Evolution of backlight compensation technology.
A modern CCTV camera is unthinkable without additional image processing systems. In order to carry out high-quality surveillance in various lighting conditions, at different times of the day, many systems for processing the signal taken from the CCD matrix are used. Among such systems, we can highlight «backlight compensation» (BLC — Backlight Compensation) and «(Super) Wide Dynamic Range» (WDR — Wide Dynamic Range). These systems will be discussed in the article.
The ideal object for a surveillance camera is one that is uniformly illuminated by a light source located behind the camera in relation to the object. In this case, the angle of reflection of the light flux from the surface of the object in relation to the camera lens should be as small as possible. In this case, we get a high-quality image with optimal brightness and contrast values throughout the frame. But this is an ideal situation, very distantly related to reality.
Usually, when organizing video surveillance, we encounter very diverse complex scenes of object illumination, which change both in time and in the physical location of the light source. The following scenes can serve as an example.
The camera is aimed at the door, behind which there is a room with a higher intensity of lighting than in the monitored room. In this case, a person entering the door will appear as a dark silhouette against the background of the doorway.
The camera surveys a room with windows. During the day, in bright sunlight, objects located between the camera and the window will be dark silhouettes.
When surveying an area at night, the lights will cast a backlight in the frame.
The frame contains both illuminated areas and unlit areas, for example, an entrance to a building, an underground passage, an arch.
When it is necessary to control and register car numbers at night, the headlights directed at the camera will overexpose the image and prevent you from seeing not only the numbers, but also the car itself.
There are other examples where uneven illumination or the light source being opposite the camera make it impossible to identify objects.
To solve this problem, video camera manufacturers came up with the BLC technology – Backlight Compensation.
Initially, the system worked according to the following algorithm. In the case of a darkened area in the frame (a certain ratio to a light area, usually not less than 20% of the total area of the frame), the video processor began to increase the brightness of the image until the dark area was «output» to a normal brightness/contrast ratio for perception. In this case, the increase in brightness was applied to the entire frame, which led to a significant lightening of the already bright areas of the frame. In this form, the BLC system existed for quite a long time until modern video signal processors appeared, and video surveillance technology did not make great strides.
The next step was the emergence of backlight compensation systems with double frame scanning and the use of wide dynamic range (WDR) technology. The WDR function (in simple terms, the dynamic range is defined as the ratio of the maximum value of image brightness to its minimum value) allows for high-quality viewing of both light and dark areas of the frame.
Double scanning of the frame with the help of fast modern processors divides the frame into two components consisting of dark and light areas of the frame. Each component is processed separately (the dark part becomes brighter, the light part remains the same or slightly decreases in brightness), then these two parts are combined into a frame. The result is a picture of equally well-processed dark and light areas. In addition to brightness, the image contrast is also automatically adjusted.
Among the advanced BLC functions, it is now possible to adjust the effect level via the menu, as well as the location of the area in the frame where the function is carried out. The modern level of operation of cameras with BLC and WDR allows for surveillance in the dark in the presence of bright light sources directed at the camera. For example, it is possible to identify license plates not only with low beam, but also with high beam headlights.
It is worth noting that not only the number is visible with low beam headlights, but also objects beyond the level of bright light sources, located at a distance of several tens of meters behind the headlights.
Low beam mode HS-BLC One of the derivatives of the BLC system is the so-called HS-BLC (Highlight Suppression Backlight Compensation or in some camera models HLC (Highlight Compensation). A system that allows you to invert super-bright areas of the image. The level and area of frame processing are also configured. In standard systems, inversion occurs in black. In more advanced systems, you can select the color to which the bright area of the frame is converted, the intensity of the inversion, as well as the time of operation of the function (around the clock or only at night).
Models of cameras from the middle level in a frameless design to professional outdoor cameras with IR illumination, which are equipped with modern chips supporting the latest video processing systems, including BLC, HS-BLC, WDR, D-WDR, etc. The cost of such cameras is slightly higher than their chipless counterparts and can provide new options for building surveillance systems for difficult-to-light objects, including entrance groups, bars and restaurants, and night-time surveillance.