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Two-Band DMD-Based Infrared Scene Simulator
Julia Rentz Dupuis, David J. Mansur, Robert Vaillancourt,
Thomas Evans, David Carlson, and Elizabeth Schundler
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 (MWIR) scene projector based on digital micromirror device
(DMD) technology; this projector is intended for training various IR tracking systems that exploit the relative intensities
of two separate MWIR spectral bands. Our approach employs two DMDs, one for each spectral band, and an efficient
optical design which overlays the scenes reflected by each through a common telecentric projector lens. Other key
components are miniature thermal sources and a series of spectral filters. Through the use of pulse width modulation,
we are able to control the relative intensities of objects simulated by the two channels thereby enabling realistic scene
simulations of various targets and projectiles approaching the tracking system. Performance projections support radiant
intensity levels, resolution, bandwidth, and scene durations that meet the requirements for a host of IR tracking test
scenarios.
In this paper we summarize the design and build and detail the system characterization of a prototype two-band
projector. System characterization results include maximum radiant intensity, radiant intensity resolution, and angular
resolution. We also present a series of projected images.
Key Words: Two-band infrared scene simulator, infrared scene projector, 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 projector to test a host of different IR threat detection technologies including missile warning
systems (MWS) and forward looking infrared cameras. Within this application, the projector 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 threats for different test
applications. Of particular importance for this development effort is the ability to simulate a change in the spectral
properties of the threat 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 (3 to 5 m) changes as a function of range relative to the
MWS. The MWS is typically trained to differentiate such targets from non-threat manmade and natural objects by
detecting the change in relative intensities between two MWIR sub-bands which are loosely called “blue” and “red” and
are in the vicinity of 3.0-4.2 m and 4.2-5.0 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 are resistive arrays 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, lasers3, light emitting diodes (LEDs)4, photonic technologies5, and micromirror arrays6. In general
these technologies project a single spectral band which may be broad in the case of resistive arrays or very narrow in the
case of laser or LED arrays. The ability to project the two spectral bands with individually controllable relative
intensities represents an advanced capability for IR threat detection system testing.
OPTRA is presently developing a two-color MWIR scene projector based on fused projected images of two digital
micromirror devices (DMDs), one for each spectral band. The system employs miniature broadband IR (thermal)
sources to illuminate each DMD with spectrally-filtered energy via two bandpass filters (BPFs) which set the blue and
red channels at approximately 3.4-4.2 and 4.2-5 m, respectively. The reflected “on” image from each DMD is fused by
Copyright 2010 Society of Photo-Optical Instrumentation Engineers.
This paper was published in The Proceedings of Technologies for Synthetic Environments: Hardware-in-the-Loop Testing XV 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.
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a dichroic beam combiner with an edge at 4.2 m. The fused two-band image is then projected by a telescope lens
which sets the field of view (FOV) of the transmitted beam as well as the maximum radiant intensity (in units of W/ster).
The relative intensities of the two bands are controlled through the duty cycle of “on” versus “off” images reflected by
each micromirror via pulse width modulation (PWM) 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 depending on the required grayscale resolution; 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 or LED simulators, resulting in a more representative target with which to challenge
an IR threat detection system. The overall approach offers the ability to realistically simulate the spectral, spatial,
temporal, and radiant intensity properties of complex scenes for IR threat detector test applications.
The following paper presents the design, build, and test of a two-band IR scene simulator prototype. Having established
the feasibility of this technology through the design, build, and test of a breadboard two band simulator7, we are building
on this experience with the development of a fieldable prototype. Our current status is that the system is integrated with
the red channel operable (the blue channel is in progress). We summarize the prototype design and present the
integrated system. Tests include maximum radiant intensity, radiant intensity resolution, and angular resolution. We
also present a series of red band images. We expect to have the blue channel operable by the time of this presentation
and will show two band images at that time.
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PHASE II PROTOTYPE DESCRIPTION
2.1 Prototype Performance Specifications
Table 1 lists the prototype performance specifications.
Table 1: Prototype Performance Specifications
QUANTITY
VALUE
Spectral Bands*
3.4-4.2 m (blue), 4.2-5.0 m (red)
Maximum Radiant Intensity in Red Band**
≥ 1 W/ster
Gray Level Resolution
10-bits
Maximum Update Rate at 10-bits
40 Hz
Pixel Count
768 diameter
Maximum Scene Duration
54 s
***
Angular Resolution
225 rad
Image Registration
One Angular Resolution Element
*
Red and blue bands are designed to be wider than that of the UUT such that the
UUT limits the spectral range.
**
Maximum radiant intensity is specified for the 4.5-4.7 m spectral range, which
is a representative band for a UUT.
***
Angular resolution is equivalent to the instantaneous field of view of five
micromirrors.
2.2 Prototype Design Summary
The DMDs
The prototype two-band IR scene projector employs two Texas Instruments Discovery 1100 DMD kits, each equipped
with a second party ALP2 daughter board to accomplish the 10-bit grayscale operation at the required update rate. Each
DMD has had its window changed to an IR transmissive calcium fluoride window prior to integration into the system.
The other key aspects of the system are the IR illumination modules, the projection system, and the mechanical
mounting system, all detailed below.
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The Optical System
The purpose of the illumination modules is to uniformly illuminate each DMD with appropriately spectrally filtered IR
light and also to maintain radiometric throughput so that no light is lost (other than by small surface reflections). Figure
1 shows the optical design for the illumination module which is a Koehler (illuminates the DMD at a field point rather
than a source image), telecentric (chief rays illuminate the DMD parallel to the optical axis) configuration. The Koehler
aspect assures that the DMDs are illuminated uniformly, and the telecentric aspect assures throughput conservation
following the reflection by the DMD. Note that the respective bandpass filters are not shown but are located
immediately following source lens 1 (SL1). The DMD is shown tilted by 24° such that the “on” reflected light is normal
to the projection optics. All lenses are MWIR AR-coasted germanium (Ge), and in practice the illumination modules are
folded between source lens 2 (SL2) and the DMD lens.
Figure 1: Illumination Leg Optical Layout
DMD lens
SL2
DMD
IR Source
SL1
FSL2
FDMD lens
FDMD lens
Figure 2 shows the projection system composed of a dichroic beam combiner and an f/3.15 germanium-silicon
achromatic doublet. The projection system is also a telecentric configuration which continues to maintain radiometric
throughput out to the UUT which in practice will be located at the exit pupil a distance Fprojector or 315 mm from the
projector lens. The beam combiner is positioned at a 30° tilt angle to minimize astigmatism of the transmitted (blue)
image.
Figure 2: Projector Optical Layout
Ge/Si doublet
Exit pupil
Dichroic
Beam Combiner
Blue
DMD
Fprojector
Fprojector
Fprojector
Red
DMD
The Mechanical System
Figure 3a shows a solid model of the two-band scene projector prototype with dimensions. Figure 3b shows the same
view as 3a but with transparency such that the interior components can be seen. Figure 3c shows a top down view with
the cover off.
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Figure 3a: Solid Model of System – Cover On
Figure 3b: Solid Model of System – Transparent Cover
Blue DMD
14.5”
Projector
Lenses
20.5”
Dichroic
Beam
Combiner
18”
Blue
Illumination
Module
Red
Illumination
Module
Red DMD
Projector
Housing
Cover
Figure 3c: Solid Model of System – Cover Off
Blue DMD
Dichroic
Beam
Combiner
Red
Illumination
Module
Blue
Illumination
Module
Projector
Lenses
Power Supply for
DMDs and IR Sources
Red DMD
2.3 Prototype Integration
Figures 4a through 4c show the integrated two-band IR projector prototype system.
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Figure 4a: Two-Band Projector Prototype – Cover On
Projector
Lenses
Figure 4b: Two-Band Projector Prototype – Cover Off
Projector
Lenses
Dichroic Beam
Combiner
Blue
DMD
Power
Supplies
Blue
Illumination
Module
Red
DMD
Figure 4c: Two-Band Projector Prototype – Cover Of, Front View
Red
Illumination
Module
Projector
Lenses
Power
Supplies
Blue
Illumination
Module
Red
DMD
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3
SYSTEM CHARACTERIZATION
3.1 Introduction to System Characterization
The performance specifications listed in Table 1 were verified either by design or by test. The maximum update rate at a
given bit depth, pixel count, and the maximum scene duration are all functions of the DMD device and/or drive
electronics and were verified by design. We accomplish 10-bit grayscale resolution by projecting four 8-bit images for
each 10-bit image at four times the required frame rate (i.e. 160 Hz); we refer to this as “quasi” 10-bit operation.
Because the FLIR SC6000 camera used for testing had a maximum integration time of about 7 ms which was not
necessarily limited by the light level and the full integration associated with 10-bit, 40 Hz operation (i.e. 25 ms) is
required to experience the full bit depth, we verified grayscale at 8-bits with the 160 Hz/6.25 ms integration time which
indirectly verifies our quasi 10-bit operation.
We verified maximum radiant intensity in the red band, grayscale resolution (at 8-bits), and angular resolution by test.
The spectral bands are set by choice of the BPFs and dichroic beam combiner and were verified by vendor data.
3.2 Maximum Radiant Intensity
Maximum radiant intensity was measured using the thermographic calibration of a FLIR SC6000 camera which
measures surface temperature based on an assumed spectral emission integrated over 3-5 m. For this test, the entire red
DMD was turned to the on state at maximum grayscale, and the beam was projected onto the FLIR camera; the per-pixel
measured temperature by the FLIR SC6000 was then converted to maximum radiant intensity according to
W
(1)
J i, j   N (, Tmeasured _ i , j )d  A proj []
ster
where i,j are the spatial indices. Figure 5 shows the maximum radiant intensity image. To reconcile the results with the
requirement listed in Table 1, we used an analytical projection of the relative radiance in the full red band (4.2-5 m) to
that in the limited red band for which the specification was written (4.5-4.7 m). This ratio is equal to 4 (i.e. there is four
times more radiance (W/(cm2·ster)) in the full red band). Given that we measured about 5 W/ster, we can state that we
exceeded the radiant intensity requirement in the limited red band.
Figure 5: Maximum Radiant Intensity Image (W/ster)
3.3 Projected Scenes
The following series of images were originally grayscale images obtained from a popular movie about F-14s8 which
were in turn projected by the red channel of the IR scene projector and recorded using the FLIR SC6000 camera with a
250 mm focal length lens. The images are still shots from a 34 second simulation. These images are intended to
demonstrate the optical performance and radiometric intensity resolution of the system; these parameters are quantified
in Section 3.4.
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Figure 6a: Projected IR Image A
Figure 6b: Projected IR Image B
Figure 6C: Projected IR Image C
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Figure 6D: Projected IR Image D
3.4 Radiant Intensity, Contrast, and Angular Resolution
An image from the simulated scenes described above was used to assess both radiant intensity resolution and angular
resolution of the IR scene projector. Figure 7 shows an image which spans the 8-bits of grayscale resolution of the
imager. Figure 8 shows a line plot across the image (illustrated in Figure 7) which shows that the full grayscale
resolution is exercised. Figure 8 also demonstrates a contrast of approximately 250:1.
Figure 9 shows a line plot across the edge of the tower in Figure 7 which demonstrates our angular resolution. The FLIR
camera with the 250 mm focal length lens has an internal field of view on 25 m pixels of 100 rad. The IR scene
projector specification is 215 rad or about two FLIR pixels which is approximately the width of the edge of the tower.
Figure 7: Image for Grayscale Resolution, Contrast, and Angular Resolution
Figure 9 Linecut
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Counts
Figure 8: Grayscale Range and Contrast
FLIR pixel index
Figure 9: Angular Resolution
Counts
Edge width is
approximately 2
FLIR pixels or
200 rad.
FLIR pixel index
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SUMMARY AND CONCLUSIONS
OPTRA is in the process of establishing the feasibility and utility of our two-band IR scene projector. We have
designed, built, and partially integrated a prototype projector and have verified most of our performance specifications
which apply to the red channel. Once we have the blue channel operable, we will verify image registration and will
project and record a series of two band images.
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The projector system described in this paper is intended to be modular from a standpoint of being able to interface with
different fore optics to provide different fields of view and exit pupil diameters for different applications. Because of the
broadband nature of the DMDs, different spectral ranges may also be accommodated using different filters or light
sources. Future work could include adapting the technology to a specific application, adding a third potentially
ultraviolet channel, or reducing the overall package size.
Acknowledgements
This research was conducted under a NAVAIR Small Business Innovation Research Phase II contract funded by the U.S.
Navy.
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