ICCD and UV Gen 3 intensifiers
UV-Visible-NIR photocathode QE considerations
Composite phosphor coatings - such as "Lumogen" - are routinely used to extend the
UV sensitivity of CCD / EMCCD / CMOS sensors. These phosphors absorb photons
of higher energy, and re-emit radiation at wavelengths close to the maximum QE of the
photocathode. A similar approach has been used successfully to extend the UV sensitivity
of Gen 3 image intensifiers down to less than 200 nm.
Generation 3 ('Gen3') photosensitive elements ('photocathodes')
offer great Quantum Efficiency (QE) up to 50% in the visible
range and good NIR sensitivity compared to Gen 2 devices. Gen 3
photocathodes can technically only be deposited on glass substrates
or glass fibre optics plates, as this material presents the only suitable
match to the photocathode thermal expansion coefficient. Gen 3
image intensifiers have therefore no sensitivity below ~ 350 nm due
to the transmission cut-off of the Glass interface.
The present technical note explores the impact of the addition of a
phosphor interface in front of Gen 3 image intensifiers in the Visible-
NIR region, weighting pros and cons to such approach.
1. Image Intensifier construction
Originally developed for night vision, image intensifiers act as ultrafast
(nanosecond-scale) optical shutters, as well as providing signal
amplification up to x1,000 that proves invaluable to the world of
research for studies of shortlived events, e.g. fluorescence decay time
measurements, plasma or combustion kinetics.
Image intensifiers consist of an evacuated tube comprising of a
photocathode, a Micro-Channel Plate (MCP) and a phosphor screen.
In brief, an incoming photon strikes the photocathode deposited
on the inside of the input window, and a photo-electron (under a
wavelength-dependant probability) is generated. This photo-electron
is then drawn towards the MCP by an electric field. Through the
glass channels of the MCP honeycomb structure, the incoming
photo-electron is accelerated to gain enough energy to dislodge
secondary electrons, which in turn will generate further electrons in a
cascade manner. At the output of the MCP, the photo-electron cloud
hits a phosphor which converts this signal back to photons detectable
by the CCD/CMOS device.
2. Construction of a UV-sensitive Gen 3 image intensifier
UV sensitivity (below 350 nm) for Gen 3 image intensifier is
achieved by adding a phosphor interface in front of the traditional
image intensifier build, as shown on figure 2.
The photocathode substrate (input window) is replaced by a fibre
optic plate in order to direct efficiently the up-converted radiation
from the 'UV phosphor' towards the photocathode.
The 'UV' phosphor (typically 'Lumogen' type) is deposited on the
input fibre optic plate, and a further protective MgF2 window is used
to protect the 'UV phosphor' from any degradation due to exposure
to ambient air and contamination.
3. Key optical characteristics considerations for 'UV phosphor'
A simplified view of the optical interface preceding the
photocathode is shown on the exploded view in figure 3.
a) Considering the transmission of the different components
involved, the first optical element in the optical path is the protective
MgF2 window. Figure 4 shows the typical transmission of such
window, which is ~ 95% over the 200 to 900 nm range of
interest for the present discussion.
b) The typical Excitation and Emission characteristics of the 'UVphosphor'
used for Gen 3 image intensifiers are shown on figure 5a
and b. Figure 5b shows the characteristics of a proprietary 'UVphosphor'
- known as 'Unichrome' - used by Princeton Instrument
for this type of Gen 3 image intensifiers with UV response.
'UV-phosphor' characteristics represented on figure 5a and 5b
show absorption characteristics from 200-500 nm and 200-400 nm
respectively, while exhibiting peak emission at 550 nm and 440 nm
c) The third key element to consider is the replacement of the glass
photocathode substrate on the standard Gen 3 intensifier by a fibre
optic plate. Figure 4 shows that the transmission of fibre optics plate
in the re-emission region of the two ‘UV phosphors’ considered
above is 25% lower than the traditional glass window, i.e sitting at ~
65 % above 400 nm.
The last element to consider is the Gen 3 photocathode itself.
Different flavours of Gen 3 photocathodes are available (GaAs,
GaAsP, GaAsP enhanced red ...) with various wavelength coverage
and peak efficiency. Given the absorption profiles of the phosphors
shown above, the photocathode of choice for this UV-sensitive
Gen 3 is the GaAs substrate, which typical QE as given by
manufacturer is shown on figure 6. Peak QE is shown to be 30% @ 650 nm
Over the GaAs photocathode useful QE range :
- MgF2 window has a transmission of ~95 %
- Fibre optic window has 25% lower transmission than traditional
glass substrate on which Gen 3 photocathodes would be
Assuming - for simplification - complete transparency of the
phosphors to VIS and NIR radiation, and no transmission impact
from the gluing interface between MgF2 window & phosphor,
and phosphor & fibre-optic plate, one should therefore expect the
QE ('UV'-sensitive photocathode, 650 nm)
QE (GaAs photocathode on glass, 650 nm)
Fibre-optic transmission impact vs. glass substrate
= 30 x 0.95 x 0.75
= ~21 % (from 30%)
4. Extended impact on Quantum Efficiency in the Visible / NIR
Figures 7 present the overall expected impact on Quantum
Efficiency in the Visible and Near-Infrared-only based on the
summary assumptions and conclusions in paragraph 3.