High resolution measurement of freeform wavefront by using self-imaging based sensor

The freeform optical surfaces are the advanced optical elements being used in the optical systems ranging from the illumination system, head up display and ophthalmic systems. So far the metrology is not well established for freeform surfaces.There are interferometric, profilometry, deflectometry and slope measurement techniques used to measure the freeform surfaces. Due to non-rotationally symmetric nature of freeform surfaces, slope measurement systems like Shack Hartman Sensors (SHS) are being explored for the measurement of freeform wavefronts. The spatial resolution of Shack Hartmann sensor is limited by the size of the lens lets used in the sensor which is typically 100 μm to 200 μm. The self-imaging based sensing uses a periodic structure which can be replicated under collimated illumination at certain distance known as Talbot distance. If there is a wavefront other than collimated light, the deviation in self-imaging pattern is observed, and this deviation can be utilised for wavefront measurements. Being a smaller pitch of the periodic structure, a high resolution data is obtained. In the present study, we have proposed a high resolution system for measurement of freeform surface using self-imaging based technique, which is having advantage of higher spatial data as compared to Shack Hartman Sensor. A simulation study is carried out and demonstrated the improved performance of the proposed sensor as compared to SHS.