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Laser Gold: Emissivity Data
Emissivity values of Laser Gold
is documented in the following report from
The Swales Aerospace Company:
The emittance of the gold plated Al samples
were measured using a Gier-Dunkel Model DB-100 Infrared Emissometer/Reflectometer.
This instrument is located at NASA Goddard.
The instrument measures near-normal incidence,
integrated room temperature, and relative emittance. That
is, the sample is kept at room temperature (~20C) and placed
over an opening through which it alternately (~13 Hz) views
two black body cavities, at near-normal incidence. These are
heated slightly above room temperature, and differ from n
another in temperature by about 15 deg. C. Standard high reflectance
and low reflectance samples are used to calibrate the instrument’s
read out scale. The reflectance (emittance) of an unknown
sample is measured relative to these known standard reference
samples.
The instrument produces a single number;
integrated reflectance that is weighted for a room temperature
black body.
(1 – reflectance_ = near-normal emittance
at room temperature.
For a pure metal, such as gold, hemispherical
emittance can be obtained from a theoretical ratio (ref. M.
Jakob, “Heat Transfer”):
Hemispherical emittance = normal emittance
x 1.3
The Gier-Dunkle DB100 instrument measurements
have a precision better than 0.005. That is, a group of samples
whose emittance vary by this amount or more can be easily
ranked by emittance. The absolute knowledge of the standard
samples, is always questionable, however instrument precision
is the key factor in distinguishing between ‘before’
and ‘after’ values. Epner’s gold sample had
a measured emittance (.02) that was equal to our gold reference
standard to within +/-0.005.
Laser Gold: Emissivity Project
University
of Hawaii, Institute of Astronomy
An emissivity report was requested by the University of Hawaii,
Institute of Astronomy prior to awarding Epner Technology,
Inc. a contract for Laser Gold® coating the radiation
shield for the Cryo-Camera on the Suburu and Gemini telescopes.
The report was produced at the Energy Materials Testing Laboratory
in Biddeford, Maine.
The laboratory measured the hemispherical
total emission which is derived from thermal emission of the
sample integrated over all wavelengths and over all directions.
This is very different from the approximation for emissivity
often used of (1 – reflectance). This approximation is
valid only if the reflectance measurement is made close to
the wavelength where most of the energy is being emitted.
More than likely this will not be the case.
The emittance of the Epner sample is about
5% at room temperature. This would normally be considered
high since the reflectance of gold at 2 microns is about 98%,
thus suggesting the emissivity should be 2%. However this
refers to a mirror surface. We consider 5% to be realistic
for gold-coated polished aluminum.
Project Comments
“We recently finished a cooled down test of our instrument
with all of the gold-coated radiation shields installed. The
test was very successful in that we achieved the temperature
that we had expected. This was a critical test of our cooling
system and we were very gratified to find it to perform as
expected. The success of this test is a tribute to the fine
gold coating produced by your company. It also confirms that
the emittance (or emissivity) as measured was about right.
The temperature achieved was close to that predicted based
upon the emittance measurements.
Finally, I should say that when all of the gold-coated shields
are installed the instrument looks so beautiful that I have
found myself staring at it in amazement. Thanks for doing
a great job.”
A.T., University
of Hawaii, Institute of Astronomy
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