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Thanks for the answers to my question on the definition of the Airy
unit. This has been quite helpful, and I appreciate the further in-
depth discussion of this and related concepts by Guy Cox, Jim Pawley,
and others. I downloaded a copy of Guy's 2004 paper and will be give
that a look-through as well.
I believe part of my confusion in the original question was confusing
the "Airy disk" for the whole "Airy pattern", and also using the term
"first order light" where I meant "zero order light". I had a look at
the exact definition of "Airy unit" given in the chapter Jim Pawley co-
authored in "Handbook of Biological Confocal Microscopy" (p. 630) and
it's pretty straightforward: "the diameter of the first minimum in the
Airy disk". Which would define the Rayleigh limit (where the zero
order peak of one Airy disk coincides with the first minimum of an
adjacent Airy disk) as 0.5 Airy unit.
Where I think things get a bit more complicated is trying to define
proper sampling limits. I'd always been taught that the ideal Nyquist
sampling rate should be at twice the resolution (that is, half the
diameter) of the Rayleigh limit, eg, 0.25 Airy unit. However, I gather
from this discussion that would be a gross oversimplification.
Peter G. Werner
Merritt College Microscopy Program
On Oct 21, 2011, Peter Werner wrote:
> I've read that the "Airy Unit" measure commonly used in confocal
> microscopy is a measure of the diameter of the first-order Airy
> disc, given the effective resolution of the microscope for whatever
> objective/mounting medium setup one is using. But I have not been
> able to find and diagrammatic representation of the relationship
> between an Airy Unit and orders of light on an Airy disc.
>
> Is 1 Airy Unit a) The diameter distance spanning the minima between
> the first and second order light in the Airy disc, or is it b) The
> diameter distance spanning the peaks of the first order light? Or
> something else entirely?
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