Does it really make any difference whether the plate is made of glass or, say, Mylar? The substrate can almost surely tolerate higher temperatures than the emulsion. Mylar holds up even at 200 degrees C. I would make no bets on gelatin at this temperature.
Can't you use a variety of films for reversal processing? I thought it works with Tri-X and maybe also Plus-X and I suspect with many others. (Perhaps the problem here is that I know of Tri-X used this way in movie film sizes and maybe the larger format emulsions share only the name but not the characteristics!) These may not yield quite the high resolution of the films intended for the job, but how much resolution does a large format negative need for projection printing on fabric? You are correct that ALL light will end up as heat, somewhere. Conversely, there is a lot of power per unit area in the useless (for this task) parts of the spectrum of the sun. Not only is the infra-red without benefit, probably anything longer than the green wavelengths contribute little to the exposure. However, it is probably easiest to find or make an anti-IR filter. (A lightly colored solution of copper sulfate between a pair of good quality glass plates will remove a lot of IR while letting, through most of the useful light. A lot of bother but a valuable method when large sheets of heat-absorbing glass are hard to obtain or pay for.) With filtration the heat load on the film will drop dramatically. Bob At 14:26 09.08.02 +0800, you wrote: >At 2002/8/9 12:44:00, you wrote: > > >I will do some tests this coming week. Would be interesting to find out > >the ratio it will take an enlarged neg to expose the paper as opposed to > >contact. I guess one could calulate it, but I will just do some real > >world tests. > > >This suggestion probably comes too late or has some elements of >impracticality, but you might > relieve some of your problems if you would work from an enlarged > negative; 8 x 10 would be >desirable, but even 5 x 7 or 4 x 5 would be a big help. > >How you you make such a negative? I haven't checked Eastman's current >product list, but they >used to make a direct-duplicating film in a wide range of sizes. If they >don't, you could use >Commercial Ortho or an equivalent emulsion and process it by reversal. > >It is not usually mentioned, but it is possible to use carbon printing >techniques to make a >transparency. (I have used carbro to make a color transparency _on >glass._) I don't say that >this is easy, but by an admittedly tedious process, you could end up with >a negative on, for >example, 1/4-inch plate glass, perhaps even Pyrex. _That_ ought to take >the heat. > >Such an enlarged negative would of course reduce the required exposure, >but just as >importantly it would reduce the radiant energy (which eventually >translates as heat) per unit >area. No matter what filtration you use or the amount of cooling air >applied to the negative, >any light, visible or in the ultraviolet region, eventually ends up as heat. > >I once wrote someone in this group a description of an enlarger that was >used to make offset >printing plates (about 20 in. x 24 in.) from negatives about 7 in. x 15 >in. The light source was a >carbon arc concentrated by a set of condensor lenses; the negative was >able to withstand this >punishment because it was a glass plate. (I never did receive an >acknowledgment of my >message, so I don't know whether it was of any help.) > > >Mac > >George Arndt > >_______________________________________________ >Cameramakers mailing list >[EMAIL PROTECTED] >http://rmp.opusis.com/mailman/listinfo/cameramakers _______________________________________________ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers
