This binocular was produced for the Dutch Royal Air Force in the late fifties. Probably 500 were made. Although since WWII the coating of optical component parts is common practice, the lenses of this 8 x 40 are uncoated. Instead of applying a coating, the firm of Old Delft had developed a different method aimed at improving the performance of lenses. It was called " Delfineren".
As to technique and assumed results, little is known. Fact is, that it involved a minute thin layer of the glass surface itself to be chemically altered. The following is my considered "best shot" at what Delfineren was all about. I assume the technique was a process resulting in a controlled measure of " iridescence" . That is: when under certain conditions glass surfaces are exposed to certain chemicals and as a result have the chemical composition of this surface altered, reflected ( white) light will no longer be reflected as "white" but instead, it will be reflected in different wavelength. Compare the colours of the rainbow we observe on the bubbles of soapy water. I assume that ( in theory?) it should be possible to modify the glass surfaces in such a way that interference and phase differentials within thus reflected light, to an extend, will "kill" a component part of this reflected light. Anyway, Delfineren must not have been a great success - the practice was dropped soon after the production of this 8 x 40. (Written by Gijs van Berne, 2008)