220 PI-MAX/PI-MAX2 System Manual Version 5.F
be reduced by cooling the intensifier (or environment) and is usually negligible in
gated applications.
Exposure Time: The period during which the ST-133 allows incoming signal to
integrate on the CCD array. For signal to be detected and integrated on the CCD
array, it must both fall in a valid gate width and in a valid exposure time.
In Shutter Mode, the exposure time is set on the Experiment|Timing dialog
box and the photocathode is biased ON for that period.
In Gate Mode with a PTG, the exposure time is determined by a PTG pulse
ensemble defined by the PTG gating setup.
In Gate Mode with a DG535, an ET = 0 setting means that the exposure time
will be determined by the External Sync signal. An ET > 0 setting determines the
exposure time.
External Sync: Readout synchronization mode where the CCD array is synchronized to
an external source, i.e., the array is scanned upon arrival of an external trigger pulse.
External Trigger mode: Trigger mode where gate pulses are delayed from external
input trigger pulses. Also a detector readout synchronization mode where the detector
is scanned continuously, i.e. Freerun mode, but data storage does not begin until the
arrival of an external trigger pulse.
Frame: The area of the CCD array that is readout after an exposure time ends. For a
512x512 array, a full-frame would consist of the entire 512X512 pixel area. In the
WinX/32 software, the number of frames to be acquired during a data acquisition is
determined by the Number of Images (or Number of Spectra) parameter on the
Experiment Setup|Main tab page. If the parameter value is greater than 1,
multiple frames of data will be acquired and stored in a single data file.
Note: Kinetics Mode results in multiple images/spectra (subframes) collected on a
single frame
Frame Rate (fps): The number of frames that can be readout per second. The effective
frame rate can be increased by defining a Region of Interest (ROI) that is smaller
than the full-frame size. This means that a selected portion of the image can be
displayed and the remainder of the accumulated charge discarded. The frame rate
generally increases with reduction in the size of the detected area. For example, a
CCD with a sensor size of 1000 x 1000 and an output rate of ten frames/second can
produce 100 frames/second if the read-out region is reduced to 100 x 100 pixels.
Full-Well Capacity: The number of electrons that can be stored in a pixel. The smaller
the pixel, the fewer electrons can be stored; therefore, either the exposure times need
to be shorter or the signal intensity must be lower. Note that binning relies on the
full-well capacity of the pixels in the serial shift register and of the output node.
Typically, a serial shift register pixel has a full-well capacity that is 2 times greater
than that of an image pixel and the output node has a full-well capacity that is about
1.5-2 times greater than that of a serial register pixel.
FWHM: Full width half-maximum. Time period from the mid-point of the leading edge
to the mid-point of the trailing edge of the gate pulse. Used to describe pulse width.
Gate Delay: The time between the beginning of the trigger pulse (either internal or
external) and the beginning of the photocathode gate pulse.
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