Zeiss LSM 880 Operating Manual page 658

Point Spread Function
In order to understand the optical performance charac-
teristics of a confocal LSM in detail, it is necessary to have
a closer look at the fundamental optical phenomena re-
sulting from the geometry of the confocal beam path. As
mentioned before, what is most essential about a confocal
LSM is that both illumination and observation (detection)
are limited to a point.
Not even an optical system of diffraction-limited design
can image a truly point-like object as a point. The image
of an ideal point object will always be somewhat blurred,
or "spread" corresponding to the imaging properties of
the optical system. The image of a point can be described
in quantitative terms by the point spread function (PSF),
which maps the intensity distribution in the image space.
Where the three-dimensional imaging properties of a con-
focal LSM are concerned, it is necessary to consider the
3D-PSF.
In the ideal, diffraction-limited case (no optical aberrations,
and homogeneous illumination at all lens cross sections –
see Part 3 "Pupil Illumination"), the 3D-PSF is of comet-like,
rotationally symmetrical shape.
For illustration, Figure 6 shows two-dimensional sections
(XZ and XY) through an ideal 3D-PSF.
From the illustration it is evident that the central maximum
of the 3D-PSF, in which 86.5% of the total energy available
in the pupil of the objective lens are concentrated, can be
described as an ellipsoid of rotation. For considerations of
resolution and optical slice thickness it is useful to define
the half-maximum area of the ellipsoid, i.e. the well-defined
area in which the intensity of the 3D PSF in axial and lateral
direction has dropped to half of the central maximum.
Any reference to the PSF in the following discussion exclu-
sively refers to the half-maximum area.
As said before the confocal LSM system as a whole gen-
erates two point images: one by projecting a point light
source into the object space, the other by projecting a point
detail of the object into the image space.
8