A light guide (15) for delivering an illumination, in particular a reference or background illumination, the light guide comprising: a mirror box (21) which defines a mirror cavity (23), which receives a light source (19) therein, and includes an illumination aperture (25) from which an illumination is delivered; a transmissive diffuser (27) which is located at the illumination aperture to render the illumination diffuse; and a shutter member (41) which is movable into the cavity to obstruct the light source and provide for control of the radiance of the delivered illumination, whereby control of the radiance of the delivered illumination is determined by a position of the shutter member in the cavity.