15 November 1989 / Vol. 28, No. 22 / APPLIED OPTICS 4727

Real time optical edge enhancement using a Hughes liquid crystal light valve Tien-Hsin Chao

Jet Propulsion Laboratory, Pasadena, California 91109. Received 12 December 1988. 0003-6935/89/224727-05$02.00/0. © 1989 Optical Society of America.

A new real time edge enhancement effect using a Hughes CdS liquid crystal light value and its use in optical correla­tion are reported.

Edge enhancement is one of the most important prepro­cessing techniques utilized in optical pattern recognition. In an optical correlator, cross-correlation signals associated with input objects that are similar to the reference image can be greatly reduced by using the edge enhancement tech­nique.

There are at least three optical methods available for performing real time edge enhancement preprocessing in real time optical correlation detection. A real time input scene can be edge enhanced by passing through a high pass filtering optical preprocessor before it enters a cascaded optical correlator. This method will inevitably increase the system length as well as the difficulty in alignment. Edge enhancement of a reference image can also be achieved dur­ing the holographic synthesis of the matched spatial filter. By decreasing the reference-to-object beam ratio, the high spatial frequency component of the Fourier image spectrum can be enhanced. The effect is equivalent to that of high pass spatial filtering. However, the output system SNR will be greatly reduced using this method. A third approach is to utilize an edge enhancing spatial light modulator.

Recently, two types of differentiating spatial light modu­lator, specifically designed to generate edge enhanced out­put, have been reported. Casasent et al. have demonstrated real-time edge enhancement using a Priz light modulator.1

The Priz light modulator is a transverse modification of the Pockels readout optical modulator (PROM) with a [111] BSO crystal cut. Armitage and Thackara2-4 have designed a BSO photoaddressed nematic liquid crystal differentiating spatial light modulator. In their design, a layer of liquid crystal is tuned in a transverse configuration to achieve the edge enhancement. The BSO crystal is used as a photoad-dressing medium. This SLM is functionally optimized as an edge-enhancing SLM.

During a recent optical computing experiment, an edge enhancement effect was observed utilizing a Hughes Cds light valve.5 The experimental setup for this edge enhance­ment system was as follows: An Air Force resolution chart was placed in contact with a Hughes LCLV and illuminated by a 514.5-nm argon laser beam. A 632.8-nm He-Ne laser beam was directed to read out this resolution chart image through a polarizing beam splitter. An imaging lens was used to project the readout image onto an output screen for observation. The several steps involved in obtaining an edge enhanced output were as follows: First, the biasing frequency was lowered (10 kHz for normal operation), and, as it approached 2 kHz, edge enhancement started to appear, evidenced by the edges of the horizontal and vertical bars becoming brighter than their central portion. Second, the contrast of the edge enhanced image was increased as the biasing voltage (10-V rms for normal operation) was reduced to ~7-5-V rms.5 Third, the orientation of the LCLV was rotated counterclockwise (observed from the readout side)

Fig. 1. Experimental results of real time edge enhancement using a Hughes CdS LCLVs: (a) edge enhanced image of an Air Force resolution chart; (b) edge enhanced image of a continuous tone image; (c) a repeated result of (a) using a different Hughes CdS LCLV.

by ~10-30°. A sharp binary looking edge enhanced resolu­tion chart was then displayed in the output plane. The average input writing light intensity was ~50 mW/cm2. To check the repeatability of this edge enhancement effect, we conducted the same experiments using two different LCLVs.

The experimental result of the edge enhanced Air Force resolution chart using LCLV 1 is shown in Fig. 1(a). The edge of the outer horizontal and vertical bars as well as the Arabic numerals is clearly outlined. LCLV 1 was biased at 500-Hz, 6 V rms, and was rotated by 30° CCW from its normal setup. The experiment was then extended to a continuous tone input. The original input contains a girl's portrait against a halolike background illumination. The edge en­hanced output image is shown in Fig. 1(b). It is seen that the profile of the portrait of the girl is clearly accented and also that the background halo was reduced to a circular ring.

The experiment was then repeated using LCLV 2. The edge enhanced Air Force resolution chart is shown in Fig.

1(c). The LCLV 2 was biased at 1.4-kHz and 7-V rms and was rotated by ~15° CCW from its orientation for normal operation. While the edge enhancement effect is very good in the second experiment the intensity of the upper horizon­tal edge of the bars was slightly lower than that of the other remaining parts.

A second set of experiments was conducted to utilize this edge enhancement effect in real time correlation detection. We built a conventional VanderLugt optical correlator using the Hughes CdS LCLV (1) as the input SLM. Real time input was fed into the LCLV using a TV camera and monitor. Matched spatial filters were recorded on a thermoplastic camera (NRC model HC-300). In our experiments, four models cars, including a cargo van, jeep, sedan, and sports car, as shown in Fig. 2(a), were selected as the input objects. It is worth noting that to test the capability for discrimina­tion between two similar input objects, with and without edge enhanced inputs, we intentionally selected two very

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Fig. 2. Experimental results of optical correlation using continuous tone input. (a) Input objects containing four model cars. The sedan in the lower right corner is the reference image. (b)-(d) Cross-correlation signals of the van, jeep, and sports car, respectively. (e) Autocorrela­

tion signal of the sedan.

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Fig. 3. Experimental results of optical correlation using edge enhanced input. (a) Edge enhanced version of Fig. 2(a). The edge enhanced sedan in the lower right corner is the reference image. (b)-(d) Cross-correlation signals of the van, jeep, and sports car, respectively. (e) Auto­

correlation signals of the sedan.

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similar objects, the sedan and sports car. In the first experi­ment, a matched filter was holographically synthesized using the sedan as the input object.

The output correlation signals corresponding to the four objects shown in Fig. 2(a) are shown in Figs. 2(b)-(e). We see that the cross-correlation signal of the van and jeep is much lower than the autocorrelation of the sedan. However, the cross-correlation signal of the sports car is almost as strong as the autocorrelation signal of the sedan. Obviously, the high similarity between the two objects makes their clear distinction very difficult. We then biased the LCLV in its edge enhancement mode and repeated the experiment. The edge enhanced input objects are shown in Fig. 3(a). First, the edge enhanced sedan image was used to generate a MSF. The corresponding correlation signals are shown in Figs. 3(b)-(e). It is clearly seen that, compared with the correla­tion signals shown in Figs. 2(b)-(e), all three cross-correla­tion signals shown in Figs. 3(b)-(d) are decreased by the edge enhanced input. Moreover, the autocorrelation/cross-cor­relation ratio between the sedan and sports car has increased from 1, without using edge enhancement, to 1.6, with edge enhancement. ■ This experiment illustrates that the dis­crimination capability of an optical correlator can be signifi­cantly improved by using the edge enhancement effect ob­tained from a Hughes LCLV.

One possible explanation of this edge enhancement effect is that by changing the operating condition of the LCLV, a level slicing effect is achieved. As seen from the edge en­hanced Air Force resolution chart [Figs. 1(a) and (c)], the boundary between the fully on and fully off areas is enhanced as a result of the level slicing. A second possible explanation is that a LCLV is normally operated under a longitudinal configuration. When the biasing frequency is lowered, the LC molecules gradually align themselves along the direction of the applied electric field. Thus the LC is operated partial­ly in a transverse configuration. As described in a previous section, in a transverse SLM, the readout light intensity is proportional to the spatial gradient of the input light intensi­ty and results in edge enhancement.4 This effect is opti­mized by lowering the bias voltage to increase the contrast of the LCLV and rotating the LCLV slightly from its orienta­tion of normal operation to suppress the longitudinal mode.

We have reported the discovery of an edge enhancement effect in using a Hughes CdS LCLV. An edge enhanced version of the input writing image can be directly obtained by operating the LCLV at a lower bias frequency and bias voltage. Experimental conditions in which this edge en­hancement effect can be optimized are described. Experi­mental results show that the SNR of the readout image using this technique is superior to that obtained using high pass filtering. The repeatability of this effect has also been con­firmed by obtaining an edge enhancement result using two different Hughes LCLVs. We have also experimentally demonstrated the applicability of this effect in optical pat­tern recognition because of improved discrimination capa­bility. Through this experiment, we discovered that the Hughes LCLV can be used in both continuous tone and edge enhancing modes by simply adjusting its bias conditions.

The research reported in this paper was performed by the Jet Propulsion Laboratory, California Institute of Technol­ogy, as part of its Innovative Space Technology Center, which is sponsored by the Strategic Defense Initiative Orga-nizationAnnovative Science and Technology through an agreement with the National Aeronautics and Space Admin­istration (NASA). The work described was also cospon-sored by a RTOP of NASA OAST.

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References 1. D. Casasent, F. Caimi, M. Petron, and A. Khomenko, "Applica­

tions of the Priz Light Modulator," Appl. Opt. 21, 3846-3854 (1982).

2. D. Armitage and J. I. Thackara, "Liquid-Crystal Differentiating Spatial Light Modulators," Proc. Soc. Photo-Opt. Instrum. Eng. 613, 165-170 (1986).

3. D. Armitage and J. I. Thackara, "Optical Preprocessing and the Differentiating Spatial Light Modulator," in Technical Digest, Topical Meeting on Machine Vision (Optical Society of America, Washington, DC, 1987), pp. 58-61.

4. D. Armitage and J. I. Thackara, "Photoaddressed Liquid-Crystal Spatial Light Modulators," in Technical Digest, Topical Meet­ing on Spatial Light Modulators and Applications (Optical Soci­ety of America, Washington, DC, 1988), pp. 7-10.

5. J. Grinberg et al., "New Real-time Non Coherent to Coherent Light Image Converter: The Hybrid Field Effect Liquid Crystal Light Valve," Opt. Eng. 14, 217-225 (1975).