Characterization and calibration of a CCD detector for light engineering

This paper describes the methodology developed for characterizing a commercial charge-coupled device (CCD) camera as a luminance meter for analyzing lighting systems and especially for measurements in road light plants. Today, several luminance meters based on commercial CCD cameras are on the market. They are very attractive for the lighting engineer: The availability of a high number of closely spaced small detectors (pixels) on a single chip permits analyses almost impossible with a traditional luminance meter. These commercial-industrial CCD cameras are sold at prices lower than scientific grade ones. They are factory equipped with a dedicated filter to reach the correct photopic sensitivity V(λ), and they are factory calibrated in luminance SI units. The main counterparts in using these cameras are in the difficulties to define the measurement accuracy and the influence of the environment luminance on the measured values of the framed scene, in the low resolution of their A/D converter (usually 8 or 12 bit), and the higher noise level (usually the CCD chip is not cooled). To reach the measurement accuracy required by lighting norms, it is necessary to characterize metrologically a camera and quantify all the possible external influences which could degrade its performances, in real measurement situations, and which could affect the measurement results. A carefully controlled measurement set up and operating procedure could limit the causes of errors and improve the accuracy of measurements obtained in operating conditions. In this way, the measurement uncertainties might be evaluated completely, and considerations on the results could suggest particular operating practices to limit the causes of error due to measurement setup and environmental conditions.