v10308u Hamamatsu Photonics, K.K.,, v10308u Datasheet - Page 4

• Manufacturer Part Number: v10308u
• Description: Image Intensifiers
• Manufacturer: Hamamatsu Photonics, K.K.,

3
GSTRUCTURE AND OPERATION
GSTRUCTURE AND OPERATION
Figure 1 shows the structure of a typical image intensifier. A photocathode
that converts light into photoelectrons, a microchannel plate (MCP) that
multiplies electrons, and a phosphor screen that reconverts electrons into
light are arranged in close proximity in an evacuated ceramic case. The
close proximity design from the photocathode to the phosphor screen
delivers an image with no geometric distortion even at the periphery.
Types of image intensifiers are often broadly classified by "generation".
The first generation refers to image intensifiers that do not use an MCP
and where the gain is usually no greater than 100 times. The second
generation image intensifiers use MCPs for electron multiplication. Types
using a single-stage MCP have a gain of about 10000, while types using
a 3-stage MCP offer a much higher gain of more than 10 million.
A variety of photocathodes materials are currently in use. Of these,
photocathodes made of semiconductor crystals such as GaAs and
GsAsP are called "third generation". These photocathodes offer extremely
high sensitivity. Among the first and second generation image intensifiers,
there are still some inverter types in which an image is internally inverted
by the electron lens, but these are rarely used now because of geometric
distortion.
Figure 1: Structure of Image Intensifier
Figure 2 shows how light focused on the photocathode is converted into
photoelectrons. The number of photoelectrons emitted at this point is
proportional to the input light intensity. These electrons are then
accelerated by a voltage applied between the photocathode and the MCP
input surface (MCP-in) and enter individual channels of the MCP. Since
each channel of the MCP serves as an independent electron multiplier,
the input electrons impinging on the channel wall produce secondary
electrons. This process is repeated hundreds of times by the potential
gradient across the both ends of the MCP and a large number of
electrons are in this way released from the output end of the MCP. The
electrons multiplied by the MCP are further accelerated by the voltage
between the MCP output surface (MCP-out) and the phosphor screen,
and strike the photocathode which emits light according to the amount of
electrons. Through this process, an input optical image is intensified about
10 000 times (in the case of a one-stage MCP) and appears as the output
image on the phosphor screen.
Figure 2: Operating Principle
An image intensifier can be gated to open or close the optical shutter by
varying the potential between the photocathode and the MCP-in. Figure
3 shows typical gate operation circuits.
When the gate is ON, the photocathode potential is lower than the MCP-
in potential so the electrons emitted from the photocathode are attracted
PHOTOCATHODE
(PHOTONS
LOW-LEVEL
LIGHT IMAGE
INPUT WINDOW
PHOTO-
CATHODE
(LIGHT
PHOTOELECTRONS)
MCP
(ELECTRON
MULTIPLICATION)
ELECTRONS)
INPUT
WINDOW
OPERATING PRINCIPLE
GATE OPERATION
VACUUM
MCP(ELECTRON MULTIPLICATION:
1000 to 10000 TIMES)
STRUCTURE
PHOSPHOR SCREEN
ELECTRONS
OUTPUT
WINDOW
(FIBER OPTIC
PLATE)
PHOSPHOR SCREEN
(ELECTRONS
OUTPUT WINDOW:
FIBER OPTIC PLATE
(ELECTRON
INTENSIFIED
LIGHT IMAGE
LIGHT)
PHOTONS)
TII C0046EA
TII C0051EB
by this potential difference towards the MCP and multiplied there. An
intensified image can then be obtained on the phosphor screen.
When the gate is OFF however, the photocathode has a higher potential
than the MCP-in (reverse-biased) so the electrons emitted from the
photocathode are forced to return to the photocathode by this reverse-
biased potential and do not reach the MCP. In the gate OFF mode, no
output image appears on the phosphor screen even if light is incident on
the photocathode.
To actually turn on the gate operation, a high-speed, negative polarity
pulse of about 200 volts is applied to the photocathode while the MCP-in
potential is fixed. The width (time) of this pulse will be the gate time.
The gate function is very effective when analyzing high-speed optical
phenomenon. Gated image intensifiers and ICCDs (intensified CCDs)
having this gate function are capable of capturing instantaneous images
of high-speed optical phenomenon while excluding extraneous signals.
Figure 3: Gate Operation Circuits
Gate ON at point (a)
Gate OFF at point (b)
SIT (silicon intensified target) cameras and image intensifiers using a one-
stage MCP have been used in low-light-level imaging. However, these
imaging devices cannot capture a clear image when the light level is lower
than 10
analog quantity is difficult due to limitations by the laws of physics, but
detecting light by counting photons is more effective. Image intensifiers
using a 3-stage MCP are ideal for photon counting.
Image intensifiers with a 3-stage MCP can be considered high-sensitivity
image intensifiers. However, these have two operation modes, one of
which is completely different from normal image intensifier operation. At
light levels down to about 10
operate in the same way as normal image intensifiers by applying a low
voltage to the MCP. A continuous output image can be obtained with a
gray scale or gradation. This operation mode allows the 3-stage MCP to
provide a lower gain of 10
On the other hand, when the light intensity becomes so low (below 10
that the incident photons are separated in time and space, the
photocathode emits very few photoelectrons and only one or no
photoelectrons enter each channel of the MCP. Capturing a continuous
image with a gradation is then no longer possible. In such cases, by
applying about 2.4 kV to the 3-stage MCP to increase the gain to about
10
corresponding to individual photoelectrons will appear on the output
phosphor screen. The gradations of the output image are not expressed
as a difference in brightness but rather as differences in the time and
spatial density distribution of the light spots. Even at extremely low light
levels when only a few light spots appear per second on the output
phosphor screen, an image can be obtained by detecting each spot and
its position, and integrating them into an image storage unit such as a still
camera and video frame memory. The brightness distribution of this
image is configured by the difference in the number of photons at each
position. This operation is known as photon counting mode.
Since image intensifiers using a 3-stage MCP can operate in both analog
mode and photon counting mode, they can be utilized in a wide spectrum
of applications from extremely low light levels to light levels having motion
images.
6
, light spots (single photon spots) with approximately a 60 µm diameter
0
-5
0
PULSE
GENERATOR
PULSE
GENERATOR
GATE ON
PULSE
(a)
lx. At such extremely low light levels, detecting light as an
PHOTOCATHODE
PHOTOCATHODE
(b)
V
G
LIGHT
PHOTON COUNTING MODE
–200 V
+30 V
LIGHT
PHOTOELECTRONS
C
C
R
R
PHOTOELECTRONS
V
V
B
2
B
to 10
0 V
0 V
MCP
V
-4
V
ELECTRONS
MCP
MCP
MCP
lx, these 3-stage MCP image intensifiers
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and is called "analog mode".
PHOSPHOR SCREEN
V
V
S
S
PHOSPHOR
SCREEN
LIGHT
ex.: V
V
B
G
= +30 V
= -230 V
V
V
V
V
MCP
S
B
G
......MCP-in TO MCP-out VOLTAGE
......MCP-out TO
......BIAS VOLTAGE
......GATE PULSE
PHOSPHOR SCREEN VOLTAGE
TII C0047EA
TII C0048EA
-5
lx)