1 Airy Unit: what does this actually measure?

[Peter Werner]

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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?

Thanks in advance,
Peter G. Werner
Merritt College Microscopy Program

[Julio Vazquez]

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Hi Peter,

I agree that there is some obfuscation around this topic. One straightforward answer is given by Shinya Inoué in the Confocal Handbook, 2nd Edition, Chapter 1 page 1:

The radius of the Airy disc, or Airy unit,  is the radius of the first dark ring, (i.e. the distance from the center of the major peak to the first minimum). This is not necessarily equal to the full-width at half-max,  often used to measure beam diameter, although I must admit the math here is beyond my abilities.

Traditionally, two point sources are considered resolved if the center of the intensity peak of one is no closer that the first minimum of the second one, i.e. if the distance is equal to or greater than 1 Airy unit (Rayleigh criterion).

You may want to check the relevant articles in the Confocal Handbook, or the Wikipedia articles:

http://en.wikipedia.org/wiki/Airy_disk

http://en.wikipedia.org/wiki/Angular_resolution


I should point out that in one of the otherwise very nice educational websites that was mentioned (link below), the diagram shows the "Airy pattern radius" as the distance from the center of the major peak to the edge of the third ring... I don't think this corresponds to anything...


http://www.olympusmicro.com/primer/java/microscopy/airydiscs2/


--
Julio Vazquez
Fred Hutchinson Cancer Research Center
Seattle, WA

http://www.fhcrc.org
 




On Oct 21, 2011, at 11:36 AM, Peter Werner wrote:

> *****
> To join, leave or search the confocal microscopy listserv, go to:
> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> *****
>
> 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?
>
> Thanks in advance,
> Peter G. Werner
> Merritt College Microscopy Program