Richard M. Sillitto

Development of a spatial light modulator: a randomly addressed liquid-crystal-over-nMOS array

The construction of a 50- x 50-pixel spatial light modulator based on an active silicon backplane and using the hybrid field effect in nematic liquid crystals as the light modulating process is described. The design and electrical evaluation of the pixel array, which is fabricated in 1.5-microm nMOS and has an individual memory cell within each pixel, are discussed. The performances of a 16 x 16 prototype SLM and the new 50- x 50-pixel device are compared to provide an indication of progress toward high performance spatial light modulators with onboard pixel memory.

A New 'Analytic' Method for Computing the Optical Transfer Function

Using a series expansion in Zernike polynomials to express the pupil function of an optical system, a means for computing the Optical Transfer Function has been found which avoids explicit numerical integration.

High-performance spatial light modulator

The development of a ferroelectric liquid-crystal-over-single-crystal-silicon spatial light modulator is described. The reflective SLM has an array of 176 X 176 pixels over a clear aperture of 5.28 mm X 5.28 mm. Prototype devices driven from a specially designed high speed frame store have been operated at frame rates of approximately equals 1 kHz.

Edge-ringing in Partially Coherent Imaging

A simple condition is shown to be necessary and sufficient to suppress edge-ringing in optical imaging. This criterion is valid in the presence of aberrations and apodization, and for all cases of (spatially stationary) partially coherent illumination. The edge-ringing tendencies of an optical system may be assessed through the use of two new performance functions which form a Fourier-transform pair. One of these performance functions is related to the ‘transmission cross-coefficient’ which appears in the theory of partially coherent imaging. In the coherent limit this performance function reduces to the amplitude transfer function (pupil function), and in the incoherent limit it reduces to the Optical Transfer Function (OTF). The use of this performance function for the general assessment of partially coherent imaging systems is suggested.

A Simple Fourier Approach to Fraunhofer Diffraction by Triangular Apertures

The amplitude distribution in the scalar far-field approximation is derived as a simple expression, involving only sinc functions, in terms of two non-orthogonal coordinates related to the lengths and directions of two sides of the triangle. Photographs are juxtaposed with computed contours to show the real and the imaginary parts of this expression.

Two-point Resolution Criteria in Partially Coherent Imaging

The Rayleigh criterion has been investigated as a performance criterion for optical systems imaging in partially coherent light. In numerical tests with moderate amounts of primary spherical aberration—up to 18 u maximum wavefront deviation—the Rayleigh criterion proves to be insensitive to the amount of aberration. This insensitivity appears characteristic of two-point criteria generally.

The analysis of gamma-ray spectra obtained with a NaI(Tl) scintillation spectrometer by a technique which takes account of coincidence summing

Abstract A technique is described for analysing a pulse height spectrum from a NaI(Tl) scintillation spectrometer placed close to a source emitting cascades of γ rays, using the line shapes associated with each γ ray cascade. Each cascade line shape is constructed using the line shapes appropriate to the component γ rays (obtained by an energy interpolation technique from the line shapes for monoenergetic γ rays emitted from calibration sources) and the coincidence-sum shapes which result when a pair of γ rays in the cascade are detected simultaneously in the NaI(Tl) crystal. A practical method of calculating the coincidence-sum distribution due to two γ rays is discussed with reference to the γ rays from a 60Co source. The pulse height spectrum due to the detection in a NaI(Tl) crystal 5″ dia. by 4″ long of the γ rays from the 3.58 MeV level in 10B was obtained using a n-γ coincidence time-of-flight spectrometer to study the 9Be(d,nγ)10B reaction, and this spectrum is analysed using cascade line shapes.

Reduction and Removal of Replication in an Optical Processor by Randomization of Pixel Positions in the Fourier Plane Filter

Abstract When a pixellated spatial light modulator (SLM) is used in an optical processor, replicated images occur in the output due to the underlying regular pixel structure of the SLM. These replicas arise through convolution with distinct spectral orders in the SLMs Fourier spectrum and in the case of a correlator can lead to false correlation signals. A means of suppressing or eliminating the spectral orders through randomization of the pixel positions is presented, with analysis of the Fourier spectrum of random arrays. Two specific randomization schemes are considered and the results are compared with that for a regular array.

An Adaptive Fourier Optical Processor.

A prototype liquid-crystal-over-silicon spatial light modulator (SLM) with 16 x 16 pixels has been fabricated and used as an adaptive filter in the Fourier plane of a coherent optical processor. Preliminary results showing images of simple objects filtered through the SLM are presented. For comparison, images of the same objects obtained using photographic transparencies to simulate the action of the SLM as a filter are also presented.

Reduction of aliasing in correlation using a pixelated spatial light modulator

Replication in the output plane of an optical correlator, due to pixelation of the Fourier plane filter, can lead to false correlation signals. This paper suggests randomization of the pixel positions of the Fourier plane filter as a solution, and demonstrates its effectiveness through computational simulations and optical correlation experiments using a custom made SLM.