# High-speed Temporal Characterization and Visualization of

# Spatial Light Modulators and Flat-panel Displays

Eung G. Paek, C. L. Wilson, J. W. Roberts, and C. I. Watson

Information Technology Laboratory

National Institute of Standards and Technology (NIST), Gaithersburg, MD
20899

Abstract **-** An apparatus that can characterize and visualize temporal
dynamics of spatial light modulators (SLM) and flat-panel displays is
constructed and evaluated using a commercially available spatial light
modulator. The apparatus is based on the stroboscopic video sampling
method and has a temporal resolution on the order of microseconds,
permitting measurement of a long event (\> 100 ms) with a high
signal-to-noise ratio. Experimental results demonstrate the
visualization of the temporal image sequencing and addressing in an SLM.

To be submitted to Optics Letters

The spatial light modulator (SLM) has been widely used for optical
information processing, pattern recognition, optical computing,
holographic storage and displays \[1\]. The SLM has a finite response
time (typically 1ms to 100 ms) depending on materials and addressing
schemes. In the normal matrix addressing scheme, two-dimensional (2-D)
information is loaded line-by-line from top to bottom, causing a time
skew. These finite response times and time skews can cause serious
problems, especially in coherent optical information processing, because
the entire two-dimensional data must be present in the light path while
processing is in progress\[2-4\]. The loaded information should not vary
during information processing or during holographic recording. Also, all
of the data of the prior frames must disappear before processing a new
frame of data. For such temporal measurements of locally varying
behavior of high speed SLM's (such as ferroelectric liquid crystal SLM's
or deformable mirror devices), a high-speed, 2-D imaging apparatus is
needed that can take a series of pictures with a time interval of
sub-microseconds to cover the entire settling time of about 100 ms until
a stable image is obtained.

To our knowledge, no previous work has been reported on the visual
characterization of rapidly varying local behavior of 2-D SLM's.
Conventional spot checks performed at each pixel are time-consuming and
do not provide information on overall image evolution. Although
high-speed photography using streak cameras or frame cameras can be used
to take pictures with a sub-microsecond temporal resolution, it is
expensive and complicated to use. Moreover, streak cameras can cover
only a limited number of frames (typically about 10 frames at a time).
Also, current high-frame-rate charged coupled devices (CCDs) cannot
reach beyond several thousand frames per second. Recently, a low-cost
method of capturing fast repetitive processes has been developed by
Moller and Bruns \[5\]. The method is based on the conventional
oscilloscopic sampling method applied to video images, as in a
stroboscope.

In this Letter, we apply the low-cost video sampling method to
characterize and visualize the rapidly varying SLM images and answer
several important issues concerning the speed of an SLM. Figure 1 shows
a schematic diagram of our apparatus: Light from a HeNe laser is focused
by lens L~1~ and is modulated by an acousto-optic modulator (AOM)
(TeO~2~ shear mode, NEOS model no. N23080) \[6\] to generate a light
pulse with a pulsewidth of approximately 50 ns. Light from the AOM is
collimated by lens L~2~ and illuminates the SLM under test and
subsequently forms an image at the output CCD plane using lenses L~3~
and L~4~. The spatial filter at the focal plane of lens L~3~ blocks all
higher-order diffracted beams originating from the grid structure of the
SLM and passes only the 0-th order diffracted beam. This avoids spurious
Moire fringing noise in the output images.

To accurately synchronize light pulses with SLM images, a precise timing
relationship must be maintained between the pulse applied to the AOM and
the video signal. In our case, a series of image patterns is generated
by a frame grabber (EPIX, model no. SVIP-4MEG) and the video images are
displayed repeatedly. The NTSC video signal is then converted to a VGA
signal by a video-to-VGA converter and is loaded on the SLM under test.
A sharp, strong pulse is added at the beginning of the series of images
as a reference mark. The vertical sync signal from the converter and the
reference mark generates a stable reference signal. The reference signal
is delayed by a delay generator (Stanford Research Systems, model DG535)
\[6\]. The delay generator has very fine resolution (5 ps) and a long
range of delays up to 1000 s. The delayed pulse is mixed with a rf
frequency and is loaded onto the AOM. Also, the output CCD is externally
synchronized with the VGA signal through the sync signals from the
converter. A stationary stroboscopic image is detected by a CCD and is
captured by a digital video recorder. After each capture of an image,
the time delay is increased incrementally by a desired amount to take a
sequence of pictures with controlled delay that sample the whole image
sequence. The images thus recorded for the entire event are displayed at
normal speed, as for slow motion videos. A conventional active matrix
twisted nematic liquid crystal SLM for projection display is used as a
sample in this experiment.

Figure 2 shows the output intensity at two different points (**A** and
**B** marked in the figure) at the output CCD plane (separated by 12 ms)
as a function of time delays. An image (letter "3") is loaded twice
(even and odd frames) in 1/30 s as in conventional NTSC video signals.
At each point in the output plane, light intensity increases to
approximately half the peak values by the first pulse of applied voltage
and continues to increase up to the peak values by the second pulse. The
output is a convolution of two impulses separated by 1/60 s and the
temporal impulse response of the liquid crystal in the linear region of
the CCD. The two curves show similar tendencies but the first one
obtained at point **A** shows less intermediate plateauing. This
difference is attributed to the non-uniformity of light intensity
illuminating the SLM. Both curves reach peaks after 27 ms and decay in
10 ms.

In Figure 3, a series of SLM images taken every 4 ms shows dynamic
evolution of the images. A series of input images (the numbers 1, 2, 3,
and 4) is presented repeatedly at video rate. Each input image lasts for
1/30 s (interlaced) and blank images are added before and after the
numbers to allow measurement of writing and decay time without being
interrupted by neighboring images. The delay times are measured with
respect to the loading of the first pixel (on the left top corner) of
the first image "1". The first image appears from top to bottom until a
full image is revealed after 28 ms. The second image begins to appear
before the first image decays and the two images overlap for about 24 ms
(from 40 ms to 64 ms). A reasonably isolated image appears only for 8
msec before the next image begins to overlap. The intermediate
transition of images from one to the other and the resultant crosstalk
is evident in the pictures.

For high temporal resolution, the pulsewidth of illuminating light needs
to be short. In this case, contrast of an acousto-optic modulator is one
of the most important parameters and is related to the
signal-to-crosstalk ratio (SCR) of an output image by

$\text{SCR}\  = \ \frac{\Delta t}{T} \times \text{CR}$, (1)

where t, T and CR refer to pulsewidth, duration of total event, and
contrast ratio of the optical switch, respectively. The AOM used in this
experiment has a contast ratio of approximately 4,000 (measured) to
permit T = 4000 t for SCR=1.

Another important parameter as pulsewidth becomes narrower is the
intensity of the light illuminating the SLM (I) which is related with
the minimum detectable light intensity of a CCD (S) as

$I\  \times \ \frac{\Delta t}{T}\  \times \ L\  \geq \ S\ ,$$$ (2)

where L is the sum of insertion losses from AOM, SLM, and other optical
components. The minimum detectable intensity of the CCD used in this
experiment (Pulnix, model TM-7EX)\[6\] is approximately 0.01 W/cm^2^ at
the HeNe wavelength of 633 nm. When the light level is weak, the
integration (averaging) method can be used to compensate for the reduced
SCR, as in conventional sampling oscilloscopes.

The bulky laser and the acousto-optic modulator used in this experiment
can be replaced by a high-power, pulsed laser diode or a light emitting
diode (LED) to make the system both compact and inexpensive. In
addition, the laser diode can have better contrast ratio than an AOM due
to its threshold nature. Also, a high-speed LED can be used for better
images without speckle noise.

In conclusion, we have constructed a stroboscopic video sampling system
to characterize and visualize the temporal dynamics of SLM's and certain
kinds of flat-panel displays such as liquid crystals displays or
deformable mirror devices. The system was used to visually characterize
active-matrix twisted-nematic liquid crystal displays, which turned out
to be unsatisfactory for real-time video-rate processing. The use of the
system for other displays, including an array of self-luminous smart
pixels, is currently being investigated.

# Acknowledgment

This work was supported by the Advanced Technology Program of the U.S.
Department of Commerce and the Information Technology Laboratory at
NIST.

**References :**

1.  *Spatial light modulator technology, materials, devices and
    applications*, edited by U. Efron, Marcel Dekkar, Inc. New York,
    1994.

2.  D. Casasent and C.L. Wilson, "Optical metrology for
    industrialization of optical information processing," NIST-IR 6060,
    Sept. 1997.

3.  J.C. Kirsch, D. Gregory, M.W. Thie and B.K. Jones, "Modulation
    characteristics of the Epson liquid crystal television," Opt. Eng.
    31 (5), pp. 963-970, 1992.

4.  C. Soutar, S.E. Monroe and J. Knopp, "Measurement of the complex
    transmittance of the Epson liquid crystal television," Opt. Eng. 33
    (4), pp. 1061-1068, 1994.

5.  M. Moller and H.-J. Bruns, "Video sampling: a new low-cost method of
    capturing fast repetitive process," SPIE vol. 2549, pp. 2-11, 1995.

6.  Certain commercial equipment or components are identified in this
    paper only to specify the experimental procedure adequately. Use of
    these equipment or components does not constitute an endorsement by
    NIST or any other agency of the Department of Commerce.

**Figure Captions:**

Figure 1. Schematic diagram of a system for stroboscopic temporal
characterization and visualization of SLM's.

Figure 2. Output intensities at two different points at the output plane
(marked with "A" and "B" and are separated by 12 ms in time with each
other) as a function of time delays. An image (letter "3") is loaded
twice (even and odd frames) in 1/30 sec.

Figure 3. Temporal development of a series of images loaded on a typical
active matrix twisted nematic liquid crystal SLM. A series of images
(the numbers 1, 2, 3, and 4) are presented at video rate, and blank
images are added before and after the numbers to show writing and decay
of the images. The snapshot images are taken every 4 ms, and the delay
times are measured with respect to the initial loading of the first
pixel (on the left top corner) of the first image "1".