Noise equivalent irradiance

Radiant sensitivity noise equivalent irradiance (NEI) 1 x lO ph/cm /sec (because the applications for this type of instrument measure radiant power rather than temperature, sensitivity is expressed in NEI rather than NETD)... 

Noise-equivalent power (NEP) The minimum IR power a detector can accurately see (There are other similar figures of merit noise-equivalent irradiance (NEI), noise-equivalent differential temperature (NEdT))... 

The basic test for any IR detector includes measurement of the output (DC or AC) and noise, as well as the calculations of responsivity and the composite figures of merit noise-equivalent irradiance (NEI), noise-equivalent power (NEP), or D. If an assembly includes only a few elements, and if only a few assemblies are to be tested, it is reasonable to make these measurements with simple voltmeters or wave analyzers, and report the results in a tabular form. Higher volume production requires automated data acquisition equipment and graphical and statistical reporting - much like the testing of FPAs. 

Consider the requirement to test a 0.25-in.-long array of PV InSb detectors at a background of 5 X 10 photons/(cm s). To determine whether we will have adequate signal and signal-to-noise ratios, we will need the expected responsivity and noise-equivalent irradiance (NEI) values for the detector. If these are not available from previous tests, we can predict them using the methods of Chapter 4. For our example, assume that the responsivity is 15 x 10 V/[photon/(cm s)] and NEI is 2 X 10 photons/(cm s). 

The fundamental performance parameter of any detector is its noise equivalent power (NEP). This is simply the input irradiance power necessary to achieve a detector output just equal to the noise. This NEP is dependent on a number of detector and signal variables such as modulation frequency and wavelength (the input signal is defined as sine wave modulated monochromatic light), and detector area, bandwidth and temperature. 

An alternative method for excitation of nuclei over a range of chemical shifts is by irradiation with a weak, noise-modulated radio-frequency, instead of with strong r.f. pulses. In one realization of this method, protons were irradiated with repetitive sequences of noise that was truly random,162 and, in another,163 fluorine nuclei were excited by pseudo-random noise generated by amplitude modulation of the r.f. with maximum-length sequences of pulses from a computer or shift register (a series of flip-flop devices connected by feedback loops). With the carrier wave suppressed, the latter process is equivalent to phase modulation of the r.f. by+7r/2 radians when the pulse is turned on, and by —ir/2 radians when it is turned off. This method is identical with that used in most broadband, heteronuclear, noise decouplers, except that greater power is required for decoupling. 

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