Understanding Wide Dynamic Range

In many practical surveillance applications, the intensity of illumination with a scene can vary excessively. Images taken by standard cameras always seem to have an overexposed foreground or too dark a background due to the limitation of a camera sensors’ sensitivity. Over the course of a day, the situation can change with different areas of the scene being over or underexposed. What is required in this situation is a Wide Dynamic Range (WDR) camera.

The imaging luminance of a monitored object is mainly dependent on the intensity of illumination radiating or reflecting from the object that is captured by the sensor.

The unit of measure for illumination intensity is the lux unit. For a non-light-source object being monitored, the reflection intensity is also affected by its surrounding illumination, reflecting ratio and other factors.

In a real situation, the illumination of different areas may differ greatly even in the same surveillance scene. For example, on a bright sunny day, the surrounding illumination in the outdoor area can reach 100,000 lux, while the illumination at the desk indoors is approximately 1000 lux while the shadowed area below the desk is as low as 10 lux. Assuming that all those areas have similar reflection capability, the reflection ratio of the indoor shadowed area to the indoor normal lighting area to the outdoor area is 1:100: 10,000.

If we want to preserve at least 5 grey scales in the dark area so as to distinguish between objects with different reflectivity, the capability required to acquire the image information of the three parts mentioned above is 10,000 / 0.2 = 50000 times, corresponding to a dynamic range of 16bit (94db); however, most common image sensors can’t reach this value. The dynamic range of the current common CMOS sensor is around 12 bits, or 72db. When a CMOS sensor is used in such a scene, the image details either in the bright or dark areas will get lost.

What's inside?

• What is Dynamic Range?
• WDR Imaging - Multiple Exposure and Per-pixel Exposure
• Tone Mapping - Retains realistic colour & contrast
• Digital Wide Dynamic Range - Enhances details
• Hikvision WDR Technique - Features LATM and SVCE