Princeton Instruments PI-MAX 3 System, Manual, Page: 38 / 150

38 PI-MAX
®
3 System Manual Version 1.A
Exposure with an Image Intensifier
PI-MAX3 cameras use an image intensifier both to gate light on and off and to greatly
increase the brightness of the image. In these cameras the image intensifier detects and
amplifies the light, and the CCD is used for readout. The exposure programmed by
software in this case refers to duration of gating of the intensifier.
The MCP (microchannel plate) of the intensifier is composed of more than 10
6
individual
miniature electron multipliers with an excellent input to output spatial geometric
accuracy. Intensifier gain is varied by adjusting the voltage across the MCP or the voltage
across the MCP output and the phosphor. This second parameter is a factory adjustment,
as it affects both the gain and the resolution of the intensifier.
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).
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.
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.
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 (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 is
acquired, save it to disk.
After storing the background data to disk, you have two choices for background
subtraction: automatic or post-processing.

Automatic: This approach requires that you activate "Background" and specify the
background filename on the
Acquisition|Experiment Setup…|Data Corrections
tab before acquiring an image. When you acquire an image, the specified background
file data will automatically be subtracted from the raw image data before the
corrected data is displayed and is available for storage to disk.

Post-Processing: If you prefer to acquire and preserve the raw image data, make sure
that "Background" is not active on the
Acquisition|Experiment Setup…|Data
Caution