Kodak began experimenting with lens coatings in the later 1930s, as described on the Ektar page. Kodak used the term 'Lumenizing' to identify hardcoated lenses, and identified Lumenized lens with following the serial number on the lens ring.

You can find additional information about the history of lens coating   and a large collection of posts about lens coatings.

Based on Richard Knoppow's inquiry with Kodak , Kodak records of when they started applying coatings to their lenses are no longer accessible, so I am adopting the empirical approach. If you will send me those items of information listed below for your lenses, I will add them to the table; no information about the owners will be published.

0 - EI 1630¶
('48)

Ektar 50mm f/1.9 (Ektra)

• In general, it would appear that Kodak began Lumenizing lenses in 1946, since none of the 1945 lenses above were "Lumenized," but most except the Ektra lenses on or after 1946 had the Lumenizing mark .
• Both Michael Briggs and I (at least) have seen lenses that were hard coated and that were produced prior to 1946, but without the . He works with Aero Ektars that were produced for the military and these often have a yellowish-brown cast, possibly due to the radioactivity in the glasses that were used for lens elements. I have noticed a faint bluish tinge to some pre-'46 lenses, not there when directly compared to other Kodak lenses of the same type and year. Rudolf Kingslake has said that light brown is the most effective coating color for single coated lenses, so it is a mystery to me why most early Kodak lenses have a bluish tinge.
• All Ektra Ektars, the Ektars in Medalists, the Ektars in Bantam Specials made after about 1940, and large format Eastman Ektars made after about 1940 had soft calcium fluoride coating applied to inner surfaces.
• Kodak research showed that lens baffle design was as important as lens coating in reducing lens reflections that decreased contrast so cameras designed in the late 1930s and throughout the 1940 had improved baffles and paint materials.
• The single example of an Ektra Ektar made on or after 1946 was not hard coated. Ektra Owners: More data would obviously be useful. The serial numbers on Ektra Ektars are not on the lens ring, but are stamped in the metal on the shank of the lens that is placed in the camera's lens mount. So far it isn't clear whether Kodak embossed the on hardcoated Ektra Ektars.
• How were the serial numbers assigned? Sequentially by year or sequentially by lens model? Given the relatively high serial numbers of some lenses, and that Kodak probably wanted a unique number on each lens, it appears to me that a single annual sequence was used. To have assigned numbers by model, would have produced many duplicate numbers and made public use of these as identifying numbers very confusing. If lens rings were produced in a single shop, they likely would have been produced in batches. Perhaps, if the shop were going to make 50 rings for 80mm WF Ektars, they took the last number used and started their run from that point. If production was more dispersed or done in parallel operations, perhaps they assigned ranges of numbers based on projected production. Note the close proximity of number in many cases for lenses of the same type and year.

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