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Two-Band DMD-Based Infrared Scene Simulator
Julia Rentz Dupuis, David J. Mansur, George Genetti
OPTRA, Inc.
461 Boston St., Topsfield, MA 01983
phone: (978) 887-6600 fax: (978) 887-0022
[email protected]
www.optra.com
ABSTRACT
OPTRA is developing a two-band midwave infrared scene simulator based on digital micromirror devices (DMD). The
simulator is intended for testing a number of different infrared tracking technologies. Our approach allows for the
relative intensities of the two spectral bands to be varied for realistic simulations of an approaching target. The system
employs a broadband IR (thermal) source whose energy is spectrally filtered via a series of bandpass filters acting as
dichroic beamsplitters prior to being imaged onto two DMDs – one for each spectral band. The “on” reflected images
from the two DMDs are then fused, expanded by a telescope, and transmitted towards the unit under test. The relative
intensities of each spatial element of the two bands are controlled through the duty cycle of “on” versus “off” of the
related micromirror.
In this paper we present a breadboard design, build, and test which establishes the feasibility of our approach. A
description of the opto-mechanical system is given along with radiometric performance projections. Results from
breadboard testing, including maximum radiant intensity and radiant intensity resolution, and a series of simulated
images are shown.
Key Words: Two-band infrared scene simulator, digital micromirror device
1
INTRODUCTION
Under a U.S. Navy SBIR solicitation, a need was identified for the development of a fieldable two-color midwave
infrared (MWIR) scene simulator to test a host of different IR target tracking technologies including missile warning
systems (MWS), forward looking infrared (FLIR) cameras, and night vision systems. Within this application, the
simulator would be used at various ranges to illuminate the aperture of the unit under test (UUT) with a dynamic IR
scene simulation. The system must be able to simulate the spectral, spatial, temporal, and radiant intensity
characteristics of various targets for different test applications. Of particular importance for this development effort is
the ability to simulate a change in the spectral properties of the target over the duration of the event. A sample
application is the simulation of a missile flight where the spectral distribution of the emission in the MWIR region
(approximately 3.8 to 4.7 m) changes as a function of range relative to the MWS. The MWS is typically trained to
differentiate such targets from blackbody thermal radiators by looking at the change in relative intensities between two
MWIR sub-bands which are loosely called “blue” and “red” and are approximately 3.8-4.1 m and 4.5-4.7 m,
respectively.
A number of technologies have been developed to simulate the spectral, spatial, temporal, and radiant intensity
characteristics of these plumes. The most prevalent technology is a resistive array where current is driven through
individual pixels to make each radiate at an individually addressable intensity.1 Other technologies include those based
on liquid crystals2, light emitting diodes3, photonic technologies4, and micromirror arrays5. However, none of these
technologies in their current format offer the key ability to simulate the two MWIR bands with controllable relative
strength.
OPTRA is undertaking the development of a two-color MWIR source simulator based on fused projected images of two
digital micromirror devices (DMDs), one for each spectral band. The system employs a broadband IR (thermal) source
whose energy is spectrally filtered via a bandpass filter (BPF) centered on the blue band prior to being imaged onto each
DMD. The “on” reflected image from each DMD is then recombined by a second BPF centered on the red band, and the
fused beam is expanded by a telescope and transmitted towards the UUT. The relative intensities of the two bands are
Copyright 2008 Society of Photo-Optical Instrumentation Engineers.
This paper was published in The Proceedings of Technologies for Synthetic Environments: Hardware-in-the-Loop Testing XIII and
is made available as an electronic preprint with permission of SPIE. One print or electronic copy may be made for personal use
only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any
-1material in this paper for a fee or for commercial purposes, or modification
of the content of the paper are prohibited.
Approved for public release
controlled through the duty cycle of “on” versus “off” images reflected by each micromirror in the same manner that a
commercially available digital light projector (DLP) controls intensity. Because we are not changing the IR source
temperature, response is fast relative to resistive based simulators; in the same vein, thermal management issues are less
complex than with resistive arrays whose time constant depends on thermal management. Simplified thermal
management may ultimately result in a lower power, more fieldable system. At the same time, this approach provides a
broadband simulation, unlike laser simulators, resulting in a more representative target with which to challenge an IR
tracker. The overall approach offers the ability to realistically simulate the spectral, spatial, temporal, and radiant
intensity properties of complex scenes for IR tracker test applications.
2
2.1
DESCRIPTION OF THE TECHNICAL APPROACH
Breadboard Requirements
The goal of the SBIR Phase I effort was to design, build, and test a breadboard simulator thereby demonstrating the
feasibility of our approach. The breadboard made use of a single DMD, using the two halves for the two spectral
channels. The functional requirements of the breadboard system are tabulated below.
Table 1: Breadboard Simulator Functional Requirements
QUANTITY
VALUE
Spectral bands (approximate)
3.8 – 4.1 m (blue) 4.5 – 4.7 m (red)
Source maximum radiant intensity in red band
≥ 0.04 W/ster*
Grey level resolution
≥ 1000 levels
Switching time
≤ 20 ms
Pixel count
512x768
Image registration
one micromirror
Scene duration
≥10 s
Beam divergence
47 mrad
*
Corresponds to 10-6 W/cm2 at 2 m range.
2.2
Breadboard Description
Figure 1 shows the layout of the breadboard simulator, making use of a single DMD for the two channels.
The optical path is as follows. The IR
source is collimated by a lens (L1) and
projected into the simulator module.
The light is spectrally filtered by the
first dichroic beamsplitter (DBS1)
which is actually the blue channel BPF
(centered at approximately 3.9 m) that
transmits the blue band and reflects all
out of band energy; the transmitted
portion reflects off a fold mirror
(FM1b) and is focused by a lens (L2b)
to an image of the source on one half of
the DMD with a 15° angle of incidence
in the plane of the page and a 24° angle
of incidence in the plane perpendicular
to the page. The reflected portion from
DBS1 reflects off the opposing fold
mirror and is focused by a lens (L2a) to
an image of the source on the other half
of the DMD with a -15° angle of
incidence in the plane of the page and a
24° angle of incidence in the plane
perpendicular to the page. The DMD is
Figure 1a: Breadboard Simulator
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oriented such that the “on” pixels reflect the incident light up out of the page by 24° where it is collimated by a second
set of lenses (L3a and L3b) which are above the L2a and L2b (Figure 1b). Note that each light cone also obeys the law
of reflection in the plane of the page (due to the +/-15° incidence in this direction). The collimated light in each arm
then reflects off the respective fold mirror and is recombined at a second DBS(2) which is actually the red channel BPF
(centered at approximately 4.5 m) which is above DBS1. The result is the fused two-color scene of which we can
control the relative intensities by varying the duty cycle of the modulating micromirrors. Figure 1c illustrates how the
BPFs act as DBSs.
Figure 1b: Side View of L2 and L3
Figure 1c: Bandpass Filters used as Dichroic Beamsplitters
L3
(above
page)
L2
(below
page)
This concept was laid out using Zemax
design software in non-sequential ray
tracing mode. Figure 2 shows the Zemax
layout where only the chief ray and a ray
from the edge of the field are shown. An additional lens is shown (to the left in Figure 2) which is used to image the
fused scene onto an IR camera for calibration and test. The Zemax model shows that we can use a single DMD to create
the two images for the breadboard (i.e. they perfectly overlay); note that with the compound angles of incidence on the
DMD, the collimated fused image is not perfectly antiparallel with the incoming light from the IR source.
Figure 2: Simulator Optical Layout using Zemax Non-Sequential Ray Tracing (top view)
Fold mirrors
(x4)
Lens for IR
camera
IR camera
focal plane
IR source
IR source
collimating
lens
Focusing
lenses (x2)
DMD
Collimating
lenses (x2)
Blue BPF DBS (below)
Red BPF DBS (above)
Figure 3 shows the mechanical solid model of the breadboard. The simulator module containing all of the lenses, filters,
and fold mirrors are assembled into a single housing which is held at a 24° incline relative to the table top. The DMD is
held on a stage with z, x, y, tip, tilt, and yaw adjust although the yaw adjust is not shown. We had originally intended
the IR camera and lens to be held at 45° relative to the optical axis so they are square with the DMD, however, our test
plan evolved from using a small uncooled microbolometer camera (shown in the solid model) to a research grade FLIR
indium antimonide camera which was not practically held in this angle. We therefore simply simulated images rotated
by 45°.
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Figure 3: Breadboard Solid Model (side view)
IR Camera
DMD
Simulator module
tip/tilt
x
z
y
45°
24°
Figure 4 shows a solid model of the simulator module mount. All
of the lenses mount in bores and are adjustable for focus. The
fold mirrors face mount to the housing. Alignment of one image
to the other is accomplished by adjusting in position, tip, and tilt
of DBS2 while looking at the fused images with the IR camera.
This was done using a vacuum chuck on an xyz stage with tip/tilt
mount to hold and position DBS2 (Figure 1a).
Figure 4: Simulator Module Mount
DMD
Focusing
and
collimating
lenses
During the breadboard design phase we identified a Dell digital
light projector (DLP), model 3300MP, which contains a Texas
Instruments Discovery 1100 DMD board for use in the
breadboard system. While bare board development kits are
available from secondary distributors, they are cost prohibitive for
an SBIR Phase I, particularly given the fact that we would have
needed to purchase an additional daughter board to project
grayscale images. Used DLPs, on the other hand, are relatively
inexpensive and already come ready for grayscaling. We
therefore left the board development kit purchase for the Phase II
effort.
Fold mirrors
(x4)
IR source mount
DBS (x2)
In addition to stripping down the DLP and extracting the DMD
board from the projector, we also machined the existing window
off the DMD and replaced it with an IR transmissive window,
specifically calcium fluoride (CaF2). The details of this task are
given in section 2.4.
2.3
Outgoing
light
Radiometric Projections
The projected radiance over the red and blue spectral bands is given by equation 1.
R (Tsource )   s _ red / blue   red / blue 
 N(, Tsource )  d []
W
(1)
cm  ster
s_red/blue is the emissivity of the IR source in the red and blue bands as reported by the source manufacturer and is
approximately 0.9 and 0.96, respectively. red/blue is the radiometric efficiency of the system in the red and blue bands
respectively; the values are projected to be 0.30 and 0.44, respectively, based on a surface and vignetting loss
calculation. N is Planck’s spectral blackbody distribution given by
W
8hc
1
(2)
[ ] 2
N(, T)  5  hc kT

 1 cm  ster  m
e
 is wavelength (m), T is the blackbody temperature (K), h is Planck’s constant (6.6310-34Js), c is the velocity of light
(31010cm/s), and k is Boltzmann’s constant (1.38110-23J/K). This expression is integrated over the appropriate
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red / blue
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spectral band in Equation 1. As the IR source is throughput matched to the projecting optics (in this case the two lenses
following the reflection from the DMD), the maximum projected radiant intensity is given by
W
(3)
J (Tsource )  R (Tsource )  A projector _ lens []
ster
Based on these projections, the maximum radiant intensities we anticipate from the breadboard system in each channel
for the two source temperatures we used for the performance testing are tabulated below. For these projections,
Aprojector_lens = 3.46 cm2 where the lens diameters are both 2.1 cm.
Table 2: Projected Maximum Radiant Intensity Values
Tsource ≈ 875°C
Tsource ≈ 825°C
Red
Blue
Two-color
Red
Blue
6.80×10-2
W/ster
2.4
1.33×10-1
W/ster
2.03×10-1
W/ster
7.80×10-2
W/ster
1.56×10-1
W/ster
Two-Color
2.34×10-1
W/ster
Breadboard Build and Integration
Figure 5 is a photograph of the integrated simulator breadboard.
Figure 5: Breadboard Simulator System
Laptop for passing
DMD images
DMD Mount (x, y, z, tip, tilt,
and yaw adjust
Simulator
Housing
Projector
Components
(behind baffling)
DMD
MWIR (InSb)
FLIR Camera
Laptop for
Collecting IR
Camera
Images
In order to operate the DMD board we extracted from the Dell projector, we had to maintain all of the DMD board’s
electrical connections and interlocks it had while in the projector. This meant mounting all of the other projector boards
along with the lamp and filter wheel near the DMD such that the connections could be made. We interfaced with the
projector through the standard video port on a laptop and passed the DMD images primarily using Microsoft Power
Point in slide show mode. Still bitmap images were generated using Corel Draw, and dynamic images were generated
using either Matlab or the animation tools in Power Point.
Prior to system assembly, we swapped out the visible window on the DMD for an infrared transmissive window. This
was accomplished by removing the DMD chip housing from the board and milling the weld that holds the window
mount to the chip housing. We then put the housing in a nitrogen purged environment and broke the weld, and thus the
seal, by hand. We then replaced the window mount with a custom made window mount with a CaF2 window. We used
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Varian torr seal, a well known, low outgassing vacuum epoxy, to seal the new window mount onto the chip housing.
The epoxy was let to cure in the nitrogen purge environment for approximately two hours and then the rest of the way in
air for 24 hours.
3
3.1
TEST DESCRIPTION AND RESULTS
Test Description
All tests were performed using a FLIR Systems SC4000 indium antimonide (InSb) camera with the standard
thermographic calibration. Software packages used included Researcher Professional for radiometric measurements and
ExaminIR for grayscale imaging. The camera had a 25 mm silicon and germanium f/2.3 lens. During all testing, the
FLIR camera and simulator were not synchronized, and we experienced some degree of the expected aliasing between
the simulator and the camera. Future (Phase II) testing will be done with the IR camera synchronized to the simulator.
In general, the final solution will offer considerably more flexibility that the breadboard since we will have the DMD
development kit with high speed port at our disposal. Not only will we be able to synchronize the camera and simulator,
if necessary, we will also be able to correct for any rolling update issues of the DMD because the high speed port enables
phased array operation. It is not clear at this point, however, whether the rolling update of the DMD even requires
correction.
Maximum Radiant Intensity
Maximum radiant intensity was determined using the thermographic calibration of the FLIR camera. This calibration,
which assumes that energy is integrated over the 3 to 5 m spectral range, is reported to be accurate to ±2°C, according
to the FLIR literature. For these measurements, all micromirrors for the channel under test were turned into the “on”
position and the maximum temperature of the image was recorded. The radiance was then calculated by integrating
Planck’s function over the 3 to 5 m spectral band (Equation 2), and the radiant intensity was calculated by multiplying
the radiance by the area of the limiting aperture which is set by the diameter of the collimating lenses following the
reflection from the DMD (L3a and L3b).
5

W
(7)
J measured (Tsource )    N(Tmeasured , )  d   A projector _ lens []
ster
3

This measurement was made for two difference source temperatures determined from the manufacturer’s temperature
versus drive power plot. The results were reconciled with the radiometric projections.
Radiant Intensity Resolution
Figure 6: Resolution Test Image
Radiant intensity resolution was shown in two ways. The first was a
qualitative demonstration of the grayscaling capability of the system where
we simulated an image of a block with 25 subregions of increasing
grayscale. We recorded the simulated image with the FLIR camera and
present the results. Figure 6 shows the image that was passed to the DMD
for projection.
The second test was dynamic. We simulated an image of a square of
uniform grayscale level and dynamically increased the grayscale level
through 1000 levels. We recorded the response with the FLIR camera.
Simulated Images
We simulated a series of still binary images and recorded red, blue, and the two-color image with the FLIR camera.
Figures 7a through 7l show the images that were passed to the DMD for projection. Note that the DMD is rotated by 45°
with respect to the optical axis of the simulator, and this is why the red and blue images, also rotated by 45°, end up
overlaid. Note also that the FLIR camera is not rotated by 45°. As will be shown in the results section, the simulated
images still have the residual 15° rotation due to the compound reflection angle of the DMD. All of these geometric
artifacts are unique to the breadboard and will not be present in the final system where we will be using two separate
DMDs, one for each channel, and will not be working with compound angles. In this way, the final system will be
optically simpler that the breadboard.
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Figure 7a: Spot, 2-Color
Figure 7b: Spot, Red
Figure 7d: Ring, 2-Color
Figure 7e: Ring, Red
Figure 7f: Ring, Blue
Figure 7g: Cross, 2-Color
Figure 7h: Cross, Red
Figure 7i: Cross, Blue
Figure 7j: Grid, 2-Color
Figure 7k: Grid, Red
Figure 7c: Spot, Blue
Figure 7l: Grid, Blue
We also simulated a series of dynamic images including an approaching plume in the form of a point source which
increases in size and intensity as a function of time. The blue and red images are individually controllable based on the
bitmaps passed to the DMD. For this simulation we arbitrarily made the blue image intensity increase twice as fast as
the red image intensity. We show images of the blue, red, and two-color plume recorded by the FLIR camera. In
addition we simulated a company marketing dynamic image of overlaid red and blue OPTRA logos where each image is
in turn rotated relative to the other. In order to align the images to the reference frame of the camera, the red OPTRA
was nominally rotated -60° and the blue image was flipped and nominally rotated +30°. These angles compensate for
the 45° rotation of the DMD relative to the FLIR camera and also the 15° compound angle. Figures 8a and 8b show
sample bitmaps that were used for these simulations.
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Figure 8a: Plume Simulation
3.2
Figure 8b: OPTRA Simulation
Test Results
Maximum Radiant Intensity
The following tabulates the results of the maximum radiant intensity measurement. Teq_max is the maximum object
temperature measured by the FLIR camera whose thermographic calibration presumes to span the 3 to 5 m spectral
range. Nmax is the radiance calculated by integrating Planck’s function (Equation 2) over the 3 to 5 m spectral range for
an object of temperature, Teq_max. Jmax is the radiant intensity calculated by multiplying Nmax by Aprojector_lens (=3.46 cm2).
The accuracy of the thermographic calibration is ±2°C; the associated radiant intensity accuracy scales with Teq_max and
ranges from ±1.8×10-3 W/ster at Teq_max = 239°C to ±4.3×10-3 W/ster at Teq_max = 346°C. The breadboard requirement
for Jmax is greater than 0.04 W/ster.
Table 3: Maximum Radiant Intensity Measurements
V·I = 3.7 W, TS ≈ 825°C
Value
Red
Blue
Two-Color
Units
Teq max
239
294
332
°C
Nmax
1.96×10-2
3.85×10-2
5.72×10-2
W/(cm2·ster)
Jmax
6.80×10-2
1.33×10-1
1.98×10-1
W/ster
V·I = 4.27 W, Ts ≈ 875°C
Value
Red
Blue
Two-Color
Units
Teq max
249
307
346
°C
Nmax
2.25×10-2
4.47×10-2
6.60×10-2
W/(cm2·ster)
Jmax
7.80×10-2
1.55×10-1
2.28×10-1
W/ster
These measured values agree precisely with the projected values for the individual channels (Table 2), assuming that we
have accurate projections for the radiometric efficiency of the system in each channel. Note that the big unknowns
remain the effects of the vacuum chuck on the red channel and also the out of band reflectance of the bandpass filters.
Another source of uncertainty is the thermal uniformity of the IR source which certainly shows “hot spots”. This may
result in either an over or under reporting of temperature by the IR camera. This may explain why the measured two
color maximum radiant intensity values were slightly lower than expected if, for example, the “hot spots” in the red and
blue channels do not overlay well. With all of this considered, if we assume reasonable values for these uncertain
quantities, we get excellent agreement between measured and projected maximum radiant intensity.
Radiant Intensity Resolution
Figure 9a shows the grayscale image measured by the camera in response to our simulation. Some general comments
about the grayscale image are in order. One issue with the breadboard is that no special care was taken to address the
non-uniformity of the IR source, and this affects the quality of the grayscale image. With regard to the final system, we
will address source uniformity in two ways. First, the IR source will not be directly imaged onto the DMD; we will use
some means to create a uniform illumination using, for example, a gold coated integrating sphere. The second measure
will be applying a spatial scale factor to the bitmaps passed to the DMD that compensate for any residual non-uniformity
left in the illumination. This second measure will work well as long as the intensity pattern is fixed in time. With this in
mind, we are able to show spatial grayscaling capability with the breadboard and seen in Figure 9a. Figure 9b is a line
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cut in the direction of increasing grayscale level in a raster pattern across Figure 9a starting from the lower right. Note
that the simulated image outreached the dynamic range of the camera in that we did not register the bottom row of
grayscale seen in Figure 6. The dips in the line cut are due to the edges of the image.
Figure 9a: Grayscale Image
Figure 9b: Line Cut of Grayscale Image
Figure 10 shows the time response of the FLIR camera to the square of uniform grayscale level ramping through 1000
grayscale levels. This data is an average of nine time response curves; the jumps in the curve are an artifact of not
synchronizing the camera with the scene simulation (i.e. the aliasing effect). The curve shows a clear 1000x intensity
increase, which is the requirement.
Figure 10: Time Response of FLIR to Grayscale Ramp
Simulated Images – Still Images
Figures 11a though 11l show the still images recorded in
response to the images simulated from Figures 7a through
7l, respectively. For these images as well as for the
dynamic simulations, the IR source was somewhat
defocused to improve the spatial uniformity. For a given
shape, each image is shown on the same intensity scale.
Particularly with respect to the blue image, improvements
can be made regarding the overlay of the IR source image
with the simulated image passed to the DMD. This is
particularly noticeable in the blue cross and grid patterns.
Overall, the still images look very good, especially
considering the fact that we are imaging a surface that is
tilted by 15° relative to each optical axis (i.e. the
compound angle). Without this angle, we expect the final
system images to be even cleaner. Note that, to the extent
of the resolution of the FLIR imaging system, we appear to
be registered to about one micromirror, although we cannot
confirm this since one micromirror is imaged to
approximately 0.5 FLIR pixels.
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Figure 11a: Spot, 2-Color
Figure 11b: Spot, Red
Figure 11c: Spot, Blue
Figure 11d: Ring, 2-Color
Figure 11e: Ring, Red
Figure 11f: Ring, Blue
Figure 11g: Cross, 2-Color
Figure 11h: Cross, Red
Figure 11i: Cross, Blue
Figure 11j: Grid, 2-Color
Figure 11k: Grid, Red
Figure 11l: Grid, Blue
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Simulate Images – Dynamic Images
Figure 12 shows a still frame from the plume simulation. The simulation was recorded in three parts: first the blue
plume alone (red channel off), then the red plume alone (blue channel off), and finally the two-color plume (both
channels on). The FLIR was set to acquire at 60 Hz frame rate in each case. For this simulation we have the blue plume
intensity increasing twice as fast as the red plume, however, this is completely arbitrary. As we’ve shown with our
grayscale capability, we can simulate practically any temporal intensity profile for either channel with the simulator.
The source non-uniformity is apparent again in these images however, we have plans to address this in the future as
described in previous sections. The dynamic images show that we have exceeded the requirement for maximum scene
duration (10 s). As the videos were recorded at 60 Hz frame rate, and we are showing dynamic images which are
updating between frames, these dynamic images also show that we appear to be meeting the 20 ms switching time
requirement.
Figure 12: Still Frame from Plume Simulation
Figure 13 is a still frame from the OPTRA dynamic image.
Figure 13: Still Frame from “OPTRA” Dynamic Image
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4
SUMMARY AND CONCLUSIONS
In summary, we have successfully designed, built, integrated, and tested a breadboard simulator thereby demonstrating
the feasibility of our approach. We have successfully measured the maximum radiant intensity in both channels and
reconciled the results with our analytical model. We have shown spatial and temporal grayscale resolution and have
offered direction for improving the IR source uniformity in future development work. We have demonstrated the ability
to simulate spatial objects with good image quality with the breadboard. We have also generated a short series of
dynamic two-color scenes, including an approaching point source. Overall, the breadboard showed strong promise as
the basis for future development and applications.
Improvements we intend to make for the final system include the use of two separate DMDs thereby eliminating the
compound angle while doubling the number of spatial resolution elements in each channel relative to the breadboard
system. Image quality is expected to improve without the tilted DMD plane (relative to the optical axis). In general, the
final system will be optically simpler than the breadboard without the compound angles. Future testing will also include
the synchronization of the test camera and the simulator which will eliminate the aliasing effects. This improvement will
allow us to make a better dynamic range / grayscale resolution demonstration. We will also determine whether the
DMD will have any rolling update issues which can be addressed by phasing the images input to the DMD. Given the
frame rate of the DMD and time constant of the micromirrors, however, this may be a non-issue. The final system will
also employ projection optics which will increase the maximum radiant intensity for larger standoff ranges between the
simulator and unit under test. Finally, the final system, which will be a fieldable unit, will include a field
calibration/alignment module.
Acknowledgements
This research was conducted under a NAVAIR Small Business Innovation Research Phase I contract funded by the U.S.
Navy.
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