Study of the instrument transfer function of a free-from optics metrology system: SCOTS

Instrument transfer function (ITF) of an imaging based metrology system is usually closely related to its imaging transfer function. The Software Configurable Optical Test System (SCOTS), a highly accurate aspheric and free-form surface metrology, is an imaging deflectometry system consisting of a camera and a display screen. We demonstrate that its ITF follows the camera imaging MTF through numerical simulations, considering a diffraction limited camera. We then applied this result to the edge measurement data of a large mirror. A better agreement is achieved between SCOTS test and a test-plate interferometric test after the compensation. This suggests that more attention should be paid to the image quality of the SCOTS for high accuracy measurement.