Photonics Glossary

Find a comprehensive and useful glossary section explaining technical terms related Photonics


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.

Beam Angle – Generally specified as the off-axis angle where the output power drops to 50% of the peak value. Can be specified from 50% to 50% point, or peak to 50%. Generally speaking, if the value is referred to as Half Intensity Beam Angle or FWHM, the value is from 50% to 50% points.

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 the detector and load resistor in series. The 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.

Candela – A measurement of luminous intensity. Visible LEDs are usually specified in Candela (cd) or millicandela (mcd). The angle of measurement is critical when comparing lensed (narrow beam) products from different vendors. Value is pegged to the human eye response, making the peak wavelength a critical factor in the final value. Infrared LEDs have a value of nearly zero, because they do not emit appreciable levels of visible light.

Centroid Wavelength – The wavelength value where half of the light energy is at shorter and half the energy is at longer wavelengths. Value is stated in nanometers (nm) or microns (µ). This value is of interest to people in the test and measurement industries. Not commonly specified for standard LED products of any wavelength.

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 Current  Usually abbreviated as ID. The current flowing through a reverse biased photodiode when light is not incident upon a photodiode. Higher reverse bias voltages result in higher dark currents. Dark current is not present in zero photodiode bias circuits (see shunt resistance).

Dark Resistance (Rd) ohms – The resistance of the detector, at a given detector temperature, with no incident source radiation. 

Dominant Wavelength – The color, or perceived wavelength of a light source by the human eye. Also called the hue wavelength. Most visible LEDs are specified by the dominant wavelength.


Element Size and Area (L,W) mm and (Ad) cm 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. 

Full-Width-Half-Max – Usually abbreviated as FWHM. Used most commonly when discussing beam angle or spectral bandwidth. In both cases it refers to the distance from 50% to 50% point, or and –3db to and –3db point. Beam angle value is specified in degrees and spectral bandwidth values are specified in nanometers.


Load Resistor (RL) ohms – A resistor in the bias circuit which is connected in series with the detector. 


Junction Capacitance – P-N junctions have an inherent capacitance similar to a parallel plate capacitor. The junction capacitance is proportional to the active area of the semiconductor. In the case of photodiodes, the junction capacitance can be reduced by reverse biasing it. The junction capacitance in conjunction with the inherent series resistance of the diode is not the limiting factor for the response time of the device (see response time).


Light Emitting Diodes – Commonly abbreviated as LED, LEDs, or IRLEDs in the case of infrared light emitting diodes. Refers to any diode which when forward biased converts electrons (electrical current) to photons (light) in a non-coherent waveform.

Load Resistor (RL) ohms – A resistor in the bias circuit which is connected in series with the detector. 

Lumens – A measurement of total visible energy emitted from a point source. The output is measured in an integrating sphere with a detector whose spectral sensitivity approximates the human eye. This value is not commonly specified for LEDs.


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


Peak Wavelength – The wavelength value with the highest amount of energy radiating from the source. Most commonly specified for non-visible (infrared) LEDs.

Photoconductive Detector – Common name given to a photodiode operated in a reverse bias mode. This mode decreases junction capacitance, increases speed and linearity. It also increases noise current by introducing dark current to the circuit.

Photodiode – Generic name given to any diode used as a light detector. The device has no internal gain like a photodiode or photodarlington. Directly converts photons (light) into electrons (current). It is linear over at least 6 decades of light input. The average saturation point is 10mW/cm2. Used extensively where light must be accurately measured or higher speeds (greater than 30KHz) is required. The response is measured in Amps/Watt (A/W).

Photovoltaic Detector – common name given to a photodiode operated in a zero bias mode. This mode is commonly used in lower speed applications where rise time and junction capacitance are less important than minimizing dark current and thus reducing noise current.

Power Output – Value is expressed in Watts or milliwatts. A radiometric measurement of the total light energy radiating from an emitter regardless of wavelength. Measurement is made with an integrating sphere. This figure of merit is most commonly used with IRLEDs. It is the optical output measurement that is most easily correlated from one measurement facility to another.


Radiant Intensity Radiant measurement of on-axis intensity. This value must be known to calculate optical power incident on a detector that is greater than 6 inches from the LED. The angle of measurement is a critical component when comparing data sheets from one vendor to the next.

Response Time – Also known as rise or fall time. The period of time it takes an emitter or detector to go from the 10% – 90% point, emitting and detecting respectively, or the 90% – 10% point. RC time constant of the device is almost never the limiting factor. The speed of the device is almost always due to the transit time of the semiconductor material and the distance from the depletion region to edge of device.

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. 


Short Circuit Current – Operation of a photodiode in a condition where a voltage bias is not allowed to generate across the diode. Usually accomplished by using a transimpedence amplifier circuit.

Shunt Resistance – The zero bias resistance of a photodiode. In practical measurements, most manufacturers put a 10mV reverse bias on the photodiode and measure current. The ratio between the bias voltage and current determine the shunt resistance value. This value must be known to determine noise current generated by the photodiode in a photovoltaic, short circuit current mode circuit.

Spectral Response – The conventional method for determining the sensitivity of photodiodes. The term is expressed in Amps/Watt (A/W). The monochromatic wavelength the measurement is done at must be specified as well.

Spectral Responsivity () 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 (pk) – The responsivity value at which the detector has a 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 


* The Photonics glossary is a rendition from OptoDiode Inc. glossary sections (Glossary of Terms & Common Definitions for LEDs and Photodiodes)