Abstract: A beam of light, such as a laser, is used to scan across a lens. The beam passing through the lens is modulated by an optical encoder which controls the amount of the beam passing through the encoder to a light receptor. The received light is quantified and processed to determine any distortion caused by the lens. The encoder is comprised of a set of transparent windows upon an opaque face, the windows having a certain pattern that a beam travelling across the windows casts a specific light intensity pattern. The encoder also allows determination of astigmatism of the lens and distortion inherent in the beam itself.