The language of Optoelectronics adopts its own special terminology, born along the developement of this branch of technology.   In the following we are synthesizing in few lines the definition of the fundamental terms.  Terms constantly repeated in the web pages of this site with reference to photodetectors in all their existing kinds.   

Signal-to-Noise Ratio 

The Signal-to-Noise ratio (S/N) is expressed in terms of the noise in a particular bandwidth.  It may be alternatively expressed in: 

  • decibels,
  • ratio of a number to the standard deviation of the number in a particular count duration.

Equivalent Noise Input 

The Equivalent Noise Input (ENI) is defined as that value of input flux that produces an RMS signal current that is just equal to the root mean square (RMS) value of the noise current in a specified bandwidth.  Bandwidth usually 1 Hz.   Other parameters such as:

  • frequency, 
  • type of light modulation, 
  • temperature,

should also be specified.  Equivalent Noise Input characteristics are useful in determining the threshold of detection in equivalents of the input flux in watts or lumens.

Noise Current 

There are various sources of noise or current fluctuation which interfere with the precise measurement of the signal current.   Dark current has a random fluctuation as does signal current.   When the individual pulse can be observed, these fluctuations are manifest in the random arrival of the dark or signal pulses.   Sometimes, the noise in the associated components, such as thermal (or Johnson) noise in the coupling resistor or amplifier noise, is dominant.

Noise Equivalent Power 

Noise equivalent power (NEP) is essentially the flux in watts incident on the detector which gives a Signal-to-Noise ratio of unity.   The frequency bandwidth and the frequency at which the radiation is chopped must be specified as well as the spectral content of the radiation.   The two most common spectral specifications are for total radiation from a black body whose temperature is 500 K or monochromatic radiation at the peak of the detector response.


Detectivity is the reciprocal of the Noise Equivalent Power and expressed in 1/watt.  Has to be considered as a figure of merit providing the same information as the Noise Equivalent Power but describes the characteristics such that the lower the radiation level to which the photodetector can respond, the higher the detectivity. 

Specific Detectivity 

Because the Noise Equivalent Power for many types of IR photodetectors is proportional to the square root of the sensitive area A and to the square root of the bandwidth B of the measuring system, a specific detectivity D* has been defined which permits a comparison of sensors of different areas measured in different bandwidths.

Incident Flux 

The response of a photodetector is evaluated in terms of the flux incident on the sensitive area of the detector.   The flux may be specified as the: 

  • number of quanta per second at some wavelength, 
  • radiant flux in a narrow wavelength band, 
  • radiant flux over a wide band of wavelengths, 
  • luminous flux, 
  • luminous density, 
  • per unit area, 
  • in the plane of the device aperture,
  • in a surface. 


The term responsivity (R) describes the sensitivity of the photosensor and is the ratio of the output current or voltage to the input flux in watt or lumen. 

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