Thus, when converting a given pinhole diameter into AUs,
we need to consider the system’s total magnification;
which means that the Airy disk is projected onto the plane
of the pinhole (or vice versa).
Analogously, a sensible way of normalization in the axial
direction is in terms of multiples of the wave-optical depth
of field. Proceeding from the Rayleigh criterion, the follow-
ing expression is known as Rayleigh unit (RU):
n = refractive index of immersion liquid
with NA = 1.3,
λ
= 496 nm and n = 1.52
→
1 RU = 0.446 µm
The
RU
is used primarily for a generally valid representation
of the optical slice thickness in a confocal LSM.
In order to enable a representation of lateral and axial
quantities independent of the objective used, let us intro-
duce optical coordinates oriented to microscopic imaging.
Given the imaging conditions in a confocal microscope,
it suggests itself to express all lateral sizes as multiples
of the Airy disk diameter. Accordingly, the Airy unit (AU)
is defined as:
NA= numerical aperture of the objective
λ
= wavelength of the illuminating laser light
with NA = 1.3 and
λ
= 496 nm
→
1 AU = 0.465 µm
The
AU
is primarily used for normalizing the pinhole
diameter.
1AU =
1.22
.
NA
Optical Coordinates
II
Details
1.22
.
NA
2
1RU =
337_Grundlagen_Infoboxen_e 25.09.2003 16:17 Uhr Seite 2
