Zeiss LSM 880 Operating Manual page 681

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Optical Coordinates

In order to enable a representation of lateral and axial quan-

tities independent of the objective lens 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

= mean wavelength

with NA = 1.3 and λ  = 496 nm → 1AU = 0.465 µm

The AU is primarily used for normalizing the pinhole

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,

= 496 nm and n = 1.52 → 1RU = 0.446 µm

The RU is used primarily for a generally valid repre sentation

of the optical slice thickness in a confocal LSM.

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