Resources: Photoconductive Infrared Detectors and Emitters - Knowledge Base - Glossary of Terms
Background Temperature (TB) °K
The effective temperature of all radiation sources, exclusive of the chosen system source or target, in the detector field of view.
Bias Voltage (VB) volts
The voltage applied to the detector circuit, usually in DC volts. Total bias on CSI test data reports is split between detector and load resistor in series. Split is based on relative resistance match.
- Optimum Bias - The value of bias voltage which yields maximum signal-to-noise ratio.
- Maximum Bias - The value of bias voltage which yields maximum signal.
Cutoff Wavelength (λc) µm
The wavelength at which detector D* has degraded to one half of its peak value.
Cutoff Frequency (fc) Hz
A measure of detector response speed. It is defined as the frequency at which detector signal decreases to 0.707 of its maximum value when it is exposed to a square wave pulse of radiation.
D-Star (D*) cm Hz1/2 W-1
The figure of merit which essentially describes IR detector signal-to-noise ratio (S/N), normalized to a detector active area (Ad) of 1cm2 and a noise equivalent bandwidth Δf of 1Hz. It is also a function of source radiation temperature, chopping frequency (f), detector operating temperature, field of view and background temperature.
where signal (S) and noise (N) are in volts or amps, H is in rms watts/cm2 and is the flux density of source radiation incident on Ad. Unless specified otherwise, detector field of view is 2 steradians and background temperature is 298°K.
- Blackbody D Star (D*BB) - The D* of a detector irradiated by a blackbody source at a given temperature, and expressed, for example, as D* (500°K, 1000Hz, 1Hz), where 500°K is the blackbody source temperature, 1000Hz is the chopping frequency and 1Hz is the noise equivalent bandwidth.
- Peak D Star (D*λpk) - The D* of a detector irradiated by a monochromatic source operating at a wavelength where the detector has its maximum S/N, and expressed, for example, as D*(λpk, 1000Hz,1Hz). D*λpk = (C.F.) D*BB, where (C.F.) is a conversion factor which is a function of detector cutoff wavelength and spectral response. D*λpk is usually not a directly measured parameter in IR detector test data and is calculated using a predetermined C.F. for a given detector chemistry and operating temperature.
Dark Resistance (Rd) ohms
The resistance of the detector, at a given detector temperature, with no incident source radiation.
Element Size and Area (L,W) mm and (Ad) cm2
The length (L) and width (W) of the active area of the detector element. L is the distance between detector electrodes and is expressed first for rectangular elements (e.g. for a 1 mm x 2 mm detector, the distance between electrodes is 1 mm). The active area (Ad) of the detector generates signal and noise, and Ad = LxW.
Element Spacing and Pitch mm
Element spacing is used to describe the non-active distance between active detectors in an array. Pitch is used to denote the center-to-center distance between active detectors in an array.
Element Temperature (Td) °C
The actual detector element operating temperature. For uncooled detectors, this is usually ambient temperature.
Field of View (FOV) Steradians
The solid angular measure within which the detector can respond. Measured from the diagonal of the active area to the limiting aperture.
Load Resistor (RL) ohms
A resistor in the bias circuit which is connected in series with the detector.
Noise Equivalent Power (NEP) Watts
The amount of incident signal radiation (W) required to yield a detector signal-to-noise (S/N) ratio of one. NEP is a function of source temperature, chopping frequency, noise equivalent bandwidth, field of view and background
temperature.
where signal (S) and noise (N) are in volts or amps and W is the incident signal radiant power in watts. It is expressed, for example, as NEP (500°K, 1000Hz, 1Hz) where 500°K is the blackbody source temperature, 1000Hz is the chopping frequency and 1Hz is the noise equivalent bandwidth. Since noise is proportional to the square root of the bandwidth, if normalized to
Rise Time, Fall Time (tr, tf) Seconds
The measure of detector response time to pulses of radiation input. Rise time is the time required to rise from 10% to 90% of the maximum signal output and fall time is the time required to go from 90% to 10%.
RMS Noise (N) Volts rms or Amps rms
The electrical output of the detector with no incident signal radiation on it. It is primarily a function of detector area, background temperature, operating temperature, bias, noise equivalent bandwidth and field of view.
RMS Signal (S) Volts RMS or Amps RMS
The electrical output of the detector due to incident signal radiation. It is primarily a function of chopping frequency, operating temperature, bias, and spectral content/temperature of the source.
Spectral Responsivity (Rλ) Volts/Watts
The ratio of detector signal output to monochromatic incident radiation power. It is mainly a function of detector temperature, wavelength, chopping frequency and bias, and Rλ=S/W where the signal (S) is in volts or amps and W is the incident signal radiant power in watts.
Peak Responsivity (Rλpk)
The responsivity value at which the detector has maximum spectral response, and is expressed, for example as R (pk, 1000Hz) where 1000Hz is the chopping frequency.
Time Constant (τ) Seconds
A term usually used to describe the detector’s speed of response to a square wave pulse of radiation and
