- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
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- Dark Current
- The measured current in a detector circuit when operated with no signal radiation
incident on the detector element.
- 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.
- Background Temperature
- The effective temperature of all radiation sources viewed by the detector, excluding the
signal source.
- 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.
- Load Resistor
- A resistance element that is in series with the detector element and bias voltage;
typically matched to the detector's dark resistance.
- Open Circuit Voltage
- A DC voltage produced by a photovoltaic detector when connected to a high impedance
load.
- 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.
- 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.
- Cut-off Wavelength
- The long wavelength point where the detector responsivity has degraded to a specified
percent of the peak responsivity; usually 20% or 50% of the peak responsivity.
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