A LaJet Energy Company (LEC) concentrator facet, 60 in. in diameter, was tested for imaging quality. The following two methods were used: (1) autofocus tests with a point source of light at the facet's radius of curvature; and (2) tests with the Sun close to the horizon as a distant source. The tests of the LaJet facet indicate that all of the solar image reflected by an LEC 460 solar concentrator made of like facets should fall within a 9-in. aperture if the outer facets are carefully adjusted. Such a concentrator would have acceptable performance, but complete evaluation must be made with an assembled concentrator

Argoud, M. J.

Dennison, E. W.

English

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DOE/J PL-1060-64
(DE84008507)
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HAMPTON, VIRGINIA
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Work Performed Under Contract No. AT04-81 AL16228
November 15, 1983
JPL TESTS OF A LAJ ET CONCENTRATOR FACET
California Institute of Technology
Pasadena, California
By
E. W. Dennison
M. J. Argoud
Technical Information Center
Office of Scientific and Technical Information
United States Department of Energy
https://ntrs.nasa.gov/search.jsp?R=19840013936 2020-03-20T23:57:47+00:00Z
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JPL-PU8-S3-92 JPL-5iG5-125 DOE/JPL-10GG-64 NAS 1.25:173470 CN;~;
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Solar Thermal Power Systems Project
Parabolic Dish Systems Development
JPL Tests of a LaJet
Concentrator Facet
E.W. Dennison
M.J. Argoud
November 15, 1983
Prepared for
U.S. Department of Energy
Through an Agreement with
National Aeronautics and Space Administration
by
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California
JPL Publication 83-92
DOE/JPL-l060-64
(j PL-PUB-83-92)
(DE84008507)
Distribution Category UC-62

ABSTRACT
A LaJet Energy Company (LEC) concentrator facet, 60 in. in diameter, was
tested for imaging quality at the Jet Propulsion Laboratory using two
methods: (1) autofocus tests with a point source of light at the facet's
radius of curvature, and (2) tests with the sun close to the horizon as a
distant source. These tests of the LaJet facet indicate that all of the solar
image reflected by an LEC 460 solar concentrator made of like facets should-
fall within a 9-in. aperture if the outer facets are carefully adjusted. Such
a concentrator would have acceptable performance, but complete evaluation must
be made with an assembled concentrator.
iii
ACKNOWLEDGMENT
This report was published by the Jet Propulsion Laboratory through
NASA Task Order RE-152, Amendment 327 and was sponsored by the Albuquerque
Operations Office, U.S. Department of Energy under Interagency Agreement
DE-AT04-81AL16228 with the National Aeronautics and Space Administration.
iv
CONTENTS
I. INTRODUCTION............................. 1
II. AUTOFOCUS TESTS . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
III. SOLAR SOURCE TESTS • • • . . . . • • • • . . . . • . . . • . . . • .. 9
IV. CONCLUSION.............................. 13
Figures
1. Test Configuration for Autofocus Tests • • • • 4
2. Images of Facet with Point Source at Radius of Curvature .
3. Intercept Factor Curve Using Measured Data
4. Images of Facet Formed Through Focal Plane Apertures with a
Fresnel Lens • • . . . • .
5
7
8
5.
6.
Test Configuration for Sun Tests •
Images of Facet with Sun as Source • •
v
. . . • . . . . 10
• • • • 11

SECTION I
INTRODUCTION
This report covers the imaging quality tests of a LaJet Energy Company
(LEC) concentrator facet by Jet Propulsion Laboratory (JPL) personnel on
May 24 and June 23, 1983. The facet is aluminized polyester film stretched
over a partially evacuated round drum. The tests of this facet were performed
at the JPL Foothill facilities in Pasadena, California, and at the JPL
Parabolic Dish Test Site (PDTS), Edwards Test Station in the Mojave Desert,
California. The light sources for these tests were a point source of light
and the sun. For all of the tests, the facet focal length was 235 in.
(470-in. radius of curvature). The installed facets will have focal lengths
in the range of 210 to 250 in. The radius of curvature of each facet will be
different, but the image quality of the assembled concentrator will be very
similar to the test configuration. The purpose of these tests is to evaluate
the imaging characteristics of a sample facet and to demonstrate the type of
tests that can be conducted for quality control during facet manufacture and
for characterization of a complete concentrator.
1

SECTION II
AUTOFOCUS TESTS
For the autofocus tests, the point source of light and the image plane
were positioned close to the optical axis at a distance equal to the radius of
curvature. For a perfect spherical facet, the image will be a point. This
configuration has been used very successfully for testing of the PDTS test-bed
concentrator facets. Figures la and Ib are photographs of the test configura-
tion. Figures 2a through 2h are images made with the point source at the •
radius of curvature and a sequence of image planes near the source. These
images show the optical aberrations that are present. A perfect spherical
facet would have uniform circles for out-of-focus images. This highly sensi-
tive optical test shows details of the image errors that otherwise would not
be visible with the sun as a source. This effect can be seen by comparing the
point source and solar source images included in this report. (See Figures 4
and 6.) The autofocus images are two times larger than the images that would
be formed from a point source at infinity.
The quantitative measurements were made with a Fresnel lens in the image
plane (at the radius of curvature), which formed an image of the facet on a
translucent screen. This screen was viewed by a photometer to measure the
amount of light in the image. A series of apertures and disks were mounted on
the Fresnel lens to block the light outside or inside circular zones. The
light not obscured by these masks was measured by the photometer.
Figure 3 is a graphical representation of the measured data. Each point
indicates the fraction of the total image that falls inside of the indicated
aperture radius. This type of graph is called an "intercept factor curve."
The solid line is the mathematical representation of the measured data. This
mathematical representation was used to estimate the intercept factor curve,
which will be found with the sun as a source. If the facet imaging quality is
good, then the optical aberrations and the size of the sun will determine the
concentrator performance. Conversely, if the facet imaging quality is poor,
the size of the sun and the aberrations will have little effect on concentrator
performance. The difference between the point source curve and the estimated
solar curve indicates that the facet imaging quality is acceptable. If this
facet had somewhat larger errors, the concentrator performance would be
degraded; that is, the safety margin is small.
Figures 4a through 4e are photographs of the image of the facet formed by
the Fresnel lens through some of the focal plane apertures. These diagnostic
photographs show the areas of the facet that form the image inside the
aperture. The images indicate that the most serious image defects arise from
the ellipsoidal shape of the facet surface, which could be the result of the
biaxial character of the film or a slight imperfection in the shape of the
support ring. If the film had been rotated with respect to the ring or the
ring had been forcibly distorted, then this type of diagnostic photograph
could be used to locate the source of these imperfections. The diagnostic
photographs also show that there are some small imperfections in the facet
surface near the support ring. None of the imperfections in the tested facet
caused significant errors in the image, but a small increase in these
imperfections would lead to serious image degradation.
3
(a)
(b)
Figure 1. Test Configuration for Autofocus Tests
4
I "
(c) (d)
Figure 2. Images of Facet with Point Source at Radius of Curvature
5
(e)
(g)
(f)
(h)
Figure 2. Images of Facet with Point Source at Radius of Curvature (Cont'd)
6
4 5 6 7 8 9 10 11 12 mrad
1.0 1.5 2.0 2.5 FOCAL PLANE, in.
I I I I I
2.0 3.0 4.0 5.0 MEASURED PLANE, in.
RADIUS
32
~
Ir" 1...(. .....- .. --- --~ ~.--~--~ ",-0~ /"
Y /'V'/
/ ~ /// 0 MEASURED DATA
t / - FITTED CURVE/ POINT SOURCE/
-- APPROX. CURVE
1/ 1/ WITH SOLAR SOURCEI
r!'" /
/ /,/
/ I/
If I/V
/ ///
~)/V
o
o
o 0.5
Ir------II-----
o 1.0
10
70
20
50
90
80
30
40
60
100
#.
oZ
o
I-
u
«
I.L
I-
a..
UJ
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Z
Figure 3. Intercept Factor Curve Using Measured Data
(b) (c)
(d)
Figure 4. Images of Facet Formed Through ~ocal Plane Apertures with a
Fresnel Lens
SECTION III
SOLAR SOURCE TESTS
The final series of tests were made at the Parabolic Dish Test Site,
Edwards Test Station, using the sun near the horizon as a source. The facet
was tested in both on-axis and off-axis configurations. The on-axis
configuration represents a facet positioned near the optical axis of an
assembled LEC 460 concentrator; the off-axis configuration is for a facet at
the outer edge of the concentrator. The off-axis configuration is shown in
Figure 5.
In addition to photographing the image of the sun in the nominal focal
plane, the images were photographed 1 ft either side of the nominal focal
plane along the target/facet line. These images are shown in Figures 6a
through 6f. The white metal target had concentric circular slots with radial
increments of 1 in. As expected, the largest image was found at the off-axis
position. The smallest off-axis image appears to be a short distance behind
the nominal focal plane and has a diameter of between 8 and 9 in. As
indicated by the autofocus tests, the on-axis image is very good.
9
Figure 5. Test Configuration for Sun Tests
•
(a d)
(b
Nominal
(c f)
On Axis Off Axis
Figure 6. Images of Facet with Sun as Source
11

SECTION IV
CONCLUSION
The JPL tests of the sample LaJet facet indicate that all of the solar
image should fall within a 9-in. aperture if the outer facets are carefully
adjusted. The facet surfaces are measurably different from a spherical
surface. Although these surface imperfections are not large enough to cause
serious image degradation, it is clear that manufacturing procedures must be
carefully controlled to insure that the LaJet concentrator has an acceptable
operating performance. The tests covered by this report give an indication of
concentrator performance, but the final evaluation of performance must be made
with an assembled concentrator.
13


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JPL tests of a LaJet concentrator facet

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