between the luminance component of the respectively active LED and the luminance component of the halogen incandescent lamp 21 does not exceed a predefined maximum permissible deviation, it can be achieved that the color impression does not change perceptibly for the user of the surgical microscope, i.e. that the hue changes by the white hue of the halogen incandescent lamp 21, for example only within the MacAd ellipse. In other words, in the present exemplary embodiment, the MacAd ellipse specifies a limit to which the radiation power in the narrow spectral range can be increased without the hue changing. This limit also depends on the width of the narrow spectral range. If this is very narrow, the limit can be higher than in the case of a spectral range which is not quite as narrow. Thus, an increase in the radiant power by a specific amount in a narrow spectral range, which corresponds to 10%, for example, of the blue spectral range, has less effect on the hue than an increase in the radiant power by the same amount if the narrow spectral range corresponds to 50% of the blue spectral range.Although FIG. 2 shows a circuit diagram in which only one LED is switched on in each case, it is also possible to switch on more than one LED 23A- 23F in each case. The respectively extracted luminance component is then representative of more than one narrow spectral range. By cyclically varying the combinations of the turned-on LEDs and forming the difference between the respectively extracted luminance components, the luminance components of the individual narrow spectral ranges can then be determined. By simultaneously turning on more than one LED, it is possible to make the turn-on in such a way that the changes in color tone caused by the turned-on LEDs compensate each other. If, for example, LEDs are simultaneously turned on in a narrow red spectral range, an LED in a narrow green spectral range and an LED in a narrow blue spectral range, the sum of the turned-on LEDs can lead to light with a white hue. Compared to the connection of only a single LED, this makes it possible to increase the radiation power of the connected LEDs without leaving the MacAd ellipse by the hue of the halogen incandescent lamp 21. Increasing the radiant power of the turned-on LEDs results in a greater difference between the luminance component caused by the respective LED in the monochrome image and the luminance component caused by the halogen bulb 21 in the monochrome image, thereby reducing the noise after the subtraction of the luminance component caused by the halogen bulb 21 in the monochrome image.If digital images are generated with the surgical microscope 1 by means of color sensors, which are presented on a display, it is possible to average over the images with the LEDs 23A- 23D switched on, provided that the switching frequency with which the LEDs are switched is so high that at least 24 averaged images per second, preferably at least 30 averaged images per second, can be achieved. If such averaging is carried out, the radiation line of the LEDs can be increased, so that a greater difference between the luminance component caused by the respective LED and the luminance component caused by the halogen incandescent lamp 21 can be generated in the image of the monochrome image sensor 15. A prerequisite is that the color tone in the case of an addition of the narrow spectral ranges of the individual LEDs 23A-23D is sufficiently close to the color tone of the halogen incandescent lamp 21. The closer the hue of the addition is to the hue of the halogen bulb 21, the larger the difference between the radiation power of the individual LEDs 23A-23D and the radiation power of the halogen bulb 21 can be without the hue in the averaged image of the color sensors being outside the MacAd ellipse.Although four LEDs 23A- 33 dare illustrated in the exemplary embodiment illustrated in FIG. 2, the number of LEDs may be greater than or less than four. For example, 12 or even more LEDs emitting in narrow spectral ranges are frequently used for recording multi-spectral images.In the previous exemplary embodiment, the illumination device 13 has included both a white light source 21 emitting in a broad spectral range and LEDs 23A to 23D. In alternative exemplary embodiments, however, it can also comprise exclusively LEDs or exclusively a white light source in conjunction with a number of spectral filters. An illumination device 13 comprising only LEDs will be described below with reference to FIG. 3, an illumination device 13 comprising only a white light source in combination with a number of spectral filters, with reference to FIGS. 4 and 5.FIG. 3 shows an exemplary embodiment of a lighting apparatus 13 in which the lighting apparatus 13 includes only LEDs 33A- 33F and a controller 25 for controlling the radiation powers of the LEDs 33A- 33F. The control device 25 can be, for example, the computer from the exemplary embodiment described above or can be integrated as a dedicated control device into the surgical microscope 1 or the illumination apparatus 13. The narrow spectral ranges of the LEDs 33a-33F and the radiation powers of the LEDs 33A-33F relative to each other are tuned so that the narrow spectral ranges add up to light with a predetermined hue when operated with the radiation powers tuned to each other. The hue may be a white or a dark hue. In other words, if all LEDs 33A- 33F are operated with the radiation powers matched to one another, the predetermined color tone and thus a predetermined color impression is achieved in the image viewed through the eyepieces of the surgical microscope 1 and / or in the image recorded with the color sensors of the surgical microscope 1.In order to record the multi-spectral images, the radiation powers of the LEDs 33A- 33F can be individually adjusted by the control device 25. In this case, the radiation power of an LED 23A- 23F can be increased or reduced compared to the radiation power matched to one another. For each pixel of the monochrome image recorded during the increase or decrease, the luminance value of the pixel is increased or decreased in comparison to the luminance value during an image recording with the radiation power matched to one another. From the difference between the luminance value obtained with the increased or reduced radiation power and the luminance value obtained with the radiation powers matched to one another, the luminance component due to the narrow spectral range of the LED whose radiation power has been increased or decreased can then be extracted. If the radiation power of an LED 23A- 23F is reduced compared to the radiation power matched to one another, the amount of the difference is determined as the luminance component which originates from the corresponding narrow spectral range. The increase or decrease of the radiation power of the respective LEDs 33A- 33F can be kept so small that the change in the color impression changes only within the MacAd ellipse around the point of the predefined hue. The limit for increasing or decreasing the radiation power of the respective LEDs 33A- 33F is predefined by the MacAd ellipse and, as already mentioned, also depends on the width of the narrow spectral range.However, there is also the possibility, already described with reference to the above exemplary embodiment, of summing up the images recorded with the color sensors of the surgical microscope 1, in order to thus mean the effect which entails increasing or decreasing the radiation power of the individual LEDs 33A-33F in the color images. As has already been described with reference to the first exemplary embodiment, this can make the increase or decrease in the radiation power of the individual LEDs greater, as a result of which the noise of the extracted luminance component is reduced.Although six LEDs 33A- 33F are illustrated in the exemplary embodiment illustrated in FIG. 3, the number of LEDs may be greater than or less than six. For example, 12 or even more LEDs emitting in narrow spectral ranges are frequently used for recording multi-spectral images. In addition, the LEDs need not be arranged in a linear arrangement, as shown in FIG. 3 merely for the sake of simplicity of illustration. Rather, they can be arranged in a matrix-like arrangement. In addition, it is possible to repeat these matrix-like arrangements, so that as many LEDs as repetitions of the matrix-like arrangement are present for each narrow spectral range. This can entail advantages with regard to a homogeneous illumination of the observation object 5. Furthermore, the possibilities mentioned at the beginning for combining the wavelength range of the LEDs by means of spectral filters or by means of polarization beam splitters can also be used. The latter may be particularly useful if the LEDs comprise laser diodes having very narrow spectral ranges and spectral ranges located very close to one another.In the exemplary embodiment of an illumination device 13 shown in FIGS. 4 and 5, only one halogen incandescent lamp 41 or another broadband light source such as a gas discharge lamp or a white light LED is present. In the present exemplary embodiment, a filter wheel 42 is arranged downstream of the halogen incandescent lamp 41 in the illumination beam path, which filter wheel can be rotated by means of a controllable or regulable motor 44. This filter wheel 42 has a number of spectral filters 43A-43F, each having a filter characteristic which, apart from a narrow spectral range, has a high transmission over the entire broad spectral range. The transmission is reduced only in the narrow spectral range. The maximum reduction of the transmission in the narrow spectral range is limited in this case in that, as a result of the reduction of the transmission in the narrow spectral range, the color tone of the illumination light is intended to change only within a MacAd ellipse, with the result that the color impression for the user of the surgical microscope 1 does not change. The center-of-gravity wavelengths of the narrow spectral ranges in which the transmission is reduced differ between the individual spectral filters 43A- 43F, so that by means of a rotation of the filter wheel, illumination with different reduced narrow spectral ranges takes place one after the other. As in the preceding exemplary embodiments, the luminance values of the pixels are reduced due to the reduced radiation power at the observation object 5 in the recorded monochrome image. The luminance component attributable to the narrow spectral range can therefore be extracted by forming the pixel-by-pixel difference amount between the luminance values with the radiation power reduced in the narrow spectral range and the luminance values without reducing the narrow spectral range. In order to obtain the luminance values without reducing the radiation power in a narrow spectral range, the filter wheel 42 may also include a position in which no filter is present. In this way, with each revolution of the filter wheel 42 a reference image is also recorded which represents the luminance values without reducing the radiation power in a narrow spectral range. The limit for reducing the radiation power by means of the spectral filters 43A-43F is predefined by the MacAd ellipse and also depends on the width of the narrow spectral range in which the reduction of the radiation power takes place.Although filters 43A-43F in which transmission is decreased in a narrow spectral range have been described with reference to FIGS. 4 and 5, it is also possible to use filters 43A-43F in which transmission is increased in a narrow spectral range. Conceivable in this case are, for example, filters which have a transmission rate of less than 100% over the entire broad spectrum, for example 70% or 50%, and only a transmission increased by 10%, 20% or 30% compared to the other regions in the narrow spectral range, for example a transmission increased by 10%. In this case, the introduction of a filter into the illumination beam path has the same effect as the switching on of an LED described with reference to FIGS. 1 and 2.Furthermore, it is possible to configure the filters 43A- 43F as narrow-band filters, which only allow light in a narrow spectral range to pass and block all other light of the ready spectral range. However, this presupposes that the narrow spectral ranges defined by the individual filters 43A-43F in total lead to the predefined color tone to be achieved, and that the introduction of the filters 43A-43F into the illumination beam path takes place so quickly that a user of the surgical microscope 1 only perceives the color tone resulting from the sum.FIG. 6 shows a time-related schematic diagram as can be used for reducing the radiant power of the LEDs 33A- 33F from FIG. 3 or for reducing the radiant power of the halogen incandescent lamp 1 by means of the spectral filters 43A- 43F from FIGS. 4 and 5. Nine narrow spectral ranges are shown in the figure (three in the blue spectral range B1-B3, three in the green spectral range G1-G3 and three in the red spectral range R1-R3), which are delimited from one another by vertical lines. At the time t0, the radiation power is reduced in a first narrow spectral range of the illumination light, at a time t1 in a second narrow spectral range, etc., until finally at a time t8, the radiation power is reduced in the ninth narrow spectral range. At each time t 0 to t 8, a monochrome image is recorded, from which the luminance component resulting from the respective narrow spectral range is extracted. A multi-spectral image is then generated from the individual monochrome images, wherein the spectral information from the time of the recording of the respective monochrome image results in the respective narrow spectral range from the extracted luminance component.FIG. 7 shows an alternative switching diagram for reducing the radiation power of the LEDs 33A- 33F from FIG. 3 or reducing the radiation power of the halogen incandescent lamp 1 by means of the spectral filters 43A- 43F from FIGS. 4 and 5. At a time t 1, the radiation powers in a blue narrow spectral range B 2, a green narrow spectral range G 2 and a red narrow spectral range R 2 are then reduced again, wherein, however, the respective narrow spectral ranges differ from the narrow spectral ranges reduced at the time t 0. Accordingly, at a time t 2, the radiation powers in each case are reduced in turn in a narrow blue spectral range B 3, a narrow green spectral range G 3 and a narrow red spectral range R 3, wherein the spectral ranges differ both from the spectral ranges reduced at the time t 0 and from the spectral ranges reduced at the time t 1. Thereafter, the cycle is repeated. In the next cycle, the reduced radiation powers in the narrow spectral bands can be permuted, so that, for example, at the time t 0 of the second cycle, the reduction of the radiation power takes place in the blue spectral range, as at the time t 0 of the first cycle, and the reduction of the radiation power takes place in the green spectral range, as at the time t 1 of the first cycle. After termination of a cycle or after termination of the cycles with all permutations, an image can also be recorded in a narrow spectral range without reducing the radiation power in order to determine the luminance values for the image points in the monochrome image caused by the wide spectral range. At each time of a cycle, a monochrome image is recorded, from which the luminance component due to the respective narrow spectral range is extracted. On the basis of the permutations, the individual luminance components can then be determined for each narrow spectral range.FIG. 8 is another circuit diagram for changing the radiation power of the LEDs 33A-33F of the figure. This switching diagram is similar to the switching diagram shown in FIG. 6 in that the radiant power is reduced in a first narrow blue spectral range B 1 at a time t 0. In contrast to the switching diagram shown in FIG. 6, however, the radiation power is increased in at least one further narrow blue spectral range. In the present exemplary embodiment, the increase is even effected in two further blue spectral ranges B 2, B 3, as a result of which the increase can be smaller. By increasing the radiation power in the two further narrow blue spectral range, the hue change caused by the reduction of the radiation power in the first narrow blue spectral range can be reduced. At the time t 1, the radiation power is then reduced in the next narrow blue spectral range B 2 and simultaneously increased in the two remaining blue spectral ranges B 1, B 3 or in at least one further blue spectral range. This scheme continues until the radiation intensity in the last narrow red spectral range R3 is reduced and is increased in at least one other red spectral range R1, R2.With the switching schemes illustrated in FIGS. 7 and 8, it is possible to compensate the radiation power in at least one of the narrow spectral ranges by changing the radiation power in at least one other of the narrow spectral ranges, so that even with relatively large changes in the radiation powers in the narrow spectral ranges, only color tone changes are caused in the radiation power arriving at the object, which do not exceed the predefined maximum permissible deviation, i.e. lie within the required ellipse, for example. Besides the switching schemes shown in FIGS. 7 and 8, a person skilled in the art can realize further switching schemes which cause only hue changes in the radiation power arriving at the object, which do not exceed the maximum permissible deviation. for example, the switching schemes shown in FIGS. 6 to 8 can be combined with one another. Thus, for example, simultaneously with a reduction in the radiation power in a narrow blue spectral range and an increase in the radiation power in at least one further narrow blue spectral range, a reduction in the radiation power in a narrow green spectral range and an increase in the radiation power in at least one further green spectral range and a reduction in the radiation power in a narrow red spectral range and an increase in the radiation power in at least one further red spectral range can take place.In the previous exemplary embodiments, monochromatic image sensors were used in each case for recording the data for the multi-spectral images. As already mentioned above, however, it is also possible to record the data for the multi-spectral images by means of color sensors. A color sensor differs from a monochromatic image sensor in that it has a Bayer matrix, which assigns a color filter for each image point of the sensor, so that only light in a color channel defined by the spectral filter assigned to it is supplied to the respective image point. An example of a typical Bayer array is shown in Figure 9. The Bayer matrix has three types of spectral filters 45, the first type essentially letting red light, the second type essentially green light and the third type essentially blue light pass to the respectively assigned image points. Hereinafter, the three kinds of spectral filters are therefore referred to as red filter 45R, green filter 45G, and blue filter 45B. Typical transmission curves R, G, B of the red filters 45R, the green filter 45G and the blue filter 45B are shown in FIG. 10, which shows a diagram in which the transmission T of the red filters, the green filters and the blue filters is plotted as a function of the wavelength λ. The diagram from FIG. 10 shows that the red filters 45R, the green filters 45G and the blue filters 45B substantially have a high transmission T for red, green and blue light, respectively, and that their transmission T is significantly lower in the respective remaining wavelength ranges.As a rule, the number of green filters 45G is twice as large as the number of red filters 45R and the number of blue filters 45B. The background is that the human eye has the highest sensitivity in the green spectral range. The increased number of green filters 45G therefore facilitates realizing a white balance naturally acting for the human eye.The signals detected by the individual pixels of the color sensor each represent the luminance component of that color channel which is assigned to the respective pixel by means of a spectral filter. That is, those pixels associated with the red filters 45R substantially detect the luminance component caused by the red spectral range, those pixels associated with the green filters 45G substantially detect the luminance component caused by the green spectral range, and those pixels associated with the blue filters 45B substantially detect the luminance component caused by the blue spectral range. A monochrome image sensor, on the other hand, would register with each image point the luminance caused by the entire spectral range, i.e. the sum of all luminance components. In order to achieve a corresponding luminance with the color sensor, the luminance components missing at the respective image points are supplemented. This addition is based on an interpolation of the luminance components of adjacent pixels. For example, for a pixel associated with a blue filter 45B, the luminance component caused by the green spectral region is interpolated from those luminance components detected by adjacent pixels associated with green filters 45G. Accordingly, the luminance component caused by the red spectral range is interpolated from those luminance components detected by adjacent pixels to which red filters 45R are associated. The interpolated luminance components are then added to the luminance component detected by the pixel with the blue filter 45B. In the same way, the non-detected luminance components are interpolated from adjacent pixels also for the pixels to which a red filter 45R or a green filter 45G is assigned and are added to the luminance component respectively detected by the pixels. In this way, for each pixel of the color sensor, a luminance is obtained which corresponds to the luminance of a monochrome image sensor. The described addition can be performed either by the evaluation unit 27 or by an electronic system assigned to the color sensor.The transmission curves of the red filters 45R, the green filters 45G and the blue filters 45B shown in FIG. 10 can be used for recording the data for multi-spectral images. In order to explain this, FIG. 10 also shows, by way of example, two narrow spectral ranges I and II, the centroid wavelengths of which lie in the cyan spectral range (narrow spectral range I) and in the yellow-orange spectral range (narrow spectral range II), respectively. As can be seen from FIG. 10, the blue filters 45B and the green filters 45G have an average transmission T of approximately the same magnitude in the narrow spectral range I. The transmission T of the red filters 45R, on the other hand, is extremely low in the narrow spectral range I. For the narrow spectral range II, on the other hand, the red filters 45R have a very high transmission T, the green filters 45G have a low transmission T and the blue filters 45B have an extremely low transmission T. Light in the narrow spectral range I therefore leads to a medium signal at those pixels to which the blue filters 45B and the green filters 45G are assigned, whereas to an extremely low signal at those pixels to which the red filters 45R are assigned. Light in the narrow spectral range II, on the other hand, leads to an extremely low signal in the case of those pixels to which the blue filters 45B are assigned, to a low signal in the case of those pixels to which the green filters 45G are assigned, and to a very high signal in the case of those pixels to which the red filters 45R are assigned. It can be seen from the example of the narrow spectral ranges I and II that a narrow spectral range can be characterized by the ratios of the signals between the pixels to which the respective color filters 45R, 45G, 45B are assigned, which ratios are brought about by it. From the ratios of the signals between the image points to which the respective color filters 45R, 45G, 45B are assigned, the evaluation unit 27 can therefore determine the position of the narrow spectral range in the spectrum in a reverse context.Since, when the multi-spectral images are recorded compared to the wide spectral range, there is only an increase or decrease in the radiation power in the respective narrow spectral range, the luminance component which originates from the corresponding narrow spectral range is determined before the described determination of the position of the narrow spectral range. The luminance component which originates from the corresponding narrow spectral range can be determined, as in the case of the monochromatic image sensor, from the difference between the luminance which results with only one illumination in the broad spectral range and the luminance which results with the increase or decrease of the illumination in the narrow spectral range. On the basis of the image containing only the determined luminance component, the evaluation unit 27 then determines the position of the narrow spectral range from the ratios of the signals which are brought about by the luminance component at the pixels to which the respective color filters 45R, 45G, 45B are assigned, as described above, and assigns the position of the narrow spectral range to the image representing the determined luminance component.After an image has been produced for each narrow spectral range, which image represents the luminance component attributable to it and the position of the respective narrow spectral range has been determined, the evaluation unit 27 can combine these images to form a multi-spectral image. Since in this procedure the position of the narrow spectral range in the spectrum is determined from the image data recorded with the color sensor, the synchronization does not need to guarantee an assignment of the images recorded by the color sensor to the narrow spectral range active at the respective recording time. Rather, it merely has to ensure that during the recording of an image with the color sensor, a change is not made from one narrow spectral range to another narrow spectral range. The narrow spectral range used during the recording is then determined by the evaluation unit 27 from the recorded image.Although a Bayer array with red, green and blue spectral filters is used in the exemplary embodiment described with reference to Figures 9 and 10, the array may include other types of spectral filters, such as cyan, magenta and yellow spectral filters. Also, the number of different types of spectral filters is neither limited to three, nor must the wavelength ranges which can be passed by the spectral filters lie exclusively in the visible spectral range. For example, the matrix can also have only two different types of spectral filters or else four or five different types of spectral filters. An upper limit for the number of different types of spectral filters is given by the resolution capacity with which the data in the respective colored spectral range is actually to be acquired. In addition, it is possible that the matrix comprises, instead of spectral filters which allow light in the visible spectral range to pass, or in addition to at least one type of spectral filter which allows light in the visible spectral range to pass, at least one type of spectral filter which allows light in the ultraviolet spectral range to pass, and / or at least one type of spectral filter which allows light in the infrared spectral range to pass. The described way of determining the position of the narrow spectral range can also be used when using a matrix modified in this way.In the described exemplary embodiments, only hue changes that lie within a MacAd ellipse in the CIExy color space have been described in each case. However, in the described exemplary embodiments, if the perception of a small color change can be accepted, a color change within an ellipse whose half axes correspond to a multiple of a MacAd ellipse in the CIExy color space, for example twice a MacAd ellipse, four times a MacAd ellipse or even six times a MacAd ellipse, can also be accepted. Since color spaces can be transformed into one another, it is also possible to transform an ellipse from the CIExy color space into another color space, such as, for example, into the sRGB color space, the simple geometric shape of an ellipse then not necessarily being retained. In addition, the predefined maximum permissible deviation need not be based on a MacAd ellipse. For example, a sphere having a predetermined radius may be laid in a color space around a point representing a color tone. If the radus is small enough, only hues that cannot be distinguished by the human eye are present within the sphere. The corresponding radius can then represent the maximum permissible deviation in the color space. As a further alternative, it is possible to specify an admissible Duv value which must not be exceeded.The present invention has been described in detail by way of exemplary embodiments for the purpose of explanation. However, a person skilled in the art will recognize from this description that he or she may deviate from the exemplary embodiments within the scope of the invention as defined in the appended claims. The invention is therefore intended to be limited only by the appended claims, but not by the exemplary embodiments.

Claims

Illumination device (13) for generating multi-spectral images, comprising - a light source arrangement (21, 23A-D, 33A-F) which is designed to illuminate an object (5) with illumination light which has a broad spectral range and a hue corresponding to a specific point of a color space and which makes it possible to change the radiation powers arriving at the object (5) independently of one another in at least two narrow spectral ranges (I, II); and - a control device (25) for controlling the light source arrangement (21, 23A-D, 33A-F) such that the radiation powers in the at least two narrow spectral ranges (I, II) are changed in a time-sequential manner, wherein the time-sequential changes each contain a change in the radiant power in at least one of the at least two narrow spectral ranges (I, II), characterized in that a maximum permissible deviation from the specific point is predefined in the control device (25) for the color space, and the control device (25) is configured to control the light source arrangement in such a way that the change in the radiant power in at least one of the at least two narrow spectral ranges (I, II) in the radiant power arriving at the object (5) causes only hue changes which do not exceed the predefined maximum permissible deviation from the specific point of the color space.Lighting device (13) according to claim 1, characterised in that the light source arrangement (21, 23A-D, 33A-F) comprises a broadband light source (21) emitting over the entire broad spectral range.Illumination device (13) according to Claim 2, characterized in that the light source arrangement (21, 23A-D, 33A-F) comprises, for at least one of the narrow spectral ranges (I, II), at least one spectral filter (43A-F) which can be introduced into the illumination beam path (B) by the control device (25).Lighting device (13) according to one of Claims 1 to 3, characterized in that the light source arrangement (21, 23A-D, 33A-F) comprises at least one light source (23A-D, (23A-D, 33A-F) which can be adjusted by the control device (25) in terms of its radiation power and emits in one of the at least two narrow spectral ranges (I, II).Lighting device (13) according to claim 4, characterised in that the light source arrangement (21, 23A-D, 33A-F) comprises at least two light sources (23A-D, (23A-D, 33A-F) which can be adjusted by the control device (25) in terms of their radiation power and each emit in a different narrow spectral range (I, II).Lighting device (13) according to Claim 5, characterized in that the narrow spectral ranges (I, II) of the at least two light sources (33A-F) whose radiation power can be adjusted jointly form at least one partial range of the broad spectral range.Lighting device (13) according to Claim 5 or Claim 6, characterized in that a change in the radiation power in at least one of the at least two narrow spectral ranges (I, II) is compensated for by a change in the radiation power in at least one other of the at least two narrow spectral ranges (I, II) in such a way that the change in the radiation powers in the narrow spectral ranges (I, II) in the radiation power arriving at the object (5) causes only hue changes which do not exceed the predefined maximum permissible deviation from the specific point in the color space.Lighting device (13) according to one of Claims 1 to 7, characterized in that the control device (25) is additionally designed for controlling the light source arrangement (21, 23A-D, 33A-F) such that, when the radiation power is changed in at least one of the at least two narrow spectral ranges (I, II), only brightness changes which amount to at most 10% in the radiation power arriving at the object (5) occur.Device for generating multi-spectral images representing spectral properties of an object, having - at least one image sensor (15) for recording images of an object (5), - an illumination device (13) according to one of the preceding claims, and - an evaluation unit (27) which generates the multi-spectral images of the object (5) on the basis of the change in the radiation power reflected and / or scattered by the object (5) and arriving at the image sensor (15) resulting from the change in the radiation powers arriving at the object (5).Device according to Claim 9, characterized in that radiation power is fed to different regions of the image sensor (15) in each case via one of at least two different spectral channels (45R, 45G, 45B), and the evaluation unit (27) is designed to evaluate the differences in the radiation powers fed to the respective regions when the multi-spectral images of the object are determined.Device according to Claim 9 or Claim 10, characterized in that the evaluation unit (27) is furthermore designed to generate at least one digital image with the predefined hue during the time-sequential change of the radiation power arriving at the object (5) and to further reduce a hue deviation lying within the predefined maximum permissible deviation from the specific point of the colour space in the at least one digital image with the predefined hue by means of a digital hue adaptation.Method for generating multi-spectral images representing spectral properties of an object (5), comprising the steps of: - illuminating an object (5) with light which has a broad spectral range and a hue corresponding to a specific point of a colour space; - time-sequentially changing the radiation power arriving at the object (5), wherein the time-sequentially changing of the radiation power arriving at the object (5) in each case contains a change of the radiation power in at least one of at least two narrow spectral ranges (I, II); and - recording at least one image with the respective radiation power arriving at the object (5) after each time-sequentially changing of the radiation power arriving at the object (5); generating a multi-spectral image from the recorded images, wherein the spectral properties of the object (5) are determined from time-sequential changes in the radiation power reflected and / or scattered by the object (5), wherein the time-sequential changes in the radiation power reflected and / or scattered by the object (5) are caused by the time-sequential change in the radiation power arriving at the object (5); characterized in that the change in the radiation power in at least one of the at least two narrow spectral ranges (I, II) is effected in such a way that the change in the radiation power in at least one of the at least two narrow spectral ranges (I, II) in the radiation power arriving at the object (5) causes only hue changes which do not exceed a predefined maximum permissible deviation from the determined point of the color space.Method according to Claim 12, characterized in that a change in the radiation power in at least one of the at least two narrow spectral ranges (I, II) is compensated for by a change in the radiation power in at least one other of the at least two narrow spectral ranges (I, II) in such a way that the change in the radiation powers in the narrow spectral ranges (I, II) in the radiation power arriving at the object (5) causes only hue changes which do not exceed a predefined maximum permissible deviation from the specific point in the color space.Method according to Claim 12 or Claim 13, characterized in that the recording of the at least one image comprises recording image signals in at least two different spectral channels (45R, 45G, 45B) and, when generating the multi-spectral images, differences between the image signals in the at least two different spectral channels (45R, 45G, 45B) are evaluated.Method according to one of Claims 12 to 14, characterized in that the object (5) is illuminated in such a way that, when the radiation power is changed in at least one of the at least two narrow spectral ranges (I, II), only brightness changes which amount to at most 10% in the radiation power arriving at the object (5) occur.Method according to one of Claims 12 to 15, characterized in that, in addition, during the time-sequential change of the radiation power arriving at the object (5), at least one digital image having the predefined hue is generated, and in that, in the digital image having the predefined hue, a hue deviation lying within the predefined maximum permissible deviation from the specific point in the color space is further reduced by digital hue adaptation.

Images