Abstract
Purpose: :
Measurements of the wavefront phase (WFP) from human eyes are commonly made using a Shack–Hartmann sensor (SHS). The SHS samples the WFP gradient from which the phase is recovered. But the recovery is sensitive to opacities, retinal and intra–ocular scattering, speckle, bias due to sampling geometry and to the degree at which the object is extended. The curvature sensor (CS), in contrast, samples the Laplacian of the WFP within the pupil and the radial derivative at the pupil edge. In practice one compares the intensity in two planes along the propagation direction at equal distances before and after either the image or pupil plane. Phase recovery is generally done using either a numerical solution to the Poisson equation or a Gerchberg–Saxton algorithm (GSA). Since CSs make differential intensity measurements, opacities, scattering and speckle are expected to have a less adverse effect on the fidelity of the reconstruction, and since the sensor is based on a geometrical concept, extended objects are not problematic.
Methods: :
A novel set–up has been designed and implemented for curvature wavefront sensing in the human eye allowing for images in two planes to be taken simultaneously on a single detector. A modified GSA was used to recover the WFP. Calibration was done using a phase plate (PP) encoded with aberrations from a real eye. An interferogram of the WFP of the PP from the CS was compared to the interferogram from a Mach–Zender interferometer (MZI). Similarly, interferograms from the CS for real eyes were compared to that obtained from a SHS (in a WASCA Wavefront Analyser (WWA)).
Results: :
The interferograms for the PP obtained from the CS and the MZI matched very well. Likewise, the interferograms for real eyes from the CS and the WWA matched well, but in addition, the CS revealed higher frequency detail that was absent from the SHS, since the WWA measured only to 4th order.
Conclusions: :
A relatively simple curvature sensor has been shown to successfully reconstruct the WFP of human eyes. Advantages include design simplicity, the ability to easily alter the sensor’s sensitivity, and the ability to capture both required images simultaneously onto a single detector.
Keywords: optical properties • refraction