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TERMINOLOGY

Photocathode
A photoelectric surface which emits photoelectrons in
response to light. Different spectral response characteristics
can be selected by an appropriate combination of the photo-
cathode and window materials.

Photocathode Sensitivity
Luminous: The photoelectric current from the photocathode
per the input light flux from a tungsten lamp at 2856K. This is
expressed in microamperes per lumen (µA/lm)
Radiant: The photoelectric current from the photocathode per
the power of the input monochromatic light. This is expressed
in milliamperes per watt (mA/W) at a wavelength of interest.

Spectral Response
The photocathode sensitivity varies with wavelength. This rela-
tionship between sensitivity and wavelength is called the spec-
tral response characteristic.

Phosphor Screen
A screen which emits light when bombarded by electrons.

Fiber Optic Plate
Fiber optic plates are optical image transmission elements of
bundles of fine optical fibers in a parallel array.

MCP (Microchannel Plate)
An MCP is a secondary electron multiplier consisting of an
array of millions of glass capillaries (channels) fused into the
from of a thin disk 0.48 mm thick and 25 mm in diameter. The
diameter of each channel is 12 µm. As shown in the figures
below, when an electron enters and hits the channel wall, sec-
ondary electrons are produced from the channel wall. These
electrons are then accelerated by the potential gradient and
strike the opposite wall to produce additional secondary elec-
trons. As a result of these collisions repeated many times
along the channel, a large number of electrons are produced,
up to several thousand times the input number.
An MCP contains approximately 1.5 million channels, each of
which corresponds to a picture element and is simultaneously
intensified.

Illuminance
This is the luminous flux density on a photocathode, and is usu-
ally expressed in units of lumen per square meter (lm/m

). 1

lm/m

equals to 1 lx.

Photocathode Illuminance and Object Illuminance
When viewing a perfect diffusion surface of reflectivity R using
a lens of brightness (F-number) F

and transmittance TL, the

photocathode illuminance E is given by the following formula.

where E

is the object illuminance.

When F

= 2.8, T

= 0.6 and R = 0.5, the ratio of the photo-

cathode illuminance is:

Resolution
The ability to project a fine, clear image on a monitor screen. If
resolution is low enough, the output image shades off. Resolu-
tion is expressed in how many line pairs of thin black and white
stripes, whose pitch is predetermined, can be seen in 1 mm.
Each line pair is expressed in lp, so the unit is lp/mm.
In the case of TV, black and white stripes are counted individu-
ally so they are not called a line pair but called TV lines as the
unit. TV resolution is divided into horizontal resolution and verti-
cal resolution. The number of stripes is counted with respect to
the vertical height of a TV screen as the vertical resolution.
The pitch of these stripes is simply used as the horizontal reso-
lution.
The limited number of TV lines to which those lines become
invisible as stripes is called limited resolution.
Spatial resolution of an image intensifier changes according to
incident light level to a photocathode as shown in the figure
below. When a photocathode illuminance is below 10

-4

lx, even

if a gain of the image intensifier is raised, light spots on a
phosphor screen becomes brighter, but spatial resolution is not
improved. For improvement of spatial resolution, it is essential
to increase incident light to photocathode by using a brighter
input optics. At an illuminance darker than 10

-5

lx, it is effective

to use photon counting imaging using C5909-08 with two
MCPs to capture the output image with high resolution.

CHANNELS

CHANNEL WALL

INCIDENT
ELECTRON

OUTPUT
ELECTRON

Structure of MCP

Resolution Characteristic

Multiplication Principle of MCP

TMCPC0002EC

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