which is the expression known as Schottky's formula.

In photovoltaic mode, the resistive (Johnson) noise contribution is generally negligible compared to that due to the photon flux P. If the background contribution to the power at the detector is also negligible compared to that originating in the source, i.e. P « Ps, then the noise current is

while the noise current in while the signal current is qe2Ps ts = — (9.36)

so the signal-to-noise ratio, in terms of dwell time t0, is

in V hv

The noise equivalent power, defined as the signal power that results in a signal-to-noise ratio of unity, is then

n nto

This device, used in a mode where the only important source of noise is that intrinsic to the photon stream being measured, is described as an ideal photon detector.

9.5 Detector arrays and charge coupled devices

Detectors are increasingly being supplied as arrays, i.e. assemblies of more than one sensitive element on the same substrate. Area arrays with m x n elements are mostly used for infrared imaging, and may contain tens of thousands of elements. Linear arrays, with nx 1 elements, can be scanned across a target ('push-broom' imaging), as a cheaper and more easily manufactured alternative, if the extra time is available. An alternative use of linear arrays is to measure (or 'multiplex') different wavelengths simultaneously by locating them in the plane of dispersion of a spectrometer. A problem with arrays was the large number of pre-amplifiers required, if each detector element has its own. The pre-amplifiers are normally an integral part of the detector, mounted near the sensitive element to reduce the possibility of noise generation at a point of small signal and high impedance. Clearly installing 10000 of them on a 100 x 100 element array measuring less than 1 cm2 represents a challenge, which has led to the rapid adoption of the charge coupled device or CCD. These can read the signal from many detectors through a single pre-amplifier.

9.6 Properties of IR systems

The three 'dimensions' of an infra-red device are represented by its spectral, geometrical and radiometric properties. It is useful to extend the concept of the responsivity of a detector to complete systems. The responsivity Rv is then the ratio between the change in output voltage, AV, and the change in target radiance, AI, hence

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