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Radiation Instruments Institute of Arctic and Alpine Research University of Colorado, Boulder

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Radiation Instruments Institute of Arctic and Alpine Research University of Colorado, Boulder David M. Gates 2 radiation instruments Institute of Arctic and Alpine Research University of Colorado, Boulder 1. Introduction pyranometer, actinometer, net radiometer, and balance Radiant energy is always present, at all times, and in meter. Many of these terms are synonymous and lack all natural environments. Radiation may originate with definition. Few of the terms will be used here. a high temperature source such as the sun or a tungsten filament light, or it may originate with low temperature 3. Types of measurement sources such as the surface of the earth, the walls of a Radiant energy may be detected in a number of ways, building, or the leaves of a crop or forest. Radiation may some of these being: a) the warming of a suitable re- be emitted coherently from lasers or non-coherently from ceiving surface, b) the response of a photoelectric sur- most other sources; it may be monochromatic or it may face, and c) the photochemical change of a substance. span a broad range of wavelengths; and it may be strong Usually the receiving surface is flat; however, there are or weak in radiant intensity. The exchange of radiation sometimes reasons for a sphere, cylinder, or other shape. between an object or organism and its surroundings is a If one is concerned with the incident radiation on a very important factor in the energy budget of the object crop or a single leaf, a flat receiving surface is the most or organism. For this reason the measurement and de- suitable. The instrument containing a flat surface may tection of radiation is a task of utmost importance. But be mounted with the surface horizontal, or vertical, or because of the vast differences in wavelength and in- at an angle parallel to a plant leaf or ground surface. tensity which exist for various fluxes of radiation one Although a spherical or hemispherical receiving surface cannot expect to measure all of them with the same conforms to the geometry of the sky, the utility of ob- instrument. Many types of instruments are required, servations of this type is difficult to see. Sometimes for some of which are described here. The calibration of the observation of radiation for human or animal physi- radiation instruments is one of the most difficult of all ology, a spherical receiver is used, e.g., Bond and Kel- standardization tasks. ley (1955). Two types of radiation flux exchanged at a natural 2. Accuracy of measurement surface have been described. Solar radiation when Accurate measurement of the incident radiant flux on filtered by the atmosphere is composed of wavelengths a surface is made extremely difficult by instrument cali- between 0.3 and 3.0/u, while thermal radiation from bration and stability. The difficulty is partially absolved, surfaces at about 300K is comprised of wavelengths however, when one recognizes the variability of solar and between 4.5 and 50/*. Because solar and thermal radia- thermal radiation incident on a natural surface and tion represent distinctly different spectral regions, they realizes an absolute accuracy better than 10 per cent is can be measured with instruments of different design. probably not necessary. The variability of solar and Just as with a plant or leaf surface, the surface of an thermal radiation incident at the ground is the result instrument assumes a definite temperature according of atmospheric changes, such as cloud, wind, air tem- to the exchange of energy between the surface and the perature, and precipitation. If one insists on knowing environment. If the temperature of a suitable surface the incident radiant flux on a surface to better than or sensor within an instrument is measured, then pre- 10 per cent, then the question must be asked, "Why?" sumably the energy flow by radiation, convection, and Although accurate measurement of the direct incident evaporation between this surface and the environment or outgoing radiant flux from a surface is difficult, the is obtained. If the surface is dry, then only radiation accurate measurement of the net radiation flow at a and convection contribute. If the influence of convec- surface is much easier. However, one must decide tion on the temperature of the sensor can be excluded, whether radiant flux or net radiation is desired and then only the exchange of radiation influences its tem- then select the instrument accordingly. Radiation in- perature. The challenge to the designer of a radiation struments are given various names including photome- measuring device is excluding the influence of moisture ter, radiometer, pyrheliometer, solarimeter, pyrgeometer, and convection upon the temperature of the sensor with- out seriously impeding the detection of radiation. Three 1 This survey has been adapted especially for this issue techniques are used to eliminate the influence of con- from Sec. 14, Chapter 1, of Agricultural Meteorology (Meteor. Monogr., 6, No. 28, pp. 1-26). vection during detection: a) to cover the sensor or re- 2 Presently at Missouri Botanical Garden, St. Louis. ceiver surface with a semitransparent dome; b) to venti- Bulletin American Meteorological Society 539 Unauthenticated | Downloaded 10/04/21 11:25 AM UTC I'ol. 46, No. 9, September 1965 late the exposed receiver surface with a strong but to construct their own instruments. The following is an constant flow of air; and c) to use two identical receiver abbreviated discussion of radiation instruments. surfaces exposed to the air in such a manner as to com- pensate for energy exchange by convection. If a glass 4. Solarimeters and pyrheliometers: electrical dome is used as a convection shield then the radiation and shielded transmitted to the sensor is limited to 0.28 to 3.0^. Since Solar radiation instruments with glass hemispherical the spectral distribution of sunlight reaching the earth's domes receive energy from 0.28 to 3.0^ and are called surface is largely contained within this wavelength span, solarimeters or pyrheliometers. The term solarograph or a glass enclosed sensor is suitable for measuring solar rad- pyrheliograph is used if the instrument is self registering. iation flux. The most frequently used instrument in the United If a dome is constructed of very thin polyethylene, an States is the Eppley pyrheliometer which was originally enclosed sensor will receive a broad spectrum, including described by Kimball and Hobbs (1923), and by Karoli, ultraviolet, visible, and infrared. Polyethylene is semi- Angstrom and Drummond (1960). The glass dome is transparent to wavelengths greater than 3n and has mod- hermetically sealed to the body of the instrument. The erate or weak, narrow absorption bands at 3.5, 6.8 and sensor is a thermopile formed of alloys of platinum- 13.7m- Hence, a radiometer constructed with a fragile rhodium and gold-palladium wires. The "hot" junctions polyethylene dome can detect solar and thermal radia- are in thermal contact with the blackened silver ring tion and is therefore useful both day and night, while a surrounding the central white disc, and the "cold" junc- radiometer with a durable glass dome is useful only in tions are in thermal contact with the white concentric the daytime for the solar radiation component. Since ring. The output is of the order of 8.2 mv per cal cm-2 -1 any instrument with a dome covering the sensor is in- min , resistance about 100 ohms, and the time con- fluenced by weak convection, some are constructed with stant 20 seconds. The instrument is manufactured by double concentric domes to reduce the convection as the Eppley Corporation, Newport, Rhode Island. much as possible. A similar instrument, which is used much in Europe, The most convenient sensor is a blackened thermo- is the Moll-Gorzynski solarimeter manufactured by Kipp pile, which is a set of thermocouples connected in series. and Zonen, Delft, Holland. The thermopile is of man- Alternate junctions become hot and cold, a voltage de- ganin and constantan wires. The Stern pyranometer velops, and a very small current flows in the circuit. All manufactured by Philip Schenk, 40 Voltagasse, Vienna the "hot" junctions are grouped together and placed in 21, Austria, operates in much the same fashion. good thermal, but not electrical, contact with a black- All of these instruments may be used for long periods ened surface. In an instrument measuring the incident in the field with very little servicing, except to clean flux, the "cold" junctions may be maintained at ambient dust off the glass dome. The output from all may be temperature by placing them in thermal contact with read directly with a millivoltmeter or potentiometer, or a heat sink, a block of metal. In a net radiometer the the output may be recorded with a pen and ink poten- "cold" junctions are grouped together and placed in tiometer recorder. In field use their accuracy is not better thermal contact with a second blackened receiving sur- than about 5 per cent. Trickett, Moulsley, and Edwards face. The net radiometer will have a blackened receiver (1957) report that individual Kipp and Zonen solarime- facing outward through one hemisphere and the second ters received at their laboratory from the manufacturer receiver facing diametrically opposite into the other agreed within 1 per cent when compared in the labora- hemisphere. The output from a thermopile is voltage, tory. Monteith (1959a) has described a simply made which for the range of temperatures encountered, is thermopile solarimeter with an output of 10 to 15 mv 2 -1 measured in millivolts. The advantages of a thermopile per cal cm" min , and Monteith and Szeicz (1962a) de- are its stability, linearity, accuracy, and ease of registra- scribe a system for recording and integrating the out- tration. In contrast, thermistor calibration changes with put.
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