How Bright Is the Moon? Recovering and Using Absolute Luminance Values from Internet Images

The human visual system differs from a camera in various aspects such as spatial resolution, brightness sensitivity, dynamic range, or color perception. Several of these effects depend on the absolute luminance distribution entering the eye which is not readily available from camera images. In this paper, we argue that absolute luminance is important for correct image reproduction. We investigate to which extent it is possible to recover absolute luminance values for any pixel in images taken from the Internet, extending previous studies on camera calibration in laboratory settings that are much less challenging. We use the Moon as a calibration target to estimate the remaining error. We then evaluate this error in the context of perceptual tonemapping for low dynamic range images.