Archived posting to the Leica Users Group, 1997/11/16
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]The chemistry, as I remember, behind the decreased reciprocity failure reflects the nature of the latent image. Altho it theoretically takes only one photon to turn a silver halide crystal into a silver latent image, the fledgling latent image is very unstable. Any random electron "noise" can dislodge the image. Of course, the warmer the film, the more energy is available to undo the image. Remember, we are talking about extremely low light levels from distant astronomical sources. By lowering the film temperature closer to absolute zero, the latent image can collect several photons of energy, creating a stable latent image before random electron energy can undo the effect. At least, that's how I remember my photographic chemistry from college almost 30 years ago. Gary - ---------- > > >> > >>In astronomy they sometimes use special cameras in which the emulsion is > >>chilled with liquid nitrogen. I was under the impression that film became > >>more sensitive (presumably to both visible light and cosmic rays) at very > >>low temperatures. Do any of the amateur astronomers on the LUG know > >>anything about this? > >> > > > >I'm not sure about what you mentioned, but the super-cooled instruments > >are used in astronomy are typically composed of electronic sensors, not > >film emulsions. [snip] > > Cooling photographic emulsions helps eliminate reciprocity failure when > making long exposures of very faint objects. It doesn't necessarily > increase the sensitivity of the film, though it may appear to do so. > > Dan C.