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photocathode. This dim light consists of energy in both the visible
and near-infrared portions of the electromagnetic spectrum.
2. When the light impinges on the photocathode (the light sensitive
portion of the image intensifier) electrons are emitted with
amplitude determined by the photocathode's spectral responsivity
and the amount of light energy. Because of the image intensifier's
internal electrical field, these electrons are accelerated toward the
microchannel plate assembly, the image intensifier's amplification
mechanism.
3. The microchannel plate is a glass plate with millions of tiny
closely-spaced channels bored through it. The plate is coated with a
special substance that produces secondary electron emission when
impinged by an electron. Due to the potential difference across the
plate, an incident electron enters a channel and frees other electrons
from the channel wall. These electrons are accelerated along the
channel in turn striking the channel surface again and again, giving
rise to more and more electrons. Eventually this cascade process
yields a cloud of several thousand electrons, which emerge from the
rear of the plate.
4. Electrons exiting the microchannel plate strike a phosphor that
emits light proportional to the amount of electrons hitting it. The
image is green because the selected phosphor glows green when
charged. The green color is selected because the human eye can
differentiate more shades of green than any other color.
5. Because the image is inverted, a fiber optic "twister" is used to
rotate the image 180°.
6. A special relay optic focuses the image properly to match the
image plane requirements of video and 35mm SLR cameras.
Image intensifiers exhibit several key features designed to optimize
their use, as follows:
Automatic Brightness Control (ABC): An electronic feature that
automatically reduces voltages to the MCP to keep the image
intensifier’s brightness within optimal limits and protect the image
intensifier. The effect of this can be seen when rapidly changing
from low-light to high-light conditions; the image gets brighter and
then after a momentary delay, suddenly dims to a constant level.
Bright-Source Protection (BSP): An electronic function that
reduces the voltage to the photocathode when the image intensifier
is exposed to bright light sources such as room lights or car lights.
It protects the image intensifier from damage and enhances its life.
Equivalent Background Illumination (EBI): This describes the
amount of light you see from the output of the image intensifier
when there is no light input. It increases with increasing
temperature. EBI is measured in lumens per square centimeters and
determines the light level, which equals the image intensifier’s
noise level. Below this light level, objects will be masked by EBI.
The lower value of EBI, the better.