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Hello:

The best troubleshooting I can recommend is to put a mirror at the focus of the
objective.  Track the reflected beam using an IR card or IR viewer.  Our system
has a dichroic which is intended to block the IR beam when I use the Mira and
the internal PMTs.  Extra filters are used in the non-descanned path.  Check
your default settings and your external detector path.  Maybe something is not
transmitting or not reflecting 750nm.  Are you sure that you external PMT can
detect 750nm?

By the way, many bandpass filters will transmit IR light (usually 800 - 100nm).

Good luck!
Sophie
____________________________________________________
Sophie M. K. Brunet, Ph. D.
Research Officer
Optical Spectroscopy, Laser Systems and Applications
Chemistry 112 sessional lecturer
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306-966-1719 (office)   306-966-1702 (fax)
____________________________________________________
Saskatchewan Structural Sciences Centre
University of Saskatchewan
Thorvaldson Bldg.
110 Science Place
Saskatoon, Sk   S7N 5C9
____________________________________________________


Quoting Jerry Sedgewick <[log in to unmask]>:

> Search the CONFOCAL archive at
> http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal
>
> 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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