Measurement method, measurement setup and computer program product

Abstract

Measurement method for determining the time response of an infrared camera (1), which comprises the following procedure steps: Providing an infrared camera (1); Providing an area (2) with a temperature different from the surroundings; Providing a switchable cover device (3) configured to temporarily cover or uncover the area (2) in order to take an image of the uncovered area (2) with the infrared camera (1); defining a switching behavior for the cover device (3) and Defining an exposure interval for the infrared camera (1); Providing reference pixel values ​​representing the timing behavior of the covering device by measuring the timing behavior of the covering device with a reference camera different from the infrared camera being measured, whose trigger delay and integration time are known; Controlling the switchable cover device and taking a plurality of pictures with the infrared camera (1), wherein different time delays occur between the times of the camera pictures and the times of switching the cover device (switching signal); Determining pixel values ​​obtained by recording with the infrared camera (1); Determining a deviation between the measured pixel values ​​and the reference pixel values; and Determining the timing behavior of the infrared camera (1) based on the deviation.

Description

[0001] The invention relates to a measurement method for determining the time response of a camera, in particular an infrared camera for the mid- and long-wavelength spectral range in the infrared. Furthermore, the invention relates to a computer program product comprising program code for executing the measurement method according to the invention. Finally, the invention relates to a measurement arrangement for determining the time response of an infrared camera, in particular for the mid- and long-wavelength spectral range in the infrared.

[0002] Manufacturers of infrared cameras do not specify their timing characteristics. This includes any trigger delay, unequal exposure times or exposure durations for different pixel regions, and decay constants of microbolometer detectors.

[0003] It is not known from the current state of the art how the timing behavior of cameras, and especially infrared cameras, can be measured. Works that interpret camera recordings implicitly assume that the exposure begins instantaneously with the trigger or strobe signal and ends instantaneously at the end of the exposure time.

[0004] BUDZIER Helmut; GERLACH, Gerald: Thermal Infrared Sensors. Fundamentals for Users. ISBN 978-3-527-40960-0. Weinheim: Wiley-VCH, 2010. Chapter 6: Thermal Infrared Sensors, Sections 6.1 - 6.3, pp. 151 - 178 discloses the cyclic illumination of an infrared detector, but neither the object of the present invention nor the solution now found for determining the trigger delay of an infrared camera.

[0005] LOMHEIM, Terrence S.; SCHUMANN, Lee W.; KOHN, Stanley E.: “Experimental characterization, evaluation, and diagnosis of advanced hybrid infrared focal plane array electro-optical performance”, Proc. SPIE 3379, Infrared Detectors and Focal Plane Arrays V, 520 (July 22, 1998) deals with the characterization of a focal plane array detector. The electro-optical performance is to be tested, but not the trigger delay. Blackbody radiation is to be directed periodically onto a test detector using a chopper. However, it is not shown how to introduce a varying delay between the individual images of the test object or the detector readout and the chopper, in order to accurately measure the opening and closing of a shutter.

[0006] SCHAFF, Fred L.: Transient Response Analysis Of The Thermal / Electrical Response Of Pyroelectric Detectors For Sampled Data Applications, Proc. SPIE 0172, Instrumentation in Astronomy III, 190 (May 3, 1979) shows the measurement of transients of a pyroelectric detector. For this purpose, the detector is cyclically illuminated with infrared radiation. This illumination time is determined by a second detector.

[0007] HOPKINSON, Gordon; ROJAS, Luis Gomez; SKIPPER, Mark; MEYNART, Roland: Testing of InGaAs, microbolometer and pyroelectric detectors in support of the EarthCARE mission, Proc. SPIE 7106, Sensors, Systems, and Next-Generation Satellites XII, 71061O (October 9, 2008) discloses investigations on three different detectors: InGaAs diode arrays, microbolometers, and pyroelectric detectors. The aim of these investigations is to determine the suitability of the different detector concepts for use in the satellite-based EarthCARE mission.

[0008] It is therefore the object of the present invention to provide, for the first time, a measurement method for determining the temporal behavior of infrared cameras and an associated measurement setup for carrying out the method. In particular, the invention should allow for the quantitative investigation of any trigger delay, unequal exposure times or exposure durations for different pixel regions, and decay constants of microbolometer detectors.

[0009] The problem is solved by a method according to claim 1, a computer program according to claim 8, and a measuring arrangement according to claim 9. Advantageous embodiments of the invention are found in the dependent claims.

[0010] According to a first aspect of the invention, it relates to a measurement method for determining the time response of an infrared camera, in particular for the medium and long-wavelength spectral range in the infrared, which comprises the following method steps: Provide an infrared camera; provide an area with a temperature different from the surroundings; Providing a switchable cover device configured to temporarily cover or uncover the area in order to capture an image of the uncovered area with the infrared camera; defining a switching behavior for the cover device and an exposure interval for the infrared camera; controlling the switchable cover device and taking multiple images, with different time delays between the camera images and the times the cover device is switched (switching signal); determining pixel values ​​obtained from the infrared camera image; determining a deviation between the measured pixel values ​​and reference pixel values ​​and determining the timing behavior of the infrared camera based on this deviation.

[0011] The infrared camera to be measured using the measurement method according to the invention can be a known electronic camera, i.e., the camera outputs an electrical signal representing the image. The measurement method is not limited to use with infrared cameras; cameras for other spectral ranges of the electromagnetic spectrum can also be measured. However, the invention solves for the first time the problem of measuring the time response of cameras for the infrared spectral range.

[0012] In some embodiments of the invention, a thermal infrared camera is used for measurement, which, for example, covers the wavelength range above approximately 5 µm. Different transducer materials can be used for the image sensor contained in the infrared camera. These materials can detect infrared radiation based on various physical effects. Examples include the photoelectric effect in semiconductor sensors, such as GaAs sensors. Alternatively, in microbolometers, the change in electrical resistance due to the radiation absorbed in the bolometer, which leads to a temperature change, can be used.

[0013] The provided surface, with a temperature different from the surroundings, represents a target to be detected by the infrared camera being measured. This target emits radiation in a wavelength range to which the infrared camera is sensitive. A uniformly heated surface is advantageous, as this improves the accuracy of the measurement method. In some embodiments of the invention, the surface is flat. In some embodiments, the surface normal points in the direction of the infrared camera. If the camera being measured is sensitive in a different spectral range, a different target emitting corresponding wavelengths can be used instead of the surface with a temperature different from the surroundings. In the visible spectral range, for example, a homogeneously luminous surface or a color chart can be used.

[0014] The switchable cover device can be a precise and sufficiently fast-acting cover device. For example, it can be electrically controlled and incorporate a mechanical locking mechanism, a spatial light modulator, a Kerr cell, or a Pockels cell.

[0015] In some embodiments, the time required to open the cover device can be between approximately 1 ms and approximately 50 ms, or between approximately 3 ms and approximately 15 ms, or between approximately 5 ms and approximately 10 ms. Similarly, in some embodiments, the time required to close the cover device can be between approximately 1 ms and approximately 50 ms, or between approximately 3 ms and approximately 15 ms, or between approximately 5 ms and approximately 10 ms. In some embodiments of the invention, the cover device can be triggered by a periodic trigger signal and thus switch periodically. For example, the cover device can be switched at a repetition rate or time interval of between approximately 500 ms and approximately 5 s, or between approximately 800 ms and approximately 1.5 s.

[0016] The cover device can have an opening whose size is adapted to the exposure method of the infrared camera being measured. In some embodiments of the invention, the area exposed by the cover device can be chosen to be large enough to illuminate all pixels of the electronic image sensor of the infrared camera. In other embodiments, only a subset of the pixels of the electronic image sensor of the infrared camera is illuminated; that is, the opening of the cover device appears smaller in the image of the infrared camera. For example, a surrounding border may be visible in the image. These two embodiments can be particularly useful if the infrared camera is configured to expose all pixels of the image sensor simultaneously. If the infrared camera is configured to expose individual rows or columns of the electronic image sensor sequentially, a smaller area can be selected.In some embodiments, the opening of the cover device can be mapped onto a partial area of ​​between approximately 3 and approximately 30 pixels. From this description, it can be seen that the size of the opening of the cover device depends on the size of the image sensor of the infrared camera to be measured, the size of the area provided, the viewing distance, and the focal length of the lens.

[0017] The switchable cover device can operate according to different locking principles. For example, a central shutter or a slotted shutter is possible. Other locking techniques are also feasible. The higher the repeatability of the opening and closing of the shutter, the more accurate the measurement results.

[0018] According to the invention, a timing behavior for the covering device and an exposure time for the infrared camera are defined. The timing behavior of the covering device refers to the time required for opening and closing the covering device and the time between the opening and closing processes. This time interval and the exposure time will generally be of different lengths. Crucially, the timing behavior of the covering device can be precisely defined or determined. It is not absolutely necessary to be able to adjust it at will. The only decisive factor is the precise knowledge of the timing behavior of the covering device.

[0019] In some embodiments of the invention, a measurement is performed by periodically controlling the switchable cover device and periodically taking a plurality of photographs. The individual photographs from the plurality of photographs can, in some embodiments, be taken at a frequency of approximately 5 Hz to approximately 50 Hz or from approximately 20 Hz to approximately 30 Hz. Due to the different control frequencies of the cover device and the camera shutter, different time delays occur between the times of the camera photographs and the times of the switching of the cover device. As a result, the cover device is completely closed in some images, completely open in some images, and partially open in some images, with the degree of opening varying.

[0020] In some embodiments of the invention, the signal to start the camera recording (recording signal) and the signal to switch the cover device (switching signal) can be generated by a trigger device, such that both signals have a defined time delay relative to each other. The sequence between the recording signal and the switching signal is not fixed. It is possible for the recording signal to occur before the switching signal, or vice versa. Of course, it is also possible for both signals to occur simultaneously. The defined time delay can therefore have different signs. This also results in the cover device being completely closed in some images, completely open in some images, and partially open in some images, with the degree of opening varying.

[0021] The pixel values ​​obtained from the infrared camera are now being measured and determined. Depending on the extent and duration of the area being visible to the camera at temperatures differing from the surroundings, different pixel values ​​will be obtained.

[0022] According to the invention, reference pixel values ​​are also provided. The reference pixel values ​​describe the time response of the covering device and can be determined by measurement.

[0023] According to the invention, a deviation between the measured pixel values ​​and the reference pixel values ​​is then determined. The temporal behavior of the covering device is thus described twice: once by the reference pixel values ​​and once by the measured values ​​of the infrared camera under test. The temporal behavior of the infrared camera is then determined from this deviation.

[0024] The invention thus teaches the use of a mechanical cover device as a measurement reference, which, however, cannot be expected to have a short opening time compared to the exposure time. Therefore, the change in the size of the uncovered opening during opening and / or closing is used as the measured quantity. This temporal change is referred to in this description as the time response.

[0025] If the camera being measured has an exposure and / or integration time that is short compared to the switching time of the cover device, the measurement result can be directly compared with the reference pixel values. If the integration time is longer, or if the detector used in the camera being measured has a decay characteristic, the reference pixel values ​​can be convolved with a function that describes the integration or decay curve of the infrared camera. If the integration time is very long compared to the switching time of the cover device, in some embodiments of the invention, the portion of the time between the opening and closing processes that falls within the integration time can be evaluated to determine the temporal behavior.

[0026] According to the invention, the reference pixel values ​​can be obtained by means of a reference measurement. The reference measurement is performed with a reference camera that is different from the infrared camera being measured. The reference measurement describes the temporal behavior of the cover device. The reference camera can operate in a different wavelength range than the infrared camera being measured. It is essential that the temporal behavior of the reference camera, i.e., its trigger delay and integration time, is known. The temporal progression of the opening and / or closing of the cover device can be measured using the reference measurement. In some embodiments of the invention, a reference camera with a high frame rate and / or short exposure time can be used for the reference measurement. The result of the reference measurement is the reference pixel values.

[0027] According to one embodiment of the invention, the reference pixel values ​​can be convolved with a function that describes the integration or decay curve of the infrared camera to determine the time response of the infrared camera. The term "convolution" is to be understood in the mathematical sense and denotes a product of two functions. Uneven exposure within the exposure interval occurs, for example, in microbolometers, which exhibit exponential decay behavior. In such cases, convolving the measured values ​​or the measurement function with a function describing the exposure sensitivity facilitates the determination of delay values ​​or the trigger delay of the camera.

[0028] According to one embodiment of the invention, the switchable cover device can be positioned at different locations for different shots, so that the image of the area exposed by the cover device is captured at different positions within the camera image. Infrared cameras, or thermal infrared cameras, generally have a lower spatial resolution than cameras in the visual spectral range. Therefore, in this embodiment, it is advantageous to position the cover device of the measurement setup at a considerable distance from the infrared camera so that different positions within the camera image can be captured. This embodiment is particularly useful when the infrared camera uses a system that integrates different image lines or columns with a time offset (so-called "rolling frame" or "rolling shutter"). For different image regions, or...An individual time shift can then be determined for each image line. This again relies on the known reference measurement.

[0029] In some embodiments of the invention, the reference camera can have a frame rate between approximately 5 kHz and approximately 20 kHz or approximately 8 kHz and approximately 12 kHz. The reference camera can have an exposure time between approximately 50 µs and approximately 150 µs. In some embodiments, the reference camera can have an optional ring buffer to minimize trigger delays.

[0030] According to a preferred embodiment of the invention, the time response of a microbolometer is determined.

[0031] According to another aspect of the invention, it relates to a computer program product with program code for executing at least some steps of the measurement procedure as described above in the various embodiments.

[0032] According to a further aspect of the invention, it relates to a measuring arrangement for determining the time response of an infrared camera, in particular for the medium and long-wavelength spectral range in the infrared, using the measuring method described above in one of its embodiments, wherein the measuring arrangement comprises the following: an infrared camera or a holding device for an infrared camera to be measured; a surface which can be brought to a temperature different from the environment; a switchable cover device which is designed to temporarily cover or uncover the surface so that an image of the uncovered surface can be recorded with the infrared camera; and a trigger generator which is designed to control the switchable cover device and the infrared camera for recording purposes in such a way that the signal to start the camera recording (recording signal) and the signal to switch the cover device (switching signal) have a defined time delay from each other.

[0033] Regarding the conceptual definition of features of the measuring arrangement, explicit reference is made to the statements made concerning the measuring method according to the invention.

[0034] The switchable cover device is preferably a shutter mechanism that can be opened and closed electronically. The trigger generator can be integrated into the camera or an external one. Many cameras already have a strobe output that can also be used to trigger the cover device. Alternatively, the camera can be controlled externally via a trigger input. An external trigger signal can then also be used to control the switchable cover device.

[0035] The preferred embodiments of the invention described above can be combined in whole or in part, provided that no contradictions arise. The invention is not limited to explicitly described combinations of features.

[0036] The invention is explained in more detail below with reference to exemplary embodiments and figures. It shows Fig. 1 a measuring arrangement according to the invention for carrying out the measuring method according to the invention. Fig. Figure 2 shows a graphical representation of measurement and reference measurement.

[0037] The following reference symbols are used in the figures: 1 infrared camera 2. Surface with a temperature different from the surroundings 3 Switchable cover devices 4 Shutter Controllers 5 Trigger generator.

[0038] Fig. Figure 1 shows an exemplary measuring arrangement according to the invention for carrying out the measuring method according to the invention. The temporal behavior of the infrared camera 1 is to be determined. This can be, for example, a thermal infrared camera with a semiconductor sensor or a microbolometer. The infrared camera 1 has a means of setting or determining a nominal acquisition time for an image. It can, for example, have a trigger input or a strobe output. In the example shown, it has a trigger input. This is controlled by a trigger generator 5.

[0039] The infrared source, or target to be imaged, is a surface 2 with a temperature different from that of its surroundings. In the example shown, this is a uniformly heated and flat surface 2, whose surface normal is aligned with the axis of the entire experimental setup, i.e., the camera axis. A switchable cover 3 in the form of a shutter with a central closure is located in front of the surface. This shutter can, for example, have spring-loaded, curved blades that open and close radially. The mechanical cover is controlled by a shutter control unit 4. The cover 3 itself thus operates by mechanical cover but is electrically controlled.

[0040] The shutter controller 4 receives the signal to open or close the cover device 3 via the trigger generator 5. This measuring arrangement therefore makes it possible to selectively control the switchable cover device 3 and take an infrared photograph, whereby the signal to start the camera recording (recording signal) and the signal to switch the cover device (switching signal) have a defined time delay. This defined time delay is set via the trigger generator 5.

[0041] By recording camera data with varying delays between the recording signal and the switching signal, the opening and closing process of shutter 3 is determined from the perspective of camera 1. For this purpose, the pixels of the shutter opening and as few unchanged pixels as possible are summed.

[0042] To evaluate the measurement results, a reference measurement is then used. This is exemplified in... Fig. 2 illustrated.

[0043] Fig. Figure 2 shows the closing process of the cover device against time. The opening of the shutter is plotted on the ordinate and the time on the abscissa. The graph is normalized to 1 for the fully open shutter 3.

[0044] Shown are a measurement using an infrared camera, a reference measurement, a folded reference measurement and a time-shifted measurement.

[0045] The reference measurement may, for example, have been performed with a high-speed camera. Such a camera has a very well-known timing behavior. By comparing the two results, the timing behavior of the camera under investigation is determined. A trigger delay of the infrared camera currently under investigation manifests itself as a shift in the data relative to different delay values. In the case of uneven exposure within the exposure interval, the opening and closing of the cover device 3 is convolved with a function describing the exposure sensitivity. This is the case, for example, with microbolometer cameras, which exhibit exponential decay behavior. As a result, the shifted measurement series then lies on top of the convolved reference series.

[0046] At the in Fig. The camera used in the measurement is a microbolometer with a decay constant of 4 ms.

[0047] To account for the decay constant of the microbolometer and to make the measurement data of the reference camera comparable with the measurement data of the infrared camera, the reference data were convolved with a function describing the properties of the microbolometer before comparison. In this case, the reference data were convolved with an exponential function.

[0048] If the decay constant and the trigger delay are unknown, both parameters can be determined simultaneously in a least-squares calculation.

[0049] If an infrared camera other than the microbolometer is used for measurement, its behavior can be utilized by means of a suitably adapted convolution function. For example, the reference data can be convolved with a rectangular curve of appropriate width if the infrared camera has an integration interval of 25 ms. If the infrared camera has an integration interval of 250 µs, a point-like sampling of the opening and closing curves of the scanning device is practically sufficient, and the convolution with a convolution function can be omitted.

[0050] For evaluation, the pixels of a rectangle that completely contains the shutter opening are summed. In some embodiments of the invention, this can be the entire image. The pixel sums of the frames with a partially open shutter are normalized using the adjacent frames (in which it is fully open or closed), so that the result always lies within the interval [0, 1]. This means that a shutter opening percentage is determined from each image, which, depending on the delay, ranges between 0% and 100%. This opening percentage is independent of the exact temperature of the background surface and the shutter characteristics. Furthermore, the opening percentage can be determined independently of the pixel value range and the pixel bit width of the camera. This normalized value contains only the information about how far the shutter is open relative to its maximum opening.Therefore, it is suitable for comparing measurements with different cameras and thus also with the reference measurement.

[0051] Using the method according to the invention and the associated measuring arrangement, it has become possible for the first time to precisely measure the temporal behavior of infrared cameras. The method according to the invention is applicable and useful wherever time-critical processes are controlled or regulated based on infrared camera data. A trigger delay or the explicit temporal behavior must be factored in, and uneven exposure can also be taken into account during data processing. In addition to radiometric and geometric calibration, the method according to the invention represents a further aspect of determining the behavior of infrared cameras. Therefore, the method according to the invention is applicable to the evaluation of all systems containing infrared cameras.

[0052] Naturally, the invention is not limited to the embodiment shown in the figures. The foregoing description is therefore not to be considered limiting, but rather explanatory. The following claims are to be understood as meaning that a named feature is present in at least one embodiment of the invention. This does not preclude the presence of further features. Features from different embodiments of the invention can be combined at any time to obtain further embodiments of the invention.

Claims

[1] Measurement method for determining the time response of an infrared camera (1) comprising the following procedure steps: Providing an infrared camera (1); Providing an area (2) with a temperature different from the surroundings; Providing a switchable cover device (3) configured to temporarily cover or uncover the area (2) in order to take an image of the uncovered area (2) with the infrared camera (1); defining a switching behavior for the cover device (3) and Defining an exposure interval for the infrared camera (1); Providing reference pixel values ​​representing the timing behavior of the covering device by measuring the timing behavior of the covering device with a reference camera different from the infrared camera being measured, whose trigger delay and integration time are known; Controlling the switchable cover device and taking a plurality of pictures with the infrared camera (1), wherein different time delays occur between the times of the camera pictures and the times of switching the cover device (switching signal); Determining pixel values ​​obtained by recording with the infrared camera (1); Determining a deviation between the measured pixel values ​​and the reference pixel values; and Determining the timing behavior of the infrared camera (1) based on the deviation. [2] Measuring method according to claim 1, characterized by , that the time behavior of the cover device represents the time required for opening and closing the cover device and the time between the opening and closing process. [3] Measuring method according to one of claims 1 to 2, characterized bythat the switching behavior of the cover device and / or the exposure time of the infrared camera are kept constant. [4] Measuring method according to any one of claims 1 to 3, characterized by , that to determine the time behavior of the infrared camera (1) the reference pixel values ​​are convolved with a basis function which describes the integration or decay curve of the infrared camera (1). [5] Measuring method according to any one of claims 1 to 4, characterized by , that a plurality of images are taken with the infrared camera (1), wherein the switchable cover device (3) is placed at different positions in the image of the infrared camera (1). [6] Measuring method according to any one of claims 1 to 5, characterized by that the reference camera includes or consists of a high-speed camera. [7] Measuring method according to any one of claims 1 to 6, characterized by, that the time response of a microbolometer is determined. [8] Computer program product comprising program code for performing at least some process steps of a measurement method according to any one of claims 1 to 7. [9] Measuring arrangement for determining the time response of an infrared camera (1) with a measuring method according to one of claims 1 to 7, comprising: a holding device for an infrared camera to be measured; a surface (2) which can be brought to a temperature different from that of the surroundings; a switchable cover device (3) which is configured to temporarily cover or uncover the area (2) so that an image of the uncovered area (2) can be recorded with the infrared camera (1); and a trigger generator (5) which is configured to control the switchable cover device (3) and the infrared camera (1) for recording purposes such that the signal to start the camera recording (recording signal) and the signal to switch the cover device (3) (switching signal) have a defined time delay to each other. characterized by , that the device further comprises a reference camera different from the infrared camera to be measured, with which reference pixel values ​​can be determined by measuring the time behavior of the cover device (3), wherein The trigger delay and integration time of the reference camera are known. [10] Measuring arrangement according to claim 9, characterized by , that the infrared camera (1) contains a microbolometer. [11] Measuring arrangement according to claim 9 or 10, characterized by , that the defined time delay is variable. [12] Measuring arrangement according to one of claims 9 to 11, further comprising a reference pixel memory in which reference pixel values ​​can be stored which describe the time behavior of the switchable cover device (3).

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