Adaptive camera optics based on a liquid crystal spatial light modulator

Liquid Crystal Spatial Light Modulators (LC SLMs) can be used as dynamic diffractive elements which can be controlled using for instance a computer. A wide range of possible applications exist, and LC SLMs have proven to be useful in for example laser beam steering. The aim of this study was to investigate if capabilities such as autofocus and Field of View tracking (FOV-tracking) found in a human eye can be transferred to a camera system based on a LC SLM.

A setup was constructed using a 512 times 512 pixels phase modulating LC SLM from Boulder Nonlinear Systems (BNS) placed in the fourier plane of a 4f-system. To go beyond what could be studied experimentally a simulation software was developed in Matlab. With this software the effect of optical phase delays, not achievable in the lab, could be studied.

As expected from previous research, image quality was a large issue with the system. In FOV-steering and coherent illumination, undesired ghost images degraded the image at large steering angles (0.5 degrees). In polychromatic illumination and FOV-steering, the images got heavily blurred. The situation was much better when the SLM was used for focusing and quite good image quality was achieved in that case, both in coherent and incoherent polychromatic illumination. The broadening of an intensity peak in polychromatic illumination was only a few pixels. This can be compared to FOV-steering where the broadening was about ten times larger. Simulations showed that the image quality would be improved if the SLM could generate larger phase delays.

Autofocus and FOV-tracking were implemented successfully. The FOV-tracking system could reject disturbances below approximately 0.4Hz. The autofocus algorithm established focus from a largely defocused position within a few seconds.

From this work, one can conclude that both autofocus and FOV-tracking can be implemented in a camera system based on a LC SLM and hence without any moving parts. FOV-tracking is difficult in polychromatic illumination while autofocus can be achieved in both coherent and incoherent polychromatic light. In future development of the system several improvements can be made concerning image quality and speed.

Author: Garpebring, Anders

Source: Lulea University of Technology

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