Choosing
a Detector
Sensitivity
Sensitivity to the band of interest is a primary consideration when choosing
a detector. You can control the peak responsivity and bandwidth through
the use of filters, but you must have adequate signal to start with.
Filters can suppress out of band light but cannot boost signal.
Another consideration is blindness to out of band radiation. If
you are measuring solar ultraviolet in the presence of massive amounts
of visible and infrared light, for example, you would select a detector
that is insensitive to the long wavelength light that you intend to filter
out.
Lastly, linearity, stability and durability are considerations.
Some detector types must be cooled or modulated to remain stable.
High voltages are required for other types. In addition, some can
be burned out by excessive light, or have their windows permanently ruined
by a fingerprint.
Silicon Photodiodes
Planar diffusion type silicon photodiodes are perhaps the most versatile
and reliable sensors available. The P-layer material at the light
sensitive surface and the N material at the substrate form a P-N junction
which operates as a photoelectric converter, generating a current that
is proportional to the incident light. Silicon cells operate linearly
over a ten decade dynamic range, and remain true to their original calibration
longer than any other type of sensor. For this reason, they are used
as transfer standards at NIST.
Silicon photodiodes are best used in the short-circuit mode, with zero
input impedance into an op-amp. The sensitivity of a light-sensitive
circuit is limited by dark current, shot noise, and Johnson (thermal) noise.
The practical limit of sensitivity occurs for an irradiance that produces
a photocurrent equal to the dark current (NEP = 1).
Solar-Blind Vacuum Photodiodes
The phototube is a light sensor that is based on the photoemissive effect.
The phototube is a bipolar tube which consists of a photoemissive cathode
surface that emits electrons in proportion to incident light, and an anode
which collects the emitted electrons. The anode must be biased at
a high voltage (50 to 90 V) in order to attract electrons to jump through
the vacuum of the tube. Some phototubes use a forward bias of less
than 15 volts, however.
The cathode material determines the spectral sensitivity of the tube.
Solar-blind vacuum photodiodes use Cs-Te cathodes to provide sensitivity
only to ultraviolet light, providing as much as a million to one long wavelength
rejection. A UV glass window is required for sensitivity in the UV
down to 185 nm, with fused silica windows offering transmission down to
160 nm.
Multi-Junction Thermopiles
The thermopile is a heat sensitive device that measures radiated heat.
The sensor is usually sealed in a vacuum to prevent heat transfer except
by radiation. A thermopile consists of a number of thermocouple junctions
in series which convert energy into a voltage using the Peltier effect.
Thermopiles are convenient sensor for measuring the infrared, because they
offer adequate sensitivity and a flat spectral response in a small package.
More sophisticated bolometers and pyroelectric detectors need to be chopped
and are generally used only in calibration labs.
Thermopiles suffer from temperature drift, since the reference portion
of the detector is constantly absorbing heat. The best method of
operating a thermal detector is by chopping incident radiation, so that
drift is zeroed out by the modulated reading.
The quartz window in most thermopiles is adequate for transmitting from
200 to 4200 nm, but for long wavelength sensitivity out to 40 microns,
Potassium Bromide windows are used.
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Copyright © 1997
International Light, Inc.
Alex
Ryer 26.September.1997