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I only have an opinion on your question #4. There are a few important
differences between the amplifier gain and the voltage. 1) With voltage
you can vary sensitivity in a much wider range, since sensitivity
depends on V nonlinearly (as a power function) 2) Voltage doesn't affect
the background, the gain does. 3) Not all confocal systems allow gain
control (I think Leica doesn't, at least some models). 4) It seems to me
that S/N gets much worse with voltage increase than with the gain
increase.

Michael Model, Ph.D.
Confocal Microscopy Core
Dpt. Biological Sciences
Kent State University
Kent, OH 44242
tel. 330-672-2874

-----Original Message-----
From: Confocal Microscopy List [mailto:[log in to unmask]] On
Behalf Of Quoc Thang Nguyen
Sent: Thursday, March 13, 2008 1:23 PM
To: [log in to unmask]
Subject: PMT folklore

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	Dear all,

I have encountered a good deal of folklore regarding PMT operation in 
scanning microscopy. Maybe someone in the list can confirm or debunk 
these informations.

1) Some insist that PMTs should never be turned on and off during an 
experiment, but instead that their voltage should be turned way down 
when not in use. Is that true?

2) We have routinely exposed accidentally our PMTs to ambient light 
(sometime for a few minutes at time). The dark noise should have 
increased. Is this irreversible? Does sensitivity take a hit as well? 
We use Hamamatsu PMTs with multialkali (R6357) or GaAs  photocathodes 
(H7422-40).

3) Some microscope vendors claim that they hand-pick their PMTs from a 
large lot. Does this make a difference?

4) What is the advantage of changing the PMT voltage except for 
providing a crude gain control? Would it be better to always set the 
voltage to an optimal value for best signal to noise ratio and amplify 
the output current?

5) Why is photon counting not widely available on commercial scopes, 
since computers would deal easily with digitized counts from the PMT?

Thanks to all.


Quoc Thang NGUYEN, Ph.D.
Assistant Project Scientist
Physics Department, UCSD
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Cell Phone: (949) 246-8143
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