A common way to test high-quality aspherical lenses is to use a measurement system based on a set of null corrector and
a laser interferometer. The null corrector can either be a combination of spherical lenses or be a computer generated
hologram (CGH), which compensates the aspheric wave-front being tested. However, the null optics can’t be repeatedly
used once the shape of tested optics changes. Alternative active null correctors have been proposed based on dynamic
phase modulator devices. A typical dynamic phase modulator is liquid crystal spatial light modulator (LCSLM), which
can spatially change the refractive index of the liquid crystal and thus modify the phase of the input wave-front. Even
though the measurement method based on LCSLM and laser interferometer has been proposed and demonstrated for
optical testing several years ago, it still can’t be used in the high quality measurement process due to its limited accuracy.
In this paper, we systematically study the factors such as LCSLM structure parameters, encoding error and laser
interferometer performance, which significantly affect the measurement accuracy. Some solutions will be proposed in
order to improve the measurement accuracy based on LCSLM and laser interferometer.
The invention of spatial light modulator (SLM) promotes the development of aspheric surface test. SLM has the advantage of real-time and low-cost in comparison with the etching computer-generated hologram(CGH) plate. The pixel-structure of SLM, which includes the pixel pitch, gray-level number and black matrix, has great significance on the reconstructed wave-front quality. In this paper, the effects of SLM pixel pitch, gray-level number and black matrix are analyzed by Fresnel diffraction theory and computer simulation. In the simulation, a concave spherical wave-front with a radius of 1000 mm is generated by the SLM with different pixel pitch, gray-level numbers and aperture ratios, respectively. The results show that the quality of the reconstructed wave-front gets poorer as the pixel pitch increases, the gray-level number decreases or the aperture ratio reduces. This work can guide the selection of the SLM in the aspheric surface test.
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