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The
purpose of a photomultiplier tube (PMT)
is to measure very weak light.
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Photomultiplier
tube
The
purpose of a photomultiplier tube (PMT) is to
measure very weak light. The tube multiplies the
effect of the light that strikes it and converts
photons of light into electrical signals so that
the light can be precisely measured.
Each
photon (bit of light) strikes a photocathode,
ejecting an
electron. The electrons are accelerated toward
a secondary electrode called a dynode, which is
held at a more positive potential so that each
electron gains enough energy to eject several
electrons from the dynode. This is the electron
"multiplier." By using a series of dynodes,
the PMT creates a cascading effect-the system
creates 100,000-10,000,000 electrons for each
photon hitting the first cathode. The amplified
signal can be collected and measured at the end.
Fluorometer
A fluorometer measures the intensity of
fluorescing molecules in a sample of blood
or body tissue. Individual molecules are too
small to be seen. However, when hit with ultraviolet
light, these same materials can be seen with
a fluorometer, because now the substances
fluoresce, or glow. Scientists can use the
fluorescent properties to see things that
are otherwise too small to be visible.
The
fluorometer measures the amount of fluorescent
radiation produced by a sample when the
sample is exposed to monochromatic light.
Light focuses in on the cuvette.
In early fluorometers, fluorescent light
entered the vacuum phototube through the
secondary filter at right angles to the
exciting light. The photocurrent is recorded
by an electric amplifier connected to a
galvanometer, which detects and measures
a small electric current by movements of
a magnetic needle of coil in a magnetic
field.
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A
fluorometer measures the intensity of fluorescing
molecules in a sample of blood or body tissue.
The Coleman filter fluorometer measures
the total fluorescence emitted by all materials
in a sample.
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