[RE-wrenches] Antireflective coatings

[Peter Parrish]

This comment comes from a physicist, so I may be more critical that the
average bear.

 

To begin with, antireflective coatings have been around for a long time. The
trick with antireflective coatings is as follows:

 

(1)     Even if the material (e.g. low iron glass) absorbs ~1% of the light
incident on it, at the "boundary" between the air and glass, light will be
reflected due to a "mismatch in the index of refraction". This reflection
depends on the angle of incidence, and depends on the polarization of the
light (direct sunlight is unpolarized).

(2)     For example a 3 mm (1/8") thick sheet of low iron glass will average
89% transmittance and 8% reflectance of "solar energy" (over that part of
the spectrum that Si solar cells are most sensitive). If a single layer is
used, the material used for the antireflective coating has to have an index
of refraction that is half way between 1.0 (air) and glass (~1.45) or 1.23
and be 110 nm thick. Although not a perfect match, MgF2 (index equal to
1.38) is commonly used. Teflon is an excellent coating but it is easily
scratched and difficult to apply in films as thin as 110 nm.

(3)     Coating the glass has to be tightly controlled and I doubt this
would work as a DIY project.

 

So I am pretty sure that this coated glass would improve the transmission,
but I am very skeptical about it protecting underlying glass and I would be
very surprised if it more scratch resistant that regular float glass. On the
contrary, I would expect the coated glass to require special handling and be
LESS environmentally robust.

 

- Peter

Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
CA Lic. 854779, NABCEP Cert. 031806-26
 <mailto:peter.parrish at calsolareng.com> peter.parrish at calsolareng.com  
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885


 

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Keith Cronin
Sent: Saturday, December 05, 2009 10:09 AM
To: RE-Wrenches
Subject: [RE-wrenches] Antireflective coatings

 

http://www.ecmag.com/?fa=article
<http://www.ecmag.com/?fa=article&articleID=10827> &articleID=10827

 

Do any of you have any comments on the efficacy of these types of products?

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