The laser light illuminating a particular detail of
the specimen in order to excite fluorescence is
focused by the objective into the focal plane.
Fluorescence excitation and emission are most
efficient within the focal volume. Therefore, the
fluorescence from labeled structures in the focal
plane forms a sharp image. The laser light, whilst
less efficient, is still intensive enough to also excite
fluorescently labeled structures above and below
the focal plane. Light emitted there would be
superimposed onto the sharp focal plane image
and blur it. This is prevented by a pinhole
diaphragm arranged in the ray path, which only
permits light emitted in the focal plane to reach
the detector.
The pinhole is essential to the generation of sharp
images and for the optical sectioning capability.
The very designation of confocal laser scanning
microscopy refers to the pinhole, as this is in a
plane conjugated to that of the focal plane (con-
focal plane). The thickness or Z dimension of an
optical section can be set by motor-driven adjust-
ment of the pinhole diameter. Fluorescence light
from the focal plane, having passed the pinhole, is
then detected by a photomultiplier. As an LSM
image is formed sequentially, i.e. pixel by pixel,
the detector does not require any spatial resolu-
tion. It merely measures the fluorescence intensity
as a function of time. The image proper is formed
only when the intensity measured by the detector
is assigned to the corresponding site of the laser
focus in the specimen. The laser beam is directed
by the two independent scanning mirrors to scan
the specimen in a line-by-line mode. The result of
the scanning process is an XY image that repre-
sents a two-dimensional optical section of the
specimen.
Excitation light path
The laser focused through the objective
forms a double cone of excitation light
inside the specimen. While the excitation
intensity is strongest at the center of the
double cone (in the focal plane), it is suffi-
ciently high in the planes above and below
the focus to excite fluorescence there, too.
Detection light path
The only fluorescence that
reaches the detector is that
emitted in the focal plane.
Light originating from
other planes is blocked by
a pinhole diaphragm.
5
1
Fiber (from laser source)
2
Motor-driven collimators
3
Beam combiner
4
Primary dichroic beam splitter
5
Scanning mirrors
6
Scanning lens
7
Objective
8
Specimen
9
Secondary dichroic beam splitters
10 Pinholes
11 Emission filters
12 Photomultipliers
13 META detector
14 Gray filter
15 Monitor diode
16 Fiber output