The basic concept of camera dynamic range
The so-called wide dynamic reality means that the camera can also see the ratio of the illuminance of the brightest and darkest parts of the image. The “dynamic range†broadly refers to the range of possible changes in a changeable object, that is, the region between the lowest end of the change value and the highest end of the pole. The description of this region is generally between the highest point and the lowest point. Difference. The “dynamic range†of the camera refers to the ability of the camera to adapt to the reflection of the light in the shooting scene, specifically the range of brightness (contrast) and color temperature (contrast). That is, the range of the camera's most “dark†and “brightest†adjustment of the image is the ratio of the brightest tone to the darkest tone in the still image or video frame. While hue can present precise details in an image or frame, as a ratio of two tones, the unit of dynamic range can be decibels, bits, or files, or simply expressed as a ratio or multiple. The conversion method between various units is shown in Table 1.
Table 1 only lists the 20-speed dynamic range because it covers almost all the dynamic ranges that the human eye can discern. Exceeding the dynamic range of these gears has little practical significance. The human eye can distinguish such a wide dynamic range, because the pupil, the iris, the retina and the related muscles interact and dynamically adjust when the person is watching the real scene. At the same time, the brain integrates all the “exposure elements†into A coherent image, with extremely accurate reflection of the very bright or very dull colors in the real scene.
Compared with the human eye, the exposure time (collecting photons) of all the photosensitive cells is the same for standard CCD and CMOS image sensors. The photosensitive unit collects more photons for the bright part of the scene and less for the dark part. However, the number of photons that can be collected by the light-sensing unit is limited by the well capacity. Therefore, the light-sensing light-sensing unit that captures an object may overflow or saturate. To prevent this, you can reduce the exposure time. However, if you do this, the light-sensing unit that captures the darker tone of the object may not be able to collect enough photons. Therefore, for a typical single-exposure image sensor, the upper limit of the dynamic range is limited by the well capacity of the photosensitive unit, and the lower limit is limited by the signal-to-noise ratio of the photosensitive unit. Therefore, the dynamic range of a CCD camera device refers to the ratio of the saturation voltage of the output to the peak-to-peak voltage of the noise in the dark field, ie,
Dynamic Range = Usat/UNp-p(1)
(1) In the formula, Usat is the output saturation voltage; UNP-P is the peak-to-peak noise.
Obviously, the dynamic range can also be defined and calculated in such a way that the ratio of the maximum amount of charge that can be stored in the CCD well and the amount of charge determined by the noise; the value is also the peak value of the signal at the output and the rms noise voltage Ratio (usually expressed in dB), ie dynamic range = USp-p/UNp-p (2) Formula (2) USp-p is the output signal peak voltage.
Therefore, wide dynamics means that particularly bright parts and particularly dark parts of the scene can be seen particularly clearly at the same time. The wide dynamic range is the ratio of the brightest signal value to the brightest signal value that the image can distinguish.
Obviously, there are two main ways to determine the dynamic range of the camera's imager: one is to use the relevant information of the basic circuit in the sensor and the image processor to calculate from the above formula; the other is to use the grayscale test card and experimental instrument. To collect and observe images, and measure the image level. Although computational methods can be used to theoretically calculate the limits of the dynamic range, people generally tend to use measurement methods because they reflect the user's actual experience with the imaging effect of the camera. The following describes the actual testing method of the dynamic range of the camera by three major foreign manufacturers.
JVC's Dynamic Range Test Method
The test method for camera dynamic range may not be the same, but the test principle is similar. Here is a basic test method of JVC:
The equipment and conditions required to test the dynamic range of the camera The following equipment is required to test the dynamic range of the camera:
Transmission gray card and reflection gray card;
·Brightness adjustable backlight light box and adjustable illumination light source;
Video monitor and waveform monitor;
Light meter or illuminometer;
· Standard internal lens and so on.
Conditions for testing the dynamic range of the camera: It is necessary to perform in a dark room.
Basic methods and steps for testing camera dynamic range
· The first step: Install two sets of dual grayscale test cards in the same vertical plane of a table in a darkroom. One set of transmission grayscale cards uses a backlight source with adjustable brightness as a constant reference to adjust the backlight source brightness to ensure that The front center confirms that the divergent illuminance of the white block surface is 2500Lx; another set of reflective grayscale cards uses a brightness-adjustable irradiation light source located on the front surface thereof to determine the critical value of the dynamic range;
· The second step: set the height of the camera to be tested and the center of the grayscale test card to the same level, and maintain an angle of 90° with the vertical plane of the grayscale test card, and at the same time enable the camera lens view angle to cover 2 sets of grayscale test cards;
Step 3: Connect the camera's output signal to the video monitor and waveform monitor;
· Step 4: After the camera is powered on, turn on the extended dynamic range function to adjust the brightness of the front illumination source to 2500Lx. Obviously, under this illumination is overexposed (factory standard illuminance is mostly 2000Lx), at this time, the white end stripes of the reflective grayscale card may appear gradation mixing, that is, there are 2 or more grayscale bars showing the same White, and can not distinguish the difference in brightness;
• Step 5: Slowly reduce the brightness of the light source continuously and observe and record the top level of the reflected grayscale test card waveform from the waveform monitor. When the top level begins to decrease due to the decrease of the illuminance of the light source, the illuminance value (such as L1) at this time is recorded. This illuminance value is the upper limit of the dynamic range of the camera. The camera at this time should exactly show the brightness level difference between the bright white stripes;
• Step 6: Then continue to slowly lower the brightness, and continue to observe and record the top level of the reflected grayscale test card waveform from the waveform monitor. When the top level is no longer decreasing due to the decrease of the illumination of the light source, the illuminance value (such as L2) at this time is recorded. This illuminance value is the lower limit of the dynamic range of the camera. At this time, the brightness level difference between the two dark gray and black stripes in the grayscale card image captured by the camera should just disappear and be mixed into a black one.
Test result calculation
The formula for calculating the dynamic range using the above actual test method is as follows:
Dynamic range = 20logL1/L2 (dB) (3) The JVC CCD wide dynamic camera TK-WD310EC measured the top level change from 2200Lx to 1.1Lx by the above test method, and the dynamic range was obtained from equation (2). 20log2200/1.1=66dB
Using the above test method, the dynamic range of the top level change of a CCD wide dynamic camera of a company from 1500Lx to 5Lx is measured as dynamic range=20log1500/5=49dB.
JEITA's Dynamic Range Test Method
JEITA is an abbreviation of the Japan Electronic Information Technology Industry Association. Its grayscale test card is also used for measuring the dynamic range of a camera. In fact, whether it is possible to accurately determine the dynamic range, an important limiting factor is whether the grayscale test card can effectively measure the full value of the dynamic range. For example, in the Kodak Q-14 test card, the scale difference between adjacent grayscale grids is 1/3 aperture (f-stop) and can only measure a dynamic range of 5.66 stops or about 34 dB.
When using the JEITA method to measure the dynamic range and dynamic range expansion ratio, the gamma value of the grayscale test card is specified as 2.2. There are a total of ten grayscale levels, and the dynamic range that can be measured is basically the same as the Q-14 grayscale test card. According to the specification of this method, two gray-scale test cards are placed side by side, with the screen between them. Then use two different illumination sources to illuminate the test cards on both sides of the screen, as shown in Figure 2.
The JEITA method stipulates that the brighter end of the test card is to increase the illumination intensity or increase the aperture until the two brightest gray levels can be distinguished, and then the darker end of the test card is continuously reduced in intensity until the most The light grey level (white) reaches 50 IRE. The expansion ratio (dB) of the dynamic range can be calculated by using the formula (3) in the aforementioned test method, that is, the dynamic range expansion ratio (dB) = 20 log (L3/L4)
Although this method calculates the dynamic range expansion value, it completely ignores the ability of the imager to capture midtones. The JEITA method does not overcome the major drawback of the double-exposure CCD sensor because the method only focuses on distinguishing different grayscale values ​​within the upper and lower tonal ranges.
The drawback of the JEITA method is that all test devices are not clearly specified; there is no indication of how to place the lighting device, what type of light is used, or even how to accurately measure the light intensity. This means that changes in the experimental setup and measurement conditions will affect the final measurement results. It is worth noting that the JEITA method measures the dynamic range extension value rather than the overall measurement range because the method does not specify how to determine the reference dynamic range or the overall dynamic range.
Pixim's Dynamic Range Measurement Method
In order to eliminate the above deficiencies and make the measurement experiment repeatable so that all the tone levels can be observed and compared at the same time during the measurement process, Pixim uses a set of customized instrumentation to measure the dynamic range. The kit contains a light box that uses a 700 watt incandescent light to backlight the transmissive step card. The step wedges used for the measurements were all manufactured by SinePatterns LLC. The two wedges overlapped to a maximum density range of 0.1 to 6.1 or approximately 120 dB.
It is obviously not easy to accurately display a wide dynamic scene on a computer monitor or in a printed document, but Pixim DPS technology can well capture ultra-wide dynamic images while presenting monotonous grayscale and intermediate grayscale responses.
The same picture taken with the CCD camera using the double exposure method, please note that although the camera claims to have a high dynamic range, in fact it is still possible to highlight the part of the scene in the picture and the cross-talk phenomenon of the middle tone Observe its obvious limitations. In addition, the response of the camera to the midtones is too flat. Compared with the monotonous response of the Pixim DPS camera, it is easy to see who is superior or not.
Conclusion
The concept of dynamic range and three specific test methods are described above, which can be used by users for reference. From the wide dynamic performance of the two cameras they tested, CCD wide dynamic cameras are not as good as CMOS wide dynamic cameras. Although the sensitivity of the CCD is high, the response speed is low and it is not suitable for the high resolution progressive scan method adopted by the high-definition surveillance camera. Therefore, the high-definition surveillance cameras use CMOS imaging devices. And because CMOS imaging devices have the advantages of wide dynamic range, high-speed digital readout, no column readout noise or fixed pattern noise, faster operating speed, and lower power consumption, it can more easily realize network and intelligence. Change. Obviously, CMOS cameras have great potential, and their excellent performance in terms of dynamic range, etc. will gradually replace CCD cameras and occupy the market in the future.
The so-called wide dynamic reality means that the camera can also see the ratio of the illuminance of the brightest and darkest parts of the image. The “dynamic range†broadly refers to the range of possible changes in a changeable object, that is, the region between the lowest end of the change value and the highest end of the pole. The description of this region is generally between the highest point and the lowest point. Difference. The “dynamic range†of the camera refers to the ability of the camera to adapt to the reflection of the light in the shooting scene, specifically the range of brightness (contrast) and color temperature (contrast). That is, the range of the camera's most “dark†and “brightest†adjustment of the image is the ratio of the brightest tone to the darkest tone in the still image or video frame. While hue can present precise details in an image or frame, as a ratio of two tones, the unit of dynamic range can be decibels, bits, or files, or simply expressed as a ratio or multiple. The conversion method between various units is shown in Table 1.
Table 1 only lists the 20-speed dynamic range because it covers almost all the dynamic ranges that the human eye can discern. Exceeding the dynamic range of these gears has little practical significance. The human eye can distinguish such a wide dynamic range, because the pupil, the iris, the retina and the related muscles interact and dynamically adjust when the person is watching the real scene. At the same time, the brain integrates all the “exposure elements†into A coherent image, with extremely accurate reflection of the very bright or very dull colors in the real scene.
Compared with the human eye, the exposure time (collecting photons) of all the photosensitive cells is the same for standard CCD and CMOS image sensors. The photosensitive unit collects more photons for the bright part of the scene and less for the dark part. However, the number of photons that can be collected by the light-sensing unit is limited by the well capacity. Therefore, the light-sensing light-sensing unit that captures an object may overflow or saturate. To prevent this, you can reduce the exposure time. However, if you do this, the light-sensing unit that captures the darker tone of the object may not be able to collect enough photons. Therefore, for a typical single-exposure image sensor, the upper limit of the dynamic range is limited by the well capacity of the photosensitive unit, and the lower limit is limited by the signal-to-noise ratio of the photosensitive unit. Therefore, the dynamic range of a CCD camera device refers to the ratio of the saturation voltage of the output to the peak-to-peak voltage of the noise in the dark field, ie,
Dynamic Range = Usat/UNp-p(1)
(1) In the formula, Usat is the output saturation voltage; UNP-P is the peak-to-peak noise.
Obviously, the dynamic range can also be defined and calculated in such a way that the ratio of the maximum amount of charge that can be stored in the CCD well and the amount of charge determined by the noise; the value is also the peak value of the signal at the output and the rms noise voltage Ratio (usually expressed in dB), ie dynamic range = USp-p/UNp-p (2) Formula (2) USp-p is the output signal peak voltage.
Therefore, wide dynamics means that particularly bright parts and particularly dark parts of the scene can be seen particularly clearly at the same time. The wide dynamic range is the ratio of the brightest signal value to the brightest signal value that the image can distinguish.
Obviously, there are two main ways to determine the dynamic range of the camera's imager: one is to use the relevant information of the basic circuit in the sensor and the image processor to calculate from the above formula; the other is to use the grayscale test card and experimental instrument. To collect and observe images, and measure the image level. Although computational methods can be used to theoretically calculate the limits of the dynamic range, people generally tend to use measurement methods because they reflect the user's actual experience with the imaging effect of the camera. The following describes the actual testing method of the dynamic range of the camera by three major foreign manufacturers.
JVC's Dynamic Range Test Method
The test method for camera dynamic range may not be the same, but the test principle is similar. Here is a basic test method of JVC:
The equipment and conditions required to test the dynamic range of the camera The following equipment is required to test the dynamic range of the camera:
Transmission gray card and reflection gray card;
·Brightness adjustable backlight light box and adjustable illumination light source;
Video monitor and waveform monitor;
Light meter or illuminometer;
· Standard internal lens and so on.
Conditions for testing the dynamic range of the camera: It is necessary to perform in a dark room.
Basic methods and steps for testing camera dynamic range
· The first step: Install two sets of dual grayscale test cards in the same vertical plane of a table in a darkroom. One set of transmission grayscale cards uses a backlight source with adjustable brightness as a constant reference to adjust the backlight source brightness to ensure that The front center confirms that the divergent illuminance of the white block surface is 2500Lx; another set of reflective grayscale cards uses a brightness-adjustable irradiation light source located on the front surface thereof to determine the critical value of the dynamic range;
· The second step: set the height of the camera to be tested and the center of the grayscale test card to the same level, and maintain an angle of 90° with the vertical plane of the grayscale test card, and at the same time enable the camera lens view angle to cover 2 sets of grayscale test cards;
Step 3: Connect the camera's output signal to the video monitor and waveform monitor;
· Step 4: After the camera is powered on, turn on the extended dynamic range function to adjust the brightness of the front illumination source to 2500Lx. Obviously, under this illumination is overexposed (factory standard illuminance is mostly 2000Lx), at this time, the white end stripes of the reflective grayscale card may appear gradation mixing, that is, there are 2 or more grayscale bars showing the same White, and can not distinguish the difference in brightness;
• Step 5: Slowly reduce the brightness of the light source continuously and observe and record the top level of the reflected grayscale test card waveform from the waveform monitor. When the top level begins to decrease due to the decrease of the illuminance of the light source, the illuminance value (such as L1) at this time is recorded. This illuminance value is the upper limit of the dynamic range of the camera. The camera at this time should exactly show the brightness level difference between the bright white stripes;
• Step 6: Then continue to slowly lower the brightness, and continue to observe and record the top level of the reflected grayscale test card waveform from the waveform monitor. When the top level is no longer decreasing due to the decrease of the illumination of the light source, the illuminance value (such as L2) at this time is recorded. This illuminance value is the lower limit of the dynamic range of the camera. At this time, the brightness level difference between the two dark gray and black stripes in the grayscale card image captured by the camera should just disappear and be mixed into a black one.
Test result calculation
The formula for calculating the dynamic range using the above actual test method is as follows:
Dynamic range = 20logL1/L2 (dB) (3) The JVC CCD wide dynamic camera TK-WD310EC measured the top level change from 2200Lx to 1.1Lx by the above test method, and the dynamic range was obtained from equation (2). 20log2200/1.1=66dB
Using the above test method, the dynamic range of the top level change of a CCD wide dynamic camera of a company from 1500Lx to 5Lx is measured as dynamic range=20log1500/5=49dB.
JEITA's Dynamic Range Test Method
JEITA is an abbreviation of the Japan Electronic Information Technology Industry Association. Its grayscale test card is also used for measuring the dynamic range of a camera. In fact, whether it is possible to accurately determine the dynamic range, an important limiting factor is whether the grayscale test card can effectively measure the full value of the dynamic range. For example, in the Kodak Q-14 test card, the scale difference between adjacent grayscale grids is 1/3 aperture (f-stop) and can only measure a dynamic range of 5.66 stops or about 34 dB.
When using the JEITA method to measure the dynamic range and dynamic range expansion ratio, the gamma value of the grayscale test card is specified as 2.2. There are a total of ten grayscale levels, and the dynamic range that can be measured is basically the same as the Q-14 grayscale test card. According to the specification of this method, two gray-scale test cards are placed side by side, with the screen between them. Then use two different illumination sources to illuminate the test cards on both sides of the screen, as shown in Figure 2.
The JEITA method stipulates that the brighter end of the test card is to increase the illumination intensity or increase the aperture until the two brightest gray levels can be distinguished, and then the darker end of the test card is continuously reduced in intensity until the most The light grey level (white) reaches 50 IRE. The expansion ratio (dB) of the dynamic range can be calculated by using the formula (3) in the aforementioned test method, that is, the dynamic range expansion ratio (dB) = 20 log (L3/L4)
Although this method calculates the dynamic range expansion value, it completely ignores the ability of the imager to capture midtones. The JEITA method does not overcome the major drawback of the double-exposure CCD sensor because the method only focuses on distinguishing different grayscale values ​​within the upper and lower tonal ranges.
The drawback of the JEITA method is that all test devices are not clearly specified; there is no indication of how to place the lighting device, what type of light is used, or even how to accurately measure the light intensity. This means that changes in the experimental setup and measurement conditions will affect the final measurement results. It is worth noting that the JEITA method measures the dynamic range extension value rather than the overall measurement range because the method does not specify how to determine the reference dynamic range or the overall dynamic range.
Pixim's Dynamic Range Measurement Method
In order to eliminate the above deficiencies and make the measurement experiment repeatable so that all the tone levels can be observed and compared at the same time during the measurement process, Pixim uses a set of customized instrumentation to measure the dynamic range. The kit contains a light box that uses a 700 watt incandescent light to backlight the transmissive step card. The step wedges used for the measurements were all manufactured by SinePatterns LLC. The two wedges overlapped to a maximum density range of 0.1 to 6.1 or approximately 120 dB.
It is obviously not easy to accurately display a wide dynamic scene on a computer monitor or in a printed document, but Pixim DPS technology can well capture ultra-wide dynamic images while presenting monotonous grayscale and intermediate grayscale responses.
The same picture taken with the CCD camera using the double exposure method, please note that although the camera claims to have a high dynamic range, in fact it is still possible to highlight the part of the scene in the picture and the cross-talk phenomenon of the middle tone Observe its obvious limitations. In addition, the response of the camera to the midtones is too flat. Compared with the monotonous response of the Pixim DPS camera, it is easy to see who is superior or not.
Conclusion
The concept of dynamic range and three specific test methods are described above, which can be used by users for reference. From the wide dynamic performance of the two cameras they tested, CCD wide dynamic cameras are not as good as CMOS wide dynamic cameras. Although the sensitivity of the CCD is high, the response speed is low and it is not suitable for the high resolution progressive scan method adopted by the high-definition surveillance camera. Therefore, the high-definition surveillance cameras use CMOS imaging devices. And because CMOS imaging devices have the advantages of wide dynamic range, high-speed digital readout, no column readout noise or fixed pattern noise, faster operating speed, and lower power consumption, it can more easily realize network and intelligence. Change. Obviously, CMOS cameras have great potential, and their excellent performance in terms of dynamic range, etc. will gradually replace CCD cameras and occupy the market in the future.
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Formica have a history of hundreds of years, and they launch a large number of popular designs every year, which are loved by designers all over the world.
Sinowolf keeps up with the design trend, and has developed most of the popular designs of Formica. Authoritative certifications such as British ITS, various testing standards, etc., achieve the same color and surface treatment, and provide you with a one-step edge sealing solution!
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