Princeton Instruments PI-MAX 3 System Manual page 13

Chapter 1
the output of the image intensifier is translated to the input of the CCD at the same size.
After being detected by the CCD, the image is read out to the Controller, where it is
digitized and transferred to the computer for processing via a high-speed data link.
The sequence below steps through the process by which photons are converted to data
that can be displayed on a computer monitor. For the sake of simplicity, triggers and gate
pulses are not mentioned and it is assumed that a high speed (GigE) serial interface card
is installed in the host computer. When reading through the sequence, keep in mind that
electrons are attracted to more positively charged surfaces and are repelled by more
negatively charged surfaces. This principal is used to control electron flow through the
intensifier tube: changing the photocathode voltage with respect to the voltage at the
MCP input is used to switch (gate) the intensifier on and off.
1. Incident photons pass through the intensifier input window, strike the photocathode,
and release electrons. (See Figure 2 above.)
2. Assuming that the intensifier is gated ON (the photocathode is more negative than the
MCP input), these electrons will be attracted to the MCP input. Gating acts like a
shutter in that gating the intensifier on allows the CCD to "see" light and gating the
intensifier off prevents the CCD from seeing light.
3. Since the voltage at the MCP output is much more positive, most of the electrons
accelerate into the MCP channels and, if they hit the channel walls, will generate
additional electrons, resulting in electron gain. The amount of gain is adjusted by
increasing or decreasing the voltage at the MCP output.
4. When the electrons exit the channels they are further accelerated by a constant high
voltage (5-6 kV) and strike the phosphor coating on the fluorescent screen causing it
to release photons. Because of the MCP gain, there are now many photons for each
photon that struck the photocathode surface.
5. The photons released by the coating are transferred to the surface of the CCD (via
fiberoptic or lens) and produce charge at the pixels they strike. Note that fiberoptic
coupling is not only the most efficient coupling possible, but lens-coupling effects
such as vignetting are eliminated.
6. Charge accumulates in the pixel wells until the intensifier is gated off (the
photocathode is more positive than the MCP input).
7. At that point, the accumulated charge is shifted to the serial register where it is read
out to an on-chip amplifier that converts the charge to an analog voltage.
8. This voltage is input to the selected analog-to-digital (A/D) converter(s) where it is
digitally encoded. The conversion speed and the quality of the data are dependent on
the effective ADC rate.
9. The digitized information is transmitted from the camera head through the Ethernet
cable to the interface card in the host computer where it is stored in RAM.
10. The application software retrieves the information from RAM, processes it, displays
it, and/or stores it to a file according to user-defined settings.
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Units having a tapered fiber optic bundle may also be available. Contact the factory for
information.
Introduction 13
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