Appendix H Glossary 221
Gate Mode: PI-MAX intensifier mode in which the photocathode is biased on only for
the time that each gate pulse is applied. In this way, the array can be exposed to
multiple images during a single exposure time. As a result, the tolerance to room
light is higher in gated operation, but the risk of damaging overload from intense
light sources such as lasers remains. In fact, intense light sources in gated
experiments can cause spot damage that would be undetected by the alarm circuit.
Gate Width: The time during which light will be detected by the intensifier, intensified,
and applied to the CCD. Basically, the intensifier controls what the chip 'sees' during
the exposure time. For signal to be detected, it must both fall in a valid gate width
and in a valid exposure time.
Input Windows: The intensifier and the CCD array both have input windows.
MgF
2
: High vacuum UV transmission (between 100 nm and 200 nm).
Quartz: Excellent transmission over 190 nm - 1100 nm.
Clear glass (BK7): Visible (400 nm -700 nm) and NIR (700 nm - 2500 nm).
Anti-Reflection (A/R) coatings may be added to input windows to reduce signal loss and
glare caused by reflection.
Intensifier-CCD Coupling: Transmission of the emitted photons is either through a
fiberoptic bundle or with a lens. The drawback to lens coupling is lower throughput
(5%-10%) and increased stray light in the camera system. The advantages are that the
intensifier can be removed and the camera can be used as a standard CCD imager
conversely an intensifier can be added to an existing camera.
Fiberoptic coupling results in a throughput of >60%, are capable of sensitivities
approaching single-photoelectron detection, and have a much better signal-to-noise
ratio (SNR) than lens-coupled devices. Disadvantages are that the fiberoptic bundle
is permanently attached to the CCD array and that the camera must be operated in a
dry, non-vacuum inert environment.
Intensifier Gating Speed: Temporal resolution in a PI-MAX is made possible by
switching the intensifier on and off (gating) very rapidly. Typical fast-gate
intensifiers have minimum gate widths (FWHM=full width at half-maximum gate
pulse) of approximately 2 nsec. For slow-gated devices the FWHM is about 50 nsec.
Fast-gating is achieved by adding a nickel (Ni) underlayer to photocathode. However,
this layer may produce an effective QE reduction of as much as 40%. Slow-gate
intensifiers have neither the Ni layer nor its effects on QE.
Intensifier On/Off Ratio: The ratio of light output when the intensifier is gated on and
off: The higher the ratio, the better the gating. A high on/off ratio is necessary to
eliminate the background and to faithfully reproduce transient events. In the visible
region on/off ratios exceeding 10
6
:1 is typically achieved. In the UV region, the
on/off ratio is typically much poorer (10
4
:1) though with MCP Bracket Pulsing (see
page 85) ratios in the UV region can be improved dramatically (10
7
:1).
Intensifier Size: 18 mm dia. and 25 mm dia. Generally speaking, the larger diameter
gives a larger field of view at the surface of the CCD array. The coupling of the
intensifier to the CCD array is also a factor in determining the field view. A
fiberoptic reducing taper of 1.27:1 will increase the field of view, while a taper of 1:1
will have no effect.
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