lllumination refers to the light falling on a scene. Strickly speaking,
illumination is not a camera function; however, it's a critical issue when considering
a camera for a given area. Adequate illumination is essential to acquiring
images that allow security personnel to monitor an area (detection), observe
activity at the location (recognition) and identify specific actions, objects,
or persons (identification).
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 object identification. 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 a wide variation in light level
(such as occurs when the sun is obscured by clouds), you need an automatic-iris
camera system. Typically, scene illumination measured in foot-candles
(fc) can vary over a range of 10,000 to 1 (or more).
The chart below summarizes the light levels that occur under daylight and low-light
conditions. The equivalent metric measure of light level (lux) compared
with the condition (fc) is given.
The "golden rule" for deciding which camera to use for a given lighting condition
is not to choose one that merely delivers a minimally usable picture.
Try to give the camera approximately 10 times its quoted minimum scene illumination.
Most cameras will be able to handle the excess light. The major problem
is when they do not have enough light to produce a picture.
