Robin van Spaandonk wrote: >Michael Foster wrote:
>>Conversely, imagine this cheap stuff stapled to a cheap >>wooden or maybe tubular plastic frame. You blast it with >>a heat gun or perhaps even hold it over an open fire >>and the film shrinks tight as a drum, giving you a flat >>lens probably more rigid than if you sandwiched it between >>thicker materials. Cheap, cheap, cheap. Did I mention cheap? > Ok, but doesn't stretching it play havoc with the focal point? Again, you ask exactly the right question. Tentered polyester films owe their amazing optical clarity to having been stretched greatly in both directions. They call this biaxial orientation. You can heat the stuff up and it shrinks very uniformly, unless you really go to extremes with a very hot spot. And now it's going to sound like I'm making this up as I go along. Although this was done for entirely different reasons, the fresnel films I make are particularly well adapted to this sort of heat shrink application; because even though the substrate is thermoplastic polyester, the actual tiny conical facets of the fresnel lens are of a highly heat resistance thermoset (doesn't melt) polymer. This prevents any local heat distortion of the fresnel structure itself. To answer your question more succinctly, if you attach the film to the frame in such manner as not to require too much shrinkage and you are reasonably careful in applying the heat, there should be no significant effect on the focal point. In fact, you don't really need to shrink the film at all for optical reasons, I was thinking more of tensile strengthening of the frame. M. _______________________________________________ Join Excite! - http://www.excite.com The most personalized portal on the Web!
