Wide dynamic function of security camera

Name and definition

Application scenario

In the field of security video surveillance, the wide dynamic range of the camera has always been a very important function. Especially in some special industries or scenes, the wide dynamic function must be used. For example, a miniature pinhole camera installed in a bank ATM machine to monitor users, because the camera is generally irradiated from indoor to outdoor, with a strong contrast between light and dark, it is difficult for ordinary cameras to see faces clearly, and a camera with a bandwidth dynamic function must be used. . For another example, a camera installed in a hotel lobby illuminates the entrance and exit passage from the indoor to the outdoor lobby. If you want to see the characteristics of the people entering and exiting, especially the facial features, you must use a camera with a bandwidth dynamic function.
Typical scenes used in wide dynamic monitoring:

  1. Entrance with daylight outside and dark indoor environment.
  2. Parking lots or tunnels with daylight outside but low indoor brightness.
  3. Outdoor scenes with direct sunlight and dark shadows.
  4. Office buildings or shopping centers with lots of windows reflecting light.

Definition

The general experience is that when the camera shoots video imaging, if the light of the entire surveillance scene is complicated, some places are very dark, and some places are very bright, then the picture presented by the camera may be dark, and nothing can be seen, or The bright place is exposed to transition, and nothing can be seen clearly. In order to describe the recognition ability of the camera when shooting scenes with strong light and dark contrast, we introduced the concept of dynamic range, and the wide dynamic and narrow understanding shows the ability of the camera to recognize light and dark contrast scenes when shooting.

Other names

Wide dynamic range, WDR, English is wide dynamic range, that is, wide dynamic range. After the security sensor entered the CMOS era, wide dynamic is also called HDR, that is, High-Dynamic Range (high dynamic range). Especially in the film and television industry, the display field, mobile phone screens, photography, etc. are generally called HDR. The security industry also refers to wide dynamics as HDR, which is considered to be in line with these industry sectors.

Sony, which leads the development trend of security sensors, has these names for wide dynamic functions: DOL-HDR or DOL-WDR, both have the same meaning.

DOL(digital overlap) WDR stands for digital overlap wide dynamic range, it’s the next generation Sensor-based WDR technology exclusively developed by Sony.

FROM SONY WEBSITE.

Difference from backlight compensation(BLC)

The problem that the backlight compensation solves is that the monitoring target light is too dark because the background light is too strong, and the problem is not clear. After turning on the backlight compensation, the target objects in the foreground can be seen clearly, but the previously bright areas will be overexposed.

Wide dynamic is to make the whole picture can be clearly visible, there is no over-exposed area, and no obvious darkened area.

120dB

Regarding the wide dynamic value of 120dB, this is actually a ratio, which means that the camera can recognize the ratio of the illuminance between the brightest part and the darkest part of the shot is 1 000 000:1, which is 106:1.

106: Why is 120dB corresponding to 1? Our artificially defined wide-dynamic ratio formula:

dB = 20 log (illuminance of the brightest part [lx] / darkest part [lx])

20log(1 000 000/1)=20 X 6=120dB

In fact, dB is a ratio, a pure counting method, without any unit indication. It has different names in different fields, so it also represents different practical meanings. Such as sound size, signal strength, etc.

The meaning of dB is actually very simple, that is, a very large (followed by a long string of 0s) or a very small (preceded by a long string of 0) numbers are expressed relatively briefly. For example, the wide dynamic range expressed as 120dB is obviously more intuitive and concise than using 1 000 000:1.

For other fields, for power, dB = 10Xlog(). For voltage or current, dB = 20Xlog().

120dB is also a special ratio, and the dynamic range that the human eye can recognize is also about 120dB. (Panasonic believes that the dynamic range of the human eye is about 80-110dB)

Method to realize

In the analog and standard definition era, a well-known wide-dynamic solution product is SONY’s effio-p solution. I still have SONY effio-p wide-dynamic video video. Now it seems that the wide-dynamic effect is very general, but this effect was the same in the past. It is already very good.

In the analog era, it is difficult to implement wide dynamic functions. Due to the limitation of processor performance, CCD sensors must support wide dynamic functions, that is, dual-speed CCD sensors must be used.

Entering the era of network HD, wide dynamic technology has further developed. Processor performance and functions are continuously enhanced; more CMOS sensors can support the linear superposition of multi-frame images, and the wide dynamic effect is getting better and better. To sum up, the realization of wide dynamic technical effects is mainly in the following ways:

  1. Use a dual-speed/high-speed sensor, that is, use a sensor with a wide dynamic function (Sensor Build-In WDR). Capture multiple images at the same time (for different effects of brightness and darkness, Sony CMOS sensor generally uses 3 frames), and multiple frames are superimposed. This wide dynamic effect is generally called true wide dynamic;
  2. Wide dynamic based on DSP processor. At present, most processors on the market can do it (the ISP used by analog high-definition cameras, the encoding chip used by network cameras), because the technologies accumulated by various manufacturers are different, and the wide dynamic effects presented are also different. This wide dynamic is generally called digital-WDR/dWDR.

Measurement methods

The public safety industry standard GA/T 1127—2013 “General Technical Requirements for Security Video Surveillance Cameras” sets out a method for measuring the wide dynamic capabilities of cameras. Through the number of resolvable gray scales, dynamic range, gray scale linearity, gray scale of gray scales, the number of resolvable color areas, tailing impedance, edge contrast, square array definition and signal-to-noise ratio, etc. nine To comprehensively evaluate the wide dynamic capabilities of the camera. This method is scientific and reasonable, and is introduced as follows:

Equipment required for testing

  • Wide dynamic test card. Use a wide dynamic range test card, including grayscale, grayscale area, color square and various graphics. The collected pictures are then used to calculate and measure various performance parameters of the wide dynamic camera with analysis software.
  • Light box. The light intensity of the light box light source should reach 70 000 lx~100 000 lx where the test card is placed; the color temperature of the light box light source should be between cold fluorescence and D50, that is, between 3 500 K and 5 100 K color temperature.
  • Lens. Manual iris lens. It is recommended to use F1.4/12.5mm lens.
  • testing software. The specific analysis software used is not provided in the standard. It is recommended to use ColorSpace, Imatest, IQL, IE, DNP, DxO and other image testing software tools.

Test Methods
Set up the camera, adjust the lens, and capture a still picture. Then use analysis software to calculate and analyze the wide dynamic performance of the camera.

Measure and calculate the resolvable order of gray scale, dynamic range, linearity of brightness response, gray in gray scale domain, the number of resolvable color squares, trailing impedance, edge contrast, square array definition, signal-to-noise ratio 9 items .

Weighted calculation
Assign weights to the above 9 different items and then calculate:

project nameWeights
Dynamic range (DR)16
gray_steps16
color_patches16
linearity16
grayness16
smear_resistance16
edge_contrast16
grid_clarity16
SNR16
Wide dynamic performance allocation weight table

According to the weights, we can calculate a comprehensive comprehensive evaluation, the calculation method is as follows:

Comprehensive Evaluation = 16 X (DR/100) + 16 X (gray_steps/2l) + 16 X (color_patches/84) + 16 X linearity + 16 X grayness + 16 X smear_resistance + 16 X (edge_contrast/200) + 16 X grid_clarity + 16 X (SNR/60)

The problem

After using the wide dynamic function, it will affect the authenticity and color of the image, which is unavoidable. Common problems are as follows:

  • Displacement afterimage. Moving objects in the scene too fast or the exposure time is too long, resulting in moving afterimages.
  • Ghost. When using multiple exposures to generate an image, the same moving object may be acquired at different positions. After being superimposed, the picture will show a ghost effect.
  • Flicker caused by other light sources. In the program settings, it is usually assumed that the lighting conditions remain unchanged. Modulated light sources such as fluorescent lamps usually cause problems, producing artifacts that appear as bars or visible pulses.
  • Fringe noise. A certain amount of randomly distributed noise in the image is usually acceptable. However, when digital processing is performed, visible streak noise may sometimes be generated.
  • Cartoonization and over-sharpening. WDR images may contain too many repetitive tones and enhanced details to be difficult to display on a standard camera. As a result, the image may appear obvious cartoonish and unnatural style.
  • Color cast. Different pixel processing methods may cause color reproduction artifacts, such as wrong colors or too many colors.
  • Purple fringe. Because of the chromatic aberration of the lens, the effect of purple fringing or blue fringing may be seen beside the sharp edges of the image.The light is refracted differently in the lens, so there will be a slight offset or out of focus on the sensor. This effect is usually stronger at the edge of the sensor. The sensitivity of WDR cameras to chromatic aberration is higher than that of traditional cameras, because the hue of the dark part of the image has a greater impact on the image.
  • Lens flare and halo. When light enters the lens, some of the light will scatter or sparkle in the lens, thus reaching the wrong position on the sensor, causing different types of artifacts.

The most common artifact is lens flare, which occurs when most lenses are facing strong light sources such as sunlight. Another effect called halation reduces the contrast and color saturation in a larger area of the image. These two effects are more serious when there are strong light sources in the image, scenes with wide dynamic range, dirty lenses, or dust in the lenses. It is possible to reduce glare and halo by installing a dust cover on the camera, but WDR cameras that wish to achieve a larger dynamic range are still limited by the scattered light in the optical system.

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