Yuri Kudinov wrote- >Now, a couple of questions to experts. (1) I need to achieve uniform light >distribution in order to shoot DNA bands on Polaroid film. Any ideas on how >to diffuse the flashlight beam evenly over the 4x5-inch area? The Home Depot >style solutions are particularly welcome. (2) Any ideas on what can be used >as a good longpass filter? How about amber-glass beer bottle? :o) The basic problem with LED's is that even the high-brightness ones aren't that bright; the emitting surfaces are relatively small, and illuminating a larger area with a single diode means that the number of photons going through a small portion of the larger area becomes very small. That's why diodes are used in arrays (e.g., in most new green traffic lights) when large illuminated areas are necessary. Bob Hoffman of B-D had an interesting poster at the ISAC Montpellier meeting last May in which he described using high brightness LED's as light sources for flow cytometry. His results with a UV LED on DAPI-stained cells were about as good as mine (also reported at Montpellier and due out in the June issue of Cytometry) with a violet laser diode; the LED is, of course cheaper, but somewhat harder to use, and puts out a good deal less light. The "gels" used for stage lighting include some long pass filters - they are probably no more expensive than the sheet plastic filters available from photographic supply houses, and both should be cheaper than long pass filter glass. You just have to check to see whether filter fluorescence is a problem. -Howard
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