;When taking a picture of a light source, the conversion relationship between the grayscale and the luminance of the camera will be interfered by those defocused light sources. To solve the de-focusing problem, we develop a novel calibration process. By exploring the dependence of the blurred edges of an object-image boundary upon the imaging distance, the distance of the projecting light source can be determined in calibrated imaging system. Then the measuring luminance of the light sources within the focusing range can be refined by removing the contaminated signals from the defocusing light sources. The calibration process has also been verified in practical measurements and analysis.