Posted by
Benjamin Smith on
URL: http://confocal-microscopy-list.275.s1.nabble.com/Power-at-Immersion-Lenses-tp7589305p7589312.html
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A few quick notes:
1) Be careful comparing between objectives. Not only is the transmission
efficiency different, but the area of the back aperture is also different,
meaning the amount of light going into an objective also varies. Also,
unless the light intensity profile at the back aperture is a perfectly
uniform top hat, the difference in light going into an objective won't be
as simple as the ratio of the back aperture areas.
2) With high NA objectives, you run into an additional problem of
increasing amounts of light reflecting off of the power meter (think
Brewster's angle). As such, a lot of the marginal photons will reflect,
reducing the overall measurement.
3) As already mentioned, the back reflection due to the glass-air interface
at the objective lens will also reduce the amount of transmitted light
compared to a glass-oil interface, and even more so if the objective lens
is scratched or dirty.
4) Power meters can saturate and become non-linear if all the light is
shined on it in a very small area. Ideally, you will want to cover as much
of the sensor as possible, especially at higher intensities.
With this in mind, a quick and dirty solution would be to get the objective
as close to (if not lightly touching) the sensor area as possible, and then
gradually increase the light intensity until you get a stable measurement.
Then gradually pull the objective away until you reach a peak power
reading. The objective may need to be able to get within a millimeter of
the sensor to have the marginal rays stay within the sensor area. Due to
the issues stated above, this will be a slight underestimation, but it will
definitely get you in the ball park.
If you need a much more accurate measurement, an oil immersion compatible
power meter seems the best (although fairly expensive) option:
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_ID=2191One other option that comes to mind would be if you had an oil immersion
condenser, you could use it to recollimate the light from the first
objective, and then place the power meter at the back aperture of the
condenser. With everything lined up correctly, and without any scattering
material between the objective lenses, the losses from the condenser should
be purely transmission losses, allowing you to accurately back-calculate
the intensity at the sample plane. Obviously, make sure the aperture stop
is wide open.
Cheers,
Ben Smith
On Fri, Mar 15, 2019 at 12:14 PM Jim Haley <
[hidden email]> wrote:
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> Are you trying to measure relative light output at the various
> proximities or absolute light output in terms of units such as Lux?
>
> Thanks And Have a Great Day!
>
> Jim Haley
>
>
>
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> On 3/15/2019 11:08 AM, Claire Brown, Dr. wrote:
> > *****
> > To join, leave or search the confocal microscopy listserv, go to:
> >
http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy> > Post images on
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> posting.
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> > Hello All,
> >
> > We are trying to measure the power of a light source through a 63x oil
> immersion lens. With the short focal distance the power we measure is
> highly dependent on the position of the sensor relative to the lens.
> >
> > Can the power be measured this way or do we need to get a sensor that we
> can put oil on and use oil immersion to really get a proper measure of the
> power at the sample?
> >
> >
> > Sincerely,
> >
> > Claire
> >
>
--
Benjamin E. Smith, Ph. D.
Imaging Specialist, Vision Science
University of California, Berkeley
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Berkeley, CA 94720-3200
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Fax (510) 643-6791
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