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Quoting carl <[log in to unmask]>:
> Search the CONFOCAL archive at
> http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal
>
> Hi Robert,
> Thanks very much for the reply. However my background doesn't include the
> wherewithall to gleen from your reply the answer to my question.
>
> I will rephrase: At a given gain setting, do pixel values from 0-255 in an
> image imply a linear increase in brightness in the sample?
Yes, PMT's are generally very linear devices. Thus the output current = k *
photon flux + k * dark current. k is what is varied by voltage. You should take
care to adjust the black level so that k * dark current is a small =ve constant
in the output. In this case, linearity is pretty much assured. The only caveat
is that at very low C-d1 voltage there is insufficient field to cause d1
secondary emission at low photon flux rates. This should not be a regime you
will be operating in though.
Cheers Mark
> If not, how
> does
> one correct for the non-linearity? When the gain is changed to a different
> setting, is there a change in the slope/shape of the sensitivity curve?
> The
> user that prompted this question wants to measure greyscale values and
> wants
> to know if a range of low values have the same relationship to each other
> as
> mid-values have to each other and high pixel values have to each other. I
> guess the next question is, "Is there a simple answer to this question?"
>
> You identified several unknowns regarding how the PMT is being used in the
> system. These questions could probably be best be answered by the Nikon
> folks who made the system, however I've received no response yet from a
> direct query. Is anyone out there?
>
> Thanks again,
> Carl
> Carl A. Boswell, Ph.D.
> Molecular and Cellular Biology
> University of Arizona
> 520-626-8469
> FAX 520-621-3709
> ----- Original Message -----
> From: "Robert Atkinson" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Tuesday, November 09, 2004 3:59 AM
> Subject: Re: PMT sensitivity
>
>
> > Hi Carl,
> > I'm not sure what exactly you are asking. If you mean that if you change
> > the anode voltage from 400V to 500V and the gain goes from 2Alm to
> > 10A/lm, will changing it from 500 to 600V give a gain of 50A/lm then the
> > answer is no the gain will be 30A/lm, you'd need to go to 700V to get
> > 50A/lm. The graph "Anode Luminous Sensitivity and Gain Characteristics"
> > on the data sheet (fig 2 on my copy) shows this. It unfortunately a very
> > cluttered graph with non linear axes.
> > The other important question is what "mode" the PMT is being used in,
> > I.E single photon or current mode. If the light level is so low that you
> > are getting individual pulses (one for each photon) from the PMT then
> > adjusting the gain will not affect the number of pulses, just their
> > amplitude. If you are in current mode then the gain will directly affect
> > the signal level.
> > I do not know how the PCM2000 operates, but may instruments use a
> > current amplifier with a relatively long time constant. This when
> > individual pulses are present it integrates them and changes to current
> > mode when the out put becomes continuous. Have you tried imaging a
> > dilution series at different gain settings. Again I don't know the
> > PCM2000 but normally the "gain" control just adjusts the PMT anode
> > voltage. The other unknown is whether the ratio between the gain
> > "control" and the voltage is linear. Some instruments scale the voltage
> > change so that 10% change in the control really is 10% change in gain. A
> > dilution series check should show this.
> >
> > HTH,
> > Robert.
> >
> > -----Original Message-----
> > From: Confocal Microscopy List [mailto:[log in to unmask]] On
> > Behalf Of carl
> > Sent: 09 November 2004 00:04
> > To: [log in to unmask]
> > Subject: PMT sensitivity
> >
> > Search the CONFOCAL archive at
> > http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal
> >
> > Hi All,
> > Does anyone have info on the linearity of the sensitivity of the
> > Hamamatsu
> > R928 PMT that we have in our Nikon PCM2000 confocal? Does it change
> > with
> > gain settings? I have a 4-page spec sheet on this thing but I can't
> > decipher what it is telling me.
> > Thanks very much,
> > Carl
> >
> > Carl A. Boswell, Ph.D.
> > Molecular and Cellular Biology
> > University of Arizona
> > 520-626-8469
> > FAX 520-621-3709
> >
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