The only one I'm personally acquainted with is the Tecan Infinite 200 PRO. I don't know the general market, so I'm not able to compare it to other products. I've seen a used one listed for sale at about $4000 USD. I don't know if it's realistic to produce a knockoff at that price, but it would be plausably within purchasing range of both persons and small labs doing DIY bio, at least. If the price would go higher, then I guess it would be better to go hunting for a used instrument
The essential features would be:
*Ability to load 96-well plates.
Ability to perform fluorescence measurements using adjustable excitation and emission wavelengths
*Ability to measure absorbance at 600 nm.
Nice to have features:
*Temperature control/heating
*Shaking
*Rapid measurements (ability to perform measurements on every well within, say, 5 minutes)
An accuracy to within 1 % should be sufficient, but I imagine the achievable accuracy would depend heavily on signal strength.
With the Tecan reader mentioned you can either use automatic gain
optimization (to get the highest accuracy), or fix the gain manually to
allow comparison between different measurements and when running a
time-series experiment (incubating a sample in the reader), there's an
automatic gain adjustment option which automatically decreases the gain
in case of saturation. I've always used a control sample (blank growth medium and/or wildtype cells). I don't know about any way to relate absolute fluorescence to protein concentration (for example) in cells, so measurements are always relative/measured in arbitrary units anyway.
An internal control reference sounds like a good idea. But I have no idea about how such things work, really.
For reference, here's a page on the Infinite m200 :
http://www.biocompare.com/20131-Microplate-Reader-ELISA-Plate-Reader/343356-Infinitereg-M200/I'm speaking only from my own, very limited experience. I'm sure there are some people on this list who knows a lot more about this kind of instruments.
Again, I'm sure there are lots of complex details about signal detection and processing, but I don't know anything about that.
So, I know about what the *practical* need is, but I think someone else would be better able to describe the technical requirements.
Cheers,
JP