Background
Temperature
The effective temperature of all radiation sources viewed by the
detector, excluding the signal source.
Bias Voltage
The voltage applied to the detector circuit, normally DC volts;
sometimes called optimum bias for values which give optimum
signal-to-noise ratios and maximum bias for values which produce
the maximum signal voltage output, it is called reverse bias
when applied to the P-N junction of solid crystal detectors in a
reverse mode to increase the speed of response or to increase
the long wavelength response.
Dark Current
The measured current in a detector circuit when operated with no
signal radiation incident on the detector element
Dark Resistance
The ratio of the DC voltage across the detector to the DC
current through it when no radiation is incident on the
detector.
D-Star (D*)
A relative measure of sensitivity used to compare the detecting
capabilities of different detectors. D* is the signal-to-noise
ratio at a specific electrical frequency with a 1 Hz bandwidth
when radiant power is incident on the detector active area.
Load Resistor
A resistance element that is in series with the detector element
and bias voltage; typically matched to the detector's dark
resistance
Noise-Equivalent-Power (NEP)
The amount of required signal radiant power on the detector
element area to yield a signal-to-noise ratio of one, and
indicates the minimum detectable radiation level; the smaller
the NEP value, the better the performance.
Photoconductive Detector
A photon detector which exhibits increased conductivity with
incident radiant power.
Photovoltaic detector
A photon detector with a p-n or p-i-n junction which converts
radiant power directly into electrical current; also called a
photodiode.
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Resistivity
The square areas resistance of a thin film detector, where L and
W are equal; L being the separation between the electrodes and W
is the length of the detector active area. Resistivity is a
function of the detector element temperature and the level of
irradiance.
Responsivity
A value indicating signal output from radiation incident on the
detector element. The value where the detector has a maximum
spectral response is called peak responsivity. It is a function
of detector area, wavelength, and circuit parameters.
Rise Time-Fall Time
Rise time and fall time are the times in seconds required for
the signal response to rise from 10% to 90% and fall from 90% to
10% of the maximum observed signal value. This happens when
detectors are exposed to pulses of signal radiant power.
RMS Noise Voltage or Current
The element of the electrical output (voltage or
current) which is incoherent with the signal radiant power,
usually measured with no signal radiation incident on the
detector element and is related to the detector area. It is the
function of frequency response, noise equivalent bandwidth,
operating temperature, other circuit parameters such as the load
resistor, and in some cases detector solid angle and background
temperature.
RMS Signal Voltage or Current
The element of the electrical output (voltage or current) which
is coherent with the monochromatic or blackbody input signal
radiant power. It is a function of electrical frequency, signal
power, spectral characteristics, operating temperature, and
other circuit parameters such as the load resistor and bias
voltage.
Spectral Response
Most of the time this is shown as D* vs. Wavelength, usually
presented as a graph showing relative signal as a function of
wave length of the incident radiant power.
Time Constant
A measurement of a detector's speed of response when the
detector is exposed to a square wave pulse of radiation. The
rise time constant is the time required for the signal voltage
to reach 0.63 times its asymptotic value. The decay time
constant is the time required for the signal voltage to decay to
0.37 of the asymptotic value. This can also be measured by
determining the chopping frequency at which the signal response
is 0.707 of its maximum value. |
