1380 APPLIED OPTICS / Vol. 25, No. 9 / 1 May 1986

Low-cost LCD spatial light modulator with high optical quality Anthony M. Tai

Environmental Research Institute of Michigan, P.O. Box 8618, Ann Arbor, Michigan 48017. Received 22 November 1985. 0003-6935/86/091380-03$02.00/0. 1986 Optical Society of America. Many optical processing applications are hindered by the

lack of a low-cost real-time spatial light modulator (SLM). Together with several other groups,1-3 we have looked into the use of a commercial LCD pocket TV for optical process-ing. One obvious shortcoming of the commercial device is its poor optical quality; the thin glass substrates exhibit large phase errors. The poor optical quality has led some people to doubt its usefulness in coherent optical processing. In this Letter we show that, with minimal effort, a commercially available LCD TV can be converted into a SLM with high optical quality. Some possible applications for the device are also suggested.

The LCD TV we used in our experiments is the Radio Shack model 16-151 LCD Pocketvision TV. The space-bandwidth product of the 6.86-cm (2.7-in.) diagonal screen is quite modest, composed of 122 X 148 pixels. Even though LCD TVs with larger space-bandwidth products are avail-able, we have chosen this unit for the following reasons: (1) it is extremely low cost (under $150) and readily available; (2)

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Fig. 2. Interferogram obtained with a Mach Zehnder interferome-ter.

Fig. 1. Structure of the LCD SLM (a) before and (b) after modifica-tion.

it is designed to be used in a transmission mode, making modification much easier; (3) the unit has an input jack for direct video input, permitting the SLM to be written with any video source such as a TV camera or a computer. The structure of the LCD TV is illustrated in Fig. 1(a). Two polarizing sheets are attached to the substrates with adhe-sive, one acting as the polarizer and the other as the analyzer. A plastic diffuser provides diffused illumination on one side and a clear plastic window protects the device from dust on the opposite side. The conversion procedure is quite straightforward. (1) Open the case and break the hinge stops so that the LCD screen can be opened fully. (2) Disas-semble the aluminum frame of the LCD screen and remove the plastic diffuser and window from the frame. (3) Peel off the two polarizing sheets from the glass substrates. (4) Clean the substrates carefully with acetone. (5) Bond opti-cal flats with optical epoxy onto both sides of the LCD screen as shown in Fig. 1 (b). The last step is necessary because the thin glass substrates possess poor optical quality. By bond-ing two optical flats to the LCD screen, the optical quality of the device becomes as good as the flats that are used. In keeping with the goal of a low-cost system, we stripped off the gelatin of discarded Kodak Microflat -in. photographic plates and cut them to size. The optical epoxy should be of a type that can be cured at room temperature to avoid damag-ing the unit. We used the Norland Optical Adhesive Type 61 which is UV-cured.

We examine the optical quality of the modified SLM in a Mach Zehnder interferometer. The output fringe pattern is shown in Fig. 2. Even though we did not use high quality optical flats, the optical quality of the modified SLM is quite good, exhibiting a halfwave phase error over the full aper-

Fig. 3. (a) Radial grating written on the LCD SLM; (b) its output spectrum.

ture. Much of it, however, is quadratic phase errors caused by the bending of the Microflat plates when they were cut. If we compensate for the quadratic phase errors by slightly adjusting the focal plane, the device is close to being diffrac-tion-limited. In fact, due to its very simple structure, the optical quality of this low-cost SLM can be superior to de-vices such as the Hughes LCLV which requires a multilayer structure. In Fig. 3 we show a radial grating that was written on the SLM with a personal computer and its Fourier spec-trum obtained with an optical processor. A radial grating

1382 APPLIED OPTICS / Vol. 25, No. 9 / 1 May 1986

can be considered as an on-axis computer-generated holo-gram of the letter 0. The standard composite video output of the computer can address individual horizontal TV lines but not individual columns, resulting in aliasing between the pixel display of the SLM and the bit map display of the computer. The outer four diffraction orders are due to diffraction by the grid electrodes; they also represent the sampling frequency of the SLM.

Because of its low cost and modest space-bandwidth prod-uct, the primary use of this LCD SLM may be as a research and educational tool. It is affordable by almost any institu-tion, including small university laboratories. However, it is also sufficiently capable to be used in some important real world applications. The most prominent of these is correla-tion detection. This low-cost real-time SLM when used with a video camera can be the heart of an industrial robotic vision system for parts recognition.4,5 In such applications, the parts to be recognized have fairly simple shapes and they are presented to the vision system one at a time on a conveyor belt. The space-bandwidth product required for unambig-uous detection and recognition of these parts is quite small. The built-in compact video electronics make this SLM par-ticularly attractive. The electronics needed to drive other commercially available SLMs with a video camera would themselves cost many times more than this LCD SLM. An-other possible application is in optical computing including matrix multiplication6 and some numerical optical comput-ing schemes. The SLM can be used as a programmable mask or as a transducer in a feedback processor. To be used in such applications, however, the built-in analog composite video electronics must be abandoned and new electronic interfaces have to be fabricated to access each individual pixel directly. While marginal at this time, this device may also be used to write computer-generated holograms (CGHs) in real time for optical testing. In optical testing of optical components, especially aspherics, a CGH is used to generate the reference wave front. It is not always necessary for the space-bandwidth product required for the CGH to be very large. A conventional optical element such as a spherical lens can be used to match the aspheric as closely as possible and it would only be necessary for the CGH to generate the residual phase variations. If the element to be tested is only weakly aspheric, a real-time SLM may provide enough space-bandwidth product to create the desired wave front.

The cost of this real-time spatial light modulator is more than 2 orders of magnitude lower than other commercially available units. The LCD TV that it is based on costs only $150, and the modification can be accomplished in ~ 4 h. The built-in video driving electronics in particular makes the device immediately usable. However, to be able to address individual pixels directly, alternative addressing circuits have to be designed and fabricated. While the 122 X 148 space-bandwidth product is modest, it is still competitive with other electronically addressed devices such as Litton's LIGHT MOD or the Hughes CCD addressed LCLV. The optical quality can be excellent, determined mainly by the quality of the optical flats used in the modification. Among all the technologies being applied to implement real-time SLMs, the twisted-nematic LCD is by far the most devel-oped. The research and development effort to create the LCD SLM with smaller size and larger space-bandwidth product will likely be pressed forward due to the demands in the commerical market for compact TVs and computer dis-plays. We can therefore look forward to new low-cost LCDs in the consumer market that can be taken advantage of by the optical computing community. For now, we expect this low-cost SLM to find wide applications as a research and educational tool and also in some selected industrial applica-tions.

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

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

2. J. A. McEwan et ah, "Optical-Processing Characteristics of a Low-Cost Liquid Crystal Display Device," in Technical Digest, Optical Society of America Annual Meeting (Optical Society of America, Washington, DC, 1985), paper TUE3.

3. A. M. Tai, "A Low Cost Real Time LCD Spatial Light Modula-tor," ERIM IR&D Report 655207-1-F, July 1985.

4. A. D. Gara, "Real-Time Optical Correlation of 3-D Scenes," Appl. Opt. 16, 149 (1977).

5. B. D. Guenther, C. R. Christensen, and J. Upatnieks, "Coherent Optical Processing: Another Approach," IEEE J. Quantum Electron, QE-15, 1348 (1979).

6. J. W. Goodman, A. R. Dias, and L. M. Woody, "Fully Parallel High Speed Incoherent Optical Method for Performing Discrete Fourier Transforms," Opt. Lett. 2, 1 (1978).