Princeton Instruments PI-MAX 3 System Manual page 139

Appendix H
Photocathode Coatings: Coatings on the photocathode convert a portion of the incident
photons into electrons. Any photons that are not captured by the photocathode are
lost from the final signal produced by the intensifier. Therefore, the kind of coating
and the resulting QE of the photocathode is very important. The choice of coating
determines the most effective spectral range for the intensifier. For example GaAs
(gallium arsenide) has high QE in the VIS and NIR regions. Multi-alkali coatings
have fair photoconversion in the visible (VIS) and ultraviolet (UV) but have
relatively limited response in the near IR (NIR).
Pulse Ensemble: PTG term. Consists of a Gate Start pulse, a Gate Stop pulse, and an
Auxiliary pulse. At the end of the ensemble, the photocathode is gated off, phosphor
decay time elapses to allow for phosphor decay, and then the CCD array is readout.
QE: Quantum Efficiency. The percentage of incident photons converted to electronic
charge. The throughput of the input windows, the spectral sensitivity of the
photocathode and the CCD array, the illuminated surface (front or back) of the CCD
array, the intensifier on/off ratio, the MCP resolution, the MCP gain, and the
intensifier-CCD coupling all contribute to the total system QE.
RAM: Random Access Memory used to store data such as experiment parameters.
Region of Interest (ROI): A square or rectangular set of contiguous pixels on the CCD
array that is usually smaller than the full frame. Using an ROI to acquire data results
in a faster readout of the array since data from pixels outside of that ROI is discarded.
Safe Mode: PI-MAX3 intensifier mode in which the photocathode is continuously biased
off.
Saturation: Caused when a pixel well is completely filled with charge. Once a pixel is
saturated, additional charge will spillover (bloom) into adjacent pixels. Ways to deal
with saturation include lowering the array temperature (to reduce the dark charge
component), shortening the exposure time (to reduce the signal component), and
decreasing the gain (also to reduce the signal component).
Scan or Scanning: The process of reading out the contents of a CCD array.
Vertical Shift Time (µs): Reports the speed (in microseconds) at which a single row will
be shifted vertically. This information is based on the value in the Vertical Shift box.
The higher the value in that box, the longer the vertical shift time. This information
appears for the Frame Transfer.
Vertical Shift: Determines the speed of the image transfer from the exposed area of an
array to the masked area. Setting a lower value increases the shift speed. A higher
value gives a slower shift. If the shift is too fast, not all of the charge will be
transferred. If too slow, image smearing will be increased due to the exposure that
takes place while the transfer is in progress. The default value gives good results in
most measurements.
Glossary 139