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Explanation: It appears that these surveillence cameras "compensate" for varying levels of light.
Take a look at:
"GBC CCD600PH - $239.00ea
"Pincam" ultra-miniature CCD camera. Measures only 1 1/2" W x 1 1/2" D x 3/4" W. Features built-in 3.6mm lens, electronic shutter automatically compensating for lighting changes, 400 line res., only .2 lux sensitivity and more. Simply mount this camera behind any 1/16" hole to obtain a clear picture! Complete with power supply."