Andor Technology iXon Ultra 897 User Manual page 148

TUTORIAL 1
Alternatively, neutral density filters may be inserted in front of the light
source, or the aperture on the lens may be lessened in order to reduce the
intensity of light reaching the sensor. If the signal appears too low, relative
to the surrounding noise floor, the exposure time may be increased
accordingly.
Staying in video mode, increase the gain to the maximum empirical value
that is being used within the experiment and measure the values from same
pixels within the brightest region of the image – this can readily be done
using the Gain Slide Control Bar. If at this setting, the amplified signal in
these brightest pixels saturates the CCD, the light intensity will need to be
reduced by either inserting neutral density filters in front of the LED (in the
absence of such filters, thin paper or filter paper may be used to reduce the
intensity) or adjusting the lens aperture. Once the signal level has been
reduced sufficiently, the signal at 0 gain needs to be re-checked in order to
ensure that it can still be confidently observed above the noise floor.
If this cannot be satisfied, measurements can be taken at varying exposure
times for different gains, providing that this is corrected for when
calculating the gain factor at each setting, e.g. if the exposure has to be
halved at the maximum selected gain setting relative to the exposure used at
0 gain, the measured value at maximum gain will have to be doubled in
order to determine the expected value had it been possible to maintain that
longer exposure time. However, in order to ensure that the minimum
exposure time employed in these measurements remains significantly larger
than the readout time (therefore aiding accuracy), it is recommended not to
use exposures less than 100ms.
You should now be ready to make measurements. Measurements must be
recorded with background subtraction for each gain setting – from the
Acquisition menu on the Main Window, select Setup Data Type, then
click on Counts (Bg corrected). Then from the Acquisition menu, select
Setup Acquisition. In the Setup CCD tab, set gain to zero and the
exposure time to optimal value determined for this gain setting. Select
Single Acquisition Mode. Turn off the light source, and from the
Section 7. 1
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