17
Fig. 12 Pointwise sampling of a continuous signal
T = spacing of two consecutive sampling points
t = time of signal detection (t<<T)
As a rule, object information is detected by a pho-
tomultiplier tube (PMT). The PMT registers the spa-
tial 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 PMT con-
verts optical information into electrical informa-
tion. The continuous electric signal is periodically
sampled by an analog-to-digital (A/D) converter
and thus transformed into a discrete, equidistant
succession of measured data (pixels) (figure 12).
Types of A/D conversion
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 inten-
sities over a certain percentage of the pixel time
known as pixel dwell time. To avoid signal distor-
tion (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 advan-
tage over point sampling with regard to the signal-
to-noise ratio (SNR). Therefore, Carl Zeiss confocal
LSM systems operate in the integration mode, as a
rule. The absolute integration time can be modi-
fied by varying the scanning speed, which also
means a change of the pixel time.
Signal Processing
Part 2
t
T
Time
Intensity
0
50
200
150
100
337_Zeiss_Grundlagen_e 25.09.2003 16:16 Uhr Seite 20
