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Adjust the high voltage controls gradually whilst observing the signals – an oscilloscope is probably the
most convenient means, but the data acquisition system can be used also – but with the laser still off.
o As the applied voltage is increased, the signal baseline may shift slightly positive, but it
should not move by more than a few mV in a properly set-up microscope. The oscilloscope
must be set for DC input coupling to observe this.
o There should be no sign of any 100 Hz or 120 Hz ripple in the signal (if observed, this would
indicate the presence of stray room light operated by the local AC supply).
o As the gain is increased, the signal should begin to show more and more short spikes;
these are individual photons being detected by the PMTs and also dark noise. The baseline
should still be near zero: if not, there may be a light leak in the system.
Return the gain controls to minimum before turning on the laser for imaging; this will prevent accidental
overload of the system.
SAFETY NOTE
Whilst the system is in operation the detectors can be damaged if overloaded. Do not
turn on room lights or remove any light shield over the experiment whilst the detectors
are on (even if the gain is set to minimum). Visible light reflected off the sample area will
reach the detectors with great efficiency even if there are no other light leaks in your
system! Remember that the maximum output signal will be reached when the PMTS are
delivering 50% of their maximum safe output current.
6.1.3 Temporary switch-off procedure
If you need to make a quick change to the experiment that will not change (increase) the signal levels
significantly, simply move the master safety toggle switch on the controller to the “Safe” (upwards) position
without adjusting the gain controls. This will cut off the supply to both PMTs rendering them safe from
overload. When you are ready to resume the experiment with the same sensitivity settings, return the master
safety toggle switch to the “Enable” (down) position and continue, keeping a check on the signal levels.
If you need to work on the experiment with the lights on for a longer period, use the safe switch-off procedure
below.
6.1.4 Safe switch-off procedure
To shut the system down for a longer period, or to carry out extended adjustments on the experiment with the
lights on:
Turn both gain controls anti-clockwise to minimum; the meters should both indicate zero volts.
Then move the master safety toggle switch on the controller to the “Safe” (upwards) position.
Withdraw the input dichroic mirror from the optical path by moving the silver lever on the front of the
optical block to the left (anti-clockwise). This has the effect of blocking the optical collection system
with a solid shutter, preventing any light reaching the PMTs even when they are off. This ensures that
the PMT noise levels remain as low as possible.
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