1. science
2. physics
3. optics

Electron-beam-addressed membrane mirror light
modulator for projection display
Thomas N. Horsky, Craig M. Schiller, George J. Genetti, Todd N. Tsakiris,
Daniel M. O'Mara, Robert Jurgilewicz, Sin-Doo Lee, and Cardinal Warde
The performance of a prototype, reflection-mode projection display based on an electron-beam-addressed
membrane mirror light modulator (e-MLM) is described. The e-MLM converts electronic video
information into a two-dimensional phase object, that is then schlieren imaged onto a screen.
High-contrast dynamic projection of images is demonstrated over a broad range of wavelengths, from the
visible to the midinfrared. As such this device is expected to find applications in large-screen visible
displays and dynamic infrared scene projectors.
Key words: Deformable mirror, spatial light modulator, infrared scene projector, high-definition
display.
1.
Introduction
Since the work of Preston,' the membrane light
modulator (MLM), which incorporates a deformable
membrane mirror as the light modulating element,
has emerged as an attractive technology for adaptive
optics, projection display, and optical signal processing applications. 2 4 MLM's exhibit fast response
times, can be read out with high optical efficiency,and
may contain a large number of resolution elements.
Various means of addressing a two-dimensional
deformable membrane have been demonstrated, including electron-beam addressing,5 optical addressing,3 6 8 and electrical addressing by means of integrated circuits.1 9"10 The construction of an MLM is
such that a membrane is deposited over an array of,
recessed wells with an addressable electrode at the
bottom of each well. The exterior surface of the
membrane is coated with a highly reflective, conduc-
tive material that is biased by a programmable voltage source.
A pixel, which may span one or more wells, is
activated by establishing a potential difference between the corresponding well electrode and the membrane surface, which causes the membrane to deform
into the well in response to electrostatic forces. By
the application of a spatially varying pattern of
voltages to the array of well electrodes, a twodimensional pattern of phase retardation is presented
to an incoming optical wave front. This phase modulation of the reflected readout light can be converted
to intensity modulation by the schlieren tech-
nique 4 ,5,9,111 2 and projected onto a screen.
Because of the broadband reflectance of the mirror
surface, this technology is suitable for both ultraviolet and IR scene projection, as required by hardwarein-the-loop target simulators. The combination of
high reflectance and high optical readout efficiency
also makes possible large-screen displays, as well as
projectors for wide field-of-viewflight simulators.
When this work was performed, the authors were with Optron
Systems, Inc., 3 Preston Court, Bedford, Massachusetts
07130.
C.
Warde is also with the Department of Electrical Engineering and
Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139. D. O'Mara is now with Rosenstiel
Basic Medical Sciences Research Center, Brandeis University,
Waltham, Massachusetts 02254; T.N. Horsky is now with Balzers
High Vacuum Products, 8 Sagamore Road, Hudson, New Hampshire 03051; S.-D. Lee is now with the Department of Physics,
Sogang University, C.P.O. Box 1142, Seoul 100-611, Korea.
Received 13 August 1991.
0003-6935/92/203980-11$05.00/0.
© 1992 Optical Society of America.
3980
APPLIED OPTICS / Vol. 31, No. 20 / 10 July 1992
2.
A.
Description and Operating Principles of the e-MLM
Device Architecture
The electron-beam-addressed membrane mirror light
modulator (e-MLM), which is illustrated in Fig. 1,
consists of an addressing electron gun and a special
deformable membrane mirror anode assembly. This
anode assembly consists of a planar grid and a
charge-transfer plate (CTP) to which a deformable
membrane mirror is bonded. The mirror material is
PIXEL
WELL
COLLECTOR
, - GRID
SIGNAL
Fig. 1.
Architecture
of the e-MLM.
chosen for its high reflectivity in the wavelength band
of interest.
The CTP is a wafer of electrically insulating material embedded with a dense, regular array of conductive pins precisely oriented normal to the wafer
surface, in a hexagonal array.13 The CTP derives its
name from its ability to serve as a high-density,
multifeedthrough vacuum interface that is capable of
transferring a two-dimensional charge distribution
from vacuum to air. Our laboratories produce 25mm-diameter, 4-mm-thick CTP's with 10-[im conductors on 14-pim centers, 25-im conductors on 35-[um
centers, and 50-[um conductors on 70-jim centers.
The resulting surface area fill factor of these closepacked arrays is approximately 50%. A photomicrograph of a CTP surface with 25-jim pins is shown in
Fig. 2.
In one version of the device, the CTP is polished to
yield an optically flat surface, and material is removed
from the pins on one side of the plate so as to form a
regular array of recessed wells. We have also fabricated devices by using photomask and etching techniques to deposit an array of wells directly onto the
CTP surface such that the wells are oversampled by
the CTP pins, as depicted in Fig. 1. In this manner
the depth of the wells, the pixel diameter, and both
the fill factor and the symmetry of the array can be
chosen according to the wavelength of the light to be
processed and the specific system function of the
spatial light modulator (SLM). A polymeric membrane is then deposited over the wells such that a
reliable bond between the two dielectric surfaces is
established by van der Waals forces.
B.
Electronic-Beam Addressing
Electrostatically, the e-MLM may be viewed as a
triode consisting of a cathode biased at Vk, a collector
grid at V = 0, and an anode (CTP surface) referenced
to the membrane potential Vm(t). The thermionic
cathode at Vk < Vm(t) < 0 emits a primary electron
beam that strikes one or more pins of the CTP. The
local potential of the CTP, which determines the
landing energy Ep of the primaries, is initially close to
Vm(t). Ep is given by
Ep =
e(Vk -
V),
(1)
Fig.2. 150x photomicrograph of a CTP surface. Currently used
in visible displays, it features 25-pm pins on a 35-,umpitch. The
1-in.- (2.54-cm-) diameter plate contains approximately
4 x 105
pins.
where e is the electronic charge and V, is the local
potential of the anode surface being addressed.
When the device is initially in its uncharged state,
Vs = Vm,while, in general, Vm < V, < 0 when image
information is being written. The ratio of emitted
secondary electrons14 to primary electrons incident
on the CTP surface, which is defined as 8, is also
determined by Ep. The value of 8 strongly influences
the efficiencywith which pixels can be charged, and so
affects the maximum framing speed of the device.
When the CTP is uncharged, essentially all the
voltage applied to the membrane surface is coupled by
capacitive division across the gap between the CTP
and collector grid. Since there is no potential difference across the pixel, the membrane remains in its
undeformed state. As the electron beam scans across
a conductive pin of the CTP, secondary electrons are
emitted from that pin so that the accumulated charge
establishes a potential difference between the CTP
and the grid. If the value of Ep is such that 8 exceeds
unity and the pin is at a lower potential than the
collector grid, a net positive charge accumulates on
the pin. If the electron beam continues to address
that pin, charge accumulates until the pin potential
stabilizes to the grid potential, i.e., to ground.
C.
Framed Operating Mode
In the framed mode of operation, a negative dc bias is
applied to the membrane and the electron beam is
rastered across the anode while the value of its
current is modulated by the video signal applied to
the cathode control grid. This achieves gray scale in
the manner of cathode-ray tubes' 5 (CRT's). The
beam current can be limited such that no anode pixel
is allowed to saturate to the grid potential during one
10 July 1992 / Vol. 31, No. 20 / APPLIED OPTICS
3981
video frame. Since the resulting potential of each
addressed pixel is simply proportional to the beam
current at that pixel, a continuously varying twodimensional charge image can be written onto the
anode.
The anode's large ( 1 pF) pixel capacitance, and
hence a long (minutes) charge storage time, requires
that the image be erased between frames. Erasure
is easily accomplished by grounding the membrane
electrode during electron-beam addressing. Setting
the membrane potential equal to the collector grid
potential switches all pixels charged with image information to a more positive potential than that of the
collector. Since a large fraction of the secondaries
emitted from these pins cannot overcome the repulsive field between the anode and collector, they fall
back to the anode and neutralize the net positive
charge, erasing the image. A simple means of incorporating an explicit erase cycle is by utilizing an
RS-170 format such that the image is erased during
the second field of each frame; this, however, reduces
the duty cycle of the projector to < 50% and results in
image flicker. This duty cycle can be greatly increased by instead flooding the anode with a defocused beam of electrons for an erase time that is
much less than a field period. We have shown that
this approach can decrease the required erase time by
an order of magnitude and increase the duty cycle to
almost 90%. Replacing the raster by a flood erase
takes advantage of the large parallel capacitance of
the CTP pins,16 i.e., driving them to ground in parallel
requires much less total charge than driving them to
ground serially.
The frame address or write time Tr of the e-MLMis
given by
NCV
t
ip(8 - 1)T
(2)
where n is the number of pixels in the array, C is the
pixel capacitance,V is the potential difference required for deflecting the membrane to full-contrast
modulation, ip is the primary electron current assumed to be confined within a pixel, 8 is the secondary
electron emission ratio of the CTP, and T is the
collector grid transmission. The beam currentlimited frame rate R is given by
R =(
+
-1,
(3)
where T is required frame erase time, and Te < Tw.
D.
Flickerless Operating Mode
The flickerless operating mode also is achieved by
choosing Ep such that 8 > 1, but the negative-going
video signal is applied directly to the membrane
surface rather than to the control grid of the electron
gun.' 7 In this case, if the pin voltage is below the
collector potential, secondary electrons generated at
the pin are collected by the grid, and the pin voltage
increases. Similarly, if the potential of the pin is
above that of the collector, secondary electrons emit3982
APPLIED OPTICS / Vol. 31, No. 20 / 10 July 1992
ted from the pin are returned by the electric field
between the pin and collector so that the pin voltage
falls as electrons are deposited. Thus, in this mode,
which is termed the grid-stabilized mode, the voltage
at the pin that is being addressed by the electron
beam always tends toward the collector potential.
By synchronizing the video signal with a constantcurrent raster address of the anode by the electron
beam, an arbitrary charge image can be written onto
the CTP. Given sufficient beam current, the potential between the membrane and well electrode is thus
simply equal to the instantaneous video signal.
Also, the charge-storage characteristics of the CTP
result in image information being maintained between frames. Previous frames are simply updated
on a pixel-by-pixel basis as the electron beam scans
the anode to write new frames. Since an explicit
erasure cycle is not required, flickerless operation is
achieved. We note that during the process of grid
stabilization the grid ceases to act as an efficient
collector of secondaries as the pin potential approaches the grid potential. Thus landing errors of
emitted secondaries onto neighboring pins can degrade spatial resolution and reduce image contrast.
Fortunately this problem can be alleviated by spacing
the collector mesh within a pixel pitch of the anode
surface; this ensures that returning secondaries (except those emitted at large angles from the surface
normal) land close to the addressed pixel.
E. Speed of Operation
One of the attractive attributes of the MLM technology is its fast response time,' 6 which makes high
frame rates possible. In the e-MLM, the limiting
factor in determining frame rate is the beam current
of the addressing electron gun. For example, since
the flickerless grid-stabilized mode requires about
half the beam current as the framed mode of addressing, it can support twice the update (or frame) rate.
By manipulation of Eq. (2), a lower limit to the
effective frame rate may be expressed as the throughput Q of the e-MLM for grid-stabilized operation:
QN =
ip(8T, CV
)T)
(4)
where Q may be expressed in pixels/s. Given the
following device-dependent parameter values that are
consistent with empirical studies, i.e., V = 50 V, 8 =
1.5, C = 0.5 pF, ip = 250 A, T = 0.8 yields Q 4 x
106 pixels/s. While a reduction in pixel capacitance
C would result in increased speed, we also note that a
substantial increase in the delivered beam current
may be effected by replacing our present vidicon tube
technology by dispenser cathode, laminar-flow-based
diode guns such as those incorporated in high-output
projection CRT's. 8" 9 In addition, for IR displays
that permit larger pixel dimensions, a larger beam
spot is acceptable. This relaxes the space charge
effects that fundamentally limit the current density
at the target, resulting in a higher deliveredcurrent
and, hence, a higher throughput.
F.
Image Formation
The voltage pattern capacitively written onto the
MLM anode is converted to a phase object by the
deformation of the reflective membrane into the pixel
wells. As the readout light is reflected from the
membrane surface, the light is also diffracted into
discrete orders because of the periodicity of the
close-packed pixel array of the modulator surface.
With the e-MLM in its off state, the mirror surface is
undeformed, and only the specular, or zeroth-order,
reflection contributes to the diffracted wave. Diffraction into the higher orders increases as the pixels are
activated. The phase-modulated readout beam can
be Fourier transformed by a lens to produce a diffraction pattern consisting of an array of bright spots
with the sixfold symmetry of the hexagonal pixel
array. Thus, in a Fourier plane, the diffracted orders provide spatial carrier frequencies for phase
information. Being separated in space, they can be
spatially filtered such that any combination of orders
are passed and the others blocked. The passed diffraction order(s) then creates an intensity-modulated
image of the phase object at the image plane. This is
equivalent to the technique of schlieren imaging.
Figure 3 illustrates the optical train of the schlieren
projector. One method of spatial filtering in the
Fourier plane is presented, in which the zeroth order
or dc component of the Fourier transform is blocked,
permitting only the nonzero orders to construct the
image. This results in a uniformly dark off state,
and hence a potentially high-contrast image. Alternatively we have used an inverse-filtering approach,
in which the dc component is passed and all others are
blocked. This results in a contrast-reversed image,
i.e., dark pixels on a bright field. A high contrast
ratio is harder to achieve in this case, since the
off-state intensity now depends on the pixel fill factor
and on both the wavelength and the spectral bandwidth of the readout light.
theoretical diffraction efficiencies and contrast ratios
for various pixel geometries and Fourier-filtering
schemes. The results described below may be compared with a similar analysis for the case of rectangularly shaped pixels described previously.9
Figure 4 depicts a cross-sectional view of a single
pixel of diameter a, in which an applied voltage across
the air gap has deflected the membrane a maximum
distance do into the well. The shape of the deflected
membrane may be modeled as a circularly symmetric
paraboloid, so that the deflection as a function of
position is given by
< a 2 /4
otherwise,
p(x, y) = f
low
(5)
where the x and y axes are in the plane of the
undeflected membrane surface and the coordinate
system's origin lies at the center of the pixel.
We consider the case in which all pixels are deflected to the same positions. The deflection function, ignoring for the moment the finite aperture of
the e-MLM, can be written as
d(x,y) = p(x,y) *
y
-
IXB
k=-co
3-- (k
-
(k + 1),
2=-m
- ) ,
(6)
where c is the center-to-center spacing of the pixels, k
and are integers, and * denotes the convolution
operation. The double summation in Eq. (6) defines
a hexagonal array of impulses located at the pixel
centers. The corresponding phase-delay function,
which is seen by an incident monochromatic wave
front of wavelength X,is
4
Ill. Device Modeling
A.
ifx 2 + y 2
do[4(x2+ y2 )/a2 - 1],
7T
(x,y) = - d(x, y).
(7)
Diffraction Efficiency for a Uniformly Deflected
Two-Dimensional Array of Pixels
For a device of diameter D, the function
We have modeled the complex transfer function of the
pixelized membrane surface in order to determine
_ LIGHT
LENS
)
Fig. 3.
Readout
otherwise
(8)
N
ZEROTH
ORDER
STOP
optical train of the schlieren
s(x, y) =
defines the active area aperture. If the membrane is
assumed to have a uniform reflectance of 1 across its
surface, then, within a constant phase term, an
incident wave front is multiplied by the complex
'SOURCE
PROJECTION
ifx 2 + y 2 < D2 A4
IMAGE
PLANE
projector.
One
form of Fourier-plane spatial filtering, the zeroth-order stop, is
illustrated.
a
Fig. 4. Cross section of an e-MLM pixel of diameter a showing
paraboloidal deflection of peak amplitude do.
10 July 1992 / Vol. 31, No. 20 / APPLIED OPTICS
3983
function
m(x, y)
=
s(x, y)exp[j+(x,y)]
(9)
after reflection.
It is desirable to determine the optical Fourier
transform of m(x, y) to evaluate diffraction efficiency,
contrast ratio, and other performance parameters.
Unfortunately the transform does not have a simple
analytic form. Instead we calculate the Fourier
transform numerically for a range of pixel fill factors
and the membrane deflections.
For the purpose of computing its discrete Fourier
transform, m(x, y) was sampled on a hexagonal grid,
and a linear coordinate transformation was used to
align the hexagonal grid of samples onto a rectangular grid:
x' =x,
(10)
y'
(11)
y/\/3.
Figure 5 shows how application of this coordinate
transformation aligns the centers of the pixels onto a
rectangular grid. The inverse transformation is applied to the discrete Fourier-transform result to
restore hexagonal symmetry.
In Fig. 5, a dotted rectangle demarcates the unit
cell, which is the smallest cell that can be replicated
along the x and y axes to produce a hexagonal grid of
pixels. In the right-hand figure of Fig. 5, the unit
cell can be sampled on a square grid to give a
discretized function u[m, n]. The discrete Fourier
transform of u[m, n], which is denoted by Uk, 1],
when convolved with the continuous Fourier transform of the aperture function s(x, y) gives M(u, v),
which is the continuous Fourier transform of m(x, y).
M (u, v) has the form of a hexagonal array of impulselike intensity peaks [see Fig. 6(a)]. For the optical
Fourier transform taken by a nonbandwidth-limiting
lens of focal length f, the spacing and width of peaks is
as shown in Fig. 6(b). As long as the device aperture
is much larger than the spacing between adjacent
pixels (D >>c), M(u, v) will be a nearly hexagonal
array of impulses. In this case one can consider
M(u, v) as a set of discrete diffracted orders in which
y
0-- 0
l
(
0,'
C)N
K>
I CD C)
y
D
0
I
r-
,
(>
\>
S..
D
(b)
Fig. 6. (a) Photograph of the optical Fourier transform of a
uniformly deflected membrane pixel array; (b) diagram showing
the effect of pixel spacing c and device diameter D on the spacing
and width, respectively, of the diffracted orders. The separation
of adjacent orders is given by f/c, where f is the focal length of the
lens. The diffraction-limited diameter of the nonzero orders is
given by 2.44Xf/D.
D
-- (El
I
B. Dependence
Factor
C)
C)
(
x
x
Fig. 5. Coordinate transform that scales the y axis is used to fit
the hexagonal symmetry of the pixel array onto a rectangular grid,
as discussed in the text.
3984
2.44
the energy in each order is the squared magnitude of
the corresponding entry of U[k, 1].
0 0 0
(
(a)
APPLIED OPTICS / Vol. 31, No. 20 / 10 July 1992
of Intensity and Contrast on the Pixel Fill
The above model was used to calculate the fraction of
energy in an incident plane wave that is diffracted
into the central (zeroth) order as a function of
membrane deflection. Deflection is normalized to
units of wavelengths and is measured at the center of
the pixel. Uk, 1]and u[m, n] were sampled on M x
N grids, and the fraction of zero-order energy, Io, was
calculated as
M-1 N-1
Io = IU(0,
0)
2/ E
X I U(k, 1)12.
h= 10
(12)
50-
70 -
1
10
50-
0
01
02
0.3
04
0.5
06
0.8
0.7
1
0.9
DEFLECrON(WAVELENGTHS)
Fig. 7. Fraction of total energy in the zeroth order as a function of
membrane deflection for four pixel spacing-diameter (c/a) ratios:
1 (solid curve), c/a = 1.13 (71% fill); 2 (dashed curve), c/a = 1.23
(60% fill); 3 (dotted curve), c/a = 1.33 (51% fill); 4 (dotted-dashed
curve), c/a = 1.43 (44% fill).
Figure 7 shows the results expressed as a percentage
of the total energy. Several curves are plotted for
different pixel spacing-pixel diameter (cla) ratios.
The fill factor, which is defined as the fraction of the
device active area covered by pixels, is related to c/a
by
fill factor = 0.907/(c/a)2 .
(13)
It can be seen from Fig. 7 that as c/a approaches 1.0,
it is possible to diffract a greater fraction of the light
out of the zero order. The curves of Fig. 7 may be
used to determine a theoretical upper limit on the
contrast ratio of an e-MLM imaging system that
passes only the zero order at the Fourier plane by
taking the ratio of the maximum (always 1.0) to the
minimum values along the curve corresponding to
the device's fill factor. For this device, the lower the
c/a
ratio, the better the contrast ratio. For c/a
=
1.03, the theoretical contrast ratio is 150:1.
Figure 8 is a similar plot, but it shows the sum of
the energy diffracted into the six first-order peaks
closest to the zeroth order. For an e-MLM imaging
system passing only these orders, maximum light
throughput is reduced to less than 60%, but there is
no theoretical limit on the contrast ratio since an
undeflected membrane diffracts zero energy into the
first order.
Once the Fourier transform is calculated, we may
simulate the spatial filtering that yields contrast in
the image plane. For example, we may set the dc
component of the Fourier transform U[0, 0] to zero,
which simulates the effect of incorporating the zerothorder stop of Fig. 3. Inverse transforming the filtered Fourier transform and taking the squared
magnitude of the result then reconstructs what one
would see in the image plane. For purposes of
illustration, Fig. 9 shows six samples of the image
plane for a uniformly deflected array of pixels with
deflections do for 0.1X < do < 0.7X. The assumed
pixel fill factor was 40%. The assumptions of a
perfectly coherent, monochromatic light source and a
parabolic deformation profile yield interference effects that result in circular rings within the pixels for
do > 0.3A. The extent to which these coherent
artifacts would be present in a real display depends on
the spectral bandwidth and spatial coherence of the
light source, and on the fidelity of the imaging
system. In any case, the display would typically be
operated only within the range 0 < do < 0.3A, since
this range covers the device's full range of gray-scale
modulation.
C.
Assumptions
This analysis makes the assumptions that pixels
experience paraboloidal deflection under an applied
voltage, and that the pixel diameter is large compared
with the wavelength used but small compared with
the diameter of the device's total active area. We
also assume that the Fourier-transform lens has an
aperture that is large enough so that all diffracted
orders are collected. Another significant assump-
60
501140
(a)
(b)
(d)
(e)
(C)
0
4,
20o
10
0.3
0.4
0.5
0.6
0.7
(WAVELENGTHS)
DEFLECTION
(ff)
Fig. 8. Fraction of total energy in the six first diffraction orders as
a function of membrane deflection for four pixel spacing-diameter
1 (solid curve), c/a = 1.13 (71% fill); 2 (dashed curve),
(c/a) ratios:
Fig. 9. Simulated image of the unit cell of a 40% active area
e-MLM for a range of deflections d, as described in the test. The
1.33 (51% fill); 4
(a) do = 0.Ak, (b)
schlieren optical train of Fig. 3 was assumed:
do = 0.2A, (c) do = 0.3 X,(d) do = 0.4X, (e) do = 0.5X, (f) do = 0.7A.
= 1.23 (60% fill); 3 (dotted curve), c/a
(dotted-dashed curve), c/a = 1.43 (44% fill).
c/a
=
10 July 1992 / Vol. 31, No. 20 / APPLIED OPTICS
3985
tion is that the deflection of a pixel's membrane is
small enough compared with its diameter so that no
significant focusing takes place as light traverses the
additional path length created by membrane deflection, i.e.
IV.
A.
Experimental Results
Demountable
e-MLM
For prototype development, the e-MLM was configured within a stainless-steel ultrahigh vacuum test
system that provided a base pressure of 10-9 torr.
The mechanical design consisted of demountable
modules incorporated into a compact, actively pumped
system. This approach provided ease on construction, accommodated incremental modifications and
improvements to the e-MLM design, and provided in
situ diagnostics of the electron-beam spatial resolution and associated drive electronics. The demountable e-MLMwas composed of three separate elements:
a vacuum cell, an electron gun, and the MLM anode
assembly. The anode assembly was enclosed within
a 2.75-in.- (6.985 cm) diameter conflat flange and
bolted directly onto one end of the vacuum cell; a
flange-mounted electron gun was bolted onto the
opposing end. The cell was isolated by a gate valve
from an ion pump that was mounted beneath an
optical table fitted with a through hole to accommodate conductance to the pump. The cell was enclosed by a layer of mu-metal to eliminate magnetic
field distortion of the electron-beam trajectory.
The membrane was environmentally protected by
an end window and held in vacuum. The design
utilized a conflat window flange that allowed for the
substitution of the BK-7 glass window by an IRtransmissive ZnSe window, which was antireflection
coated for a 3-5-im readout.
Two different types of electron gun have been used
in this system. The first was an Apex Electronics
406-HT flange-mounted all-electrostatic gun commonly used in electron diffraction; it incorporated a
thoriated Ir ribbon thermionic electron source.2 0 21
Its imaging properties were characterized by the use
of a flange-mounted phosphor screen that bolted into
the position normally occupied by the anode assembly.2 2 The gun reliably produced a current of 250 nA
and a beam diameter of < 150 pm at a beam energy of
300 eV. Since the spatial resolution and current
density delivered by this gun were insufficient for our
requirements when operated at low beam energies,
we replaced it with a high-resolution, low energy
vidicon gun.2 3 The unit was a magnetic-focus, electrostatic raster, Teltron Model 1301 vidicon gun fitted
with a ribbon filament emitter to make it vacuum
demountable.2 4 The gun delivered 2 A of beam
current in a 70-R m spot at 200 eV.
B.
Control Electronics
An electronic controller generated the input signals
to the e-MLM. For the first demountable prototype,
the control electronics included a vector address
system that incorporated a 256 x 256 frame buffer,
3986
APPLIED OPTICS / Vol. 31, No. 20 / 10 July 1992
an optoisolated digital-analog converter interface,
and three high-voltage amplifiers. The user interface was an IBM personal computer that permitted
the user to specify the image information to be
displayed by the projector. The image information
consisted of three 128 x 128 arrays that defined thex,
y position and z (intensity) values assigned to each
element of the image array. Once defined, the image
information was serially output by the frame buffer
to three separate digital-analog converters, and amplified to drive the x, y electrostatic deflectors and z
control grid of the electron gun. The outputs were
able to modulate the voltage applied to the video grid
with a resolution of 8 bits.
Once the higher-performance vidicongun was incorporated into the test system, the vector control
electronics were also replaced by custom video electronics that accepted an RS-170 input. This input
was supplied by either a frame grabber (for static
images) or a video cassette recorder (for dynamic
images). Our initial mode of addressingwas a framed
mode, in which the voltage applied to the membrane
was controlled by the vertical retrace signals so that a
separately variable (but integral) number of fields
could be used for the write and erase cycles. It was
found that a 3:1 ratio between the write and erase
dwells gave good performance without excessive
licker, albeit at an overall frame rate of 15 Hz.
Nonstandard video rates would be required for a
30-Hz operation with unequal write and erase times.
Since the flickerless address mode does not require
an erase cycle, 30-Hz video rates can be maintained
while also simplifying the control electronics and
reducing the beam current requirements of the electron gun. We now implement this mode of addressing in our video-rate display systems.
C. Projector Performance Characteristics: Coherent
Visible Readout
Over the course of its development the e-MLM was
read out by both coherent and incoherent light
sources. Figure 10 illustrates the image plane irradiance of a typical 50-lm pixel within a group of
uniformly deflected pixels, read out by a He-Ne laser
at a wavelength of 632.8 nm. The data were obtained by passing only the zeroth order in the Fourier
plane, and the photodetector was apertured during
the measurement to define the pixel. A contrast
ratio of greater than 100:1 was recorded within this
pixel.
The active fill factor of this device was
70%.
Figure 11 shows the corresponding modeled intensity
versus deflection but for an ensemble of uniformly
deflected pixels of a 70% fill device, similar to the solid
curve of Fig. 7, extended to 8r of phase shift. By
comparing the curve of Fig. 11 with the data of Fig.
10, membrane deflection as a function of bias voltage
may be determined as shown in Fig. 12.
It can be shown6 that the membrane deflects
1. 0
-
I-
8z 0.
0
U
z
0
w
LU
-J 0. 6
ULU
z3
LU
wU
z0 0. 4cc
IM
LU
0.: 2
if
80
120
MEMBRANEBIAS (VOLTS)
Fig. 10.
Measured image plane irradiance
/II
I 1 1 I I 1-
-
-
of a single 50-pLmpixel
versus membrane bias for the e-MLM of Fig. 1, obtained with a
zeroth-order schlieren readout. The readout wavelength was
0
632.8 nm.
Fig. 12.
40
80
120
160
MEMBRANE BIAS (VOLTS)
Membrane deflection versus membrane bias, as obtained
by comparison of the irradiance data of Fig. 10 with the model of
Fig. 11.
according to the relation
a 2E0 V
°
2
32Ts2
(14)
where do is the amplitude of membrane deflection, a
is the pixel diameter, V is the operating voltage, T is
the membrane tension, s is the well depth, and E is
the permittivity of free space.
While visual comparison of Figs. 10 and 11 indicates good agreement between their lineshapes, it is
evident from Fig. 12 that the deflection sensitivity of
the membrane decreases with the applied voltage so
that it ceases to be quadratic for displacements
greater than 0.4 [um. Although the reasons for
this behavior are not known at present, we note that,
if T were to increase nonlinearly with do, it would
certainly reduce the voltage sensitivity of Eq. (14) for
large values of do.
D.
IR Readout
Characteristics
This devicewas also read out in the IR (see Ref. 25) by
replacing the visible laser with a 4-mW He-Ne laser
operating at 3.39 rim, followed by an IR-transmissive
beam expander and projection optics; the visible
input window of the e-MLM was also replaced by a
demountable ZnSe window. Figure 13 shows an
image of a girl's face produced by this system by using
a zeroth-order schlieren readout at a mirror bias of
220 V. The 128 x 128 input image was written onto
the MLM anode by the vector-addressed Apex Electronics electron gun. The picture was obtained by
photographing the CRT display of an Inframetrics
600 self-scanning HgCdTe video camera, which we
used as an imaging radiometer.2 6 The radiometer
indicated that the simulated temperature range of
the projected image was 200'C.
Encouraged by the initial IR performance, we
100
90
50-
70D
~60
50
O
30-
U
P
0
20
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
DEFLECTION (WAVELENGTHS)
Fig. 13. Photograph of a CRT monitor displaying the output of a
The e-MLM projected this image
Fig. 11. Modeled zeroth-order readout intensity vesus deflection
HgCdTe 3-5-,um video camera.
for the device of Fig. 10.
of a girl's face at a readout wavelength of 3.39 p±m.
10 July 1992 / Vol. 31, No. 20 / APPLIED OPTICS
3987
proceeded to modify the pixel array dimensions to
increase the phase dynamic range of the e-MLM for
IR scene projection.2 7 While, for visible displays, the
50-jim diameter CTP pins themselves could be used
to fabricate the mirror pixels by etching away a few
micrometers of the pin material to form individual
wells, this was not adequate for a much longer
wavelength readout, so the diameter of the pixels was
increased. This requirement stems from the fact
that avoiding collapse of the membrane requires that
the pixel well depth s be at least three times the
maximum mirror deflection.9 Moreover, Eq. (14)
indicates that, for a given pixel diameter, the operating voltage V scales linearly with the well depth and
as the square root of the deflection amplitude, i.e., Vcx
sd 2. This results in unacceptably high operating
voltages for 50-jim pixels with wells that are deep
enough to accommodate midwave IR or long-wave IR
radiation. For example, since a deflection of do <
X/3 is sufficient for gray-scale image modulation, one
could argue that s > is a reasonable limiting design
parameter. For a fixed pixel diameter, then, the
operating voltage V for a device optimized to modulate the light of wavelength Xmay then be scaled from
the known operating voltage V of a device optimized
to modulate the light of wavelength Xoby the relation
V V(X/Xo)3 12. Since the device of Fig. 10 incorporated pixels with a 3-jim well, it follows that a 12-jim
well device of the same diameter built to modulate
light in the 8-12-jim band would require 8 times the
mirror bias.
Several e-MLM anodes were produced for our
demountable midwave IR test system, which featured
a 10-jim well depth, with 159-jim diameter pixels on
214-jim centers in a hexagonal array; the pixel fill
factor was 50%. The pixel arrays were fabricated
directly on the polished CTP surface by using a
photolithographic process. Figure 14 shows the zeroth-order optical readout intensity versus the mirror
bias for this device. The crosses represent measured
data, while the solid curve is the behavior predicted
by the model of Section 3. The two should be
directly comparable, since the data were collected
with a large-area cooled PbSe photodetector positioned in the image plane, which averaged the combined signal from a group of 6 pixels within a large
array of uniformly deflected pixels. Figure 15 shows
similar data and theory for a dark-ground filtering
technique of a zeroth-order stop. The modeled curve
represents the addition of the first through fourth
diffraction orders, since we calculate that the clear
aperture of our projection lens was such that only
these orders were collected by the lens. The scale of
the ordinate is normalized to the image plane intensity obtained by inserting a mirror in place of the
SLM, and removing the Fourier-plane filter. The
75% peak irradiance level at 160 V is thus the optical
efficiency of the SLM at this wavelength. This number can be increased by enlarging the numerical
aperture of the projection lens so that a greater
number of diffraction orders are collected. In practice some losses are also expected because of surface
reflections from the refractive optics, and finite absorption by the lens material. These losses are partly
responsible for the peak amplitudes of the theoretical
curves of Figs. 14 and 15 being larger than those of
the measured data.
Figure 15 demonstrates both the gray scale and the
dynamic range of the device. Contrast ratio, which
is defined as the ratio of maximum to minimum
intensity, was about 130:1 after subtracting a background irradiance measured with the commercial
mirror in place; we attribute this background to
scattering from the uncoated projection lens. This
background represents
30% of the overall off-state
signal, and the remainder is presumably due to the
SLM. A maximum deflection of >2 jim was attained, which is sufficient to modulate an 8-jim
readout light.
100
9s0
0
50
~40
30
+
U20
o
1io
0
50
100
150
200
250
MIRROR BIAS (VOLTS)
MIRROR BIAS (VOLTS)
Fig. 14. Image plane intensity versus mirror bias for an eMLM
incorporating 159-pim-diameter pixels at a readout wavelength of
3.39 pum. The device was read out by passing the zeroth order in
the Fourier plane. The scale of the ordinate has been normalized
to the throughput of the optical train. Crosses, measured data;
curve, theoretical model.
3988
APPLIED OPTICS / Vol. 31, No. 20 / 10 July 1992
Fig. 15. Measured and theoretical intensities analogous to those
of Fig. 14, but read out with the zeroth-order stop illustrated in
Fig. 3. The scale of the ordinate has been normalized to the
throughput of the optical train so that the peak intensity of 75% is
the optical readout efficiency of the SLM. Crosses, measured
data; curve, modeled intensity of the sum of the first four nonzero
orders.
E. Sealed-Tube e-MLM: Broadband Incoherent Readout
Having successfully determined many of the required
operating parameters for the vacuum-demountable
e-MLM, we proceeded to move toward a commercially
attractive version that incorporated state-of-the-art
camera tube technology. Working in collaboration
with Teltron, Inc.,24 we sealed vacuum-tight CTP
anodes onto focus-projected scan (FPS) vidicon camera tube2 3 bodies in place of the customary photoconductive PbO targets. The FPS vidicon gun tubes
provide many features that make it suitable for the
e-MLM. Mechanically, they are compact, lightweight, and rugged. The linearity of the scan is
excellent, and the beam diameter is small, 20 jim.
These vidicons are designed to operate at low electronbeam energies, which is an important consideration
since the optimum beam energy for an efficient
secondary electron emission from the CTP is a few
hundered eV. Finally, the FPS vidicon can deliver a
high current density on the order of 1 A/cm2 to the
target.
We produced a number of these sealed and gettered
e-MLM tubes, which were then fitted with 0-ringsealed readout windows to provide a vacuum environment for the membrane. The readout windows had
a clear aperture of 32 mm, and a 10° wedge to help
compensate for the phase error induced by the offnormal incidence angle of the readout beam. The
active diameter of the e-MLM was 20 mm. The
device was tested as a black-and-white
visible display
by utilizing a spatially filtered Hg-Xe arc lamp as the
light source.
Figure 16 shows photographs of images projected
by the e-MLM onto a screen, using zeroth-order
(a)
(c)
(b)
Fig. 16. Photographs of white light images projected onto a
screen by the vidicon-based e-MLM, with a Hg-Xe arc lamp as the
light source. (a) Text, (b) checkerboard, (c) American eagle.
readout in a framed operating mode. The spatial
resolution of these images is better than that in Fig.
13, primarily because of the smaller diameter of the
addressing electron beam. The pixel geometries of
the two devices are the same, 50-jim pixels on a
70-jim pitch. Because of, in part, the capacitive
point spread of the CTP,16 the limiting resolution of
this deviceis 3.5 line pairs/mm, or 100 television
lines.
V. Summaryand Discussion of Results
We have demonstrated the operation of a MLM as a
projection display with coherent visible, coherent
midwave infrared, and white-light readout sources.
Both 30-Hz video rates and flickerless operation have
been achieved. Several forms of Fourier-plane spatial filtering were explored, but the preferred mode is
the dark-ground technique, which consists of a central zeroth-order stop. This method yields a uniformly dark off state and a high optical readout
efficiency,even for broadband light.
By using an improved means of fabricating customsized mirror pixel arrays, we optimized a modulator
for the 3-5-jim wavelength band and achieved a
contrast ratio of > 100:1 with an optical readout
efficiency of 75% by using the dark-ground readout
technique. This means of fabrication yields uniform
and highly periodic arrays of pixels, which minimizes
high-frequency noise in the Fourier plane. The principal sources of off-state irradiance in the e-MLM are:
(1) quiescent pullback of the membrane mirror into
the wells, (2) aperiodicity of the pixel array, and (3)
diffuse scattering from defects in the mirror surface.
The fact that the image information is impressed onto
spatial carrier frequencies in the Fourier plane means
that much of the diffuse scattering located between
the orders can be blocked by constructing a spatial
filter that passes the six first-order spots only, for
example.16 We also note that the contribution of
these background effects is dependent on the readout
wavelength, so that an IR readout of a particular
device should give a much darker off state, and,
hence, a higher contrast ratio than when read out
with visible light.
A two-dimensional calculational model that accurately predicts the intensities of the various orders as
a function of membrane deflection was constructed.
It also predicts the form of the projected image for the
various forms of Fourier-plane spatial filtering, at
least for the special case of an array of uniformly
deflected pixels. Work is currently in progress to
extend this model to include arbitrary two-dimensional images, and to study the optical effects of
deflecting a few pixels only. The ability to model
accurately device performance is useful in providing
guidance to both device and system design. The
dependence of the output irradiance on the pixel fill
factor, which is illustrated by Figs. 7 and 8, is one
example of the utility of the model.
Since CTP technology is scalable, we are planning
to fabricate large-area CTP's to increase the number
10 July 1992 / Vol. 31, No. 20 / APPLIED OPTICS
3989
of resolution elements. For example, increasing the
device diameter to 2 in. (5.08 cm) and reducing the
pixel pitch to 32 m would yield 1024 x 1024 pixels in
a square array format. If we consider two pixels in
each dimension as constituting one resolution element, the device should support full 512 x 512
imagery. Although a finer pixel pitch places more
constraints on the electron-beam diameter and on the
device fabrication process, it also increases the optical
throughput of the projector. This is due to the fact
that the higher orders in the Fourier plane become
more separated from the zeroth order as the pitch is
reduced, relaxing the phase-space-limited constraints
on light source extent and beam collimation.2 7 Once
full video resolution is obtained, a full-color display
may be constructed by using three SLM's and a
beam-combining system with the filtered output of a
white-light source.
Portions of this research were sponsored by the
U.S. Strategic Defense Initiative Organization and
managed by the U.S. Army Strategic Defense Command under contract DASG60-90-C-0056, the U.S.
Air Force Systems Command under contract F1962891-C-0170, the U.S. Army Missile Command under
contract DAAH01-89-C-A014,and the National Science Foundation under the Division of Industrial
Science and Technology Innovation award 8961378.
We acknowledge the excellent engineering work of
W. S. Hamnett, W. K. Loizides, and L. C. Lerman.
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