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Fundamentals of Intensified
CCD(ICCD)
The Speaker: Leiting Pan
The
Tutor: Jingjun Xu
TEDA Applied Physics School
Leiting Pan, Nankai University
Content
ICCD.1 Image Intensifier
ICCD.2 Characteristics of Intensified
Cameras
ICCD.3 Photometric Image
ICCD.4 System Components
Leiting Pan, Nankai University
ICCD.1 Image Intensifier
Basics of ICCD Cameras
Intensified CCD camera are equipped with one or more(cascaded)
image intensifier(s) that are mounted in front of the CCD camera either
fiber optically or lens coupled. The image intensifier is gateable and
acts as fast shutter. Its gain is adjustable.
The combination of CCD and image intensifier has several advantages
compared to using only CCD:
Ultimate sensitivity: it is possible to measure
single photons.
UV extended spectral sensitivity down to 200
nm.
The most important: an extremely short
shutter.
Leiting Pan, Nankai University
ICCD.1 Image Intensifier
Basics of ICCD Cameras
The camera head contains three main components: image
intensifier, lens coupling, CCD sensor.
Fig.1 The figure shows a cross section of ICCD with lens coupling between
intensifier and CCD sensor.
Leiting Pan, Nankai University
ICCD.1 Image Intensifier
Image Intensifier
Fig.2 Cross section view of a single stage
proximity focused image intensifier
including operation circuit.
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Input window capable of
transmitting light over the
range near UV visible to
near IR with gateable photo
cathode deposited on its
inner surface.
Micro channel plate
(MCP) to provide electron
gain.
Output window on which
a suitable luminescent
screen (phosphor) in
deposited.
ICCD.1 Image Intensifier
ICCD.1.1 photo cathode
Wavelength Range The wavelength range to which
the tube is sensitive depends on the selection of the
photo cathode material and the input window
material. Within the Lavision ICCD cameras S20 or
S25 photo cathodes and quartz is usually used. The
covered range is typically from 190nm~900nm.
Sensitivity During its lifetime the photo
cathode sensitivity decrease. From manufacturer
side an expected lifetime of >1000h is given
(CW-operation). For gated operation this
lifetime has to be divided by data cycle (gate
time).
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Fig.3 The photo cathode
sensitivity curve for 23493 photo
cathode.
ICCD.1 Image Intensifier
ICCD.1.1 photo cathode
Gating considerations
The intensifier gate is achieved giving a pulse combing from a high
voltage module. The output level if the HV-pulse module is usually
+50V to block the image intensifier and drops to –180V during
exposure time t . Due to this pulse shape photoelectrons escape the
photo cathode only during t ,i.e., the camera is only active
during t .
Gate on pulse
+50V
-180V
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ICCD.1 Image Intensifier
ICCD.1.1 photo cathode
Because of the close proximity of the photo cathode and the entrance
side of the MCP, only relatively small change in photo cathode voltage is
required to prevent the emitted photoelectrons from entering the MCP.
This characteristic, together with the high conductivity of the photo
cathode, allows the intensifier to be gated as quickly as 5ns.
The strength of the applied negative field determines the spatial
resolution of the image intensifier. Higher voltages yield higher spatial
resolution. On the other hand, for very high negative voltages the
electrons generate positive ions on the MCP. These ions are pulled back
to the photo cathode (ion feedback) and may damage it. Therefore, an
optimum voltage is –180V.
The conductivity of S20 photo cathode allows gate times of typically
100ns. To get shorter gates the conductivity of the cathode is increased
by an additional metal layer.this allows a faster charge and discharge of
the photo cathode but with a loss in quantum efficiency. For 18nm and
25nm diameter tubes gate times of 5ns realized.
Leiting Pan, Nankai University
ICCD.1 Image Intensifier
ICCD.1.2 Micro channel plates
Micro channel plates (MCPs) are
compact electron multipliers of high
gain. They have been used in a wider
range of particle and photon detection
systems perhaps more than any other
kind of detector.
A typical MCP consists of about
10,000,000 closely packed channels of
common diameter which are formed by
drawing, etching and so on. Typically,
the diameter of each channel is ~ 10
Fig.4 The diagrammatic
microns. Each channel acts as an
sketch micro channel
independent, continuous dinode
plates
photomultiplier.
Leiting Pan, Nankai University
ICCD.1 Image Intensifier
ICCD.1.2 Micro channel plates
Amplification (Gain) Every electron entering a channel in the MCP collides
with the channel wall and produces secondary electrons. These electrons are
accelerated through the channel by means of the high potential gradient
applied to the MCP and by further collision with the channel wall, produce
further secondary electrons is an electron gain up to 106 – 108. The l/d
parameter (length/diameter) of the channels determines the maximum gain.
A typical value is l/d=40.
Fig.5 the schematic diagram
of the application for the
MCP.
Leiting Pan, Nankai University
ICCD.1 Image Intensifier
ICCD.1.3 Output window/Phosphor screen
The output window consists of a glass or fiber optical base, covered
by the luminescent phosphor. Various phosphor are used. They differ
in their luminescence color, efficiency, delay time and grain size.
The phosphor is covered by a aluminum surface which acts as a
mirror in both directions. This guaranties for a very low
transparency of the image intensifier for two reasons:
It reflects the back emitted light of the phosphor into the
direction of the exit plane.
It reflects residual light from the entrance surface that
passed through the photo cathode and the MCP.
Leiting Pan, Nankai University
ICCD.1 Image Intensifier
ICCD.1.3 Output window/Phosphor screen
Electron-to-light Conversion at the phosphor is realized by the
high potential of 5-6KV referring MCP output. After leaving the
MCP the electron cloud is pulled by this voltage onto the
phosphor. Due to the light electron conversion (at the photo
cathode) and re-conversion (at the phosphor) the color
information of the original image is lost.
Fiber optical coupling VS. Lens coupling By fiber optical
coupling the most effective light transmission from the image
intensifier to the CCD can be realized. The efficiency of lens
coupling depends strongly on the demagnification factor and on
the F/# of the lenses used, but also in worst cased the fiber
coupling is at least 2 times more effective.
Leiting Pan, Nankai University
ICCD.1 Image Intensifier
ICCD.1.3 Output window/Phosphor screen
Phosphor considerations Different types of phosphor are used
within image intensifiers. The selection depends on the phosphor
emission color, its efficiency and in the phosphor delay time. The
table below shows some typical data.
Usually P43 phosphor are used. These have highest efficiency
in combination with a reasonable short decay time.
Leiting Pan, Nankai University
ICCD.1 Image Intensifier
ICCD.1.4 High Voltage Electronics
The operating voltage of the imaging tube is generated by
two special HV-modules.
HV-MCP Supply The HV-MCP supply module generates the
phosphor acceleration voltage of 6KV and the voltage across the
MCP. The gain of the image intensifier tube is set by variation of
the MCP-voltage form 0~900V. The MCP voltage is externally
controlled by a regulation voltage of 0~3V.
Leiting Pan, Nankai University
ICCD.1 Image Intensifier
ICCD.1.4 High Voltage Electronics
HV-Pulse Supply The HV-pulse supply module switches the
voltage between photo cathode and MCP (ground) from +50V
(gate off) to -180V (gate on) with respect to the level of a TTL
signal.
Two different HV-pulse supplies are used by LaVision:
A fast switch model with a minmum gate time of 5ns. Its
maximum repeition rate is 4KHz. This pulser is usually
used within standard ICCD cameras as “Flamestar”.
Another model allows shortest gate of 50ns, but may be
gated with 2MHz within burst mode. This pulser is used
within High speed & time resolved cameras as “Streakstar”,
“speedstar” and IRO module for “FlowMaster”.
Leiting Pan, Nankai University
ICCD.2 Characteristics of ICCD
ICCD.2.1 Camera Sensitivity
Cathode sensitivity The right
graph shows a typical curve of
an S20 cathode. It gives the
photo response in mA/W
(upper trace) as well as the
quantum QE in % (lower
trance). Form the photo
response the quantum
efficiency can be calculated as
QE [%]=Pr[mA/W]*124/ 
(nm)
Fig.6 Typical photo cathode sensitivity for
S20 photo (graph & table).
Leiting Pan, Nankai University
ICCD.2 Characteristics of ICCD
ICCD.2.1 Camera Sensitivity
Sensitivity (MCP voltage) The 2nd
graph shows the photo electron
amplification as function of gain
settting (or MCP voltage):
Nphoton=S[cnts]/Sens[cnts/ph.el]/QE
Example: For a gain setting of
40 and light wavelength of
450nm a signal of 1000counts
corresponds to
N=1000
cnts*(10cnt/ph.el)-1/0.2
=500photons
Leiting Pan, Nankai University
Fig.7 Sensitivity curve. It give the
counts/photo electron versus MCP gain
setting (~MCP voltage).
ICCD.2 Characteristics of ICCD
ICCD.2.2 Noise Consideration
The measurement precision strongly depends on noise. In low
light application we have to consider mainly three aspects:
photon statistical noise of the measured signal, dark noise and
amplification noise on the detection side.
Photo statistical noise known as photonic or photon shot noise, is a
fundamental property of the quantum nature of light. The total
number of photons emitted by a steady source over any time interval
varies according to a Poisson distribution. The electrons generated
by the photo cathode exhibit the same Poisson distribution.
For example, a measured signal of N=100 photo electrons has an
inherent uncertainty of N  10 , the signal-to-noise ratio is
10%.
Leiting Pan, Nankai University
ICCD.2 Characteristics of ICCD
ICCD.2.2 Noise Consideration
The signal-to-noise ratio will not change if the intensifier gain is
changed. Photon (shot) noise is unavoidable and is always present in
imaging system; it is simply the uncertainty in the data.
Fig.7 For low gain (e.g. 0.1
cnt/photo electron) the best
Signal/Noise ratio is measured
(lowest trace in figure). For high
gain the noise is close to the
measured signal. In this case the
camera has almost no dynamic
and can be considered as event
counting device.
Leiting Pan, Nankai University
ICCD.2 Characteristics of ICCD
ICCD.2.2 Noise Consideration
The table below shows the relative error and the detection limit
for different light levels accuracy=SNR
Leiting Pan, Nankai University
ICCD.2 Characteristics of ICCD
ICCD.2.2 Noise Consideration
Intensifier Noise The electron amplification inside the MCP of 2nd .
Gen. intensifiers induces an additional statistic noise that relates to
the Nr. of collision of the electron. A thumb rule estimation gives an
additional noise factor of 2.
Dark Noise The dark image can be subtracted from the measured
image. But its noise component can not be isolated and has to be
taken into account, especially in low light application.
Leiting Pan, Nankai University
ICCD.2 Characteristics of ICCD
ICCD.2.2 Noise Consideration
Random Emission from the photo cathode (EBI) Random
emission from the photo cathode can limit low light detection. The emission is
due to the thermal energy distribution Ekin~kT of the electrons inside the photo
cathode. Some have enough energy to overcome the work function of the
cathode material and are amplified in the same way as photo induced electrons.
This noise signal is expressed as EBI (Equivalent Background Illumination).
If the camera is gated down to the microsecond time scale, this noise is usually
negligible in comparison to photon statistical noise.
The EBI rate is higher for red sensitive photo cathode, because if their lower
the work funciton.
The EBI can be reduced by a factor of 100 by cooling the photo cathode.
Leiting Pan, Nankai University
ICCD.2 Characteristics of ICCD
ICCD.2.2 Noise Consideration
Read Out Noise the final bottle neck of low light detection can be
the read out noise of the CCD, also called preamplifier noise. This
noise is drastically decreased in the slow scan operation of
LaVision ICCD cameras. Still, the read out noise limits low light
detection, whenever it is of the same intensity as the detected signal.
This table lists the
noise sources and
classifies them
into their
relevance.
Leiting Pan, Nankai University
ICCD.2 Characteristics of ICCD
ICCD.2.3 Signal Dynamic of Image Intensifiers
Limited Channel Capacity As outline previously the 2nd gen.
image intensifier realizes its high signal amplification by electron
multiplication inside the MCP. But to transport the image
information through the intensifier tube the MCP channels have a
small diameter of ca. 10um or less. The length of a channel is
typically 400um. Inside these physical dimensions each channel
hold a finite capacity that are available for the amplification of
the photo electron.
At a given MCP voltage a certain electron multiplication V is
realized if N electrons enter a single channel. We have V*N
electron at exit of the MCP channel.
Leiting Pan, Nankai University
ICCD.2 Characteristics of ICCD
ICCD.2.3 Signal Dynamic of Image Intensifiers
If V*N is close to the total capacity of the channel the MCP channel
is discharged! The signal amplification can’t hold for further
electrons, until the system is recharged.
A discharged MCP does not give a linear signal amplification; it
turns out that the discharging of the MCP can influence the signal
registration of ICCD cameras if high intensities are measured！
The recharge time depends on the MCP resistance, which is in the
range of several 10th of M . Test showed, that it takes several ms to
recharge a discharged MCP.
Leiting Pan, Nankai University
ICCD.2 Characteristics of ICCD
ICCD.2.4 Image resolution of ICCD
It is evident that the combination of imaging modules as CCD, image intensifiers
and fiber optical tapers or lenses, for which each components has a finite image
resolution. Must reduce the image resolution compared to CCD alone. Especially
2nd gen. proximity focused image intensifiers have limited resolution of max.
50lp/mm3 (=10um/line). At this resolution the intensifier has a contrast value of
5%.
Fig.8 left image CCD
alone. Right image:
intensified CCD with
2nd gen. intensifier. The
image resolution of left
image is superior.
Leiting Pan, Nankai University
ICCD.3 Photometric Image
Photometric image are characterized by quantitative relations
between the detected intensity and the light signal emitted by the
measured object. Therefore, they are important for scientific
applications.
The image sampled with slow scan cameras consist of high
dynamic and linearity. Furthermore, due to the pixel synchronous
read out all the image information is correctly transferred to the
resulting digitized image. But the images of each CCD show
some characteristics that have to be taken into account for image
analysis:
Dark signal
Photo response non-uniformity
Leiting Pan, Nankai University
ICCD.3 Photometric Image
ICCD.3.1 Dark Image
Apart from the photo charges also thermal charges are created
within the photo elements. These thermal charges are always
produced, also if the camera entry is covered and no photons enter
the camera. Therefore, we call these charges the “dark image”.
They contribute an offset to each acquired image.
The amount of thermal charge is proportional to the integration
time and a highly dependent function of temperature: it doubles
for every 8 to 100 centigrade (above -250C). Therefore, the dark
image is image is reduced by cooling the CCD sensor’s operation
temperature.
Leiting Pan, Nankai University
ICCD.3 Photometric Image
ICCD.3.2 Photo Response Non-Uniformity
Local variations in the different layer thickness or in the
geometry of the pixels generate modifications in the quantum
efficiency along the photosensitive area. For intensified CCD
camera with fiber optical coupling further inhomogeneities due
to chicken wire s and varying thickness of phosphor and photo
cathode material superimpose the CCD inhomogeneities. The
result is photo response non-uniformity.
This non-uniformity can be measured by sampling an image of
uniform illumination.
Leiting Pan, Nankai University
ICCD.3 Photometric Image
ICCD.3.3 Calculation of Photometric Images
A photometric image can be calculated from the measured
image with aid of the two calibration images: the dark image
and a uniformly illuminated reference image. The photometric
image may be calculated by:
I ( x, y ) p  [ I ( x, y ) m  I ( x, y ) d ] I ( x , y ) r   I r 
Ip(x,y)=photometric image Im(x,y)=measured image
Id(x,y)=dark image
Ir(x,y)=reference image
<Ir>=Avg reference image
This calculation must be done for each pixel. It is performed
very fast by the data processing software DaVis.
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ICCD.4 System Components
The whole system is under control if Personal Computer
(PC) via the PCI camera interface board. Image
acquisiton as well as image processing is performed under
control of laVision’s DaVis software.
Fig.9 Cabling between
Camera Head, HRI
controller and computer.
The positions of the
interface boards at the
delivered computer may
differ from this figure.
Leiting Pan, Nankai University
ICCD.4 System Components
ICCD.4.1 Camera Head
The camera head contain three main components: image intensifier,
lens coupling, CCD sensor
Peltier Cooling The CCD is cooled by a two-stage peltier cooler. The
fixed final temperature is appr.-150C.
Water instead if ventilator In extremely critical optical settings
possible vibrations can be prevented. Optionally, the camera is
delivered with water connectors instead of the ventilator. In this
case water circulating through the heat exchanger removes the heat.
Cleaning Method for FOL the connectors and the fiber itself
should be cleaned only by dry dust free air. Again, after
disconnecting, replace the respective protection caps on camera
and cable immediately.
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ICCD.4 System Components
ICCD.4.2 High Rate Image Intensifier Controller
The controller unit which drives the image intensifier head is housed
in a single box. The box contains five section: Low voltage power,
High voltage tube bias supply, Signal conditioning, Gate pulse driver,
Micro controller.
Fig.10 The diagrammatic
sketch High Rate Image
Control Unit
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ICCD.4 System Components
ICCD.4.3 Picosecond Delay Unit
The PS Delay unit provides 20nsec of adjustment in 25psec step.
It may be used at trigger rates>100MHz.
Fig.11 PicoSecond Delay Unit
front and rear panel.
Leiting Pan, Nankai University
ICCD.4 System Components
ICCD.4.4 Computer Hardware
The computer is an IBM
compatible AT-computer
with windows operating
system (95 or NT). It
contains at least 32MB
RAM.
Fig.12 Computer slots with
inputs and outputs.
Leiting Pan, Nankai University
ICCD.4 System Components
Fig.13 The color graph of
the system component.
Leiting Pan, Nankai University
Appendix-GOI module
The Gated Optical Imager (GOI) module is an extension for
PicoStar camera system for shortest gate pulses down to 80ps.
Fig.14 The figure left shows
the camera system with
camera head, HRI and GOI
pulser.
Leiting Pan, Nankai University
THE END
THANK YOU!
Leiting Pan, Nankai University