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I know this had been discussed in the past, but I didn't follow the thread.  I
seem to be getting more engineer types wanting to use our confocal.  Because
they often have ancillary equipment they need to set up to replace the 
stage, I
am cornered into providing a spot for them on a custom-built 
multiphoton.  They
are forever wanting to see surfaces or reflection from particles: if my memory
serves me well, reflection cannot be had unless the illumination wavelength
matches the reflected wavelength.  But I'm starting to wonder whether another
phenomenon I am not aware of creeps in, for, on this custom 
multiphoton, I have
replaced a cube with a 50/50 beamsplitter so that the reflected wavelength is
the same as the illumination wavelength, and removed filters from in front of
the PMT detector (sensitive out to 800nm or so), set the pulsed laser 
at 750nm,
and even placed a 1/2 waveplate and glan polarizer in the laser path with
another polarizer in front of the PMT to play with these (I believe the
polarizer in front of the PMT, however, is a circular polarizer as the
reflected light never quite extinguished with cross-polarization). 
Inexplicably, I got no reflection off surfaces.

The custom multiphoton has two paths, one for the external detectors 
(where the
50/50 beamsplitter sent the reflected light) and internal PMTs in a Fluoview
300 confocal unit.  Strangely, if I sent the reflected light to the internal
PMTs, I was able to get a reflection, if I was willing to live with a bright
spot in the middle of the field.  This wasn't supposed to happen because the
reflected light was (supposedly) blocked by bandpass filters, but, after
placing another 650sp filter in the reflected light path, the reflection went
away.  (Clearly, a portion of the laser light bled through).

In any event, it is confounding to get the reflection in one instance, and to
not get it in the other even when optimized.  Any ideas?

Jerry Sedgewick
University of Minnesota





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