Teledyne Princeton Instruments PI-MAX 3, System Manual

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Chapter 7 Operation 71
Detection of extremely weak Continuous Wave (CW) signals (e.g., luminescence and
Raman scattering from solid state samples,) is typically limited by the dark current of the
intensifier's photocathode, usually referred to as the equivalent background
illumination (EBI).
7.6.2 Saturation
When signal levels in some part of the image are very high, charge generated in one
pixel may exceed the well capacity of the pixel, spilling over into adjacent pixels in a
process called "blooming." If this is the case, you could reduce the intensifier gain or
read out more frequently, with signal averaging to enhance S/N (Signal-to-Noise ratio)
accomplished through the software.
CAUTION!
!
Signals large enough to bloom may damage the intensifier.
For signal levels low enough to be readout-noise limited, longer exposures, and
therefore longer signal accumulation in the CCD, will improve the S/N ratio
approximately linearly with the length of exposure time. There is, however, a maximum
time limit for on-chip averaging, determined by either the saturation of the CCD pixels
by the signal or the loss of dynamic range due to the buildup of dark charge in the
pixels.
7.7 Background Subtraction
Each CCD has its own dark charge pattern or background that can be subtracted from
the total acquired signal. By subtracting this background, you can eliminate the dark
charge, which might otherwise hide low-intensity signal.
When setting up for background subtraction, set up the experiment conditions for
acquiring the actual image (i.e., camera temperature, gating sequences, region of
interest, timing mode, etc.,) and then, while blocking the incoming signal from the
array, acquire a dark charge background image under those conditions. Once the
background image has been acquired, save it to disk.
After storing the background data to disk, you have two choices for background
subtraction:
• Automatic
In WinX, this approach requires that you activate Background and specify the
background filename on the Acquisition
►
Experiment Setup…
►
Data
Corrections tab before acquiring an image.
In LightField, you must check the Apply Background Subtraction box on the
Online Corrections expander and select the appropriate background file or
acquire a new background. When you acquire an image, the specified
background will be subtracted automatically from the raw image data before
the corrected data is displayed and is available for storage to disk.