A device includes a housing, illumination means, a reflective plate, and a detector. The housing defines an aperture. The illumination means is for providing illumination (natural or artificial) along an optical axis that passes through the aperture. The reflective plate is movable, relative to the housing, between a retracted and a deployed position. The positions are defined so that during such movement, the optical axis traces a line across the reflective plate. The detector is aligned to detect illumination from the illumination means after light through the aperture is reflected from the reflective plate. A method is also described. The device is particularly suitable for moving the reflective plate temporarily in front of a pushbroom or whisk broom type sensor for calibration because the line traces across a first portion of the diffusing surface that is subject to sunlight degradation and a second portion that is always shielded from sunlight.