Contrast strong and dynamic camera technology development

Dynamic technology is a technique that allows the camera to see the characteristics of the image under very strong contrast. When a high-luminance area illuminated by a strong light source (daylight, lamps, or reflection, etc.) and an area with relatively low relative brightness such as shadow and backlight are simultaneously present in the image, a bright area of ​​the image output by the camera will appear white due to overexposure. The dark area becomes black due to underexposure, which seriously affects the image quality. The performance of the camera in the brightest and dark areas in the same scene is limited. This limitation is usually referred to as the "dynamic range."

In some situations where the contrast between light and shade is too large, the general camera is limited by the CCD's photosensitivity characteristics, and the captured image tends to be too bright or the foreground is too dark. In response to this situation, wide dynamic technology came into being and solved this problem better. Before this, the traditional camera will generally adopt the backlight compensation function to adapt to the occasion with large light contrast.

When the object in the field of view of the conventional camera is in high-brightness background light, it needs to look at the object at the entrance or outside the window. Usually, the central backlight compensation (BLC) mode is used, which is mainly to improve the brightness of the central part of the field of view and reduce the field of view. Part of the brightness to achieve the purpose of seeing objects in the central location.

Backlight compensation, also known as backlight compensation, divides the picture into several different areas, each of which is exposed separately. In some applications, the field of view may contain a very bright area, and the included subject is surrounded by a bright field. The picture is dark and has no hierarchy. Since the signal level detected by the AGC is not low at this time, the gain of the amplifier is very low, and the brightness of the screen body cannot be improved. When the backlight compensation is introduced, the camera only detects one subfield of the entire field of view. The average signal level of this area determines the operating point of the AGC circuit. Because the average level of the sub-area is very low, the AGC amplifier will have a higher gain, so that the amplitude of the output signal is increased, so that the subject on the monitor is clear, greatly reducing the subjective brightness difference between the background picture and the subject picture. The visibility of the field of view is improved. Although backlight compensation improves the brightness of the subject, the image quality deteriorates more or less.

The technique of wide dynamics is to expose twice at the same time, once fast, once slow, and then combined to make it possible to see bright and dark objects on the screen at the same time. Although both are measures taken to overcome the problem of seeing the target in a strong backlight environment, the backlight compensation is at the expense of the contrast of the screen, so in a sense, the wide dynamic technology is the backlight compensation. upgrade.

Since the natural light arrangement is from 100,000 Lux to 0.00035 Lux at night, the indoor illumination is 100 Lux, and the illumination of the outside scenery may be 10,000 Lux, so when the camera looks out of the room from the window, the contrast between the two is 10000/100=100:1. This contrasting eye can easily see that because the human eye can handle the contrast ratio of 1000:1, the traditional security surveillance camera cannot. It has only 3:1 contrast function.

Therefore, when a security surveillance camera is used in a scene where the contrast between light and darkness is large or the backlight is turned on, the bright area in the entire image is overexposed and the dark area is underexposed, while the brightest and darkest areas of the image cannot be clearly seen. Such as in the bank savings, import and other important places. Since the strong light from the outside of the window and the soft light from the fluorescent lamp on the ceiling may cause difficulties in capturing the indoor and outdoor scenes at the time, it is not possible to clearly photograph indoor and outdoor scenes with great contrast at the same time.

The images they take will have a background that is too bright. The foreground is too dark, or the background is clear. The foreground is too dark and the foreground is suitable for too bright backgrounds. The earliest solution is to use backlight compensation technology or to set up two cameras indoors or outdoors to accommodate larger light contrasts, but the effect is not ideal. The technology that expands the dynamic range arises at the same time, namely the wide dynamic WDR technology that today says.

The wide dynamic representation is expressed in "multiples" or "dB". When 100IRE is used as the standard, the conversion formula is: N dB = 20log (V2/V1). The wide dynamic value of an ordinary camera (called V1) is 10dB, such as a wide dynamic of 48 dB, and the difference between the common camera and the ordinary camera is 38 dB, and V2/V1=80, which means that the difference between the width and the dynamic width of an ordinary camera is 80 times. Panasonic The three-generation wide dynamic camera is 54 dB, and V2/V1=160 times. The ISD-A10 camera on the pool has a typical dynamic range of 95 dB, V2/V1 = 17782 times, a maximum wide dynamic range of 120 dB, and V2/V1 = 316227 times. From the "multiplier" point of view, cameras with Pixim DPS technology have a much wider dynamic range than the CCD's wide dynamic range.

The minimum illuminance of a wide dynamic camera camera is that when the brightness of the subject is low to a certain extent, the amplitude of the video signal output by the camera is reduced to 50%--33% of the standard amplitude 700mV (video nominal value is 1V, The standard value is 700mV): The other minimum illumination is the illuminance on the CCD, which is the sensitivity of the CCD. The illuminance of the CCD is much lower than the minimum illuminance of the camera. Many unscrupulous traders label the lowest illuminance of the CCD as the lowest illuminance of the camera in order to deceive the uninformed person. This is particularly reflected in some domestic OEM products and some No-name, low-end camera products.

The CCD wide dynamic technology uses a special DSP circuit to perform the most suitable shutter speed exposure for the bright area, and then exposes the dark sector to the most suitable shutter speed, and then recombines the two images for DSP processing to make it bright. Departments and dark sectors can be seen. Although this 160 times dynamic range technology has a large wide dynamics and realizes the optimization of image gray scales and other advantages, it has a high requirement for DSP performance. At present, due to the limitations of CCD characteristics, even with multiple exposure sampling methods, the camera's wide dynamic range can only reach 66dB.

Obviously, because of the limitation of the CCD's sensitometric characteristics, it is difficult to make major breakthroughs in technology. The CMOS camera is because of its excellent performance of the image sensor itself, or has a prominent performance. Such as the general linear output mode of the dynamic range up to 40 ~ 60dB;

Therefore, it can be foreseen that the future surveillance cameras belong to wide dynamic cameras, while the wide dynamic technology belongs to CMOS.