Glossary of Terms

Background Current (IBG)
The DC current produced by background radiation on a photovoltaic detector. Measured across a low impedance load.

Bias Current (IB)
The constant current applied across a photoconductive detector for proper operation.    

Blackbody (BB)
An ideal radiator whose radiant emittance, W, follows the Stefan-Boltzmann law.  The relationship is shown below.

BLIP
A detector is referred to as BLIP (background limited) when its D* is limited by the noise associated with photons from the background radiation and not by intrinsic detector noise.

Cutoff Wavelength (lco)
The long wavelength point at which the detector responsivity has fallen to a specified percent of the peak responsivity.  Cutoff wavelength is usually specified at 20% or 50% of peak.

D-Star (D*)
A relative sensitivity parameter used to compare performance of different detector types. D* is the signal-to-noise ratio at a particular electrical frequency and in a 1 Hz bandwidth when 1 Watt of radiant power is incident on a 1 cm2 active area detector. The higher the D* value, the better the detector.

D*(cmHz1/2W-1) =  [Active Area (cm2)]1/2 / NEP(W/Hz1/2)

Dark Current (ID)
The current through a photodiode when a specific reverse bias voltage is applied, with no incident radiant power. Also referred to as Reverse Current.

Johnson (Thermal) Noise
When operated in the PV mode at 0 volt bias, a photodiode will generate Johnson noise current due to its shunt resistance (RD) according to:

i2 =   4KTDf / RD

where K is the Boltzman constant, T is temperature in °K, Df is the noise measurement bandwidth and RD is the detector shunt resistance.

Junction Capacitance (CD)
The p-n junction of a photovoltaic detector has a capacitance proportional to the active size.

Maximum Reverse Voltage (VR)
A photovoltaic detector can be damaged by applying more than the maximum reverse voltage VR.

NEP (Noise Equivalent Power)
The radiant power that produces a signal-to-noise ratio of one at the output of the detector. Defined with respect to a particular chopping frequency, wavelength and effective noise bandwidth.

NEP @ lp =  Noise (A/Hz1/2) / Responsivity @ lp (A/W)

Open Circuit Voltage (Voc)
The DC voltage generated by a photovoltaic detector when connected to a high impedance load.

Photoconductive Detector (PC)
A photon detector which exhibits an increase in conductivity as a function of radiant power.

Photovoltaic Detector (PV)
Any photon detector utilizing a p-n or p–i–n junction to convert radiant power directly into electric current. Also referred to as a photodiode.

Responsivity (R)
The detector photocurrent (or voltage) output per unit incident radiant power at a particular wavelength. Expressed in units of amps/watt (or volts/watt).

Shot Noise
Random generation of current in a diode results in Shot Noise according to:

i2 = 2qIDf

where I is the total current, q is the electric charge and Df is noise bandwidth.

Shunt Resistance (RD)
The resistance of a photodiode measured at zero-volt bias with no radiant power incident on the detector. RD is the slope, dV/dl, of the current-voltage curve at zero-volt bias. Also referred to as Dynamic Impedance.

Time Constant (t)
The time required for the detector signal to reach 63% of its final value after the onset of a fixed incident power. Related to cutoff frequency (fc) as follows:

t = 1 / 2pfc

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