Determination of accuracy of measurements by NREL`s Scanning Hartmann Optical Test instrument Page: 3 of 11
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DETERMINATION OF ACCURACY OF MEASUREMENTS
BY NREL'S SCANNING HARTMANN OPTICAL TEST INSTRUMENT
Gary Jorgensen, Tim Wendelin, and Meir Carasso
National Renewable Energy Laboratory
1617 Cole Blvd.
Golden, CO 80401
NREL's Scanning Hartmann Optical Test (SHOT) instrument is routinely used to characterize the surface
of candidate dish concentrator elements for solar thernial applications. An approach was devised to
quantify the accuracy of these measurements. Excellent reproducibility was exhibited and high confidence
established in the absolute error related to individual characterizations,
The SHOT instrument was designed to allow the surface figure of large optical test articles to be
accurately specified. Such test articles are nominally parabolic shapes with an f/D ratio (in which
f = focal length and D = aperture diameter) in the range of 0.5-1.0. Recent modifications of SHOT have
extended the characterization range out to about 1/D = 3.0.
A series of experiments was designed to investigate and quantify the uncertainties associated with optical
characterizations performed by SHOT. This approach involved making a series of measurements with an
arbitrary test article positioned at a number of locations transverse to the optical axis of SHOT.
Optical measuremmnts; optical characterization; surface figure; large optics; dish concentrators.
A reliable and accurate means of quantifying the surface figure of large-aperture dish concentrators is
required to allow optical performance to he predicted. Such a capability would allow comparison of
candidate prototype designs, suggest improvement of fabrication techniques during the manufacturing
process, and provide quality control of mass-produced modules. An instrument has been developed at the
National Renewable Energy Laboratory (NREL) and has been used to test a variety of concentrators
fabricated for the U.S. Department of Energy's Solar Thermal Program (Wendelin, Jorgensen, and Wood,
The standard test configuration is shown in Fig. 1. A laser heam emanating from the center of a screen
is sequentially directed toward a user-specified number of points located on the test article. To minimize
the time required to process each point, a regularly spaced test grid pattern is used as a first-approximation
sampling scheme. To avoid biasing the data by any periodic features that may be present whose size or
spatial frequency is of the order of the sampling grid, each point is located randomly about its nominal
regular grid coordinate.
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Jorgensen, G.; Wendelin, T. & Carasso, M. Determination of accuracy of measurements by NREL`s Scanning Hartmann Optical Test instrument, article, April 1, 1991; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc1311642/m1/3/: accessed May 27, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.