When analyzing IR illumination efficiency in CCTV usually we take into account influence of equipment characteristics (video cameras, lenses, IR illuminators etc.). In this article we take into consideration the influence of an object under observation itself on IR illumination system efficiency.
Reflection power
Image sharpness and object recognition ability depend to a large extent on subject-to-background contrast and intra-subject contrast.
Reflection power of different surfaces is different in visible spectrum and close IR band where IR illumination for CCTV works. This difference is not very significant for the most of surfaces, otherwise an image in IR band would not be the same as an analogous image in the visible spectrum. This should be taken into account when projecting systems with IR illumination.
Foliage and grass have higher reflection power in IR band than in the visible spectrum. That is why the greenery in IR band looks more bright and forms more contrast image details. Water, on the contrary, has lower reflection power in IR band than in the visible spectrum. Water surface looks in IR band darker. But many objects on the water surface look more contrasting. Extraordinary high reflection power in IR band is found with certain synthetics.
Positional relationship of camera and illumination.
When projecting CCTV with illumination one can use different schemes of camera and illumination positional relationship. It is necessary to avoid pointing searchlight beams directly into the camera lens. Such situation can appear if one uses several cameras and illuminators at the same territory. At that direct pointing of spurious emission may not be reflected on the picture straightforwardly but may lead to the response of auto- diaphragm or camera’s AGC which will significantly desensitize the camera.
Location of objects in the surveillance area
If there are closely located objects in the surveillance area it can lead to camera desensitization owing to auto- diaphragm and AGC and picture-quality degradation in the background. For example, a tree situated in the foreground may lead to significant desensitization for the background objects whose lighting conditions are far inferior. Sometimes in this case it is rational to place IR illuminators at some distance from the camera to create the optimal illumination or to use several illuminators: one – near the camera and the other one – at some distance. Use of several illuminators for the surveillance area may help to avoid unilluminated zones created by shadows from objects in the foreground.