A method used for photoastronomy using CCDs, is to use a Peltier cooling
bank taking the CCD array down to between -40 - -50 C, then integrating
the exposure time over 10 to 60 seconds, in the process there is a dark
frame to zero out the detector, and a flat field process is used to
balance out the detectors photo response. This way with even a very few
photons, filters or prisms will work, unless you need high speed
detection.

Vern Jones

"Alan B. Pearce" wrote:
>
> >I am working on a similar but slightly more complex project at the moment
> >which needs to detect 4 different wavelengths of light emitted from a
> sample
> >luminescing. The amplitude ( brightness??) of the light at each frequency
> >must be measured independently so I can draw 4 graphs- each frequency
> >corresponds to a particular molecule and the graph is to show the
> >concentrations ( brightness ) with time of each. I am positive this can't
> be
> >done with filters over photo detecetors becuase the number of photons I
> need
> >to detect is so small it would be absorbed into the filter. What would be
> >ideal is a several photodetectors each of which is sensitive to a
> >(different) narrow band of wavelengths. Does anyone have any pointers where
> >I could look for such specific devices?
>
> This is exactly the way it is usually done on satellites which are used for
> earth resources research. I had dealings with an instrument being used to
> check the upper atmosphere gasses which had 21 CCD's in it, each with its
> own filter for the specific wavelength emitted by a specific gas. The
> filters were done by a university group who were part of the instrument
> team. Unfortunately I cannot give you any specific help on the subject
> except to say it is probably worth trying to contact a university which has
> involvement in this sort of field to see what expertise you may be able to
> gain from them.
>
> I would expect this sort of approach to work for you unless the spectral
> lines are extremely close together.
>
> As an after thought, have you considered trying to pass the emitted light
> through a prism, and then using a linear CCD to look at the resulting
> spectral line image? The only problem I see with this approach is that you
> seem to think the light output will be that low, that filter losses will be
> too great. If that is the case you are probably going to have to cool what
> ever detector you use to minimise the internal detector noise. The
> "standard" method on spacecraft is to use a helium pump to get close to
> absolute 0.
>  ;)
>
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