http://confocal-microscopy-list.275.s1.nabble.com/Reporting-laser-power-in-publication-tp7584120p7584126.html
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> 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").
scattering index matched media before the sensor itself. This should
give you accurate readings even with a oil immersion objective. No use