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Hi Ian, I actually helped design the low-power version of that power meter
head with Pina Colarusso. The S175C is for higher-power applications, while
the original S170C is for lower power. The 170 is more accurate for sub-mW
powers typical of confocal systems. The higher-power one is good for power
for certain non-linear processes. Both the 175 and 170 are oil and water
compatible, and I found using the proper immersion medium on the sensor was
especially critical for oil lenses.

Craig Brideau

On Tue, Aug 18, 2015 at 3:38 AM, Ian Dobbie <[log in to unmask]>
wrote:

> *****
> To join, leave or search the confocal microscopy listserv, go to:
> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> Post images on http://www.imgur.com and include the link in your posting.
> *****
>
> Johannes Helm <[log in to unmask]> writes:
>
> > *****
> > To join, leave or search the confocal microscopy listserv, go to:
> > http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> > Post images on http://www.imgur.com and include the link in your
> posting.
> > *****
> >
> > Dear Jolien,
> >
> > it is not an easy task to measure the power in the focal region of the
> > objective "exactly" and the job gets even more complicated if you want
> > to determine the power "per pixel" in the way you describe.
> >
> > 1)
> > In general:
> >
> > a)
> > Unless you have a dry objective, some power will, even - and
> > particularly - in case of "perfect" or "quasi perfect" alignment - be
> > "lost" when measuring the laser power with the detector placed under
> > the objective because of total reflection (unless you could "immerse"
> > the sensitive area of the power meter in the immersion medium, which
> > most probably would be bad for the power meter). In other words: If
> > you have an oil immersion objective and do not have any oil on its
> > front lens, the "outer ring" of light incident from the inside of the
> > objective to the front lens will be lost due to total reflection (will
> > be "sent back into the objective").
>
> Thorlabs have a nice new power meter head.
>
> http://www.thorlabs.de/thorproduct.cfm?partnumber=S175C
>
> It is in a slide form factor with a glass coverslip and then a
> scattering index matched media before the sensor itself. This should
> give you accurate readings even with a oil immersion objective. No use
> for a 1.6 NA TIRF objective but good for normal oil objectives.
>
> Ian
>