Oblique astigmatism and field curvature measured with light-field imaging.

The testing of astigmatism and field curvature of an optical system with light-field imaging is proposed. The method consists in measuring the depth map, obtained with a plenoptic camera, of the image of a test pattern formed by the optical system. To demonstrate the accuracy of the method, the virtual image formed by a plano-convex lens was tested. The resulting depth map was compared with the sagittal and tangential foci distances calculated with the Coddington theory. The proposed method could be applied to the quality testing of ocular optics, including eyeglasses, eyepieces, virtual reality goggles, and augmented reality devices.

Direct calibration of a spatial light modulator by lateral shearing interferometry.

A new interferometric technique is described to measure the complex modulation curve of a spatial light modulator. Based on a lateral shear imaging interferometer, it enables the amplitude and phase modulation for several modulation levels to be displayed simultaneously in a single interferogram. As an example of the power of this technique a heuristic optimization of input and output elliptical polarization states for a mostly-phase operation mode was obtained within a few minutes for a commercial twisted-nematic liquid crystal display.

Statistics of spatial cone-excitation ratios in natural scenes.

For some sets of surfaces, the spatial ratios of cone-photoreceptor excitations produced by light reflected from pairs of surfaces are almost invariant under illuminant changes. These sets include large populations of spectral reflectances, some of which represent individual natural surfaces but not their relative abundances in nature. The aim of this study was to determine whether spatial cone-excitation ratios are preserved under illuminant changes within the natural visual environment. A fast hyperspectral imaging system was used to obtain populations of 640,000 reflectance spectra from each of 30 natural scenes. The statistics of spatial cone-excitation ratios for randomly selected pairs of points in these scenes were determined for two extreme daylights. Almost-invariant ratios were common, suggesting that they represent a reliable property of the natural visual environment and a suitable foundation for visual color constancy.