IMPROVED SCREEN FOR REAR PROJECTION VIEWERS

• Document Type: CREST
• Collection: NGA Records (Formerly NIMA)
• Document Number (FOIA) /ESDN (CREST): CIA-RDP79B00873A001900010046-4
• Release Decision: RIPPUB
• Original Classification: K
• Document Page Count: 21
• Document Creation Date: December 28, 2016
• Document Release Date: September 6, 2012
• Sequence Number: 46
• Publication Date: December 5, 1969
• Content Type: REPORT

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CORNING GLASS WORKS
ELECTRO-OPTICS DEPARTMENT
RALEIGH, NORTH CAROLINA
IMPROVED SCREEN FOR REAR-PROJECTION VIEWERS
Technical Reports Nos. -- 47 and 48'
Date ? December 5, 1969
Periods Covered - October 10 to November 7, 1969
November 7 to December 5, 1969
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TECHNICAL REPORTS NOS. P-19-47 and 48
1. Introduction
Eight experimental scattering-type screens and one
commercial scattering-type screen have been evaluated in
terms of observed resolution and judged quality by the
Aerospace Group of the Boeing Company. Their final report
is included in this report as Appendix CG3.
In the quality tests, each of the 12" x 15" screens
was compared side by side with every other screen in a
projector using standard imagery. Observations were made
by Several experienced photointerpreterS and a quality
scale factor Z was determined for each screen depending on
how many times it was chosen as the better Screen. For .the
resolution tests, a standard USAF tribar resolution chart
was projected onto the screens and the photointerpreters
recorded the highest resolvable spatial frequencies. The
correlation of these quality and resolution judgments with
measured screen properties such as axial gain, brightness
variations, MTF, substrate transmittance, etc., was then
investigated. In general, the differences among screens
were found to be small both in judged quality and in judged
resolution. This was true in spite of the fact that signi-
ficant differences existed in measured screen properties.
These results can be understood when the following factors
are taken into account: ?
1. Projector MTF
2: Projector brightness
3. Ambient light level
Inmany of the tests these factors had the effect of
diminishing observed differences among screens.
2. Projeator 'MTF.
The highest resolution reported in CG3 is about 4 li/mm
(p. B9) for the unaided eye viewing, from a distance of about
7 inches, a high contrast target projected onto the rear-
projection screen under acceptable ambient light conditions.?
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2
On the other hand, typical square-wave MTF values obtained
by the contact method (P-19-41) for these screens were 0.97
at 5 lines/mm, 0.91 at 10 lines/mm, and 0.75 at 15 lines/mm.
If these contact MTF values are even approximately valid
for projected resolution targets, then the MTF of the pro-
jector mut have been the controlling factor in the resolu-
tion determinations of CG3.
It is possible to estimate the projector MTF from the
limit-of-resolution determinations described in 2.7.3 of
CG3, in conjunction with the square-wave response of the
eye. With screen removed, the target images in the screen
plane were observed by use of a 7X magnifier. The in-
dependently-measured contrast CT and maximum resolvable
resolution number RN for each target contrast are reproduced
here from p. 15 of 0G3. Included also are the corresponding
TABLE I
Limit-of-resolution data on targets of CG3
Resolution Spatial
Contrast Modulation Number Frequency (F)
(CT) Ovy (RN) (mm-1)
4.45
0.69
43.5
13
0.86
0.30
42.0
11.2
0.38
0.16
40.0
9
0.073
0.035
22.0
1.12
modulation of the target
MT 2 CT
and maximum resolvable spatial frequency for that target
RN/6
2
F
11.4
calculated from CT and RN. Square-wave modulation thres-:-
holds for the human eye are adapted from the data of DePalma
and Lowry" and are plotted in Fig. 1. for a viewing diStance
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3
of 7 inches. The 7X magnifier used at 7.inches effective
viewing distance provides a magnification of approximately
6. The effect of the magnifier is to reduce the spatial
frequency on the retina by a factor of 6. Thus, for a
target having modulation MT and a maximum resolvable spatial
frequency F, the appropriate point of the eye response curve
is at 6 in Fig. 1. The corresponding modulation threshold
is read from the curve.. The product of the target modulation
MT and the projector modulation Mp(F) must be equal to this
modulation threshold MTH at frequency F/6. Hence the pro-
jector square-wave MTF is
MT H (F/6)
M (F)
"T
(1)
When these calculations are carried_out for the four
resolution targets listed in Table I, the results are as
shown in Table II and in Fig. 1. The intersection of the
TABLE II
_Observed
Resolution Modulation
Target with 6X Threshold Projector
:Modulation Magnification of Eye -Nodulation
MT F/6 MTH ( 6 ) M (F)
- -
( 1mm ) ( 1mm )
0.69
13
2.17
.010
0.014
0.30
11.2
1.87
.0075
0.025
0.16
9
1.50
.0045
0.028
0.036
1.12
0.187
.0022
0.063
eye modulation threshold curve and the projector modulation
curve-falls at 4.6 lines/mm. This implies that even with a
-perfect rear-projection screen the maximum resolution would
be 4.6 lines/mm. This low projector MTF largely explains
the 4 lines/mm limit to the observed resolution and also
explains the difficulty encountered in distinguishing sig
nifitant differences in resolution and quality among the
screens.
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4
The above calculation is not highly accurate, because
of differences in experimental conditions for the eye re-
sponse measurements of DePalma and Lowry and the projector
resolution determinations.. The eye modulation threshold
depends upon the observer, the nature of the testobject,
the threshold criterion chosen,- the angular field covered
by the target, the luminance, and the condition of visual
1/
adaptation. The eye response data of Fig. 1. were adapted
from an experiment in which the target was square-wave over
a broad angular field, the luminance was 20 F.L., and the
criterion for threshold was ability to detect modulation.1/
Thus in the CG3 projector resolution measurements the
observer was different, the threshold criterion was more
stringent, and the angular field was smaller. For these
reasons, the projector MTF calculation must be considered
as an estimate.
While the above analysis shows that the projector MTF
was much lower than expected, it is also not clear from the
CG3 measurements that even the best screens did not degrade
the resolution. Direct viewing of the projected image with
a 7X magnifier gave a limit of resolution of 13 li/mm with-
out a screen. With a screen in place the limit of resolu-
tion with the 7X magnifier was about 7 li/mm for the average
screen, perhaps 8 li/mm for the best screens (CG3, p. B 16).
Thus it remains to be proved that contact square-wave MTF
values provide a realistic measure of resolution in the
projection situation.
3. Ambient Light
The ambient light level was 3 F.C. (CG3, p.7) and caused
little modulation degradation in the resolution measurements.
This was because the average film density was low for the
resolution targets and the, minimum input illumination to the
screens Was 10 F.C. (CG3, p. 13). But in quality tests,
average film density was about 1.0 and the ambient-to-projector
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illumination ratio was often greater than unity. The
approximate calculations below show a degradation of
modulation by this effect of as much as a factor of 5.
Trapped projector light was generally negligible compared
with ambient.
Reflected ambient light and trapped projector ligh-e
both have the effect of degrading the observed modulation
transfer by a constant factor for all spatial frequencies.
The ratio of the modulation, or contrast, y displayed by
the screen to the modulation yo projected onto the screen
can be calculated in an approximate fashion by reference
to Fig. 2. The displayed modulation is
Bmax
+ BT + BR - (Bmin + BT + BR
y
(Bmax + BT + BR) + 1BminD + BT + BR
(2)
min
'where Braa
and BD are the maximum and minimum brightnesses
directly transmitted through a local area of the screen, BT
is the brightness of the trapped projector light contributed
'by all parts of the screen, and BR is the reflected ambient
brightness. Since the modulation projected onto the screen
-is'
max min
BD - BD
0
Bmax Min (3)
Y =
+ BD
the transfer of modulation by the screen can be written
Y _ 1 (4)
1 +
Y 0 2B + 2B
T R
. .
Bmax ? + Bmin
D D
The trapped light in Eq. (4) can be expressed in
terms of the measured trapped light ratio
BT
,
aT B = T/- =
-N (max + Bmin)
2 iElp
(5)
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where
N
6
D
1 Bmax + Bin)
2 D
(
D
is the ratio of the average brightness over the whole
screen to thelocal average brightness.
The. reflected ambient light BR in Eq. (4) is eX7
pressible in terms of the measurable quantity RDTs.
Since the ambient reflected light suffers one diffuse
reflection and two traversals through the substrate,
the reflected brightness is proportional to RDTs2 Eamb.
The local transmitted brightness makes a single pass
through the sub trate and is thus proportional to
TS (Erna
D
x + Emin
max
The quantities ED and ED are are the
incident illumination maxima and minima in the local area
max min
correspondingtotransmitted-brightriess BD .and B
The ambient illumination is Eamb' The reflected brightness
as a fraction of the incident local average brightness is
thus approximately
RDTS2 Eamb RDT Emb
(7)
1 iRmax Brain)t 1 2T
+ S + max
.(E Emin) -E.D/N D D D D
2
where'
D IE-3D
1 (Emax
2 D + D
Emin)
1
2 (B"x
D + Bl) (8)
Equation (7) holds if the reflected and transmitted light..
;have approximately the same angular distribution.
Equations (4) - (8) can now be combined to yield
1 (9)
ED )
.0 1 +N aT +RDTS amb
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Quality test II, in which the open gate screen
brightness was limited to 10 F.L. for all screens, was
most strongly affected by ambient light. A sample calcu-
lation of y/y0 will.be made for the LS-60 screen. According
to CG3, p. 5, the projector provided a maximum of about 30 F.C.
open gate to the screen under standard conditions. In order
to reduce the brightness of screen LS-60 to 10 F.L. it was
necessary to reduce this open gate illumination to
10 F.L.
30 F.C. X F.L. 3.75 F.C.
79
since under 30 F.C. illumination this screen produced a
brightness of 79 F.L. (LBRT-I = 3.82 from CG3, p. A2).
Because the average imagery density was about 1.0, the
average illumination projected onto this screen was
E = 3.75 F.C./10. Then EaD = 3 F.C./0.375 F.C. = 8.
The assessment of image quality was made with emphasis on
dense, shadowed 'areas of the imagery where the transmission
was as low as 24 (CG3, p. 10). ?Then for an average film
transmittance of 10%, the value of,li was 5. The product
RDTS was calculated from the values in Table II of P-19-40
.for all screens except LS-60, for which a separate measure-
ment was made. The value for LS-60 was RDTs' = 4.4%, The
value aT = 0,11% can be found in Table A-1 of CG3. The
.quantity y/yo can now be calculated for this screen under
the conditions of the test. The results of such calculations
for all the screens appear in Table III.
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Table III
Parameters describing the-effect of reflected ambient
'light.and.trapped projector light on the observed MTF.
(Quality Test: ii)
aT
RDTS
E /E.
amb D
Y
Screen
(%)
(%)
YO
Z
AQ-20
0.062
2.1
1.0
0.90
1.01
AQ-17
0.081
3.1
2.8
0.70
0.67
AQ-11
0.133
4.9
3.6
0.53
0
AR-27
0.086
4.4
4.7
0.49
0.24
AQ-18
0.630
6.6
4.4
0.41
-0.08
LS-60
0.110
4.4
8.0
0.36
0.40
AL-5
0.135
9.2
4.0
0.35
-0.67
AR-28
0.240
6.6
10.7
0.22
-0.58
AL-4
0.740
14.0
5.4
0.21
-1.01
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"vow'
The quality factor Z is plotted against y/y0 in Fig. 3,
where it can be seen that the correlation is very good. The
effectiveness of substrate darkening in suppressing reflected
ambient light is well demonstrated. This is in excellent.
agreement with the correlation of -0.89 reported in CG3,
2 -
Table C-10, between Z and RDTs. Figure 4 shows this correla-
tion. The displacement of the LS-60 point from the others
prompted a remeasurement of RD T2 ' this time by a direct method.
S
The value of 2% obtained for LS-60 should replace the earlier
value of 6.3%. This change causes Ls-60 to fall in line with
the others.
When y/yo is calculated for the Quality I and Quality II
tests, the results are not so clear cut as in test II because
the ambient light was not as large relative to the illumina-
tion provided by the projector. These results are plotted' in
Figs. 5 and 6. In Quality test I, projector luminance was
held constant. Figure 5 shows the quality factor increasing
as yho increases, at low values of y/y0 in test I. But at
high values of.y/y0,_the reduced screen luminance caused a
rapid drop in judged quality. LS-60 performed best here be-
cause of its high efficiency and adequate ambient light re-
jection.
In the Quality III tests, screen luminance was maintained
constant, except for screen AQ-20. Figure 6 shows a general
dependence on Vy0 except for screen AQ-17 and AQ-20. The
reduced luminance of AQ-20 explains its low judged quality,
but no good explanation for the performance of AQ-17 is ap-
parent.
As Mentioned earlier, ambient light was of much less
influence in the resolution determinations. The lowest value
of y/y0 calculated by use of Eq. (9) for the constant-
luminance case was 0.93. Nevertheless, for the low-contrast
targets a significant correlation was noted between RN and
RD T2 (CG3, pp C13 and C15).
S
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4. Projector Brightness.
The illumination produced on the screen by the projector
affected the tests through the, ratio Eamb/ED as described
above. Also, in some cases the screen luminance fell low
enough to cause decreased visual acuity, as in Fig. 5. If
projector power had been unlimited, it would have been of
great interest to see whether the highest resolution could
be obtained by highly illuminating the very dark substrate
screens.
5. Screen Parameters
The list of screen parameters in Table A-1 of CG3 was
purposely made redundant on the chance that some unexpected
correlations might be discovered. The following list is
probably sufficient for interpreting the results:
T T
30 S
R T2 or R T
DS D5
V3.0
T
MTF
DRTHI
'The correlations found between these parameters and
resolution and judged quality are found in CG3, pp. C10 -
- C-15.
The last three parameters had negligible effect on the
outcome, although dry thickness.DRTHI correlated extra-
ordinarily well with quality in the, Quality II test and
with resolution in the resolution tests. This must be
considered as fortuitous, arising largely because the in-
efficient screens AL-4 and AL-5 had very thin layers, and
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the screen which was given the greatest substrate darkening,
AQ-20, had the thickest layer. The low projector MTF.pre--
cluded any significant.dependence on MTF values of the
screens at 6 li/mm, which were not very different anyway.
The trapped light ratio a would be important only if the
ambient light were quite low, which was not the case.
The parameter T30Ts is basically a measure of screen
efficiency and could equally Well be replaced by LBRT,
B(0)TS' or T45TS' for which the 'correlations were very
similar-. Not unexpectedly, at constant projector illumina-
tion T30 T correlates highly with quality and with resolution
S
for the high-contrast target. The correlation vanishes,
however, for low-contrast targetS.
? A significant correlation exists for RD T2 in Quality
S
test II for the reasons explained earlier. In all the other
tests the correlation is weak, although in the constant-
? screen luminance resolution test with LS-60 excluded, the
correlation may reflect a real ambient and trapped light
? effect.
Large correlations were found for the brightness varia-
tionV30 in the resolution tests and in Quality Test II.
The latter is understandable in view of the strong dependence
of V30 on B(0).. The surprisingly high correlation in the
resolution tests is at least partly fortuitous; since the
? low efficiency screens AL-4 and AL-5, which nearly always
gave inferior performance, had very high brightness variation.
6. Variation of Resolution with Viewing Angle
? This phenomenon should be investigated further. Since
it occurs for all screens, it could be a property of the
projector. Also, if the screens were being used to best.
? advantage, in a high-MTF system, the effect might be
smaller or even more pronounced.
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7. Effect of Target Contrast
One unexpected result is that the darker screens
showed lower resolution than the lighter screens for high
contrast targets, but the reverse was true for low con-
trast targets. 'Figure 3-10 of CG3 illustrates this point
for constant projector output and corresponding results
hold for constant screen luminance. The greater separation
of screens on the resolution scale for low contrast targets
can be explained by reference to the slope of the eye response
curve. At very low contrasts a given fractional change in
modulation produces a greater fractional change in detect-
able spatial frequency than at higher contrasts. However
this does not explain the observed interchange of rankings
of light and dark screens, as occurs most convincingly for
screens LS-60 and AQ-20. If this effect persists in a more
ideal projector arrangement, incorporation of heavier sub-
strate darkening may be justified.
8. Conclusions
Significant dependences on some screen parameters,
notably efficiency T30Ts and diffuse reflectance times
substrate transmittance RD TS' were established by the tests.
The more efficient screens performed best for a fixed pro-
jector output.
The projector MTF limited observed resolution to about
4.6 li/mm, whereas the screens should have been capable of
displaying considerably higher .resolution.
Quality tests were dominated by the projector MTF and
by the ambient-to-projector illumination ratio. Calcula-
tions based on the known ambient light level revealed a
strong RDTs dependence, which was one of the principal aims
of the investigation. The importance of T was underscored
by an unexplained superiority of dark screens for .the low
contrast resolution targets.
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No physical justification is apparent for the large
negative correlation between brightness variation V30 and
resolution. While it is partly fortuitous, it may be
significant.
Similarly, the reason for the observed increase of
resolution with viewing angle is obscure. This effect may.
or may not be evident under ideal projection conditions.
Ambient light was .generally high enough that the trapped
light ratio aT had little effect. Likewise, measured contact
MTF values for the screens were not sufficiently different
at 5 li/mm to have an observable influence on the results.
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REFERENCES
1. J. J. DePalna and E. M. Lowry, J. Opt. Soc. Am. 52,
328 (1962).
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Figure 1. Square-wave modulation
versus spatial frequency- Curve 1,
9._
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WM
:ill
ENV

mm
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Declassified in Part - Sanitized Copy Approved for Release 2012/09/06: CIA-RDP79B00873A001900010046-4
Declassified in Part - Sanitized Copy Approved for Release 2012/09/06: CIA-RDP79B00873A001900010046-4
? 16 P-19-47 and 48
SIDE
LOCAL LOCAL
ILL liv/3Ti5N
Figure
December 5, 1969
VIEWER
SI De
c?ivi N sgA-x, 1/1 LOCAL
)
51316 ?IFT S
AV. BPI& 117
? Geometry and nomenclature for describing
trapped projector light and reflected ambient
light.
Pfil:Lrcr
A-P-1,13LE
16 H r N
I
Declassified in Part - Sanitized Copy Approved for Release 2012/09/06: CIA-RDP79B00873A001900010046-4
Declassified in Part - Sanitized Copy Approved for Release 2012/09/06: CIA-RDP79B00873A001900010046-4
1,
P-19-47 and 48
December 5, 1969
QUALITy re:Sr it
Figure 3. Quality scale factor Z versus modulation
transfer factor.y/y0 produced by reflected
ambient and trapped projector light. Numbers
.beside points are abbreviated screen numbers.
Data for Quality Test II.
Declassified in Part - Sanitized Copy Approved for Release 2012/09/06: CIA-RDP79B00873A001900010046-4
Declassified in Part - Sanitized Copy Approved for Release 2012/09/06: CIA-RDP79B00873A001900010046-4
J8 co.) P-19-47 and 48
December 5, 1969.
ALIr resr
20
2
Figure 4. Quality Scale factor Z versus RD T for Quality
S
Teq II. Screen LS-60 has corrected,value of
RDTs.
Declassified in Part - Sanitized Copy Approved for Release 2012/09/06: CIA-RDP79B00873A001900010046-4
Declassified in Part - Sanitized Copy Approved for Release 2012/09/06: CIA-RDP79B00873A001900010046-4
? ,
19
P-19-47 and 48
December 5, 1969
Qvm-trv TEST-
Go
23
2,5 0"
?111)
-1,
17
20
Figure 5. Quality scale factor versus y/y0 for Quality
Test I.
Declassified in Part - Sanitized Copy Approved for Release 2012/09/06: CIA-RDP79B00873A001900010046-4
Declassified in Part - Sanitized Copy Approved for Release 2012/09/06: CIA-RDP79B00873A001900010046-4
20 P-19-47 and 48
December 5, 1969
.8
16-
11-
quA-Lly re_cr
ib 27
ID
0 1 1 1 1._L__11_,....._ I
2 g - .6 .8 lp
v
6D
-.6
2O
-
2O
Figure 6. Quality scale factor versus y/y0 for Quality
Test III.
Declassified in Part - Sanitized Copy Approved for Release 2012/09/06: CIA-RDP79B00873A001900010046-4