optical interference filters - SPOT Imaging Solutions

Types Of Anti-Reflective
Treatments And When To Use Them
application note Types of Anti-Reflective Treatments and When to Use Them
This is a sizeable improvement over the energy damage threshold
of most thin-film anti-reflective coatings. Because the antireflective
“coating” is made of the glass itself, it will have an energy damage
threshold similar to that of the glass from which the optic is made.
To anti-reflect glass at visible wavelengths, an equally effective
and more cost-effective anti-reflective coating can be created by
etching the glass in a random pattern. An image of the resultant
random spacing of the prominences is shown in figure 7. Treating
a fused silica surface to create this random microstructure pattern
can decrease broadband visible reflections by 80% to 90%.
Cleaning of microstructured anti-reflective surfaces poses a small
problem. Physical cleaning of microstructured surfaces must be
done carefully, if at all. The prominences that give the substrate its
anti-reflective property can be easily broken off if the cleaning is
too vigorous.
Absorptive Anti-Reflective Coatings
Another method for minimizing reflections off an optic is to make
the substrate more absorptive. If the goal is to improve transmission
through the optic, use of an absorptive optical coating generally will
not help. However, absorptive coatings can very effectively absorb
light that would otherwise be reflected.
Absorptive coatings are not usually the best solution for high-energy
applications because, rather than transmitting the light that is
being anti-reflected, that light now is being absorbed by molecules
in the optical element, inevitably leading to heating and thermal
damage.
Summary
Figure 7
SEM image of
random AR microstructures
in
glass. (Courtesy
of TelAztec, Inc.)
There are a few different options available for building an anti-reflective
optic. While no single solution fits all needs, by appropriately
selecting the right anti-reflective technique, nearly any optic
now can be anti-reflected to meet the needs of the user.
Dr. Michael Fink studied chemistry as an undergraduate at Bates
College in Lewiston, ME, where he worked in the laboratory of
Dr. Matthew Côté building a scanning tunneling microscope to
determine the feasibility of using two color-distinguished oxidation
states of tungsten oxide as a digital information storage medium. At the
University of Oregon in Eugene, OR, Mike continued his studies, earning
his doctorate in chemistry by improving the sensitivity of molecular
Fourier imaging correlation spectroscopy in Dr. Andrew Marcus’s lab at
the Oregon Center for Optics.
References
• 1. Johnson, Robert. AR coatings application note. 2006.
• 2. Hass G. 1955. Filmed surfaces for reflecting optics. J. Opt. Soc. Am.
45: 945-52.
• 3. Hobbs, Douglas S., Bruce D. MacLeod & Juanita R. Riccobono.
“Update on the Development of High Performance Anti-Reflecting
Surface Relief Micro-Structures.” SPIE 6545-34. April 12, 2007.
• 4. Ibid.
Figure 6 Percent transmission for a ZnSe window untreated and treated with
motheye AR texture on one side. (Courtesy of TelAztec, Inc.)
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