Theory Of Radiation Measurements

9.1 Introduction

Radiation measurements are crucial for understanding the climate system and monitoring its behaviour. Increasingly, the most important measurements are made from instruments in space, orbiting the Earth on platforms that offer global coverage of the atmosphere and surface beneath. These remote sensing instruments are the subject of the following chapter. Here, we consider the basic design and operating principles of the instruments that produce both remote and in-situ climate measurements.

We are interested mainly in devices that determine the intensity of the radiation as a function of its wavelength or wave number (A-1). The instruments of interest for climate research operate mostly, although not exclusively, in the infra-red part of the spectrum, which is conveniently broken down into two regimes, the near and the thermal infra-red. In the climate system, virtually all of the solar energy flux is contained within the range from about 0.2 to about 5.0 ¡m, which includes the visible and the ultraviolet range as well as the near-infra-red, while the planetary or terrestrial emission occurs between 5.0 and 100 ¡m, which is therefore defined to be the thermal infra-red region.

Most common infra-red optical systems can be categorized as either radiometers or spectrometers, although the terms are used fairly loosely and the following definitions are not always adopted in the literature. In most contemporary usage, radiometers are characterized by absolute radiometric performance and relatively low spectral resolving power. They are calibrated before and during use, so that they can measure flux or radiance directly. They do not scan the spectrum, but employ a fixed wavelength band or bands. In radiometers, typically the ratio of bandwidth to wavelength is not less than a few per cent. Spectrometers differ from radiometers in that they scan in wavelength, have high spectral resolution and are not necessarily absolute radiometric instruments. They may operate either in absorption or emission and measure the position and intensity of spectral lines in absorption on a scale of transmitted or absorbed energy versus wavelength. Calibration in wavelength is usually accomplished with reference scans of a reference gas or material, such as polyethylene, which has a number of distinctive absorption features at well-defined wavelengths. A spectrometer that is absolutely calibrated in energy units is sometimes called a spectroradiometer. The term is also used for radiometers with unusually high spectral resolution, and perhaps with some limited spectral scanning capability.

The fundamental principles by which instruments operate are best appreciated by considering simple, idealized systems first, then the practical factors that must be taken into account in reality, and finally (in Chapter 10) some examples of real instruments and their applications. Infra-red systems making geophysical measurements often have signal-to-noise ratios that are marginal for their particular application. This arises from the low flux of infra-red photons from relatively cool targets, the effects of background fluxes, especially from warmer or more extended objects internal or external to the system, as well as fundamental limitations including statistical effects in the detector and in the photon flux itself. It is necessary, therefore, to characterize the performance of a system in terms of measurable or calculable quantities in order that a device can be matched to a desired application. Failure to do so would risk ending up with an experiment in which the signal was drowned in noise, or a waste of resources through the use of unnecessarily sophisticated detectors, perhaps employing cooling where cheaper room-temperature devices would have done. Instead, we apply the known physics of the radiation field, of materials and of devices such as detectors and multilayer filters, to the design of a device with a specific application in mind.

9.2 Detectors

Detectors are the heart of any measurement system: any device that converts a stream of photons into an electrical signal can be used as an infra-red detector. For example, the nerves in the human skin act as a detector, albeit a relatively insensitive one; a blind person can tell that there is a heat source, such as a fire, in a room, and roughly how hot it is, by holding a hand towards it, with no other source of information. There are many different types of solid-state detector, operating on a variety of principles, which have been used at one time or another. We will concern ourselves here only with the types that are found in modern infra-red systems. All of these fall into one of two main categories, thermal detectors (where the heating effect of the radiation is measured) and photon detectors (where the quanta excite electronic transitions in the detector material - usually a semiconductor - directly). Before discussing how they work, we will look at some useful formulae and definitions that apply to all types.

A conceptual detector consists of a detector element of a material that responds to incident radiation by producing a voltage or current. In the scheme shown in Fig. 9.1, the element of area A cm2 produces a signal voltage Vs volts at the output on exposure to a flux F W cm~2. There will also in general be an unwanted voltage Vn at the output, consisting of random noise. The voltage responsivity, Rv (V ) of the detector is defined by

FlG. 9.1. A conceptual infra-red detector. When the element is exposed to a flux of radiation F, a voltage Vs is induced across the leads to the element. In general, a constant offset voltage Vo and a random noise voltage Vn (which may or may not be functions of F) will also be present.

so that Rv is the number of volts that appear at the output of the detector for every watt of power incident upon it, and the signal-to-noise ratio of the measurement of F is SNR= Vs/Vn. There may also be a constant offset voltage, V0, in which case the total voltage measured is

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