Apparatus and methods for calibrating an optical detector. In one embodiment an optical reference apparatus includes a body having an outer surface. At least a portion of the outer surface has a reflectance that can vary. The optical reference apparatus is adapted to be deployed with a cloud of optical sensors and has a terminal velocity that is about the same as the terminal velocity of the sensors. Further, the variation of the reflectance can be caused by a rotation of the optical reference apparatus (caused by an aerodynamic member). The sensors may be made from porous silicon and the optical reference apparatus may include a retroreflector. The apparatus also provides a method of referencing an optical signal. The method includes deploying an optical reference apparatus with a cloud of sensors and detecting a variation in its reflectance.