Filter set, system and method for observing protoporphyrin IX

Abstract

Filter set, including: an illumination light filter (84) and an observation light filter (91), wherein a transmission characteristic (105) of the illumination light filter (84) is a sum of a first partial characteristic (I) and a second partial characteristic (II), wherein the transmission of the first partial characteristic (I) within a first wavelength range (109) which lies below a cut-off wavelength (107) is greater than a first value (L1); wherein the transmission of the second partial characteristic (II) within a second wavelength range (111) which lies above the cut-off wavelength (107) is less than a second value (L2) and greater than a third value (L3), wherein a transmission characteristic (113) of the observation light filter (91) is a sum of a third partial characteristic (III) and a fourth partial characteristic (IV), wherein the transmission of the third partial characteristic (III) within a third wavelength range (115) which lies above the cut-off wavelength (107) is greater than the first value (L1); wherein the transmission of the fourth partial characteristic (IV) within a fourth wavelength range (117), which lies below the cut-off wavelength (107), is smaller than the second value (L2) and larger than the third value (L3), wherein the transmission of the illumination light filter (84) within a fifth wavelength range (118) which lies between the first wavelength range (109) and the second wavelength range (111) is less than a fourth value (L4); wherein the transmission of the observation light filter (91) within a sixth wavelength range (119) which lies between the third wavelength range (115) and the fourth wavelength range (117) is smaller than the fourth value (L4); where the fourth value (L4) is smaller than the third value (L3), the third value (L3) is smaller than the second value (L2) and the second value (L2) is smaller than the first value (L1), where the cutoff wavelength (107) is between 450 nm and 550 nm, wherein the first wavelength range (109) lies between 350 nm and the cutoff wavelength, wherein the second wavelength range (111) lies between the cutoff wavelength and 620 nm and is at least 50 nm wide, wherein the third wavelength range (115) lies between the cutoff wavelength and 800 nm, wherein the fourth wavelength range (117) lies between 410 nm and the cutoff wavelength.

Description

[0001] The present invention relates to a fluorescence observation system, a method for performing fluorescence observation and a filter set usable therefor.

[0002] Fluorescence observation is used in many areas of technology, biology, and medicine to clearly visualize different types of structures in an object. Conventionally, an illumination light filter is placed in a beam path between an illumination light source and the object to be observed. This filter essentially only allows light that can excite the fluorescence of a fluorescent dye to pass through. An observation light filter is then placed in the beam path of an observation optics. This filter allows fluorescent light to pass through and essentially blocks light that the illumination light filter allows to pass through.In an image that is perceived directly by the eye by looking into the observation optics or that is recorded by a camera via the observation optics, fluorescent structures of the object are perceptible as bright areas, while non-fluorescent structures of the object are dark, so that the structures contained therein are not perceptible.

[0003] It is also desirable that the non-fluorescent areas of the object be visible in the image in order to better capture the spatial position of the fluorescent structures relative to the non-fluorescent structures. With this in mind, DE 195 48 913 A1 proposes matching the transmission characteristics of the illumination light filter and the observation light filter such that they spectrally overlap in a wavelength range with a maximum width of 50 nm, so that the observation light filter allows a small portion of the light to pass through that is also allowed to pass through the illumination light filter. This makes both the fluorescent structures of the object that emit the fluorescent light visible, as well as non-fluorescent structures that reflect the light from the 50 nm wavelength range, visible via the observation light filter.

[0004] It has been shown that even with such a tuning of the illuminating light filter to the observation light filter, non-fluorescent areas of an object are insufficiently perceptible.

[0005] The document DE 10 2010 033 825 A1 discloses a filter system for the simultaneous observation of fluorescent and non-fluorescent areas of an object for various fluorescent dyes, including fluorescein. Accordingly, it is an object of the present invention to propose a filter set, a fluorescence observation system, and a method for conducting fluorescence observation that enable improved perceptibility of non-fluorescent areas of an object. First set of filters

[0006] According to embodiments of the invention, an illuminating light filter of a first filter set has a transmission characteristic as a function of the wavelength of the light, which is a sum of a first partial characteristic and a second partial characteristic. The transmission of the first partial characteristic is greater than a first value within a first wavelength range that lies below a cutoff wavelength. The transmission of the second partial characteristic is smaller than a second value and greater than a third value within a second wavelength range that lies above the cutoff wavelength.

[0007] The transmission characteristic of an observation light filter of the first filter set is the sum of a third partial characteristic and a fourth partial characteristic. The transmission of the third partial characteristic is greater than the first value within a third wavelength range, which lies above the cutoff wavelength. The transmission of the fourth partial characteristic is smaller than the second value and greater than the third value within a fourth wavelength range, which lies below the cutoff wavelength.

[0008] In addition, the transmission of the illumination light filter is less than a fourth value within a fifth wavelength range, which lies between the first wavelength range and the second wavelength range; and the transmission of the observation light filter is less than the fourth value within a sixth wavelength range, which lies between the third wavelength range and the fourth wavelength range.

[0009] Here, the fourth value is smaller than the third value, the third value is smaller than the second value and the second value is smaller than the first value.

[0010] The cutoff wavelength is a wavelength determined depending on the fluorescent dye used, which separates the wavelengths used to excite the fluorescent dye from the wavelengths used to detect it. For example, the cutoff wavelength is chosen such that it is greater than a maximum of the excitation spectrum of the fluorescent dye used and less than a maximum of the emission spectrum of the fluorescent dye used. However, deviations from this can also be made, and the cutoff wavelength can be chosen smaller than the maximum of the excitation spectrum or greater than the maximum of the emission spectrum, since excitation and emission spectra often overlap significantly in fluorescence processes.

[0011] The excitation spectrum of PPIX exhibits a maximum at approximately 405 nm. The emission spectrum of PPIX exhibits a maximum at approximately 635 nm and a secondary maximum at approximately 705 nm. Accordingly, the cutoff wavelength can be selected, for example, between 405 nm and 635 nm or between 405 nm and 705 nm.

[0012] The first sub-characteristic of the transmission characteristic of the illumination light filter serves to supply fluorescence excitation light to the object under investigation. Therefore, the first sub-characteristic has transmission values within the first wavelength range that are greater than the first value. This means that the transmission of the illumination light filter is greater than the first value across the entire first wavelength range. Accordingly, the first value represents a minimum value for the transmission within the first wavelength range. The first value should be as large as possible, for example, 50%, 70%, or 90%. A value of 1.0 can be the goal for optimization here, but in practice this value can only be approximately achieved.

[0013] The second sub-characteristic of the illumination light filter's transmission characteristic serves to supply a certain amount of light to the object. This light is not used to excite fluorescence, but rather to visualize non-fluorescent structures of the object. Since this light has wavelengths above the cutoff wavelength, it can pass through the observation light filter when reflected from the object and be perceived, allowing non-fluorescent structures of the object to be perceived via this light.

[0014] Since fluorescent light generated by a fluorescent object usually has a low intensity, it is desirable that the intensity with which non-fluorescent areas of the object can be perceived in a fluorescence image is not significantly greater than the intensity with which the fluorescent areas are perceived. Otherwise, the fluorescence might be difficult to perceive due to overexposure from the non-fluorescent areas. Therefore, the amount of light supplied to the object via the second partial characteristic is limited by the transmission of the illumination light filter being smaller than the second value in the second wavelength range and greater than the third value. This means that the transmission of the illumination light filter is smaller than the second value but greater than the third value across the entire second wavelength range.Accordingly, the second value represents a maximum value for the transmission within the second wavelength range; and the third value represents a minimum value for the transmission within the second wavelength range.

[0015] The second value is smaller than the first value, so that the maximum transmission in the second wavelength range is (significantly) smaller than the minimum transmission in the first wavelength range. The minimum transmission in the second wavelength range is, however, greater than the third value, which represents a significant transmission and not a transmission that is very low and is achieved in areas of the filter's transmission characteristic in which, if possible, no light should be transmitted. Such transmission values are achieved, for example, between the first wavelength range and the second wavelength range, where the transmission is smaller than the fourth value, which represents a transmission of the filter such that, if possible, no light should pass through the filter there. The second value is, for example, 10%, 5% or 1%.For example, the third value is 0.01%, 0.05%, or 0.1%. The fourth value is 0.005%, 0.001%, or 0.0001%.

[0016] Thus, in summary, the illumination light filter has at least the following properties: It is significantly penetrated by light from several separate wavelength ranges, wherein at least one of these several wavelength ranges is arranged below the cutoff wavelength and allows a large amount of light to pass through, while at least one other of the several wavelength ranges is arranged above the cutoff wavelength and allows light to pass through in a comparatively smaller but nevertheless significant amount.

[0017] The transmission characteristic of the observation light filter has the third and fourth partial characteristics. Above the cutoff wavelength, the third partial characteristic has the third wavelength range, within which the transmission has values that are greater than the first value. This means that the transmission of the observation light filter is greater than the first value throughout the entire third wavelength range. Accordingly, the first value represents a minimum value for the transmission within the third wavelength range. The third partial characteristic serves to allow the passage of both fluorescent light and light that is used to perceive non-fluorescent areas and that could reach the object due to the second partial characteristic of the illumination light filter.Due to the typically low intensities of fluorescent light, the transmission in the third wavelength range is as high as possible, i.e., values that are greater than the first value described above. This ensures that both fluorescent and non-fluorescent structures of the object are visible. This is achieved by directing a significant amount of light, which is not necessarily used to excite fluorescence, to the object and passing it through the observation light filter together with the fluorescent light.

[0018] The fourth partial characteristic has the fourth wavelength range below the cutoff wavelength, within which the transmission has values that are smaller than the second value and greater than the third value. This means that the transmission of the observation light filter is smaller than the second value and greater than the third value across the entire fourth wavelength range. Accordingly, the second value represents a maximum value for the transmission within the fourth wavelength range; and the third value represents a minimum value for the transmission within the fourth wavelength range. The fourth partial characteristic serves to allow light that is not fluorescent light and can therefore be used to perceive non-fluorescent objects to pass through the observation light filter.Since this light should not overwhelm the fluorescence, the amount of light that can pass through the observation light filter due to the fourth partial characteristic is limited by the fact that the maximum transmission in the fourth wavelength range is smaller than the second value, which is significantly smaller than the first value, which can represent transmissions optimized for maximum transmission. On the other hand, the amount transmitted by the observation light filter below the cutoff wavelength should be significant, which is why the transmission in the fourth wavelength range is greater than the third value, which is significantly greater than the fourth value, which describes transmissions that are intended to essentially block the passage of light.

[0019] With this design, light from at least two different wavelength ranges is available for observing non-fluorescent areas of the object: first, the light that is not fluorescent light and is supplied to the object due to the second partial characteristic of the illumination light filter and is transmitted through the observation light filter due to the third partial characteristic; and second, the light that is supplied to the object due to the first partial characteristic of the illumination light filter and transmitted through the observation light filter due to the fourth partial characteristic. Due to their spectral separation, these two wavelength ranges for observing non-fluorescent areas of the object have the advantage that the non-fluorescent areas do not appear merely monochrome.Rather, non-fluorescent areas can be perceived in several spectral ranges, which leads to improved detection of different structures in the non-fluorescent areas of the object compared to monochrome perception.

[0020] In this case, it is possible for the second partial characteristic of the illumination light filter and the fourth partial characteristic of the observation light filter to each allow the passage of light in several separate spectral ranges. These spectral ranges can be selected such that the light available for observing non-fluorescent areas originates from several spectral ranges, which can be selected such that this light mixes to form approximately white light. This allows a non-fluorescent white surface to be perceived as approximately white even through the system of illumination light filter and observation light filter.

[0021] The transmission of the illumination light filter is less than a fourth value within the fifth wavelength range, which lies between the first wavelength range and the second wavelength range. This means that the transmission of the illumination light filter is less than the fourth value throughout the entire fifth wavelength range. The fourth value therefore represents a maximum value for the transmission of the illumination light filter within the fifth wavelength range. The transmission of the observation light filter is also less than the fourth value within a sixth wavelength range, which lies between the third wavelength range and the fourth wavelength range. This means that the transmission of the observation light filter is less than the fourth value throughout the entire sixth wavelength range.The fourth value therefore represents a maximum value for the transmission of the observation light filter within the sixth wavelength range. This improves the white light impression achieved by the first to fourth sub-characteristics for non-fluorescent object areas.

[0022] For the fluorescent dye PPIX, the transmission characteristics explained above are chosen as follows: the cutoff wavelength is in the range from 450 nm to 550 nm, the first wavelength range is in the range from 350 nm to the cutoff wavelength, the second wavelength range is in the range from the cutoff wavelength to 680 nm, the third wavelength range is in the range from the cutoff wavelength to 800 nm and the fourth wavelength range is in the range from 410 nm to the cutoff wavelength.

[0023] In other words, the first wavelength range extends from a first wavelength to a second wavelength greater than the first wavelength, wherein the first wavelength is greater than or equal to 350 nm and the second wavelength is less than or equal to the cutoff wavelength.

[0024] In other words, the second wavelength range extends from a third wavelength to a fourth wavelength greater than the third wavelength, wherein the third wavelength is greater than or equal to the cutoff wavelength and the fourth wavelength is less than or equal to 680 nm.

[0025] In other words, the third wavelength range extends from a fifth wavelength to a sixth wavelength greater than the fifth wavelength, where the fifth wavelength is greater than or equal to the cutoff wavelength and the sixth wavelength is less than or equal to 800 nm.

[0026] In other words, the fourth wavelength range extends from a seventh wavelength to an eighth wavelength greater than the seventh wavelength, where the seventh wavelength is greater than or equal to 410 nm and the eighth wavelength is less than or equal to the cutoff wavelength.

[0027] In other words, the fifth wavelength range extends from a ninth wavelength to a tenth wavelength that is greater than the ninth wavelength, where the ninth wavelength is greater than the second wavelength and the tenth wavelength is less than the third wavelength.

[0028] In other words, the sixth wavelength range extends from an eleventh wavelength to a twelfth wavelength that is greater than the eleventh wavelength, the eleventh wavelength being greater than the eighth wavelength and the twelfth wavelength being less than the fifth wavelength.

[0029] According to one embodiment relating to the first filter set, the transmission of the second partial characteristic of the illumination light filter is less than the fourth value within an eighth wavelength range, which lies outside the second wavelength range but within the third wavelength range. Within the eighth wavelength range, the transmission of the third partial characteristic of the observation light filter is greater than the first value. The very low transmission within the eighth wavelength range of the second partial characteristic blocks light that would outshine fluorescent light from PPIX if it were directed onto and reflected by the object. The high transmission in the observation light filter enables the observation of the fluorescent light from PPIX.In addition, the low transmission in the eighth wavelength range of the illumination light filter is also relevant for achieving color-true observation of non-fluorescent object areas, as it limits the second wavelength range and the associated significant transmission of the illumination light filter in the second wavelength range. Second set of filters

[0030] According to embodiments of the invention, a second filter set, which achieves the same effect as the first filter set, comprises an illumination light filter and an observation light filter. The transmission characteristic of the illumination light filter is a sum of a first partial characteristic and a second partial characteristic. The first partial characteristic of the illumination light filter of the second filter set corresponds to the first partial characteristic of the illumination light filter of the first filter set. Accordingly, the transmission of the first partial characteristic is greater than a first value within a first wavelength range that lies below a cutoff wavelength.

[0031] The second partial characteristic of the illumination light filter of the second filter set differs from the second partial characteristic of the illumination light filter of the first filter set essentially only in that the transmission of the second partial characteristic is greater than the first value within a second wavelength range which lies above the cut-off wavelength.

[0032] The transmission characteristic of the observation light filter of the second filter set is a sum of a third partial characteristic and a fourth partial characteristic. The third partial characteristic of the illumination light filter of the second filter set differs significantly from the third partial characteristic of the illumination light filter of the first filter set. The transmission of the third partial characteristic of the observation light filter of the second filter set is smaller than a second value and greater than a third value within a third wavelength range that lies above the cutoff wavelength. Furthermore, the transmission of the third partial characteristic is greater than the first value within a ninth wavelength range that lies above the third wavelength range.

[0033] The fourth partial characteristic of the illumination light filter of the second filter set corresponds to the fourth partial characteristic of the illumination light filter of the first filter set. Accordingly, the transmission of the fourth partial characteristic is smaller than the second value and larger than the third value within a fourth wavelength range, which lies below the cutoff wavelength.

[0034] As with the first filter set, the transmission of the illumination light filter of the second filter set is also smaller than a fourth value within the fifth wavelength range, which lies between the first wavelength range and the second wavelength range.

[0035] As with the first filter set, the transmission of the observation light filter within the sixth wavelength range, which lies between the third wavelength range and the fourth wavelength range, is smaller than the fourth value.

[0036] As with the first set of filters, the fourth value is less than the third value, the third value is less than the second value, and the second value is less than the first value.

[0037] The first and second filter sets therefore differ in that the transmission of the illumination light filter of the first filter set within the second wavelength range is smaller than the second value but larger than the third value, while the transmission of the illumination light filter of the second filter set within the second wavelength range is larger than the first value.

[0038] This difference is compensated for in the observation light filter by ensuring that the transmission of the observation light filter of the first filter set is greater than the first value within the third wavelength range, while the transmission of the observation light filter of the second filter set is one value lower than the second value but greater than the third value within the third wavelength range. In both cases, the second and third wavelength ranges overlap spectrally. For the observation of non-fluorescent object areas, essentially the same effect is achieved with both filter sets.

[0039] With both filter sets, PPIX can be excited efficiently and the resulting fluorescence can be well observed, since the respective illumination light filters have a low transmission (less than the fourth value) in the emission range of PPIX and the respective observation light filters have a high transmission (greater than the first value) in the emission range of PPIX.

[0040] According to one embodiment of the second filter set, the cutoff wavelength is between 450 nm and 550 nm. Additionally or alternatively, the first wavelength range can be between 350 nm and the cutoff wavelength. Additionally or alternatively, the second wavelength range can be between the cutoff wavelength and 680 nm. Additionally or alternatively, the third wavelength range can be between the cutoff wavelength and 680 nm. Additionally or alternatively, the ninth wavelength range can be between the third wavelength range and 800 nm. Additionally or alternatively, the fourth wavelength range can be between 410 nm and the cutoff wavelength.

[0041] According to one embodiment of the second filter set, the transmission of the second partial characteristic within an eighth wavelength range, which lies outside the second wavelength range but within the ninth wavelength range, is smaller than the fourth value. The very low transmission within the eighth wavelength range of the second partial characteristic blocks light that would outshine fluorescent light from PPIX if it were directed at and reflected by the object. The high transmission in the observation light filter in the ninth wavelength range, which encompasses the eighth wavelength range, enables the observation of the fluorescent light from PPIX.In addition, the low transmission in the eighth wavelength range of the illumination light filter is also relevant for achieving color-true observation of non-fluorescent object areas, as it limits the second wavelength range and the associated significant transmission of the illumination light filter in the second wavelength range.

[0042] The embodiments described below relate to both the first filter set and the second filter set and are not explained individually for the sake of compact disclosure.

[0043] According to one embodiment, the transmission of the first partial characteristic of the illumination light filter is greater than the first value within a seventh wavelength range, which lies within the first wavelength range but outside the fourth wavelength range. Within the seventh wavelength range, the transmission of the fourth partial characteristic of the observation light filter is smaller than the fourth value. The high transmission in the illumination light filter within the seventh wavelength range enables the excitation of the fluorescence of PPIX with excitation light. The very low transmission in the observation light filter within the seventh wavelength range essentially blocks this excitation light, so that the light passing through the observation light filter in the third and fourth wavelength ranges is not outshone by the excitation light.In addition, the low transmission in the seventh wavelength range of the observation light filter is also relevant for achieving color-true observation of non-fluorescent object areas, as it limits the fourth wavelength range and the associated significant transmission of the observation light filter in the fourth wavelength range.

[0044] As an alternative to the above-mentioned numerical values for the transmission characteristics for the fluorescent dye PPIX, the following values can be used: The cutoff wavelength is between 480 nm and 520 nm. The first wavelength range is between 380 nm and 480 nm, in particular between 390 nm and 450 nm. The second wavelength range is between the cutoff wavelength and 620 nm, in particular between 520 nm and 620 nm. The third wavelength range is between 520 nm and 760 nm. The fourth wavelength range is between 410 nm and 450 nm, in particular between 410 nm and 435 nm. In other words, the first wavelength can be greater than or equal to 380 nm or 390 nm; the second wavelength can be less than or equal to 480 nm or 450 nm; the third wavelength can be greater than or equal to the cutoff wavelength.the third wavelength may be greater than or equal to 520 nm; the fourth wavelength may be less than or equal to 620 nm; the fifth wavelength may be greater than or equal to 520 nm; the sixth wavelength may be less than or equal to 760 nm; the seventh wavelength may be greater than or equal to 410 nm; the eighth wavelength may be less than or equal to 450 nm or 435 nm. The ninth wavelength range is between 610 nm and 750 nm, in particular between 660 nm and 750 nm.

[0045] According to exemplary embodiments, the first wavelength range is at least 40 nm or at least 60 nm wide; and / or the second wavelength range is at least 50 nm or at least 70 nm or at least 90 nm wide; and / or the third wavelength range is at least 50 nm wide or at least 75 nm or at least 100 nm or at least 150 nm or at least 200 nm or at least 230 nm wide; and / or the fourth wavelength range is at least 10 nm or at least 20 nm or at least 40 nm wide. The specified width denotes the difference between the second and first wavelength for the first wavelength range, the difference between the fourth and third wavelength for the second wavelength range, the difference between the sixth and fifth wavelength for the third wavelength range, and the difference between the eighth and seventh wavelength for the fourth wavelength range.

[0046] The first and fourth wavelength ranges overlap each other. In particular, the first wavelength range can encompass the fourth wavelength range. Accordingly, the fourth wavelength range can lie entirely within the first wavelength range. The overlapping or entirely encompassing ensures that a wavelength range, namely the intersection of the first and fourth wavelength ranges, exists within which light is significantly transmitted through the illumination light filter and the observation light filter. Thus, non-fluorescent object regions can be made visible.

[0047] Furthermore, the second wavelength range and the third wavelength range overlap one another. In particular, the third wavelength range can encompass the second wavelength range. Accordingly, the second wavelength range can lie entirely within the third wavelength range. The overlapping or entirely encompassing ensures that a wavelength range, namely the intersection of the second and third wavelength ranges, exists within which light is significantly transmitted through the illumination light filter and the observation light filter. Thus, non-fluorescent object regions can be made visible. The at least two spectrally separate intersections ensure that non-fluorescent object regions can be observed with essentially true colors, i.e. similar to white light.

[0048] According to exemplary embodiments, the first value is 50%, 70%, or 90%; and / or the second value is 10%, 5%, or 1%; and / or the third value is 0.01%, 0.05%, or 0.1%; and / or the fourth value is 0.005%, 0.001%, or 0.0001%.

[0049] According to exemplary embodiments, the illumination light filter and the observation light filter are configured such that a ratio of a first mean value to a second mean value is between 0.5 and 1.5, in particular between 0.8 and 1.2. The first mean value is defined as the product of the transmission of the illumination light filter with the transmission of the observation light filter and with a wavelength-dependent factor G (λ), averaged over the intersection of the first wavelength range with the fourth wavelength range and normalized to this intersection. The second mean value is defined as the product of the transmission of the illumination light filter with the transmission of the observation light filter and with the wavelength-dependent factor G (λ), averaged over the intersection of the second wavelength range with the third wavelength range and normalized to this intersection.This ensures that transmission within the wavelength ranges provided for visualizing non-fluorescent object areas in the illumination light filter and the observation light filter is selected such that the non-fluorescent object areas can be observed essentially with true color.

[0050] For example, the first mean M1 is defined as: M1=1|S1−S2|∫S1S2TI(λ)⋅TO(λ)⋅G(λ)dλ, where S1 is the smallest value of the intersection of the first and fourth wavelength ranges, where S2 is the largest value of the intersection of the first and fourth wavelength ranges, where T I (λ) is the wavelength-dependent transmission of the illumination light filter, where T O (λ) is the wavelength-dependent transmission of the observation light filter and where |·| represents the absolute value.

[0051] The second mean M2 is defined as: M2=1|S3−S4|∫S3S4TI(λ)⋅TO(λ)⋅G(λ)dλ, where S3 is the smallest value of the intersection of the second and third wavelength ranges, where S4 is the largest value of the intersection of the second and third wavelength ranges, where T I (λ) is the wavelength-dependent transmission of the illumination light filter, where T O (λ) is the wavelength-dependent transmission of the observation light filter and where |·| represents the absolute value.

[0052] Accordingly, the above condition can be formulated as: 0.5≤M1M2≤1.5 or 0.8≤M1M2≤1.2

[0053] With the wavelength-dependent factor G(λ) other relevant factors can be taken into account, whereby in the simplest case the factor can also be equal to 1.

[0054] The factor G(λ) can, for example, be the relative spectral luminosity of the eye V(λ), defined according to CIE (Commission Internationale de l'Eclairage) 018.2-1983. This takes the brightness perception of the human eye into account when spectrally configuring the light source for observing non-fluorescent object areas, which improves color fidelity when observing these areas.

[0055] The factor G(λ) can also or alternatively take into account the power spectrum of a light source L(λ) used for fluorescence observation. Accordingly, G(λ) = V(λ) or G(λ) = L(λ) · V(λ) can apply. A xenon lamp or other broadband light source can be used as a light source, for example.

[0056] According to exemplary embodiments, the fifth wavelength range, which lies between the first and second wavelength ranges and in which the transmission of the illumination light filter is less than the fourth value, comprises the cutoff wavelength. The fifth wavelength range can be at least 20 nm, at least 40 nm, or at least 60 nm wide. The specified width refers to the difference between the tenth and ninth wavelengths.

[0057] According to exemplary embodiments, the sixth wavelength range, which lies between the third and fourth wavelength ranges and in which the transmission of the observation light filter is less than the fourth value, comprises the cutoff wavelength. The sixth wavelength range can be at least 20 nm, at least 40 nm, or at least 60 nm wide. The specified width refers to the difference between the twelfth and eleventh wavelengths.

[0058] Another aspect of the invention relates to a fluorescence observation system for the simultaneous observation of protoporphyrin IX and a white-light-like image of an object. The fluorescence observation system comprises a light source for illuminating the object, observation optics for imaging the object, and a filter set as described herein. The illumination filter of the filter set is arranged in an illumination beam path between the light source and the object, and the observation light filter is arranged in a beam path of the observation optics.

[0059] Another aspect of the invention relates to a method for simultaneously observing protoporphyrin IX and a white-light-like image of an object. The method utilizes a filter set as described herein. The method comprises filtering an illuminating light beam directed onto the protoporphyrin IX-enriched object with the illumination filter of the filter set, and filtering the light emanating from the object with the observation filter of the filter set.

[0060] Embodiments of the invention are explained in more detail below with reference to figures. Fig. 1 shows a fluorescence observation system according to an embodiment of the invention. Fig. 2A to 2D show graphs for explaining a first filter set for fluorescence observation according to an embodiment of the invention. Fig. 3A to 3D show graphs for explaining a second filter set for fluorescence observation according to an embodiment of the invention.

[0061] An embodiment of a fluorescence observation system is explained below using a surgical microscope. However, embodiments of the fluorescence observation system are not limited to such surgical microscopes, but rather encompass any fluorescence observation system in which illumination light directed onto an object is filtered with an illumination light filter and light emanating from the object is filtered with an observation light filter.

[0062] With reference to Fig. 1, the fluorescence observation system or microscope 1 comprises a microscopy optics 3 with an objective 5 with an optical axis 7. An object 9 to be examined is arranged in an object plane of the objective 5. Light emanating from the object 9 is converted by the objective 5 into an image-side beam 11, in which two zoom systems 12, 13 arranged at a distance from the optical axis 7 are arranged and each pick out a partial beam 14 or 15 from the beam 11 and Fig. 1, deflecting prisms (not shown) are fed to eyepieces 16 and 17, into which an observer looks with his left eye 18 or his right eye 19, respectively, in order to perceive an enlarged representation of the object 9 as an image. Alternatively, cameras can be provided instead, which generate images of the partial beams 14, 15 via camera optics.

[0063] A partially transparent mirror 21 can be arranged in the partial beam 15 to couple out a portion of the light as beam 23, which is fed to a camera system 24. The camera system 24 can comprise one or more cameras. In the illustrated embodiment, the camera system 24 comprises a camera 32, to which light from beam 23, which passes through a partially transparent mirror 25, is fed via a camera adapter optics 31, and a camera 55, to which light from beam 23, which is reflected by the partially transparent mirror 25, is fed via a filter 57 and a camera adapter optics 53. The filter 57 can be a fluorescent light filter, which only allows fluorescent light from a fluorescent dye contained in the object 9 to pass through. Thus, the camera 32 can detect a normal light image of the object 9, while the camera 55 can detect a fluorescent light image of the object 9.Images from cameras 32 and 55 are transmitted via data connections 33 and 65, respectively, to a controller 35 and can be stored in a memory 95.

[0064] In a similar way, a partially transparent mirror 37 can be arranged in the other partial beam 14, via which a partial beam 39 is coupled out, which is fed via a camera adapter optics 41 to a camera 43, which can also detect a normal light image, the detected images of which are transmitted via a data connection 45 to the controller 35.

[0065] A display 69 is connected to the controller 35 via a data connection 67. The displayed image is coupled into the beam path to the eyepiece 17 via projection optics 70 and a further partially transparent mirror 68 arranged in the partial beam 15, so that the observer can directly perceive both the image displayed on the display 69 and the image of the object with their eye 19. Thus, the controller 35 can, for example, display data into the eyepiece 17 or images of the object, which can be detected by the cameras 32, 55, and 43 or generated by analyzing the detected images.

[0066] The images detected by the cameras can also be output by the controller 35 to a head-mounted viewing device 49, which is also referred to as a “head-mounted display,” for which purpose the device 49 comprises two displays 51, 52 for the right and left eyes of the viewer, respectively.

[0067] The microscope 1 further comprises an illumination system 63 for generating an illuminating light beam 81 directed at the object 9. For this purpose, the illumination system 63 comprises a broadband light source, such as a halogen lamp or a xenon lamp 71, a reflector 72, and a collimator 73 to generate a collimated light beam 74, which can be directed by means of one or more lenses 75 onto an entrance end 76 of a fiber optic bundle 77 in order to couple light emitted by the lamp 71 into the fiber optic bundle 77. The light is transported by the fiber optic bundle 77 into the vicinity of the object 9, exits there at an exit end 78 of the fiber optic bundle 77, and is then collimated by collimating optics 79 to form the illuminating light beam 81 directed at the object 9.

[0068] The illumination system 63 further comprises a filter plate 83, which has an illuminating light filter 84 for fluorescence observation and an illuminating light filter 85 for normal light observation. A drive 87, controlled by the controller 35, is provided to selectively arrange the illuminating light filter 84 for fluorescence observation and the illuminating light filter 85 for normal light observation in the beam 74, as indicated by the arrow 88. The illuminating light filter 84 for fluorescence observation is arranged in the beam 74 when fluorescence is to be excited and observed in the object 9, while the illuminating light filter 85 for normal light observation is arranged in the beam 74 when the object 9 is to be observed under exposure to normal light, such as white light.The illumination light filter 85 can, for example, be designed such that it does not allow infrared light generated by the lamp 71 or long-wave light close to the infrared light to pass through in order to avoid unnecessary heating of the object 9, and allows light of shorter wavelengths to pass through.

[0069] The optional arrangement of the two illuminating light filters 84 and 85 in the beam 74 can be controlled by the viewer via an input device connected to the controller 35, such as a button 97.

[0070] An observation light filter 91 for fluorescence observation is arranged in the beam path of the partial beams 14 and 15, respectively, wherein a drive 93, also controlled by the controller 35, is provided to remove the observation light filters 91 from the partial beams 14 and 15, respectively, as indicated by the arrow 94.

[0071] The observation light filters 91 are then arranged in the beam paths 14, 15 when the illumination light filter 84 for fluorescence observation is arranged in the beam 74, and they are removed from the beam paths 14, 15 when the observation light filter 85 for normal light observation is arranged in the beam 74. For this purpose, the drive 93 can be controlled together with the drive 87 by the controller 35 after the viewer has actuated the input device 97.

[0072] In the example shown, the illumination light filter 84 for fluorescence observation and the observation light filters 91 for fluorescence image observation are inserted into and removed from the beam paths by drives under the control of a controller. However, it is also possible for the filters to be provided in filter holders that are operated directly by hand by the observer to insert them into or remove them from the beam paths.

[0073] The illumination light filter and the observation light filter for fluorescence observation each have transmission characteristics that are matched to a fluorescent dye whose fluorescence is to be investigated. Properties of a first filter set for protoporphyrin IX are described below with reference to the Fig. 2A to 2D. Properties of a second set of filters for protoporphyrin IX are described below with reference to the Fig. 3A to 3D.

[0074] Fig. Figure 2A shows a graph A representing the excitation spectrum of PPIX and a graph E representing the emission spectrum of PPIX, each in a normalized representation. The maximum of the excitation spectrum A is at approximately 405 nm, and the maximum of the emission spectrum E is at approximately 635 nm. The emission spectrum E has a secondary maximum at approximately 705 nm.

[0075] Fig. Figure 2B shows a graph of a transmission characteristic 105 of the illumination light filter in a logarithmic scale of the ordinate in a wavelength range from about 350 nm to about 850 nm. This includes the visible wavelength range, which is relevant for considering the properties of the filter set. In Fig. 2B shows a cutoff wavelength 107, which in the example shown is chosen to lie between the maximum of the excitation spectrum A and the maximum of the emission spectrum E. In the present example, the cutoff wavelength is approximately 480 nm.

[0076] The transmission characteristic 105 of the illumination light filter is the sum of two partial characteristics I and II, which are defined in the entire range from 350 nm to 850 nm. At wavelengths below the cutoff wavelength 107, the partial characteristic I has a first wavelength range 109, within which the transmission is greater than a first value L1. This means that the transmission of the illumination light filter is greater than the first value L1 throughout the entire first wavelength range 109. The value L1 therefore represents a minimum value for the transmission of the illumination light filter in the first wavelength range 109. The particle characteristic I is intended to allow fluorescence excitation light to pass through the illumination light filter in order to excite the fluorescence of PPIX. Therefore, the first wavelength range 109 is selected such that the excitation spectrum A of PPIX has significant values within this wavelength range.The excitation should be as effective as possible, which is why the filter's transmission is selected to be as high as possible in the wavelength range 109. The value L1 represents a transmission value that is achieved, for example, when the filter is optimized for the highest possible transmission in the respective wavelength range. In this example, the first value L1 is approximately 50%.

[0077] At wavelengths above the cutoff wavelength 107, the partial characteristic II has a second wavelength range 111, within which the transmission is less than a second value L2 and greater than a third value L3. This means that the transmission of the illumination light filter is less than the second value L2 and greater than the third value L3 throughout the entire second wavelength range 111. Accordingly, the second value L2 is a maximum value for the transmission of the illumination light filter in the second wavelength range 111; and the third value L3 is a minimum value for the transmission of the illumination light filter in the second wavelength range 111. The purpose of the partial characteristic II is to allow light to pass through the illumination light filter, which light does not serve to excite the fluorescence, but rather to illuminate the object in such a way that its non-fluorescent regions become perceptible.The second wavelength range 111 is therefore selected such that it lies outside the wavelength ranges in which the emission spectrum E has significant values. Since the non-fluorescent regions should not outshine the fluorescent regions in perception and the fluorescence usually has a low intensity, the illumination light is passed through the illumination light filter with comparatively low intensity due to partial characteristic II. Therefore, the value L2 is smaller than the value L1. In the example shown, the second value L2 is approximately 5%. However, the transmission provided by partial characteristic II is greater than the third value L3, which in turn is significantly greater than a fourth value L4. In the present example, the third value is approximately 0.05% and the fourth value L4 is approximately 0.005%.

[0078] A transmission characteristic 113 of the observation light filter is in Fig. 2C. The transmission characteristic 113 is in turn the sum of two partial characteristics III and IV. At wavelengths above the cutoff wavelength 107, the partial characteristic III has a third wavelength range 115, within which the transmission is greater than the first value L1. The first value L1 is thus a minimum value for the transmission of the observation light filter within the third wavelength range 115. The partial characteristic III has the task of allowing both fluorescent light (between approximately 610 nm and 740 nm) and illumination light to pass through the observation light filter, which reaches the object due to the partial characteristic II of the illumination light filter (second wavelength range 111).Therefore, on the one hand, fluorescent areas can be perceived because the fluorescent light can penetrate the observation light filter due to partial characteristic III, and non-fluorescent areas of the object can also be perceived because the light reaching the object due to partial characteristic II, which is not fluorescence excitation light, can penetrate the observation light filter.

[0079] At wavelengths below the cutoff wavelength 107, the partial characteristic IV has a fourth wavelength range 117, within which the transmission is less than the second value L2 and greater than the third value L3. The second value L2 is therefore a maximum value for the transmission of the observation light filter within the fourth wavelength range 117; and the third value L3 is a minimum value for the transmission of the observation light filter within the fourth wavelength range 117. The purpose of the partial characteristic IV is to allow at least a portion of the light that reaches the object due to the partial characteristic I of the illumination light filter and is reflected or scattered by it to pass through the observation light filter in order to make non-fluorescent areas of the object visible.Similar to partial characteristic II of the illumination light filter, partial characteristic IV of the observation light filter also has a limited transmission of at most the second value L2 to prevent overexposure of the fluorescent areas by the non-fluorescent areas. To ensure that sufficient light intensity can pass through the observation filter, the transmission within the fourth wavelength range is at least the third value L3, which is significantly greater than the value L4, which is present in those areas of transmission characteristic 113 in which the passage of light through the observation light filter is to be blocked.

[0080] As in Fig. 2B, the illumination light filter has a fifth wavelength range 118. The fifth wavelength range 118 lies between the first wavelength range 109 and the second wavelength range 111. Within the fifth wavelength range 118, the transmission of the illumination light filter is less than the fourth value L4. The fifth wavelength range serves to spectrally separate the first and second wavelength ranges. As shown in Fig. 2B, the fifth wavelength range may include the cutoff wavelength 107.

[0081] As in Fig. 2C, the observation light filter has a sixth wavelength range 119. The sixth wavelength range 119 lies between the third wavelength range 115 and the fourth wavelength range 117. Within the sixth wavelength range 119, the transmission of the observation light filter is smaller than the fourth value L4. The sixth wavelength range serves to spectrally separate the third and fourth wavelength ranges. As shown in Fig. 2C, the sixth wavelength range may include the cutoff wavelength 107.

[0082] As in Fig. 2B and Fig. 2C, the illumination light filter and the observation light filter have a seventh wavelength range 120. The seventh wavelength range 120 lies within the first wavelength range 109, but outside the fourth wavelength range 117. In the first partial characteristic I of the illumination light filter, the transmission within the seventh wavelength range 120 is at least the first value L1. In the fourth partial characteristic IV of the observation light filter, the transmission within the seventh wavelength range 120 is at most the fourth value L4. Light of the seventh wavelength range, which is suitable for exciting PPIX, can pass through the illumination light filter but is blocked by the observation light filter, so that this excitation light does not outshine the light of the third wavelength range 115 and the fourth wavelength range 117.

[0083] As in Fig. 2B and Fig. 2C, the illumination light filter and the observation light filter have an eighth wavelength range 121. The eighth wavelength range 121 lies within the third wavelength range 115, but outside the second wavelength range 111. In the third partial characteristic III of the observation light filter, the transmission within the eighth wavelength range 121 is at least the first value L1. In the second partial characteristic II of the illumination light filter, the transmission within the eighth wavelength range 121 is at most the fourth value L4. Light of the eighth wavelength range essentially comprises the fluorescent light from PPIX. To prevent this from being outshone by the illumination light, the illumination light filter blocks light of the eighth wavelength range 121, while the observation light filter transmits light of the eighth wavelength range 121, thus enabling the fluorescent light to be observed.

[0084] Fig. Figure 2D shows the product of the transmission characteristic 105 of the illumination light filter and the transmission characteristic 113 of the observation light filter. This product exhibits significant values in two areas, namely, an intersection 123 of the first wavelength range 109 with the fourth wavelength range 117 and an intersection 125 of the third wavelength range 115 with the second wavelength range 111. The two intersections 123, 125 are wavelength ranges outside the fluorescence of PPIX. The high values within the intersections 123, 125 therefore contribute to the visualization of non-fluorescent object areas. In this way, in addition to fluorescent object areas that appear in the color of the fluorescent dye, non-fluorescent object areas can also be observed simultaneously with essentially true color.

[0085] An exemplary second filter set for protoporphyrin IX is described below with reference to the Fig. 3A to 3D.

[0086] Fig. 3A corresponds to Fig. 2A. Reference is made to their description.

[0087] Fig. Figure 3B shows a graph of a transmission characteristic 205 of the illumination light filter in a logarithmic scale of the ordinate in a wavelength range from about 350 nm to about 850 nm. This includes the visible wavelength range, which is relevant for considering the properties of the filter set. In Fig. 3B shows a cut-off wavelength 207, which essentially corresponds to the cut-off wavelength 107, as used in connection with the Fig. 2B to 2D.

[0088] The transmission characteristic 205 of the illumination light filter is the sum of two partial characteristics I and IIa, which are defined in the entire range from 350 nm to 850 nm. The partial characteristic I of the second filter set essentially corresponds to the partial characteristic I of the first filter set, as described in connection with the Fig. 2B to 2D. Accordingly, at wavelengths below the cutoff wavelength 207, it has a first wavelength range 209 within which the transmission is greater than a first value L1.

[0089] The partial characteristic IIa of the second filter set differs essentially only from the partial characteristic II of the first filter set in that the transmission within a second wavelength range 211, which lies above the cutoff wavelength 207, is greater than the first value L1. This means that the transmission of the illumination light filter is greater than the first value L1 in the entire second wavelength range 211. Accordingly, the first value L1 is a minimum value for the transmission of the illumination light filter in the second wavelength range 211. The purpose of the partial characteristic II is to allow light to pass through the illumination light filter, which light does not serve to excite the fluorescence, but rather to illuminate the object in such a way that its non-fluorescent regions become perceptible.The second wavelength range 211 is therefore selected such that it lies outside the wavelength ranges in which the emission spectrum E has significant values. Since the non-fluorescent regions should not outshine the fluorescent regions in perception and the fluorescence usually has a low intensity, the observation light filter described below has a significantly lower transmission in the second wavelength range 211 compared to the first value L1.

[0090] A transmission characteristic 213 of the observation light filter is shown in Fig. 3C. The transmission characteristic 213 is again the sum of two partial characteristics IIIa and IV. The third partial characteristic IIIa of the second filter set differs significantly from that of the first filter set.

[0091] At wavelengths above the cutoff wavelength 207, the partial characteristic IIIa has a third wavelength range 215, within which the transmission is less than the second value L2 and greater than the third value L3. The second value is therefore a maximum value for the transmission of the observation light filter within the third wavelength range 215; and the third value L3 is a minimum value for the transmission of the observation light filter within the third wavelength range 215.

[0092] Furthermore, the partial characteristic IIIa has a ninth wavelength range above the third wavelength range 215, which ranges from approximately 610 nm to 750 nm. Within the ninth wavelength range, the transmission of the observation light filter is greater than the first value L1. This allows the fluorescent light from PPIX to pass through the observation filter and be observed.

[0093] Partial characteristic III is designed to allow both fluorescent light (between approximately 610 nm and 740 nm) and illumination light that reaches the object (second wavelength range 211) due to partial characteristic II of the illumination light filter to pass through the observation light filter. Therefore, fluorescent areas can be perceived, since the fluorescent light can pass through the observation light filter due to partial characteristic III. Non-fluorescent areas of the object can also be perceived, since the light that reaches the object due to partial characteristic II, which is not fluorescence excitation light, can pass through the observation light filter in the third wavelength range 215.

[0094] The fourth partial characteristic IV of the second filter set essentially corresponds to the fourth partial characteristic IV of the first filter set. Accordingly, at wavelengths below the cutoff wavelength 207, it has a fourth wavelength range 217 within which the transmission is less than the second value L2 and greater than the third value L3. The second value L2 is therefore a maximum value for the transmission of the observation light filter within the fourth wavelength range 217; and the third value L3 is a minimum value for the transmission of the observation light filter within the fourth wavelength range 217.

[0095] As in Fig. 3B, the illumination light filter has a fifth wavelength range 218. The fifth wavelength range 218 lies between the first wavelength range 209 and the second wavelength range 211. Within the fifth wavelength range 218, the transmission of the illumination light filter is less than the fourth value L4. The fifth wavelength range serves to spectrally separate the first and second wavelength ranges. As shown in Fig. 3B, the fifth wavelength range may include the cutoff wavelength 207.

[0096] As in Fig. 3C, the observation light filter has a sixth wavelength range 219. The sixth wavelength range 219 lies between the third wavelength range 215 and the fourth wavelength range 217. Within the sixth wavelength range 219, the transmission of the observation light filter is smaller than the fourth value L4. The sixth wavelength range serves to spectrally separate the third and fourth wavelength ranges. As shown in Fig. 3C, the sixth wavelength range may include the cutoff wavelength 207.

[0097] As in Fig. 3B and Fig. 3C, the illumination light filter and the observation light filter have a seventh wavelength range 220. The seventh wavelength range 220 lies within the first wavelength range 209, but outside the fourth wavelength range 217. In the first partial characteristic I of the illumination light filter, the transmission within the seventh wavelength range 220 is at least the first value L1. In the fourth partial characteristic IV of the observation light filter, the transmission within the seventh wavelength range 220 is at most the fourth value L4. Light of the seventh wavelength range, which is suitable for exciting PPIX, can pass through the illumination light filter but is blocked by the observation light filter, so that this excitation light does not outshine the light of the third wavelength range 215 and the fourth wavelength range 217.

[0098] As in Fig. 3B and Fig. 3C, the illumination light filter and the observation light filter have an eighth wavelength range 221. The eighth wavelength range 221 lies above the second wavelength range 211 and can be encompassed by the ninth wavelength range 229. In the second partial characteristic IIa of the illumination light filter, the transmission within the eighth wavelength range 221 is at most the fourth value L4. Light of the eighth wavelength range essentially comprises the fluorescent light of PPIX. To prevent this from being outshone by the illumination light, the illumination light filter blocks light of the eighth wavelength range 221, while the observation light filter transmits light of the eighth wavelength range 221, thus allowing the fluorescent light to be observed.

[0099] Fig.Figure 3D shows the product of the transmission characteristic 205 of the illumination light filter and the transmission characteristic 213 of the observation light filter. This product exhibits significant values in two areas, namely in an intersection 223 of the first wavelength range 209 with the fourth wavelength range 217 and in an intersection 225 of the third wavelength range 215 with the second wavelength range 211. The two intersections 223, 225 are wavelength ranges outside the fluorescence of PPIX. The high values within the intersections 223, 225 therefore contribute to the visualization of non-fluorescent object areas. In this way, in addition to fluorescent object areas that appear in the color of the fluorescent dye, non-fluorescent object areas can also be observed simultaneously with essentially true color.

Claims

[1] Filter set, comprising: an illumination light filter (84) and an observation light filter (91), wherein a transmission characteristic (105) of the illumination light filter (84) is a sum of a first partial characteristic (I) and a second partial characteristic (II), wherein the transmission of the first partial characteristic (I) within a first wavelength range (109) which lies below a cut-off wavelength (107) is greater than a first value (L1); wherein the transmission of the second partial characteristic (II) within a second wavelength range (111) which lies above the cut-off wavelength (107) is less than a second value (L2) and greater than a third value (L3), wherein a transmission characteristic (113) of the observation light filter (91) is a sum of a third partial characteristic (III) and a fourth partial characteristic (IV), wherein the transmission of the third partial characteristic (III) within a third wavelength range (115) which lies above the cut-off wavelength (107) is greater than the first value (L1); wherein the transmission of the fourth partial characteristic (IV) within a fourth wavelength range (117), which lies below the cut-off wavelength (107), is smaller than the second value (L2) and larger than the third value (L3), wherein the transmission of the illumination light filter (84) within a fifth wavelength range (118) which lies between the first wavelength range (109) and the second wavelength range (111) is less than a fourth value (L4); wherein the transmission of the observation light filter (91) within a sixth wavelength range (119) which lies between the third wavelength range (115) and the fourth wavelength range (117) is smaller than the fourth value (L4); where the fourth value (L4) is smaller than the third value (L3), the third value (L3) is smaller than the second value (L2) and the second value (L2) is smaller than the first value (L1), where the cutoff wavelength (107) is between 450 nm and 550 nm, wherein the first wavelength range (109) lies between 350 nm and the cutoff wavelength, wherein the second wavelength range (111) lies between the cutoff wavelength and 620 nm and is at least 50 nm wide, wherein the third wavelength range (115) lies between the cutoff wavelength and 800 nm, wherein the fourth wavelength range (117) lies between 410 nm and the cutoff wavelength. [2] Filter set according to claim 1, wherein the transmission of the second partial characteristic (II) within an eighth wavelength range (121), which lies outside the second wavelength range (111) but within the third wavelength range (115), is smaller than the fourth value (L4); and / or wherein the transmission of the third partial characteristic (III) within the eighth wavelength range (121) is greater than the first value (L1). [3] Filter set according to claim 1 or 2, wherein the third wavelength range (115) is at least 150 nm wide, in particular at least 200 nm wide, further in particular at least 230 nm wide. [4] Filter set, comprising: an illumination light filter (84) and an observation light filter (91), wherein a transmission characteristic (205) of the illumination light filter (84) is a sum of a first partial characteristic (I) and a second partial characteristic (IIa), wherein the transmission of the first partial characteristic (I) within a first wavelength range (209) which lies below a cut-off wavelength (207) is greater than a first value (L1); wherein the transmission of the second partial characteristic (II) within a second wavelength range (211) which lies above the cut-off wavelength (207) is greater than the first value (L1); wherein a transmission characteristic (213) of the observation light filter (91) is a sum of a third partial characteristic (IIIa) and a fourth partial characteristic (IV), wherein the transmission of the third partial characteristic (IIIa) within a third wavelength range (215) which lies above the cut-off wavelength (207) is less than a second value (L2) and greater than a third value (L3); wherein the transmission of the third partial characteristic (III) within a ninth wavelength range (229) which lies above the third wavelength range (215) is greater than the first value (L1); wherein the transmission of the fourth partial characteristic (IV) within a fourth wavelength range (217), which lies below the cut-off wavelength (207), is smaller than the second value (L2) and larger than the third value (L3), wherein the transmission of the illumination light filter (84) within a fifth wavelength range (218) which lies between the first wavelength range (209) and the second wavelength range (211) is less than a fourth value (L4); wherein the transmission of the observation light filter (91) within a sixth wavelength range (219) which lies between the third wavelength range (115) and the fourth wavelength range (117) is smaller than the fourth value (L4); where the fourth value (L4) is smaller than the third value (L3), the third value (L3) is smaller than the second value (L2) and the second value (L2) is smaller than the first value (L1). [5] Filter set according to claim 4, where the cutoff wavelength (207) is between 450 nm and 550 nm, wherein the first wavelength range (209) lies between 350 nm and the cutoff wavelength, and / or wherein the second wavelength range (211) lies between the cutoff wavelength and 680 nm, and / or wherein the third wavelength range (215) lies between the cutoff wavelength and 680 nm, and / or wherein the ninth wavelength range (229) lies between the third wavelength range (215) and 800 nm, and / or wherein the fourth wavelength range (217) lies between 410 nm and the cutoff wavelength. [6] Filter set according to claim 4 or 5, wherein the transmission of the second partial characteristic (IIa) within an eighth wavelength range (221) which lies outside the second wavelength range (211) but within the ninth wavelength range (229) is smaller than the fourth value (L4). [7] Filter set according to one of claims 4 to 6, wherein the third wavelength range (215) is at least 50 nm wide, in particular at least 75 nm wide, further in particular at least 100 nm wide. [8] Filter set according to one of claims 1 to 7, wherein the transmission of the first partial characteristic (I) within a seventh wavelength range (120; 220), which lies within the first wavelength range (109; 209) but outside the fourth wavelength range (117; 217), is greater than the first value (L1); and / or wherein the transmission of the fourth partial characteristic (IV) within the seventh wavelength range (120; 220) is smaller than the fourth value (L4). [9] Filter set according to one of claims 1 to 8, wherein the cutoff wavelength (107; 207) is between 480 nm and 520 nm. [10] Filter set according to one of claims 1 to 9, wherein the first wavelength range (109; 209) is between 380 nm and 480 nm, in particular between 390 nm and 450 nm; and / or wherein the second wavelength range (111; 211) lies between the cutoff wavelength and 620 nm, in particular between 520 nm and 620 nm; and / or wherein the third wavelength range (115; 215) is between 520 nm and 760 nm; and / or wherein the fourth wavelength range (117; 217) lies between 410 nm and 450 nm, in particular between 410 nm and 435 nm. [11] Filter set according to one of claims 1 to 10, wherein the first wavelength range (109; 209) is at least 40 nm wide, in particular at least 60 nm wide; and / or wherein the second wavelength range (111; 211) is at least 50 nm wide, in particular at least 70 nm wide, further in particular at least 90 nm wide; and / or wherein the fourth wavelength range (117; 217) is at least 10 nm wide, in particular at least 20 nm wide, further in particular at least 40 nm wide. [12] Filter set according to one of claims 1 to 11, wherein the first wavelength range (109; 209) and the fourth wavelength range (117; 217) overlap each other; and / or wherein the second wavelength range (111; 211) and the third wavelength range (115; 215) overlap each other. [13] Filter set according to one of claims 1 to 12, wherein the first wavelength range (109; 209) comprises the fourth wavelength range (117; 217); and / or wherein the third wavelength range (115; 215) comprises the second wavelength range (111; 211). [14] Filter set according to one of claims 1 to 13, where the first value (L1) is 50%, 70% or 90%; and / or wherein the second value (L2) is 10%, 5% or 1%; and / or wherein the third value (L3) is 0.01%, 0.05% or 0.1%; and / or where the fourth value (L4) is 0.005% or 0.001% or 0.0001%. [15] Filter set according to one of claims 1 to 14, wherein the illumination light filter (84) and the observation light filter (91) are configured such that a ratio of a first mean value to a second mean value is between 0.5 and 1.5, in particular between 0.8 and 1.2, wherein the first mean value is defined as the product of the transmission of the illumination light filter (84) with the transmission of the observation light filter (91) and with a wavelength-dependent factor, averaged over the intersection of the first wavelength range (109; 209) with the fourth wavelength range (117; 217) and normalized to this intersection, and wherein the second mean value is defined as the product of the transmission of the illumination light filter (84) with the transmission of the observation light filter (91) and with a wavelength-dependent factor, averaged over the intersection of the second wavelength range (111; 211) with the third wavelength range (115;215) and normalized to this intersection, where is the wavelength-dependent factor 1 or is the relative spectral luminosity of the eye V(λ), defined according to CIE (Commission Internationale de l'Eclairage) 018.2-1983.; [16] Filter set according to one of claims 1 to 15, wherein the fifth wavelength range (118; 218) comprises the cutoff wavelength (107; 207); and / or wherein the fifth wavelength range (118; 218) is at least 20 nm, at least 40 nm or at least 60 nm wide; and / or wherein the sixth wavelength range (119; 219) comprises the cutoff wavelength (107; 207); and / or wherein the sixth wavelength range (119; 219) is at least 20 nm, at least 40 nm or at least 60 nm wide. [17] Fluorescence observation system for the simultaneous observation of protoporphyrin IX and a white light-like image of an object, comprising: a light source (71) for illuminating the object (9), an observation optics (3) for imaging the object (9) and a filter set according to one of claims 1 to 16, wherein the illumination filter (84) is arranged in an illumination beam path between the light source (72) and the object (9) and the observation light filter (91) is arranged in a beam path of the observation optics (3). [18] A method for simultaneously observing protoporphyrin IX and a white light-like image of an object using the filter set according to any one of claims 1 to 16, the method comprising: Filtering an illumination light beam (81) directed onto the protoporphyrin IX-enriched object (9) with the illumination filter (84) of the filter set, and Filtering light emanating from the object (9) with the observation light filter (91) of the filter set.

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