[rollei_list] Lens Coating discovery, note from Richard
- From: Don Williams <dwilli10@xxxxxxxxxxxxx>
- To: "rollei_list@xxxxxxxxxxxxx" <rollei_list@xxxxxxxxxxxxx>
- Date: Sun, 22 Jan 2006 20:39:48 -0800
At 10:51 PM 7/4/2003 -0700, Richard Knoppow wrote:
(In response to a comment from DAW)
This is true. The discovery was made by Harold Denis Taylor, the
inventor of the Cooke Triplet lens. Taylor noticed that some
tarnished lenses had better transmission than freshly polished ones.
He set out to determine why. He found the tarnish had formed a
coating of oxides that had an index of refraction midway between the
glass and air. When they were of the right thickness they reduced
the reflection. Taylor experimented with ways of artificially
tarnishing glass, the use of weak acids for instance, but found no
practical method.
Later investigators experimented with coatings applied with
chemical baths but these coatings were not permanent. The softest
could be simply wiped off the lens.
The use of vacuum deposited metallic salts was evidently tried
first at Zeiss about 1935. I don't know why Zeiss did not
commercially exploit this, perhaps conditions at the time were
unfavorable, or perhaps the process wasn't reliable or economical.
Vacuum deposited coatings are quite hard, some are harder than
some types of optical glass. The form a molecular bond with the
glass so they can't be removed by any simple means such as a
solvent or just rubbing.
I had some experience with vacuum deposition many years ago.
While simple in principle its not at all simple to get good results
practically. I rather think the method Zeiss came up with had to
wait for some development of vacuum deposition as a technology.
I am not sure when this method was first applied to production
lenses by anyone. Coating became available on some commercial
lenses as early as 1946. Kodak and Wollensak began offering hard
coated lenses at this time. Kodak had offered a few soft-coated
lenses in the early 1940's. The lenses for the ill stared Ektra
camera were soft coated on internal surfaces, as was the Medalist
lens, and the Eastman Ektar series, the predecessors of the Kodak
Commercial Ektar. By the mid 1950's most lenses were hard coated.
As mentioned before the principle of coatings is related to
similar impedance matching devises in acoustics and radio frequency
transmission. Those familiar with the quarter wave matching section
will recognize it as an analogue of a lens coating. Multiple
coatings approach a horn in function.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
I have to admit that although this is in my Rollei mailbox folder,
but was not posted to the group as far as I know. I belive it was
part of a private conversation between Dick and me, so maybe my
comment about bringing the subject around for a second cycle was incorrect.
Don Williams
La Jolla, CA
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