During the day the amount of illumination reaching a scene depends on the time of day and atmospheric conditions. Direct sunlight produces the highest-contrast scene, allowing maximum identification of objects. On a cloudy or overcast day, less light is received by the objects in the scene, resulting in less contrast. To produce an optimum camera picture under the wide variation in light level (such as occurs when the sun is obscured by clouds), an automatic-iris camera system is required. Typically, scene illumination measured in footcandles (FTCD) can vary over a range of 10,000 to 1 (or more), which exceeds the operating range of most cameras for producing good quality video images.
Specifications for surveillance cameras often include a minimum illuminance level in lux at which the camera will record a satisfactory image. A camera with good low-light capability will have a lower lux rating.
The chart below summarizes the light levels occurring under daylight and typical low light level conditions. The equivalent metric measure of light level (lux) compared with the FTCD is given. One footcandle ≈ 10.764 lux. One lux = 0.30482 footcandle.
|Full Day Light||10,764||1,000|
|Very Dark Day||107.64||10|
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