http://confocal-microscopy-list.275.s1.nabble.com/Preamplifier-for-fast-point-scanning-tp7583344p7583427.html
The Airyscan system is a 32 channel GaAsP PMT array. I don't believe
information on other GaAsP PMT array systems from Hamamatsu. You
crosstalk are. Looking at some of the ones on the Hamamatsu website,
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>
> This is off-topic, but I suspect the same people who replied to this
> thread could answer my question. Can someone describe the type of noise I
> should expect with an Airyscan-style detector?
>
> I'm familiar with sCMOS and EMCCD cameras, but I have almost no experience
> with megahertz few-pixel detectors. I assumed that the noise of each
> 'pixel' of the Airyscan detector would behave similarly to the pixels of a
> sCMOS: Wavelength-dependent quantum efficiency above 50%, Poisson noise
> that depends only on the number of photoelectrons generated by the signal
> light, and additive Gaussian noise that depends only on the detector
> settings. Is this true? Are there other important details I should know
> about, like dynamic range, etc?
>
> Thanks for the help!
>
> -Andrew
>
> On Wed, Feb 4, 2015 at 11:53 AM, James Pawley <
[hidden email]> wrote:
>
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>>
http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy>> Post images on
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>>
>>> @Zdenek: A SNR of 3.5 or a little bit more might be enough for some
>>> applications ... I was planning to measure the photon count per pixel
>>> (using this method :
>>>
http://labrigger.com/blog/2010/07/30/measuring-the-gain->>> of-your-imaging-system/ ), but I always was busy with other things, so I
>>> cannot give you numbers for my imaging system.
>>>
>>
>> Hi all,
>>
>> Thanks to Labrigger for working on this important topic.
>>
>> However, I have read his analysis and think that the assumption that one
>> can use this procedure to measure the number of photoelectrons (PE: i.e.,
>> detected photons) created at the photocathode (PC) of the PMT may be an
>> over-simplification.
>>
>> The analysis depends on the assumption that the only source of noise in
>> the data recorded in the "image" of a flat white field is Poisson Noise
>> associated with the small number of PEs produced at the photocathode. This
>> might be true if PMTs were free from multiplicative noise but in fact
>> Poisson Noise also affects every stage in the multiplication of a single PE
>> after it leaves the PC. In the very unusual case that the voltage between
>> the PC and the first dynode is 500-600 volts (and that this dynode has both
>> the optimal shape and the best GaAs surface), the gain of this stage may be
>> 25 +/-5 or 20% additional noise. More commonly, this gain will be closer
>> to 4 +/-2 or 50% additional noise. More noise is added at each stage and
>> even though these noise terms are not additive (they are combined as the
>> sqrt of the sum of the squares), it is not at all uncommon for this process
>> to double or even triple the variation present in the resulting signal
>> beyond what one would expect from Poisson Noise applied only to the number
>> of PE. Furthermore, this added noise will be somewhat larger if the system
>> is working at a relatively high signal level because then the PMT will be
>> turned down, the gain/stage correspondingly lower and the Poisson Noise
>> proportionally higher.
>>
>> Offsetting this error to some extent is the finite bandwidth of the entire
>> amplifier system (PMT plus the electronics between the final dynode and the
>> ADC). This bandwidth is in general unknown but may be adjusted by the
>> computer to more-or-less match what the computer estimates is needed to
>> pass the finest optical details that the system can transmit on the basis
>> of settings for wavelength, objective NA, zoom/pixel size, and even PMT
>> setting (high PMT voltage implies a noisy signal that may benefit from the
>> artificial, 1-dimensional smoothing that attends lower bandwidth).
>>
>> Clearly, because bandwidth limits the maximum excursion that can be
>> transmitted between one pixel and its neighbour, it will tend to reduce the
>> apparent noise present in the digitized signal. The magnitude of this
>> clipping is unknown but may vary with the parameters mentioned above.
>>
>> This is relevant because, unlike the optical signal, the Poisson Noise
>> signal that we are searching for shows no correlation between adjacent
>> pixels. In particular, following the blog's suggestion of using a high zoom
>> (to reduce fixed pattern noise) may cause the computer to limit the
>> bandwidth more than using a lower zoom.
>>
>> Although, as noted above, because these two factors bias the results in
>> opposite directions, their effects may cancel each other out to some
>> extent. However, we need to know a lot more about how the components are
>> actually operating before we can decide whether and to what extent this is
>> true.
>>
>> The analysis also assumes that there is no fixed patterns noise in the
>> image of a "flat white field" as might be caused, for instance, by field
>> curvature, spherical aberration, vignetting, dust or other optical
>> parameters that may change detected signal across the field of view. I
>> note that many of these sources of non-Poisson Noise can be substantially
>> reduced by recording two sequential frames and obtaining a measure of the
>> noise by subtracting one from the other.
>>
>> For the analysis to work, it is also important to set the brightness
>> control (DC - offset) so that zero signal corresponds to closely to zero
>> intensity in the image memory.
>>
>> I should note that multiplicative noise ceases to be a factor in systems
>> employing either hybrid PMT (where the first stage gain is about 10,000) or
>> effective photon-counting (i.e. a photon counting where the recorded peak
>> pixel signal is at least 10x smaller than the saturation count rate of the
>> system as set by pulse-pileup.).
>>
>> One can avoid multiplcative noise by recording the data using a CCD (but
>> NOT on an EM-CCD used with the electronic gain turned on) and the
>> record-two-then-subtract approach can again be used to reduce inevitable
>> fixed pattern noise. However, this sensor will probably work best when
>> recording a fairly large signal (at least 10% of peak?) so that read noise
>> will be relatively insignificant. And as above, the results will again be
>> limited by the finite bandwidth of the FET amplifier between the read-node
>> and the ADC. Finally, when using a CCD for quantitative measurements, it is
>> particularly important to remember that they are usually set up so that
>> zero light corresponds to 20-50 computer intensity units.
>>
>> The noise performance of sCMOS detectors is both non-Gaussian and depends
>> strongly on the extent to which the internal pixel-by-pixel variations in
>> gain and offset are detected and corrected. This will make their use for
>> this type of measurement somewhat more difficult unless the signal levels
>> are well away from the noise floor.
>>
>> Bottom line: Although the procedure may indeed give a useful benchmark
>> that we might call the "effective gain" of the signal path, the measurement
>> is subject to influence by a number of imaging parameters and will not
>> really allow one to measure how many recorded-signal-intensity-units
>> correspond to one PE.
>>
>> Jim Pawley
>> --
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