Re: [ATM] The standard aluminium coating: How much scatter?

["Scott Milligan" <starzkey@charter.net>]

Working from basic electromagnetic theory (Drude theory of light interaction
with solids), we understand that the theoretical (i.e. not degraded by
deposition parameters or weathering) best possible reflectance for bare,
un-oxidized aluminum is approximately 90%, exact value is wavelength
dependent below 1000 nm.  The other 10% is absorbed and converted into heat.
Any non-specular scatter that occurs in a real world situation subtracts
from the 90%, but scatter needs to become really bad before the loss
approaches the 10% attributed to absorption, which is fundamentally a
consequence of the metal's finite conductivity. 


Scott Milligan
 
 

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Guntram Lampert
Sent: Monday, August 14, 2006 5:49 PM
To: atm@atmlist.net
Subject: [ATM] The standard aluminium coating: How much scatter?

Hello list,

my name is Guntram Lampert. I am an ATM living in Austria, Europe. This is
my first posting on this remarkable list.
    It has long been suspected that light scatter originating with the
mirror coatings causes a noticeable decrease in image contrast. Though I did
search the literature and the internet, and asked knowledgeable people, I
rarely got hard facts. The information that did turn up was confusing at
best: Trustworthy Texereau says, in short, hardly any scatter, and backs
this up(p. 284), some authorities, either in optics or in the coating
business in Europe and the U.S. say just the opposite: Much more scatter
than even caused by complex multicoatings used for lenses, not too difficult
to see.
    So: How much of the incident light is scattered by a well made, standard
Al coating, with SiO overcoat? Is there a narrow angle scatter component?
I´m aware  that the properties of even standard coatings differ
considerably, and I would be happy even for a good "order of magnitude"
estimate, or a reference to the literature.


Thank you!

Regards,

Guntram Lampert
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