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Actually PMTs can be pretty linear devices, so long
as one is not operation at very high or low voltages.
There is an effect which boosts the gain slightly
in the midpoint of the range but proper circuitry
can eliminate that.  It's very clearly explained
at the following reference:

http://www.osa-opn.org/
view_file.cfm?doc=%24(%5C%3F-J0%20%20%0A&id=%24)%3C%3B'IP%20%20%0A

NB this is a long link and will probably have a
line break so you may need to 'reconstruct' it.

The paper mentioned is interesting in its own right,
too - it concerns the early days of lasers.

Guy


> Dear Deborah--
>
> I think that the dynamic range can be linear only if one is using photon
> counting.  I'm not sure that's even possible on some systems.
>
> Good luck!
>
> Martin Wessendorf
>
>>> When the gain and offset of the detectors are correctly set and no
>>> averaging is used, is the 0 to 256 dynamic range linear on a confocal
>>> image?
>>>
>>> Many thanks!
>>> Deborah Wessels, Ph.D.
>>> W.M. Keck Dynamic Image Analysis Facility
>>> Room 14 BBE
>>> Department of Biological Sciences
>>> University of Iowa
>>> Iowa City, IA 52245
>>>
>>> phone: (319) 335-2883
>>> FAX: (319) 335-2772
>
> --
> Martin Wessendorf, Ph.D.                   office: (612) 626-0145
> Assoc Prof, Dept Neuroscience                 lab: (612) 624-2991
> University of Minnesota             Preferred FAX: (612) 624-8118
> 6-145 Jackson Hall, 321 Church St. SE    Dept Fax: (612) 626-5009
> Minneapolis, MN  55455                E-mail: [log in to unmask]
>


--
Associate Professor Guy Cox
Electron Microscope Unit,
University of Sydney,
NSW 2006, Australia

Phone:+61 2 9351 3176    Fax:+61 2 9351 7682
http://www.guycox.net