>
> All true but I believe one disadvantage is the peak illumination is
> less for some fluorochromes such as DAPI.  We see a difference in our
> two systems that have Hg or Xe.
>

Xenon lamps produce a lot more energy in the orange and red parts of
the spectrum, as compared to Hg lamps. In one case, we had a big
problem with scattered light when viewing red samples, when using a
100W Xe lamp. Changing to a 100W Hg lamp solved the problem. This did
two things: 1) it provided energy peaks at good excitation
wavelengths; and 2) it emitted much less light in the red. Of course,
better filters would help, but even the best excitation filters are
not perfect.

The excitation spectra of organic fluorophores are very wide, so
the Hg lines do a good job even with orange and red fluorophores. So I
would use an Hg lamp unless there was a very specific reason not to.
However, systems that use monochrometers (like many excitation ratio
systems) would use a Xenon lamp because they really need a continuous
spectrum from which the monochrometer can select the desired
wavelength.

--aryeh
Aryeh Weiss                          | email: [log in to unmask]
Department of Electronics            | URL:   http://optics.jct.ac.il/~aryeh
Jerusalem College of Technology      | phone: 972-2-6751146
POB 16031                            | FAX:   972-2-6751275
Jerusalem, Israel                    | ham radio: 4X1PB/KA1PB