Glossary of Terms

Background Temperature, T_B (Kelvin) – The effective temperature of all radiation sources, exclusive of the chosen system source or target, in the detector field of view.

Bias Voltage, V_B (Volts) – The voltage applied to the detector circuit, usually in D.C. volts. Total bias on Infrared Materials’ 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, λ_c (µm) – The wavelength at which detector D* has degraded to one half of its peak value.

Cutoff Frequency, f_c (Hz) – Is 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 Hz^1/2 W^-1) – The figure of merit which essentially describes IR detector signal – to – noise ratio (S/N), normalized to a detector active area (A_d) of 1 cm² and a noise equivalent bandwidth Δf of 1 Hz. It is also a function of source radiation temperature, chopping frequency (f), detector operating temperature, field of view and background temperature.

D*=√A_d●∆f ●Responsivity = √A_d = S/N ●√∆f/A_d                          N            NEP      H
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 A_d. Unless specified otherwise, the detector field of view is 2 steradians and background temperature is 298 K. 


Blackbody D Star – The D* of a detector irradiated by a blackbody source at a given temperature, and expressed for example, as D* (500 K, 1000 Hz, 1 Hz), where 500 K is the blackbody source temperature, 1000 Hz is the chopping frequency and 1 Hz is the noise equivalent bandwidth. 


Peak D Star – Is 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*(λ_pk, 1000Hz, 1Hz). D*λ_pk = (C.F.) D* BB, where (C.F.) is a conversion factor which is a function of detector cutoff wavelength and spectral response. D*λ_pk 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, R_d (Ohms) – Is the resistance of the detector, at a given detector temperature, with no incident source radiation.

Element Size L,W (mm) and Area, A_d (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 1mm). The active area (A_d) of the detector generates signal and noise, and A_d = L x W.

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, T_d (°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, R_L (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.

 NEP = N ●W

            
 S

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, 1000 Hz, 1 Hz) where 500 °K is the blackbody source temperature, 1000 Hz is the chopping frequency and 1 Hz is the noise equivalent bandwidth. Since noise is proportional to the square root of the bandwidth, if normalized to a ∆f of 1 Hz, then NEP =√A_d/D*


Rise Time, t_r and Fall Time, t_f (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, R_λ (Volts/Watt) – 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 R_λ=S/W where the signal (S) is in volts or amps and W is the incident signal radiant power in watts.

Peak Responsivity – The responsivity value at which the detector has maximum spectral response, and is expressed, for example as R (pk, 1000 Hz) where 1000 Hz is the chopping frequency.