Glossary for Optical Detectors
Detector Characteristics
Detectivity - The NEP of a detector is dependent on the area of the detector. To provide a figure of merit that is dependent on the intrinsic properties of the detector, not on how large it happens to be, a term called detectivity is defined. Detectivity is represented by the symbol D*, which is pronounced as D-star. It is defined as the square root of the detector area per unit value of NEP.
D* = A1/2/NEP
Since many detectors have NEP proportional to the square root of their areas, D* is independent of the area of the detector. The detectivity thus gives a measure of the intrinsic quality of the detector material itself.
When a value of D* for an optical detector is measured, it is usually measured in a system in which the incident light is modulated or chopped at a frequency f so as to produce an AC signal, which is then amplified with an amplification bandwidth Df. These quantities must also be specified. The dependence of D* on the wavelength l, the frequency f at which the measurement is made, and the bandwidth Df are expressed by the notation D*(l,f,Df). The reference bandwidth is often 1 Hertz. The units of D*(l,f,Df ) are cm-Hz1/2/watt. A high value of D*(l,f,Df ) means that the detector is suitable for detecting weak signals in the presence of noise. Later, in the discussion of noise, we will describe the effect of modulation frequency and bandwidth on the noise characteristics.
Dynamic Range - measures the range of signals, which can be recorded by a detector. It is the ratio of the highest (lightest) signal, which a detector can record to the lowest (darkest) signal. The lightest signal would correspond to the brightest highlights in an image, the darkest signal to the deepest shadows.
Linearity - another important characteristic of optical detectors is their linearity. Detectors are characterized by a response in which the output is linear with incident intensity. The response may be linear over a broad range, perhaps many orders of magnitude. If the output of the detector is plotted versus the input power, there should be no change in the slope of the curve. Noise will determine the lowest level of incident light that is detectable. The upper limit of the input/output linearity is determined by the maximum current that the detector can produce without becoming saturated. Saturation is a condition in which there is no further increase in detector response as the input light intensity is increased. When the detector becomes saturated, one can no longer rely on its output to represent the input faithfully. The user should ensure that the detector is operating in the range in which it is linear.
Noise Equivalent Power (NEP) - This is defined as the optical power that produces a signal voltage (or current) equal to the noise voltage (or current) of the detector. The noise is dependent on the bandwidth of the measurement, so that bandwidth must be specified. Frequently it is taken as 1 Hz. The lower the value of the NEP, the better are the characteristics of the detector for detecting a small signal in the presence of noise.
Quantum efficiency - This is defined as the ratio of countable events produced by photons incident on the detector to the number of incident photons. If the detector is a photoemissive detector that emits free electrons from its surface when light strikes it, the quantum efficiency is the number of free electrons divided by the number of incident photons. If the detector is a semiconductor pn-junction device, in which hole-electron pairs are produced, the quantum efficiency is the number of hole-electron pairs divided by the number of incident photons. If, over a period of time, 100,000 photons are incident on the detector and 10,000 hole-electron pairs are produced, the quantum efficiency is 10%.
The quantum efficiency is basically another way of expressing the effectiveness of the incident optical energy for producing an output of electrical current. The quantum efficiency Q (in percent) may be related to the responsivity by the equation:
Q = 100 � Rd � (1.2395/l)
where Rd is the responsivity (in amperes per watt) of the detector at wavelength l (in micrometers).
Responsivity - This is the detector output per unit of input power. The units of responsivity are either amperes/watt (alternatively milliamperes/milliwatt or microamperes/microwatt, which are numerically the same) or volts/watt, depending on whether the output is an electric current or a voltage. The responsivity is an important parameter that is usually specified by the manufacturer. Knowledge of the responsivity allows the user to determine how much detector signal will be available for a specific application.
Rise time - is defined as the time difference between the point at which the detector has reached 10% of its peak output and the point at which it has reached 90% of its peak response, when it is irradiated by a very short pulse of light.
Fall time - is defined as the time between the 90% point and the 10% point on the trailing edge of the pulse waveform. This is also called the decay time. We should note that the fall time may be different numerically from the rise time.
Signal to Noise Ratio - The ratio of the overall signal output to the noise level is known as the signal to noise ratio (S/N) and can be used to determine whether noise will be a concern for a particular application.
Photon Noise & Detectivity Limits
Shot Noise - is a type of noise that occurs when the finite number of particles that carry energy, such as electrons in an electronic circuit or photons in an optical device, gives rise to detectable statistical fluctuations in a measurement. It is important in electronics, telecommunications and fundamental physics. The strength of this noise increases with the average magnitude of the current or intensity of the light. Often, however, as the signal increases more rapidly as the average signal becomes stronger, shot noise often is only a problem with small currents and light intensities.
Fluctuating Opto-Electric Quantity
Dark Current - (1) The charge accumulated within a well, in the absence of light. (2) The background current that flows in a charge-coupled device or image intensifier of a camera system. Cooling the photodetector's primary imaging surface (i.e., the CCD's photoconductor or the image intensifier's photocathode) can reduce or eliminate dark current. Also called thermally generated charge.
Dark Curent Noise - See dark noise.
Dark Noise - The statistical variation of the dark current, equal to the square root of the dark current. Dark current can be subtracted from an image, while dark noise remains. Also called dark current noise.
