8
Wherever quantitative data on the resolving
power and depth discrimination of a confocal LSM
are specified, it is necessary to distinguish clearly
whether the objects they refer to are point-like or
extended, and whether they are reflective or fluo-
rescent. These differences involve distinctly varying
imaging properties. Fine structures in real
biological specimens are mainly of a filiform or
point-like fluorescent type, so that the explana-
tions below are limited to point-like fluorescent
objects. The statements made for this case are well
applicable to practical assignments.
As already mentioned, the pinhole diameter plays
a decisive role in resolution and depth discrimina-
tion. With a pinhole diameter greater than 1 AU
(AU = Airy unit –
see Details “Optical Coordi-
nates”
), the depth discriminating properties under
consideration are essentially based on the law of
geometric optics (geometric-optical confocality).
The smaller the pinhole diameter, the more PSF
det
approaches the order of magnitude of PSF
ill
. In the
limit case (PH < 0.25 AU), both PSFs are approxi-
mately equal in size, and wave-optical image
formation laws clearly dominate (wave-optical
confocality).
Figure 7 illustrates these concepts. It is a schematic
representation of the half-intensity areas of PSF
ill
and PSF
det
at selected pinhole diameters.
Depending on which kind of confocality domi-
nates, the data and computation methods for
resolution and depth discrimination differ. A com-
parison with image formation in conventional
microscopes is interesting as well. The following
sections deal with this in detail.
Fig. 7 Geometric-optical (a) and wave-optical confocality (c) [XZ view].
The pinhole diameter decreases from (a) to (c).
Accordingly, PSF
det
shrinks until it approaches the order of magnitude of PSF
ill
(c).
Resolution and Confocality
Geometric-optical
confocality
Wave-optical
confocality
a)
PH~3.0 AU
b)
PH~1 AU
c)
PH~0,25 AU
PSF
det
>> PSF
ill
PSF
det
> PSF
ill
PSF
det
>= PSF
ill
FWHM
ill, lateral
FWHM
det, lateral
FWHM
ill,
axial
FWHM
det,axial
337_Zeiss_Grundlagen_e 25.09.2003 16:16 Uhr Seite 11