6
Point Spread Function
In order to understand the optical performance
characteristics of a confocal LSM in detail, it is ne-
cessary to have a closer look at the fundamental
optical phenomena resulting 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” corre-
sponding 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 confocal LSM are concerned, it is necessary to
consider the 3D image or the 3D-PSF.
In the ideal, diffraction-limited case (no optical
aberrations, homogeneous illumination of the
pupil –
see Details “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 are concentra-
ted, 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 of rotation, i.e. the well-
defined area in which the intensity of the 3D point
image in axial and lateral directions has dropped
to half of the central maximum.
Fig. 6 Section through the 3D-PSF in
Z direction – top, and in XY-direction
– bottom (computed; dimensionless
representation); the central, elliptical
maximum is distinctly visible.
The central maximum in the bottom
illustration is called Airy disk and is
contained in the 3D-PSF as the greatest
core diameter in lateral direction.
z
x
y
x
337_Zeiss_Grundlagen_e 25.09.2003 16:16 Uhr Seite 9