> Ok. So let me turn this into a more biological problem. You're
> imaging a live cell (which can be damaged by laser irradiation) with a
> constant level of fluorescence. You need an optical section so you're
> not going to open the pinhole. So ideally what you want is a
> detector which allows you to pick up a similar signal (with a better
> signal-to-noise ratio) but with a lower laser power. In this case, you
> have to depend on your gain (or voltage applied to the PMT) to adjust
> the intensity of the signal. Shouldn't then the sensitivity of the
> PMT be related with the gain settings ?
>
The sensitivity of the PMT is certainly related to the gain
setting. However, it is not a linear function of that
setting. Moreover, after a certain point a higher gain setting may
result in a large increase in thermal noise, thus lowering the S/N.
In addition, the PMT sensitivity at a given gain setting varies from
tube to tube. So it is hard to make a meaningful comparison of two PMTs
based on gain settings.

As an aside -- concerning optical sectioning. I just tested the axial
resolution of my MRC-1024/Axiovert combo using the Bangs labs 63nm
beads. I found that at best I was getting about 700nm full width
between the first zeros at 0.7mm iris (63x/1.4 objective). However, my
system is not vibration isolated, and the bead was in water (although I
gave it time to settle). I also tested at 2mm and 4mm iris
settings. At 2mm iris diameter I still had about 1 micron axial
resolution. At 4mm it was about 1.3 micron between the zeros.
The initial data I took were noisy, so I plan to redo it to get more
accurate results -- but the initial numbers are in the right range and
confirm what I see when imaging specimens.
The moral is, unless you need the ultimate in resolution, you can do
very well by opening up the iris (or pinhole) and sacrificing some
axial resolution. You gain a lot in sensitivity, and dont lose that
much in resolution.

--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:   972-2-6422075
Jerusalem, Israel                    | ham radio: 4X1PB/KA1PB