Response time of a liquid crystal image transducer

Bernard W. Joseph and Aaron D. Gara General Motors Research Laboratories, Physics Depart­ment, Warren, Michigan 48090. B. W. Joseph is now with GM Proving Ground, Vehicle Emission Laboratory, Milford, Michigan 48042. Received 3 November 1977. 0003-6935/78/0701-1987$0.50/0. © 1978 Optical Society of America.

Coherent optical processing systems have been developed that use photographic transparencies of objects as the input. The mechanics of film exposure and development severely limit the rate at which optical data may be acquired and processed. The recent development of electrooptical image transducers that have many features characteristic of pho­tographic emulsions, such as high spatial resolution and good image contrast, has effectively removed the temporal con­straint imposed on optical processing by photographic transparencies. We are investigating the use of a real-time image transducer, developed by Hughes Research Laboratory, for real-time coherent optical information processing.1 A summary of the main operating characteristics of the device have been previously reported.2,3 The work reported here describes the response time of the device under various op­erating conditions.

Figure 1 shows the essential elements of the transducer. The noncoherent light falling on the photoconductor varies its impedance, so that more or less voltage appears across the

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Fig. 1. Hughes liquid crystal image transducer (not to scale).

Fig. 2. Schematic diagram of the apparatus used to record the re­sponse time of the Hughes liquid crystal image transducer. The polarization of the He-Ne laser readout beam is indicated in the di­agram. The beam splitter reflects the vertical polarization compo­

nent and transmits the horizontal component.

irradiate the transducer's input side. The lamp's spectrum closely resembles that of a 3280-K blackbody. The ir filter cuts off sharply at 700 nm to suppress long wavelength ra­diation to which the photoconductor and liquid crystal are transparent. The filter has an essentially flat passband for wavelengths shorter than 700 nm to well beyond the 526-nm CdS band edge reported by Fraas.4

The irradiance on the transducer's input side was measured by substituting a radiometer. Response times were measured for input irradiances which gave the maximum output, as well as for irradiances which gave steady-state outputs of one-half and one-quarter this saturation output. Drive voltage was supplied by an audio oscillator continuously variable in voltage from 0 to 10 V rms and discretely variable in frequency at 0.5 kHz, 1 kHz, 2 kHz, 5 kHz, and 10 kHz. The irradiance required to saturate the device is an increasing function of the drive voltage frequency and varied from 20 μW/cm2 at the lowest frequency to 1000 μW/cm2 at the highest.

We define contrast as the simple ratio of output with sat­urating input to output with no input. At the recommended operating voltage of 4 V rms, device contrast is adequate for optical correlation on real objects.1 However, we found that maximum contrast was obtained with about 5.8 V rms applied. This maximum contrast voltage is a slowly increasing function of frequency. Response times were measured at both the 4 V rms and the maximum contrast voltage for each frequen­cy.

The electronically timed mechanical shutter has an opening and closing time of less than 5 msec. Neither it nor the silicon diode detector limits the measured rise and decay times of the transducer. Rise time is defined as the time to reach 90% of steady-state output from a dark baseline; decay time is the time to drop to 10% of steady-state on-value. During a series of measurements a minimum of 10 sec was maintained be­tween light pulses to let the panel recover.

Figure 3 summarizes the rise time measurements obtained for two Hughes image transducers. The lines represent the envelopes of all measurements. In general, the lower boundaries are with 10-kHz excitation frequency, and the upper boundaries are with 0.5 kHz. Rise time is a strong function of the writing light irradiance, decreasing as the ir-

liquid crystal layer according to the light intensity. The twisted nematic liquid crystal converts the linearly polarized readout light to elliptical polarization according to the voltage across the layer. An analyzer, placed in the path of the emerging beam, passes only the component of polarization that is perpendicular to the original polarization direction of the readout beam. The spatial intensity distribution of the readout beam after the analyzer will be approximately pro­portional to the spatial intensity distribution of the input image.

Since the present work was not concerned with the image-transducing quality of the device, the response time data were obtained with a uniformly irradiated panel in the apparatus shown in Fig. 2. Although Hughes measured and reported results on transducers irradiated with 525-nm writing light, our panel will be principally used on scenes irradiated with white light. A 300-W tungsten-halogen lamp was used to

Fig. 3. Rise time of two Hughes image transducers as a function of normalized writing irradiance. The full and solid lines are the en­velopes of all measurements made on the two devices. Writing irra­diance was normalized by the input irradiance which produced

saturation.

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Fig. 4. Effect of a bias irradiance on rise time and decay time of Hughes image transducer II. The drive voltage was 4 V rms at 10

kHz.

radiance increases. The shortest rise time always occurs when the irradiance saturates the output. Rise time also decreases as the drive frequency increases.

The inverse relation between rise time and irradiance is in the expected direction; but inverse relation between rise time and frequency is in the opposite direction to that expected from Hughes's study of the CDS photosensitive layer.4 Both panels consistently showed the shortest rise time with 10-kHz excitation.

Decay time varies only slightly with the writing irradiance and was generally faster at lower irradiances. Dependence on frequency is similar to that for rise time. Decay times at saturation irradiance (worst case) varied from 250 msec at 0.5 kHz to 7 msec at 10 kHz.

The over-all response time of the device is seen to be limited by the rise time. The rise time of the panel is related to the properties of the CdS-CdTe photoconductor substrate.4 The long time constant for the buildup of current at low light levels suggests a high electron trap density in the photoconductor, which inhibits the initial transfer of photoelectrons to the electrode surface.

If indeed electron traps inhibit the photoelectron transfer, the most obvious method for improving the rise time is to bias the photoconductor to fill the electron traps. To accomplish this a separate light was used to continuously flood the transducer. The rise and fall times are shown in Fig. 4 for different values of this bias illumination with the light pulse near the one-quarter saturation irradiance level. We see that biasing the device to only 1% of saturation decreases the rise time by a factor of 10. A further increase in bias produced a less dramatic decrease. At the same time, the decay time increased but remained shorter than the rise time.

The gains made in improved rise time by this method must be weighed against a loss in output image contrast. However, with no bias illumination the contrast is at least 100:1. A 1% bias will reduce maximum contrast to approximately 50:1. This contrast is acceptable in many of the applications we are investigating, particularly optical correlation.

References 1. A. D. Gara, Appl. Opt. 16, 149 (1977). 2. A. Jacobsen et al., Inf. Disp. 12, 17 (1975). 3. A. Jacobsen et al., Opt. Eng. 43, 217 (1975). 4. L. M. Fraas et al, J. Appl. Phys. 47, 576 (1976).

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