1. A focal plane detector adapted to be calibrated using an image of part of the human body, the focal plane detector having a field of view, a focal range and a depth of field configured to focus an image of objects in a scene within said field of view, focal range and depth of field onto a plurality of detector elements, the focal plane detector further comprising, a housing having an imaging aperture arranged to convey an image of a scene to the detector elements, and a manually operable switching means for activating the calibration of the focal plane detector, wherein, the manually operable switching means and the imaging aperture are arranged with respect to one another so as to ensure that, during manual operation of the switching means, the part of the human body is located so as to substantially fill the field of view of the focal plane detector and positioned outside the focal range and depth of field thereof, such that the image of the scene presented to the detector elements comprises an image of part of the human body having substantially no features therein discernible by said detector elements.