----- Original Message -----
From: "Marc James Small" <msmall@xxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Thursday, April 27, 2006 7:42 AM
Subject: [rollei_list] Re: 'Old' Zeiss glass question... HFT
and T*
It would be interesting to know just what claims are made
for the two coatings. It would also be interesting to know
exactly how they are coated. Since both are apparently
considered proprietary this last is not likely. I am always
skeptical of second or third hand information, especially if
it has been filtered through a sales department.
Vacuum coating technique is no secret, nor is the design
of optical coatings. There is a long section on coatings in
the _Handbook of Optics_ published by the Optical Society of
America through McGraw-Hill. There is also some discussion
of coating methods. Information on the basics of coating can
be found on the web site of the Society of Vacuum Coaters
http://www.svc.org Included here are a complete manual of
coating c.WW-2 and a long article discussing the history of
vacuum coating. While the process evidently originated at
Zeiss it was not perfected there as this article will make
clear.
Lens coating is related to the very large field of thin
film technology. This is a very important field because it
covers the techniques and methods of making integrated
circuits and the general use of dielectric coatings in
optics and elswhere. I doubt if there are many secrets
beyond specific proprietary methods (such as those used at
Intel and other chip makers).
Multiple coatings are used to broaden out the spectrum
over which a lens or mirror coating is effective. A single
coating really works at only one wavelength but the effect
falls off slowly enough on either side to cover the 2:1
bandwidth needed for visual or conventional photographic
purposes. By using multiple layers with appropriate indices
of refraction the bandwidth can be broadened out to the
point where the the anti-reflection property is nearly
constant over the entire spectral range of interest.
Multiple lens coatings are closely related to the use in
electronics of matching filters particularly at radio
frequencies, an area where theory has been very highly
developed for some time. Coatings are also used on mirrors
to increase reflectivity. By this means Aluminum mirrors can
be made even more efficient than Silver. Coated mirrors are
now very common in all sorts of applications.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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