Zeiss LSM 880, Operating Manual, Page: 669 / 690

The object information gained by the scanning process is,
in most cases, detected by a photomultiplier tube (PMT),
which registers the spatial changes of object properties I(x)
as a temporal intensity fluctuation I(t). Spatial and temporal
coordinates are related to each other by the speed of the
scanning process (x=t·v
scan
). The cathode of the PMT con-
verts optical information (photon) into electrical informa-
tion (photoelectron), which is intensified in the following
dynode cascade. The resulting continuous electric signal is
periodically sampled by a subsequent analog-to-digital (A/D)
converter and thus transformed into a discrete, equidistant
succession of measured data (pixels) (figure 12).
19
TypesofA/Dconversion
The quality of the image scanned depends on the type
of A/D conversion which is employed. Two types can be
distinguished:
• Sampling: The time (t) for signal detection (measurement)
is small compared to the time (T) per cycle (pixel time)
(see figure 12).
• Integration: The signal detection time has the same order
of magnitude as the pixel time.
Integration is equivalent to an averaging of intensities over
a certain percentage of the pixel time known as pixel dwell
time. To avoid signal distortion (and thus to prevent a loss
of resolution), the integration time must be shorter than
the pixel time. The highest resolution is attained with point
sampling (the sampling time is infinitesimally short, so that
a maximum density of sampling points can be obtained).
By signal integration, a greater share of the light emitted by
the specimen contributes to the image signal. Where signals
are weak (e.g. fluorescence), this is a decisive advantage
over point sampling with regard to the signal-to-noise ratio
(SNR). Therefore, Carl Zeiss confocal LSM systems operate in
the integration mode. The absolute integration time can be
modified by varying the scanning speed, which also means
a change of the pixel time.
Fig. 12 Pointwise sampling of a continuous signal
T = spacing of two consecutive sampling points
t = time of signal detection (t<<T)
t T
Time
Intensity
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PART 2
SignalProcessing