Ana Salas asked- >Which source of red >light is nowadays more suitable for flow cytometers in terms of power, >stability (noise), life, mainteinance and prize? >Facscalibur has a red diode 635nm but I think that new LSR is provided >with an He-Ne laser 633nm. The shortest answer is that whichever laser the manufacturer will sell you with some reasonable warranty should do the job. He-Ne lasers are larger, consume more power, and usually cost more per milliwatt than red diodes; they have nicer beam shapes (TEM00), and they don't have much (but do have some) long wavelength incoherent emission at wavelengths in the region of some of the fluorescence you're trying to excite with the primary beam. Noise on air-cooled He-Ne's with reasonable power is about 1% RMS. Diodes, while very small, more energy-efficient, and less expensive than He-Ne's, have ugly beams, which can be made reasonably smooth with appropriate optics, and can be made very quiet (a few hundredths of one per cent RMS noise), but they do emit long wavelength LED glow which usually requires that they be used with band pass excitation filters, and they can become unstable due to mode hopping. Diodes also vary over a range of a few nanometers in emission wavelength (635-640 nm); He-Ne's are really 633 nm, period. Both He-Ne and diode lasers should be good for over 10,000 hours of operation, but there seems to have been a higher failure rate among diodes, at least until recently. In general, the user isn't the one who puts the red laser into her or his instrument; the cytometer manufacturers do that, and they deal with the laser system manufacturers to get the specs they need. The FACSCalibur has extremely good red fluorescence sensitivity using a diode, and, if I'm not mistaken, it is a diode that is the standard red excitation source in the LSR, which also uses a He-Cd laser (*not* He-Ne) for UV - but if B-D is putting a red He-Ne into the LSR instead of the diode - possibly for more power - it should work just fine. -Howard -
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