LSM 510
ANNEX
LSM 510 META
Monitor Diode
Carl Zeiss
03/06
B 45-0021 e
8-19
Application examples:
a)
Checking the laser power
This function is not automated so far. To
qualitatively measure the laser power, the diode
can be used in such a way that the gray level
obtained in the
Line Scan
mode at a certain
setting of the whole system is stored as a text
overlay together with the image (manually done by
user). As the diode setting (Ampl. Gain, Ampl.
Offset, ND filter) is stored together with the image,
the setting is automatically reloaded when using
the
REUSE
button. If deviations can be observed it
is easy to set the laser power to the old value by
means of the AOTF transmission.
b)
Noise Reduction by Ratio
Contrary to the PMT signal, the signal of the
monitor diode is not modulated by any specimen
information. Thus it can be used to ratio the PMT
signal to get rid of the laser noise (due to any laser
as a physical fact) and thereby improve the signal
to noise ratio of the fluorescence or reflectance
image. The major condition which has to be
fulfilled to use the monitor diode for this purpose
is that the dominating source of noise is laser
noise. The signal of the monitor diode will always
be dominated by laser noise (independent of the
power set at the laser, or the transmission set at
the AOTF), whereas the dominating source of
noise in the PMT signal can also be the shot noise
of light (shot noise especially occurs in low light
fluorescence application; as rule of thumb it can be
noted that the shot noise is limiting the signal to
noise ratio, if the PMT voltage has to be set to a
value > 400 V).
Any kind of noise which can not be observed in both channels at a time will be amplified and
not reduced by the ratio process. Low or high frequency laser noise is the only source of noise
which is correlated in the PMT signal and the signal of the monitor diode.
Low or high frequency laser noise is mainly introduced if the Ar lasers are used at a tube current lower
than 8 A (Ar-Vis) or 20 A (Ar-UV) respectively.
Fig. 8-6
Scan Control window