Radiometric Camera Calibration

Abstract

AbstractRadiometric calibration is a method of capturing digital images and processing the digital image data so that the pixel values represent actual measurements of light. Radiometric camera calibration converts ordinary digital images into radiometric images. Absolute radiometric calibration means that the image pixels are calibrated with reference to physical units such as watts per steradian per square meter (W.sr−1.m−2) or photon counts. Relative radiometric calibration means that the image pixels are calibrated with reference to each other — the ratio of two image pixels is the same as the ratio of the corresponding physical measurements of light. Radiometric calibration, both absolute and relative, has a wide range of applications in science and technology, including astronomy, remote sensing, and machine vision. Methods for radiometric calibration typically involve up to four steps. First, the camera response is corrected to be linear. Second, the camera's black response is subtracted. Third, corrections are applied for spatial variations in the effective aperture of the lens and the sensor sensitivity. This correction yields a relative radiometric calibration. A final step converts the relative calibration to absolute calibration by applying an appropriate scaling factor. This article first explains the basis of radiometric calibration — the additive property of light and the formation of digital images. Techniques for radiometric calibration are then explained, focusing on the four steps of estimating the camera response function, measuring the black response, correcting for vignetting and sensitivity, and using calibration targets. Finally, applications of radiometric imaging are discussed briefly.