[LargeFormat] Kodak serial numbers and lens coatings

[Michael Briggs]

On 07-Nov-2002 Richard Knoppow wrote:
>   FWIW, the "circle-L" mark and the trade-name "Luminized"
> for hard coating appeared first about 1946 or 1947. Kodak
> _soft_ coated some lenses from about 1940. These were the
> Eastman Ektar, predecessor to the Kodak Commercial Ektar,
> the lenses for the Kodak Ektra camera, and lenses for the
> Kodak Medalist. There may have been others. Soft coatings
> are deposited in a chemical bath. They are very soft and can
> be damaged or even removed by ordinary lens cleaning. They
> were applied to surfaces in sealed cells.
>   It is diffucult to find data to date vacuume deposited
> lens coatings. The processes was developed at Zeiss as early
> as 1935 but does not seem to have been used by them, at
> least not for common optical products. U.S. manufacturers
> began hard coating in 1946 or 47, some later. I think all
> Kodak and Wollensak lenses were coated beginning in 1947.
> Wollensak's trademark was a large C with a W inside it for
> "Wocoating". There were also aftermarket coating services
> available.

Rudolf Kingslake, in a 1947 article about WWII aerial lenses that appeared in
the Journal of the Optical Society of America, states "From the very
beginning of the war, .... the glass-air surfaces of all aerial lenses were
coated with an antireflection film.  This was of soft calcium fluoride at
first, but later hard coatings were developed that offer as much resistance to
abrasion as the glass itself."

Are the chemically deposited soft coatings that Richard refers to the same as
the calcium fluoride that Kingslake mentions?    

Today the cheapest single coating is typically magnesium fluoride, and I
suppose that this was the material of the first vacumn coatings, and is likely
to be the "hard coatings" that Kingslake refers to.  A 1955 Kodak product
booklet that I have says that Kodak Lumenized lenses are coated with "a thin,
hard coating of magnesium fluoride".   A 1951 Kodak brochure defines Lumenizing
as a "Kodak process for hard-coating the glass-air lens elements" which is
"produced in an evacuated bell jar by evaporating a fluoride salt onto the
surfaces of the lens elements."

From my examination of Kodak Aero-Ektars, even ones as early as 1943 (based on
serial numbers beginning EA), I find that they are all coated.  The coatings
are in generally good condition and I think are inconsistent with soft coatings
that might be removed by ordinary lens cleaning.  The coatings appear similar
to those of coated lenses of the 1950s.  I guess that Kodak was vacuum coating
some military lenses at least as early as 1943.  These lenses lack the circle-L
trademark, but there would have been no need for the marking since the military
would have purchased based upon detailed specifications.  My guess is that
Kodak's vacuum coating facilties were reserved for the most important military
optics during the war and were used for civilian lenses after the war ended.

--Michael