Digital Camera Calibration for Luminance Estimation in Nighttime Visibility Studies

Estimation of target-to-background luminance ratios is a powerful method by which human detection of objects can be assessed. In the forensic community, evaluation of the detectability of a pedestrian to an automobile driver is often of interest. With calibration, the modern digital camera employing a CCD or CMOS light collection device can be a convenient and economical luminance estimation tool. Certain CCD or CMOS sensors will linearly report the impinging incident light pixel by pixel over a range of intensities. The device becomes nonlinear at low and high intensities; however, the linear region can be adjusted to the specific lighting conditions of interest by modifying the shutter speed, ISO setting, and aperture size. Image noise, sensor non-uniformity, temperature sensitivity, camera color sensitivity, and the spectral power distribution of the illuminant require treatment for direct comparison to the luminance. The techniques, procedure, and post-processing outlined demonstrates the use of a calibrated digital camera for the quantitative estimation of luminance.