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Laser Gold Properties
The following is a detailed listing of the
salient properties of Laser Gold.
In our experience, it answers most of the “frequently
asked questions.
Laser Damage Threshold :
Courtesy of Big Sky Laser (Data and
Graph)
Reflectivity:
Laser Gold plating reflectivity is
greater than 97% at 0.7 microns and greater than 99% at 10.6
microns when measured on a low-scatter substrate. The reflectivity
in the infrared equals and often exceeds that of a freshly
vapor-deposited gold. Because of its outstanding cleanability,
stability and reflectivity, the National Bureau of Standards,
(NIST), chose Laser Gold plating as the Standard IR reference
material (#2011). For more than fifteen years Epner Technology
has been the sole source supplier of these standards.
(Tabular
Data) | (Reflectance
Graph)
Bi-directional
Reflectance Distribution Function (BRDF):
Epner's diffuse Lasergold coating specified
primarily on integrating spheres, has been measured at Schmitt
Measurement Systems, Inc. The measurements were made at 10.6
microns on a mechanically diffused surface. (Graph)
Emissivity:
Measurements made by a variety of aerospace
companies confirm Laser Gold’s theoretically low emissivity.
While a value of .02 is consistently obtained on polished
substrates, ETI has been able to modify, electrochemically,
sufaces that are too delicate or too convoluted to permit
physical polishing. The surface of machined materials, Be
or Al, for example, can achieve a consistent emissivity of
.03 to .05.
Indeed, the ability to achieve a repeatable emissivity value,
whatever the substrate, has made Laser Gold the coating specified
on an impressive array of military and space programs.
(Emissivity
Test Data)
Hardness:
Laser Gold plating hardness exceeds 180 on the Knoop scale
when measured by the diamond-indenter method of ASTM-B-578-87.
Laser Gold passes the 50-rub test of Mil-O-13830 and paragraph
3.8 and 4.4.5 of Mil-M-13508. It will withstand judicious
cleaning … indefinitely.
Stability:
Laser Gold has been the NIST (NBS) IR Reflectance Standard
(#2011) for nearly20 years.
Overcoat:
None, and none required. It exhibits the broadband reflectivity
of a pure gold surface, except that this surface can be physically
wiped clean.
Thickness:
Laser Gold plating can be applied from 0.1 micron to 100 microns
or more. The heavier deposits have been used for single point
diamond turning directly into the Laser Gold plated surface
and for ruling diffraction gratings. The thinner coatings
are used when the optical figure is critical.
Uniformity:
Laser Gold plating can be applied with remarkable uniformity
across surfaces as large a 1-meter in diameter. Because it
is electrolytically deposited, Laser Gold can exhibit high
current density “runout” but special tooling and
years of experience permit Epner Technology to eliminate the
effect of this phenomenon. Because Laser Gold plating is not
line-of-sight dependent, it can thus coat the inside of lightpipes
or other deep cavities with parallel walls.
Temperature:
There is virtually no risk of optical distortion during processing
since metal substrates are never subjected to more than 70°C
(150°F) Non-metal optics will see temperatures that are
typical of vacuum coating chambers.
Laser Gold plating itself is qualified
to meet the thermal stresses of space.
Purity:
Laser Gold is a pure (.9999). Combining this purity with a
hardness of almost 200 Knoop is what makes Laser Gold plating
unique in the world of optical coatings.
Size:
The largest beryllium mirror (0.9 meter diameter) ever gold
coated was plated with Laser Gold. It was the Vegetation Canopy
Lidar mirror for NASA Goddard. Prior to that, the largest
mirrors gold coated at Epner were the KECK Telescope IR Secondary
Mirror and the Mars Observer Laser Altimeter Mirror both 0.5-meter
in diameter.
Polarization
Plane and Scatter:
When applied to metal optics, there is no perceptible change
in performance in either the S or P polarization plane. Laser
Gold plating does not increase scatter, in fact, certain customers
have actually reported reduced scatter after Laser Gold plating.
When Laser Gold is applied over glass, Zerodur,
BK7 substrates, differences in the S & P polarization
plane have been detected.
Certain customers have detected a difference
in the S & P polarization plane when laser gold is applied
over glass, Zerodure or BK7 substrates. In other words processes
that involve vapor deposition prior to the Laser Gold application.
Substrates:
Laser Gold is usually applied over metal substrates including,
but not limited to, aluminum, beryllium, copper, stainless,
molybdenum, titanium. Non-metallic substrates such as Zerodur,
silicon, quartz, BK7 etc. can also achieve the durability
and reflectivity of Laser Gold by first applying a binder
coating (usually chromium) by sputtering or vacuum deposition.
(see specification plating
and substrates)
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