Liquid crystal television in speckle metrology Brian Bates and Paul C. Miller

Queen's University of Belfast, Department of Pure & Ap­plied Physics, Belfast BT7 1NN, Northern Ireland, U.K. Received 24 November 1987. 0003-6935/88/142816-02$02.00/0. © 1988 Optical Society of America. It is evident from the literature that there is a growing

interest in the use of low-cost liquid crystal television (LCTV) in a variety of programs in optical information pro­cessing and pattern recognition. 1-6 Straightforward modifi­cations have been described which are needed to make these commercial devices suitable in optical performance for such applications. In particular, it is necessary to correct for the phase distortions which arise from the low optical quality of the device, and this is usually overcome by mounting the LCTV in a liquid gate. In addition, the transmittance char­acteristics may be improved by replacing the plastic polariz­ers attached to either side of the liquid crystal/electrode grid

2816 APPLIED OPTICS / Vol. 27, No. 14 / 15 July 1988

Fig. 1. Schematic diagram of the optical system for the display of speckle correlation: BS, beam splitter; SF, spatial filters; V1, V2, vidicons. The digitized speckle is displayed on LCTV1 and the

resulting fringe pattern on LCTV2.

sandwich with externally mounted high-quality polarizers.3

Although we have found that it is possible to obtain a modest but useful improvement in the intensity contrast between black and white pixels by optimizing the orientation of the two external polarizers, in the experiments described below we merely mounted the commercial LCTV in a liquid gate without disassembly of the original polarizers.

The main purpose of our investigation was to explore the application of LCTVs to the display in real time of the correlation output of a pair of speckle patterns produced by an in-plane displacement of an object. The optical arrange­ment which employs two LCTVs is shown schematically in Fig. 1. Essentially this is a similar arrangement to that which is commonly used for displaying Young's fringes in the analysis of double-exposure photographically recorded speckle patterns. In our experiment, however, a digitized double-exposure speckle pattern is displayed on LCTV1, and the fringe pattern which results is recorded with a TV camera whose output is then displayed on LCTV2. A second optical transform can then provide a display of the speckle correlation spots on a TV monitor. The output may be calibrated to give a direct displacement measurement, since the separation of the correlation spots is proportional to the object displacement.

The speckle was produced by illuminating the object (a frosted glass screen) with a He-Ne laser, and the resulting pattern was recorded in the Fraunhofer diffraction plane using a vidicon camera.7 The size of the speckle structure was adjusted using a field stop at the object to provide a good match to the spatial resolution of the overall system (see below). Using a low-cost digitizer board (Microworks Digi-sector DS-65) interfaced to an Apple He microcomputer, the intensity distribution in the speckle pattern was digitized to produce a high-contrast image which is stored on disk. The object was then displaced in-plane by a known value (typi­cally in the 50-350-μm range), and the second speckle pat­tern was digitized. The pair of speckle patterns is then coadded, and the double-exposure digitized speckle pattern can be stored on disk for subsequent display on LCTV1. Because the TV employed has no video input jack, we have connected between the computer and the LCTV aerial via an rf modulator. Subsequently, we have made modifications to the device to obtain a direct video input by passing the rf section of the TV.

Fig. 2. Top: intensity distribution which is recorded by vidicon V1 showing the LCTV1 pixel structure and the fringe pattern produced by the double-speckle exposure. Bottom: correlation output re­corded off the vidicon V2 monitor showing the dc and correlation

spots. The object displacement for these recordings is 150 μm.

Using a plane wave incident on LCTV1, the diffraction pattern is produced in the Fourier transform (FT) plane, i.e., in the back focal plane of lens L1 (ƒ ~ 500 mm). The intensity distribution in this plane consists of the diffraction pattern produced by the LCTV pixel structure together with the fringe pattern produced by the double-speckle exposure (Fig. 2). A microscope objective is used to project a magni­fied image of the fringes onto vidicon V1 whose output is then displayed on LCTV2. The magnification is a compro­mise so that the fringes are resolved by the LCTV, but the spacing is not too large to cause the correlation spots to be buried in the extended dc spot. The fringe spacings corre­sponding to the above displacements are typically in the 350-900-μm range. The intensity distribution in the focal plane of lens L2 (ƒ ~ 400 mm) is the diffraction pattern of LCTV2 pixel structure and the diffraction spots produced by the fringes. A magnified image of the central region of the diffraction pattern can be projected onto a vidicon camera (V2) to display the bright zero order (dc) and the correlation spots. An example of the output from the TV monitor for an object displacement of 150 μm is shown in Fig. 2. The range of object displacement which can be measured using this present technique is between 50 and 350 μm. For best performance it is necessary to optimize the brightness con­trol of each LCTV. This control determines the bias voltage across the LC display allowing the transmission level and contrast of the screen as a whole to be varied.

The LCTV employed (Realistic model 16-7056) has a screen size of 50 × 38 mm and 160 horizontal by 132 vertical elements, the pixel dimensions being - ~ 3 0 0 m square. It is necessary to choose an input speckle size so that after record­ing and digitization it is suitably matched to the LCTV. A convenient method for this purpose was to make a separate evaluation of resolution by imaging a test target (e.g., USAF 1951) at unit magnification onto a vidicon camera whose digitized output is then displayed on LCTV1 The resolu­tion so determined as referred to the test target input plane was ~ 8 and 5 cycles mm-1 in the vertical and horizontal directions, respectively. The Digisector output consists of 256 horizontal and 192 vertical pixels, which addresses 123 horizontal by 96 vertical elements of the LCTV. Thus there is a 2:1 match of the vertical elements permitting exact registration, while the mismatch in the horizontal direction is a principal cause of the lower resolution. The field stop which determines the speckle structure size was chosen to give a maximum spatial frequency in the speckle pattern, ~7 cycles mm - 1 , so providing a reasonable match to the spatial resolution determined for this system.

Our experimental arrangement for the display of correla­tion output between two displaced speckle patterns is similar in some respects to the joint transform correlator architec­ture described by Yu et al.6 The main difference in ap­proach lies in the technique for presenting the input data of the object scene. For certain applications where a high processing speed is not of paramount importance, our ap­proach, which requires digitization of the object scene which is then added to a stored reference yields a simple experi­mental approach requiring the use of only two LCTVs. Cur­rently we are exploring further the method applied to speckle metrology.

We wish to thank Brenda Cavanagh for providing mea­sured data on LCTV optical performance characteristics. P. C. Miller gratefully acknowledges a studentship from the Department of Education, Northern Ireland.

References 1. H-K. Lui, J. A. Davis, and R. A. Lilly, "Optical-Data-Processing

Properties of a Liquid-Crystal Television Spatial Light Modula­tor," Opt. Lett. 10, 635 (1985).

2. M. Young, "Low-Cost LCD Video Display for Optical Process­ing," Appl. Opt. 25, 1024 (1986).

3. A. M. Tai, "Low-Cost LCD Spatial Light Modulator with High Optical Quality," Appl. Opt. 25, 1380 (1986).

4. D. Casasent, S-F. Xia, A. J. Lee, and J-Z. Song, "Real-Time Deformation Invariant Optical Pattern Recognition Using Coor­dinate Transformations," Appl. Opt. 26, 938 (1987).

5. K. D. Hughes, S. K. Rogers, J. P. Mills, and M. Kabrisky, "Optical Preprocessing Using Liquid Crystal Televisions," Appl. Opt. 26, 1042 (1987).

6. F. T. S. Yu, S. JutamuHa, T. W. Lin, and D. A. Gregory, "Adaptive Real-Time Pattern Recognition Using a Liquid Crystal TV Based Joint Transform Correlator," Appl. Opt. 26, 1370 (1987).

7. R. F. Erf, Ed., Speckle Metrology (Academic, New York, 1978).

15 July 1988 / Vol. 27, No. 14 / APPLIED OPTICS 2817