What is the mysterious Airy Unit on a point scanning confocal microscope?
It's the unit used to define the size of the confocal pinhole... but what actually is this unit?
Typical user Question:
I am wondering what the definition of an Airy unit is. Is it that 1 AU is the distance d=1.22 lambda/NA?
Hopefully understandable answer:
An Airy unit is the diameter of the central maximum peak of the Airy pattern (caused by diffraction at the finite back aperture of the objective lens, NA) of a focussed beam. i.e. 2x the distance from the peak to the 1st trough in the pattern.
Abbe wrote that resolution distance, d = lambda / 2NA
Airy said the the diameter of the central peak of the pattern you see is diameter = 1.22 lambda / NA
Using the same logic, Rayleigh went on to define the arbitrary but useful criterion that you reach the resolution limit where 2 objects resolve, when the peak of the first lies in the 1st trough of the other: d = 1.22 lambda / 2 NA
So these 2 distances are not the same thing.. in fact the Airy disk diameter is 2x the resolution distance.
So the pinhole at 1 Airy unit lets through the central peak of the Airy pattern, only.. which is in fact more important for its effect in Z direction than in xy.
In theory, you get more contrast and resolution with pinhole at 0.5.. but rare is the sample that is bright enough and bleach resistant enough to get a good enough signal:noise when so few photons are allowed through the small pinhole.
Apparently, signal is proportional to the square of the pinhole radius; I guess ....since area = pi . rsqured
Real confocal pinholes are not round though.... they are square, or maybe hexagonal on some new systems.... except on spinning disk confocals... those really are round!