A Custom CMOS–based Hartmann–Shack Wavefront Sensor

Abstract

Abstract: : Purpose:Adaptive optical systems have been widely used in ophthalmology for dynamic compensation of the eye’s optical aberrations. However, the real–time tracking and compensation of the eye's dynamic aberrations demands sensing methods with high repetition rates. CMOS–based Hartmann–Shack sensors seem to be a promising cost–effective alternative to normal CCD cameras whose bandwidth is limited by software processing tasks to some tens of Hz. CMOS–based Hartmann–Shack sensors offer image capture and image processing functions on a single chip and thus allowing repetition rates in the kHz–range. A CMOS wavefront sensor for ophthalmologic applications with a light sensitivity in the nW region has been developed, allowing a bandwidth of 4 kHz. Once the sensor was available, the integration of the custom CMOS sensor to a general PC interface is necessary and the speed, sensitivity and accuracy in the wavefront reconstruction of the new CMOS wavefront sensor have to be characterized. Methods: A parallel port interface was implemented to allow easy connection of the custom CMOS sensor to every IBM compatible PC, without requiring any additional digital I/O devices. The tracking speed of the CMOS sensor at various spot powers was tested with an oscillating tip–tilt mirror. A membrane deformable mirror was used to produce arbitrary wavefronts, which were measured simultaneously by the CMOS wavefront sensor as the test device and a Twyman–Green interferometer as reference. Results: The interfaced CMOS sensor is able to measure dynamic wavefront aberrations with a repetition rate of 300 Hz at spot powers in the nW region, which is approximately an order of magnitude faster than with standard CCD–based solutions. For test wavefronts produced by the deformable mirror, the corresponding low– as well as high–order Zernike coefficients derived from the CMOS sensor agree well with the results of the Twyman–Green interferometer. Conclusions:The custom CMOS sensor allows high repetition rates at low spot powers required for ophthalmologic applications. Its precision and sensitivity in the wavefront reconstruction are comparable to the standard CCD device, but with a greatly improvement in bandwidth. The custom CMOS sensor is able to monitor the dynamic aberrations of the eye up to hundreds of Hz, allowing a broad application range from eye accommodation investigations through characterization of the dynamics of the eye’s aberrations and real–time vision acuity measurements to aberration–free retina imaging. Keywords: Wavefront, Adaptive Optics, Dynamic aberrations, Hartmann–Shack Sensor

Keywords: eye movements: recording techniques • refractive error development