Provided are equipment and a method for measuring a transmittance of a photomask. The system includes an acoustic optical deflector (AOD) substrate interposed between a light source and the photomask. The AOD is adapted to deflect a laser beam to an oblique incidence angle with respect to a surface of the photomask. A radio frequency (RF) signal source is coupled with the AOD substrate. Varying the frequency of the signal applied to the AOD substrate acts to change the refractive degree of the substrate, thereby changing an angle of deflection of the incident laser beam. A photodetector is positioned to receive the laser beam passing through the photomask and is adapted to measure an intensity of the laser beam which has penetrated the photomask. As a result, a transmittance of the photomask can be measured under off axis illumination (OAI).