Camera-Based Vital Signs Detection

Abstract

The vital signs detection system includes a camera 10 configured to acquire image frames from an examination zone 42. A signal processor 11 derives vital signs information from the acquired image frames. A lighting controller 12 controls the illumination of the examination zone, generates a time modulation of the illumination, and synchronizes the modulated illumination with the camera frame rate. The vital signs detection system of the present invention achieves an increased dynamic range and therefore an increased signal-to-noise ratio of the vital signs signal.

Description

[Technical field]

[0001] The present invention relates to a vital signs detection system having a camera configured to capture image frames from an examination zone, and a signal processor to derive vital signs information from the captured images. [Background technology]

[0002] A multi-wavelength variant of remote PPG (MV-rPPG) is desirable for camera-based pulse measurements because it improves robustness and enables SpO2 measurements. Time multiplexing is attractive to turn monochrome camera remote PPG solutions, such as in the Philips MR VitalEye, into MVs without camera hardware modifications. However, time multiplexing requires precise synchronization between optical switching and camera frame acquisition. This is typically achieved by triggering cables, which may not be possible or easy to install in existing setups such as the MR VitalEye. Summary of the Invention [Problem to be solved by the invention]

[0003] Remote photoplethysmography (rPPG) can be used to measure pulse rate and even current heart phase in real time based on a video stream of a human subject's face. Multi-wavelength rPPG, which uses several different wavelengths simultaneously for rPPG signal generation, has been proposed to improve the SNR of the rPPG signal, automatic skin selection, and robustness of this signal with respect to changes in ambient illumination and movement of the subject. A simple way to realize multi-wavelength rPPG is to use a color camera. However, color cameras have fixed RGB colors. If other combinations of wavelength bands other than these RGB colors are to be used, including for example infrared (IR) light, or if the setup already consists of only monochrome cameras, or if monochrome cameras should be used to save costs or reduce the form factor (compact design), it is possible to realize the multi-wavelength feature by using a single monochrome camera and several light sources, one for each wavelength band. This approach requires precise synchronization of the camera frame acquisition and the switching of the light sources. Typically, this is realized by triggering the camera and light source controllers with the same trigger signal. However, this requires a light controller and a high speed signal link to the camera.Such a vital signs detection system is known from US patent application 2017 / 0354334.

[0004] Known vital signs detection systems utilize camera-based measurement of a subject's vital signs. In particular, video images are acquired and processed to obtain a remote reflectance photoplethysmogram (rPPG) of a region of interest. Signal averaging is used to improve the signal-to-noise ratio of the rPPG signal.

[0005] It is an object of the present invention to provide a vital signs detection system that is more robust in generating a vital signs signal. [Means for solving the problem]

[0006] This object is achieved by a vital signs detection system according to claim 1. In terms of technical advantages (non-limiting examples), it is to have a camera and a light source that have high speed synchronization for a more efficient vital signs detection system.

[0007] A camera of the vital signs detection system acquires image frames from the examination zone, in particular of a monitored subject placed in the examination zone. The examination zone with the subject is illuminated by an illumination device configured to illuminate mainly a region of interest of the subject, in particular an area of ​​the subject's bare skin, so that the camera can acquire image frames from the subject illuminated in (non-specular) reflection. The illumination device can be configured as an in-bore illumination system. From the acquired image frames, a signal processor derives one or more vital signs signals. In particular, temporal variations in the blood volume in the tissue of the subject result in variations in the absorption and reflection of the illumination light. Thus, from the image frames, a temporal rPPG signal can be derived. The illumination has a temporal modulation (hereinafter "temporal modulation"), and the frame rate of the camera is synchronized with said temporal modulation. This allows the signal processor to remove the baseline illumination from the image frames and to separate the contribution of the vital signs (e.g. blood flow) from the image frames. Thus, the dynamic range and thus also the signal-to-noise ratio of the vital signs signals is increased. Additionally, the illumination may be generated by a lighting device that illuminates the examination room in which the vital signs detection system is located.

[0008] These and other aspects of the invention will be further elaborated with reference to the embodiments defined in the dependent claims.

[0009] In a preferred embodiment of the vital signs detection system, the signal processor can calculate the difference between successive image frames, which are selected with respect to the temporal modulation of the illumination such that the modulation phase of the illumination is different in each image frame. This eliminates the baseline contribution of the illumination from the image frames, allowing the vital signs signal to be preserved with a higher signal-to-noise ratio.

[0010] In a further preferred embodiment of the vital signs detection system, the illumination system is considered as part of the vital signs detection system and is configured to emit illumination having a spectral content within the hemoglobin spectral absorption band. This makes the image frame particularly sensitive to absorption by blood in the tissue of the subject, especially in the wavelength range of 550-600 nm. As a result, the rPPG signal is significantly enhanced, since the illumination contains a very strong spectral content around the absorption maximum of hemoglobin. This embodiment can be incorporated into a magnetic resonance examination system, in which the examination zone is illuminated by white light containing such a very strong spectral content around the absorption maximum of hemoglobin.

[0011] In practical examples, the temporal modulation frequency is in the range of 20-60 Hz. In this range, the modulation frequency is high enough to cover the temporal variations of vital sign signals, while illumination fluctuations due to macroscopic external factors are often on time scales much slower than tens of milliseconds.

[0012] In another practical example, the image frame rate of the camera is twice the time modulation of the illumination. Preferably, the camera operation is phase-locked with the modulated illumination. In this example, any pair of temporally adjacent image frames is acquired with the opposite modulation phase of the modulated illumination, e.g., one image frame of the pair is acquired with the illumination on and the other image frame of the pair is acquired with the illumination off. As a result, successive images are acquired by the camera with and without the contribution of the illumination. The difference image between the successive images of the pair contains mainly the contribution of the vital signs, since the surrounding background is cancelled. This allows the baseline illumination to be eliminated and the vital signs information to be preserved by subtracting each pair of image frames.

[0013] In another example, the vital signs detection system can be placed in a room with an external ambient light. The external ambient light can have a unique time modulation frequency that is linked to the mains frequency at which the ambient light is powered. Selecting the time modulation frequency of the illumination of the examination zone equal to the unique time modulation frequency avoids beats between the external ambient light and the (additional) illumination of the examination zone. For example, the vital signs detection system can be combined with or incorporated into a magnetic resonance examination system, where the examination zone in the magnet bore of the magnetic resonance examination system is illuminated by a bore light incorporated into the magnetic resonance examination system and is also illuminated by ambient light from the room in which the magnetic resonance examination system is installed reaching the examination zone. This embodiment of the invention avoids disturbing beats of the vital signs signal between the bore light and the ambient light from the room.

[0014] In a practical implementation, the synchronization of the camera frame rate and the illumination modulation is adjusted so that the modulation depth of the image intensity is at or near maximum, and the camera frame rate and the illumination modulation are in phase.

[0015] The cameras may be monochrome cameras since only the brightness differences due to different wavelength bands of illumination are required. In particular, synchronization does not require the formation of color images.

[0016] In another embodiment, the vital signs detection system has a signal analyzer that detects a modulation depth of the detected brightness of the image frames between successive image frames. The illumination controller is configured to generate a wavelength band modulation of the illumination of the examination zone and synchronize the camera frame rate according to the modulation depth. The image brightness is different for each wavelength band. This is due, for example, to the wavelength dependence of the absorption and reflection from the object (e.g., the patient being examined) in the examination zone. Furthermore, the wavelength dependence of the detection sensitivity of the camera for each wavelength band can be added to the different image brightness of the acquired image frames. This embodiment is based on the insight that the modulation depth depends on the synchronization of the wavelength modulation and the camera frame rate. The difference between the image brightness of successive image frames is maximum when the wavelength modulation and the camera frame rate are in phase. Then, each individual image frame is associated with an image from the illumination from one wavelength band. When the wavelength modulation and the camera frame rate are not in phase (or are in antiphase), the image brightness of each individual image frame is substantially averaged over the different wavelength bands and the difference between the image brightness of successive image frames is minimal or even zero. Thus, the present embodiment based on wavelength modulation achieves synchronization of the camera frame rate and the illumination modulation based on the captured stream of captured image frames. In this way, the synchronization is accurate and realized in a simple manner. In particular, there is no need to precisely adjust the signal transmission time due to different lengths of the signal leads to the camera and the illumination controller. In a simple implementation, the illumination controller is configured to calculate the average brightness value of the image frames (over the pixels of each image frame). The Fourier transform of the average brightness of this time series is calculated, for example, by a sliding window of a predetermined number of image frames. The average brightness value can be derived from a selected region of interest in the image frames where the difference between illuminations with different wavelengths is relatively high. Furthermore, the modulation depth of the wavelength band modulation can be measured while the speed of the wavelength band modulation is much slower than the image frame rate. The time pattern of the illumination then approaches a square waveform.This can in fact be realized in an easy way to generate wavelength band modulation by switching on / off different sets of color LEDs and using image frame rates ranging from 10 to 100 Hz, so that the modulation depth can be measured within a second. Alternatively, an additional object with marked color difference can be placed within the range of the camera. For example, hardware test images on stickers can be used. The precise synchronization of the illumination wavelength band and the camera frame rate corresponds to the dominant frequency component (i.e. with the largest amplitude) in the series of average brightness values. In a simple implementation where only two different illumination wavelengths are used, a sliding window average of the modulus of the brightness difference of subsequent image frames represents the modulation depth.

[0017] Control of the camera frame rate relative to the rate of the wavelength band modulation of the illumination can be achieved by a feedback loop in which the modulation depth of the image frames is maximized. Positive and negative phase delays in the time course of the illumination modulation can be introduced by this feedback loop, and the maximum modulation depth is then obtained at a camera frame rate that is exactly in phase with the wavelength band modulation of the illumination in a stable manner.

[0018] In a practical embodiment, the illumination wavelength band modulation and the camera frame rate are controlled by a common processor of the vital signs detection system. When the current image frame arrives at the processor, the illumination wavelength band is switched. This can also be achieved by LED-based illumination and switching on / off different (sets of) LEDs operating in the respective wavelength bands. In this way, the camera frame rate (i.e., image frame rate) and the frequency of the illumination wavelength band modulation are of equal frequency. Any phase delay that may be introduced by different transit times between the control signals for the camera and illumination can be corrected by maximizing the modulation depth of the illumination. This correction can be derived from the image frame without specific determination of the individual signal delays.

[0019] In another embodiment, an interleaved resynchronization measurement is used, where a π / 2 phase angle is introduced into the wavelength band modulation and the image frame rate. In that case, the modulation depth is highly sensitive to small phase variations in the synchronization between the image frame rate and the illumination wavelength band modulation. The sign of the variation of the modulation depth change with the small additional phase determines whether to increase or decrease the phase between the image frame rate and the illumination wavelength band modulation in order to resynchronize to the same phase synchronization.

[0020] The vital sign signal from the vital sign detection system can be used as a trigger signal for another imaging system, such as a magnetic resonance examination system or a computed tomography system. The vital sign detection system of the present invention then serves as a synchronization system for the imaging system, synchronizing the acquired image data, such as a k-space profile, an X-ray absorption profile or a sinogram, with the detected vital sign. For example, the vital sign may be the patient's heart rate, so that the vital sign signal represents the R peak in the patient's electrocardiogram and the image data is acquired in synchronization with the patient's heart rate. This makes it possible to avoid cardiac motion artifacts in the images reconstructed from the acquired image data.

[0021] A camera-based implementation of the vital signs detection system of the present invention in clinical practice achieves, inter alia, (i) better region of interest detection (or skin detection) in multi-dimensional color space, i.e., the DC color of the object / skin can be used for better skin and non-skin segmentation; (ii) better vital signs signal (e.g., PPG signal) signal quality, i.e., the pulsating component has characteristic features in multi-dimensional color space that can be used to distinguish it from distortion (e.g., body movement).

[0022] The invention also relates to a vital signs detection method as defined in claim 12. This vital signs detection method of the invention achieves an increase in the dynamic range and thus the signal-to-noise ratio of the vital signs signal. The invention further relates to a computer program according to claim 14. The computer program of the invention can be provided on a data carrier such as a CD-rom disk or a USB memory stick or the computer program of the invention can be downloaded from a data network such as the World Wide Web. When installed in a computer included in a vital signs detection system, the vital signs detection system is able to operate according to the invention and achieves an increase in the dynamic range and thus the signal-to-noise ratio of the vital signs signal.

[0023] These and other aspects of the invention will be explained with reference to the embodiments described hereinafter and with reference to the accompanying drawings. [Brief description of the drawings]

[0024] [Figure 1] 1 is a schematic side view of a tomographic imaging system incorporating an embodiment of a vital signs detection system of the present invention; [Diagram 2] Schematic diagram showing the time course of modulated illumination, the signal acquired by the camera, and the vital sign information of the presented signal. [Diagram 3] 1 is a schematic side view illustrating a tomographic imaging system incorporating another embodiment of the vital signs detection system of the present invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] In some embodiments, a triggerless synchronization based on video analysis is disclosed. It is based on the insight that for perfect synchronization, the modulation of the spatial average of the frame intensity over time is maximal. The optical switching is continuously adapted with a small delay that maximizes this modulation. Region-of-interest based analysis and other embodiments involving only two light sources are given to increase the robustness of the method.

[0026] FIG. 1 shows a schematic side view of a tomographic imaging system in which an embodiment of the vital signs detection system of the present invention is incorporated. The tomographic imaging system 40 is represented diagrammatically by its frame 41, a patient carrier 42 and an examination zone 43 defined by a gantry. For example, in the case of a computed tomographic system, the frame is formed by a gantry on which an X-ray source and an X-ray detector are rotatably mounted so as to rotate around the examination zone. In another example of a magnetic resonance examination system, the frame constitutes a structure carrying a magnet assembly, and the examination zone is defined by a volume in which the magnet assembly applies a uniform magnetic field and a precise linear gradient magnetic field. For example, the examination zone is formed by (part of) a cylindrically shaped magnet bore of the magnet assembly or by the space between the opposing pole pieces of an open magnet assembly. An illumination system 20 is provided for illuminating the examination zone 43 and comprises an illumination device 21 and an illumination controller 12. The illumination controller 12 drives the illumination device 21 to generate an amplitude modulated illumination of the examination zone. The lighting device may have an array of LED white light sources with significant spectral components in the green wavelength range of 550-600 nm. In this wavelength range, the brightness level of the LED white light sources can be higher than that of the infrared illumination without facing safety issues. The lighting controller 12 is typically configured to drive the lighting device 21 in amplitude modulation mode, with a modulation frequency in the range of 35-45 Hz, for example 40 Hz. This range of modulation frequencies exceeds the time resolution of the human visual system, so that the patient to be examined and the operator or support staff are not disturbed by flickering of the lighting system. The lighting control also communicates to the camera 10 to synchronize the frame rate of the camera, at which image frames from the examination zone (and the patient to be examined) are acquired, with the amplitude modulation of the lighting system. In this way, it is achieved that the camera acquires images at the respective modulation phases of the illumination. The image frames from the camera are provided to a signal processor 11, which derives vital sign information 13, i.e. the rPPG signal, from the image frames.In particular, the signal processor can subtract image frames of the respective modulation phases to separate the vital sign information. Good results are achieved by subtracting successive image frames from different modulation phases. This may be successive images from the examination zone with the illumination on and off. The background signal, which is mainly related to the vital sign information of the examined patient, is removed in the subtraction. Since the modulation frequency is much higher than the temporal variations of the ambient light, for example from the ambient illumination 30 of the room, or from other external factors, the residual signal in the subtraction image is small and in particular inversely proportional to the modulation frequency. The lighting controller can further comprise a phase locking circuit that phase locks the lighting system with the ambient illumination to avoid beats between the illumination of the examination zone and the periodicity of the light output of the ambient illumination. This is beneficial when the ambient illumination is driven directly at the mains frequency (50-60 Hz). This allows the vital sign detection system of the present invention to be installed in an environment where there is ambient illumination driven directly at the mains frequency. This facilitates the deployment of a tomographic imaging system using the vital sign detection system of the present invention.

[0027] FIG. 2 shows a schematic diagram of the time evolution of modulated illumination, the signal acquired by the camera, and the signal representing vital sign information. Trace I mode represents the amplitude modulation of illumination over time. This trace mode has alternating modulation phases 61, 62 of high and low brightness. Trace rPPG-BG represents the remote PPG signal added to the background signal level. This rPPG signal is high during the high illumination phases and is detected by the camera during the low (dark) illumination phases above the background signal. The signal trace rPPG is formed from the subtraction of successive image frames at alternating modulation phases of illumination modulation.

[0028] FIG. 3 shows a schematic side view of a tomographic imaging system incorporating another embodiment of the vital signs detection system of the present invention. In the embodiment of FIG. 3, the lighting controller has a wavelength modulator 51 that controls the lighting device 21 to operate at a temporal wavelength. For example, the lighting device 21 can be equipped with LEDs of different colors that are alternately switched by the wavelength modulator 51. The vital signs detection system in this embodiment is provided with a signal analyzer that is circuit-connected to receive the image frames from the camera 10. The signal analyzer 52 is configured to determine a modulation depth of the luminance values ​​of the image frames between the image frames that depends on the relative phase between the camera frame rate and the wavelength modulation of the illumination. Based on the detected modulation depth of the image frames, the lighting controller 12 generates a wavelength band modulation of the illumination emitted by the lighting device. Also, based on the detected modulation depth as a function of the relative phase, the camera frame rate is synchronized with the modulated illumination, whereby the relative in-phase of the wavelength modulation of the illumination and the camera frame rate is found at the maximum wavelength modulation depth. In particular, the signal analyzer determines the maximum modulation depth depending on the temporal modulation rate of the illumination. The camera frame rate is then set and synchronized with the illumination modulation, which is the maximum frame rate, i.e., the camera frame rate is synchronized with the illumination modulation based on the camera frame rate information, where the wavelength modulation depth between successive image frames is continuously measured by the signal analyzer 52, which adjusts the camera frame rate and the illumination modulation by maintaining the maximum wavelength modulation depth or by maintaining the wavelength modulation depth above a pre-set threshold, so that their relative phases are kept in phase.

[0029] Further details of the implementation of the invention relate to a metric for measuring the quality of synchronization between the light source (switching phase) and the camera (sampling phase). The quality metric is performed online (in a real-time manner) to evaluate the quality of each new incoming frame. If the synchronization is perfectly in phase, the contrast between the multi-wavelength DC values ​​will be maximum (e.g., steepness / large contrast between values ​​in the DC vector); if the synchronization is poor (not in phase), the contrast will be reduced since the frame is obtained from the combined parts of two wavelengths (e.g., each different wavelength contributes to a single frame). Thus, the DC vector will be flatter. In the worst case, where the synchronization is perfectly out of phase, the vector is flat. Thus, we define the DC vector, which measures the DC values ​​of the region of interest pixels at different wavelengths, as follows (e.g., here we use three wavelengths as a showcase): DC = [mean(ROI wavelength1 ), mean(ROI wavelength2 ), mean(ROI wavelength3 )]

[0030] For ease of interpretation and optimization, since the total intensity does not matter here, only the contrast, we normalize the DC vector by its total energy (e.g., L1-norm or L2-norm): TIFF2024528904000002.tif2144

[0031] This is the normalized DC vector of the multi-wavelength values. This vector is measured in real-time for every new frame.

[0032] In case of poor synchronization, the time delay (with respect to the cameras) is determined so that the vector has maximum contrast, which means that the vector should deviate as much as possible from the [1,1,1] direction (e.g. flat). Therefore, the following optimization function can be adopted: TIFF2024528904000003.tif2367

[0033] This maximizes the L2 distance between the normalized DC vector and the undesired [1,1,1] direction. Note that other distances can also be used, such as maximizing the (cosine) angle between the two vectors. The output of this optimization function is the time delay at which the camera needs to compensate for its asynchronous phase and resynchronize with the light source.

[0034] In some embodiments, the average luminance Ii of each frame i is calculated. When the synchronization is perfect, the modulation amplitude Ii is maximum as shown in FIG. 4a. When the light switching and the image acquisition are out of sync, the modulation amplitude drops as in FIG. 4(b). In some embodiments, it is proposed to continuously evaluate this modulation amplitude and preferentially adapt the phase of the light switching frequency if a drop is detected. Alternatively, a small positive or negative delay is repeatedly introduced in the time course of the light switching. One effect of this is that the light switching and the frame acquisition are kept synchronized. In some embodiments, the algorithm can be adapted in the time domain, and in other embodiments, the algorithm is adapted in the spatial domain. In the time domain, it is known that the modulation frequency of the light source (n times the camera sampling rate) gives a delay that allows the spectral peak of the video signal at the modulation frequency to be as high as possible. To optimize in the spatial domain, an optimized subset of pixels is used to increase the DC contrast between wavelength bands within this subset. This allows the delay between acquisitions to be reduced.

[0035] In some embodiments, the rPPG data collection can be changed, for example stopped for a few frames, to interleave the synchronous measurement. For the synchronous measurement in this embodiment, the phase of the optical switching can be intentionally offset by a quarter of a period to introduce a phase angle of 90° between the optical switching and the frame acquisition, thereby effectively realizing the situation between Fig. 4a and Fig. 4b. This sets the system to the operating point with the maximum derivative of the modulation amplitude with respect to the phase. This can result in the following technical effects: a) a small deviation in phase results in a maximum change in the modulation amplitude, and b) the sign of this amplitude change carries the information of whether to increase or decrease the phase. After the synchronous measurement, which can last only a few cycles, the system can switch to the same phase to continue the rPPG data collection.

[0036] One of the challenges overcome by the present invention is that the light generated by the modulator 21 is not optimal and not synchronized. In this embodiment, the light source from the modulator 21 is controlled from the same computer that finally receives the frames, and the time of frame arrival can be used for frequency synchronization. Thus, every time a frame arrives at the signal processing unit 11, the light source of the modulator 21 is switched. One of the technical advantages of this approach is that both the camera 10 and the light source operate at exactly the same frequency (without any accumulating error). However, the phase (delay) between both signals is still unknown, and the method described in the present invention can be used to determine it and perfectly synchronize the light switching and the frame acquisition.

Claims

1. A camera configured to acquire an image frame from an inspection zone, A signal processor configured to derive vital sign information from the acquired image frame, An illumination controller configured to control illumination of the inspection zone, generate time modulation of the illumination, and synchronize a frame rate of the camera with the modulated illumination, wherein the illumination controller is configured to generate wavelength band modulation of the illumination of the inspection zone and synchronize the frame rate of the camera according to the modulation depth, A vital sign detection system having, The vital sign detection system further includes a signal analyzer configured to synchronize a camera frame rate and modulated illumination, and the signal analyzer is configured to detect a modulation depth of luminance of an image frame detected between consecutive image frames.

2. The vital sign detection system according to claim 1, wherein the signal processor is configured to form a differential image from image frames continuously acquired with different modulation phases.

3. The vital sign detection system according to claim 1, further comprising an illumination device configured to illuminate the inspection zone, wherein the illumination device is configured to emit illumination including spectral components within a spectral absorption band of hemoglobin.

4. The vital sign detection system according to claim 1, wherein the time modulation is amplitude modulation having a modulation frequency in a range of 20 to 60 Hz.

5. The vital sign detection system according to claim 2, wherein the frame rate is twice the time modulation frequency of the illumination.

6. The vital sign detection system according to claim 2, wherein the camera is configured to operate in a phase-locked mode.

7. The vital sign detection system according to claim 5 or 6, wherein the time modulation frequency of the illumination is equal to the time modulation frequency of ambient light.

8. The vital sign detection system according to claim 1, wherein the illumination controller is configured to synchronize the frame rate of the camera by adjusting the frame rate of the camera to a maximum value of the modulation depth between consecutive image frames.

9. The vital sign detection according to claim 1 or 8, wherein the illumination control is configured to perform an interleaved resynchronization measurement in which a π / 2 phase angle is introduced into the wavelength band modulation and the image frame rate.

10. The vital sign detection system according to claim 1 or 8, wherein the camera is a monochrome camera.

11. A method for performing vital sign detection, comprising: applying a time modulation of illumination in an inspection zone; synchronizing the acquisition of an image frame from the inspection zone with the modulated illumination; deriving vital sign information from the acquired image frame; detecting a modulation depth of luminance of the image frames detected between consecutive image frames; generating a wavelength band modulation of the illumination in the inspection zone; synchronizing a frame rate of a camera according to the modulation depth. A method having the above steps.

12. The vital sign detection method according to claim 11, wherein the time modulation of the illumination is realized as a temporal wavelength band modulation, a modulation depth of luminance of consecutive image frames between image frames is detected, and the acquisition of the image frames is synchronized according to the modulation depth.

13. applying a time modulation of illumination in an inspection zone; synchronizing the acquisition of an image frame from the inspection zone with the modulated illumination; deriving vital sign information from the acquired image frame; detecting a modulation depth of luminance of the image frames detected between consecutive image frames; generating a wavelength band modulation of the illumination in the inspection zone; synchronizing a frame rate of a camera according to the modulation depth. A computer program having instructions for performing the above steps.

Images