[pure-silver] Re: Gelatin filters

Richard Knoppow wrote:


----- Original Message ----- From: "joe mcguckin" <joe@xxxxxxx>
To: "Pure-Silver Free" <pure-silver@xxxxxxxxxxxxx>
Sent: Monday, December 17, 2007 6:18 PM
Subject: [pure-silver] Gelatin filters



Are these made from pure gelatin or is there an acetate base?


Joe McGuckin
Gelatin filters are made from pure gelatin which is cast into sheets. They are or were available either as plain gelatin or cemented between flats of optical glass. When in sheets they have some advantage when used in an image forming light path because they are quite thin and Gelatin has a rather low index of refraction. As a result they produce very little disturbance to the light rays.
Well, not quite. I've had reason to look into the refractive index of gelatin in the past. As a matter of fact is pretty similar to normal optical glass at or slightly above approx 1.5 (from memory - it will to some extent depend on the source of the gelatin). Also, if I remember correctly, Kodak Wratten gelatin filter is protected by a thin layer of lacquer on either side.
Richard is, however, quite right in saying that they have little influence on the optical performance of the system, but that is due to their thinness (is that a word ...?) rather than their refractive index. 

- Helge Nareid

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