​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.
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 Frequency (fc)
Related to time constant as follows: fc = 1 / 2pt
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