Jacek Polski wrote- >LEDs are the future of flow cytometry (IMHO). I disagree, and I think the facts and the physics (see below) support my opinion. >Roche markets Sysmex XE2100 hematology analyzer (basically a flow >cytometer with proprietary reagents) which utilizes UV LED as a light source. All the information I have about the XE2100 says it uses a red (635 nm) laser diode; this makes sense since their reticulocyte dye solution is described as blue in color which would indicate it primarily absorbs red light. There is no question that diode and solid state lasers have a bright future in flow cytometry; but their future is bright because they are. If LED's could be made as bright as lasers, they'd be competitive, but, to date, the emitting surfaces of LED's are nowhere near bright enough to make them generally suitable as laser replacements for fluorescence excitation, and the laws of physics (the same ones, incidentally, which prevent us from building perpetual motion machines) prevent successful implementation of the optical tricks (arrays of LED's, multiple lenses and light pipes, etc.) which one might imagine could get more light from the LED's to the specimen. LED's may find a niche where scatter, absorption, and extinction measurements are concerned, because those don't need to have as many photons put through the specimen as do fluorescence measurements; the lack of coherence and relative nonmonochromaticity of LED emission may actually be advantageous. I haven't heard any reports of work on this and am hoping to get around to doing some in the next few months. -Howard
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