David M. Gates 2 radiation instruments Institute of Arctic and Alpine Research University of Colorado, Boulder

1. Introduction , actinometer, , 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, , , 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 (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. The instrument is small and has been extensively age. The thermopile is easy to manufacture and can used above and within crops for extended periods. readily be constructed large or small, with many junc- tions or with few. Gates (1964) and Monteith (1959a) 5. Actinograph: mechanical and shielded describe the manufacture of thermopiles. The bimetallic actinograph was originally designed by Radiation instruments for agricultural or meteorologi- Robitsch (1932). The instrument has a blackened bi- cal use must be rugged, reasonably weather-proof, and metallic element, receiving incident sunlight and ex- must not change calibration over a short interval of time. panding on heating. The expansion of the bimetallic Various radiometers are described by Trickett et al. element is proportional to the incident solar radiation (1957), and means of integrating the output over a pe- and tilts a pen arm through a mechanical linkage. The riod of time are described by them and by Monteith pen traces a line on a clock driven piece of graph paper. and Szeicz (1962a). Gates (1962) has described radiation As designed in Europe, the bimetallic element consists instruments in considerable detail. Relatively few are of a blackened bimetallic strip mounted at one end of commercially available and many workers have chosen two shaded strips. Designed in America, the bimetallic

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Unauthenticated | Downloaded 10/04/21 11:25 AM UTC Bulletin American Meteorological Society element consists of a blackened section and a polished with a blackened opaque hemisphere made of copper, reflecting section of metal. The entire assembly is en- then the lower surface of the radiometer will view the closed in a weather-tight case, and the bimetallic ele- interior of the opaque hemisphere as a blackbody. A ment views the upper hemisphere through a glass dome. thermocouple, in good contact with the interior of the The instrument is rugged and durable, requiring little hemisphere, records the internal surface temperature, T. servicing except to change the chart. It can run for one The "cold" junctions are receiving a flux of blackbody day or one week without attention. The charts can be radiation according to o-T4. The flux on the "hot" re- planimetered to integrate radiation. On an absolute basis ceiver, which is receiving an external flux of radiation the instrument is not accurate to better than about 15 from the sun, sky, or surroundings, can then be calcu- per cent, but on a relative basis it is more reliable. The lated from the thermopile output. instrument is available from Belfort Instrument Co., An easily constructed and inexpensive radiometer for Baltimore, Maryland; Fuess, Berlin-Steglitz, Germany; agricultural purposes was described by Suomi and Kuhn Casella, London, England; Societa Italiana Apparecchi (1958). The sensor consists of a pair of di Precisione, Bologna, Italy; and, in general, from sci- inserted between two blackened aluminum foil sheets entific supply houses. which are shielded from air currents by a polyethylene sheet stretched over a frame. The instrument has been 6. Radiometers: electrical and shielded converted to a totalizing device and applied to research Radiometers, with polyethylene domes rather than glass in forestry (Goodell, 1962). domes, have the same basic thermopile detector, but re- ceive radiation of wavelengths throughout much of the 7. Radiometers: electrical; ventilated ultraviolet, visible, and infrared. These instruments may The basic purpose of a ventilated radiometer is to ex- be used to measure solar radiation as well as long-wave pose the receiver surface or heat flow unit to the incom- thermal radiation and are therefore useful both day ing radiation without interference by glass or poly- and night. ethylene. To avoid disturbance of the sensor by erratic One of the first radiometers to use a polyethylene air movement, the sensor is mounted in a strong air dome was the instrument described by Schulze (1953). stream from a blower. One of the first commercial in- The sensor is a Moll thermopile. The instrument is struments of this nature was developed by Gier and commercially available from B. Lange Co., Berlin, Dunkle (1951). When used as a net radiometer, the Germany. instrument has upward and downward facing blackened Monteith's instrument, referred to above, may be con- receiver surfaces with a thermopile connected between verted to a total radiometer by replacing the glass dome them. When used as a radiometer, responding to the with a polyethylene dome. Funk (1959) has designed a incident radiation on one receiver surface, the instru- polyethylene-shielded net radiometer which is available ment has the other surface covered with a polished from C.S.I.R.O., Division of Meteorological Physics, Mel- metal plate blackened on its inner face so that the "cold" bourne, Australia. The instrument is small and suitable junctions of the thermopile remain at ambient air tem- for field use. perature. The instrument is manufactured by Beckman Fritschen and van Wijk (1959), and Fritschen (1960) and Whitley, Inc., San Carlos, California. The radiom- 2 1 have described a simple, small net radiometer using poly- eter has a sensitivity of 10 mv per cal cm" min" , an ethylene or polyvinyl domes and a commercial heat flow internal resistance of 440 ohms, and a response time of disc, which is actually a small thermopile, sometimes about 10 seconds. A copper-constantan thermocouple is referred to as a transducer. This thermopile is a disc of built into the sensor to measure the ambient tempera- tellurium silver alloy with copper gauze coating and is ture of the "cold" junctions. The output of the radi- blackened on top and bottom. It has an output of about ometer may be read on a precision potentiometer or 0.2 mv per cal cm"2 min"1. The heat flow disc is inex- with a recording pen and ink potentiometer. A radiome- pensive and may be obtained from National Instruments ter similar to the Gier and Dunkle radiometer was de- Laboratory, Inc., Washington, D. C. The instrument can scribed by Suomi, Franssila and Islitzer (1954). The be assembled by most instrument shops from materials instrument is well engineered, has an output of about 2 -1 readily available, once the heat flow disc is obtained. A 2.0 mv per cal cm" min , and is available from Agmet net radiometer using the thermal transducer is available Products Co., Madison, Wisconsin. Many laboratories from C. W. Thornthwaite Associates, Centerton, New throughout the world have built ventilated types of Jersey. A similar instrument has been built for this radiometers in their own instrument shops. Some of these writer by Colorado Research and Engineering, Inc., "homemade" instruments have been described by Gates Boulder, Colorado, using a thermopile sensor rather (1962). than the heat flow disc. A net radiometer, which consists of a receiver black- 8. Radiometers: electrical; exposed; unventilated ened on both faces, each with a hemispherical glass or An instrument used in forestry research has been de- polyethylene dome, may be converted to a direct flux scribed by Hofmann (1952) and Baumgartner (1962). radiometer. If the lower transparent dome is replaced It consists of upward and downward facing blackened

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Unauthenticated | Downloaded 10/04/21 11:25 AM UTC I'ol. 46, No. 9, September 1965 surfaces properly spaced and mounted in the form of a Baumgartner, A., 1952: Die Strahlungsbilanz in einer Fich- disc. The two receiver surfaces are fully exposed. A ther- tendickung. Fortswissenschaftl. Centralblatt., 71, 337-349. mopile connects the two surfaces and measures their Bond, T. E., and C. P. Kelly, 1955: Globe in temperature difference. In addition, a heater element agricultural research. Agr. Eng., 36, 251-255 and 260. Fritschen, L. J., 1960: Construction and calibration details of is mounted underneath each surface. One surface may the thermal transducer type net radiometer. Bull Amer. be receiving more radiation than the other surface, and Meteor. Soc., 41, 180-183. hence be warmer. By heating the other surface electri- , and W. R. van Wijk, 1959: Use of an economical cally until the temperatures of the two surfaces are thermal transducer as a net radiometer. Bull Amer. Meteor. equal, the power consumed is a direct measure of the Soc., 40, 291-294. net radiation received. Convection is assumed to disturb Funk, J. P., 1959: Improved polyethylene-shielded net radi- the energy budget of the two surfaces to the same extent. ometer. J. Sci. Instr., 36, 267-270. By mounting two identical instruments of this type side Gates, D. M., 1962: Energy Exchange in the Biosphere. New by side, heating electrically the upward facing surface of York, Harper and Row, 151 pp. , 1964: Energy, temperature and organisms. The Science one and the downward facing surface of the other, the No. 4. direct incident fluxes are measured. A picture of this in- Teacher, 31, Gier, J. T., and R. V. Dunkle, 1951: Total hemispherical strument and further description is given by Gates radiometers. Trans. Amer. Inst. Elec. Engrs., 70, 339. (1962). Goodell, B. C., 1962: An inexpensive totalizer of solar and Angstrom (1905) has described an instrument utilizing thermal radiation. J. Geophys. Res., 67, 1383-1387. the electrical compensation technique for radiation Hofmann, G., 1952: Ein Strahlungsbilanzmesser fur Forst- measurements. This instrument is known as an actinome- meteorologische Untersuchunger. Forstwissenschaftl. Cen- eter or pyrgeometer depending upon its use. A black- tralblatt, 71, 330-337. ened strip of manganin is exposed to the radiation and Karoli, A. R., A. K. Angstrom and A. J. Drummond, 1960: Dependence on atmospheric pressure of the response char- a second blackened strip is shielded by a shutter from acteristics of thermopile radiant energy detectors. J. Opt. the radiation. The shielded strip is heated electrically Soc. Amer., 50, 758-763. until it achieves the same temperature as the exposed Kimball, H. H., and H. E. Hobbs, 1923: A new form of strip. The incoming radiation is determined by the thermoelectric recording pyrheliometer. Mon. Wea. Rev., power consumed in warming the cooler strip. The role 51, 239-242. of the two strips is reversed by changing the shutter and Monteith, J. L., 1959a: Solarimeter for field use. J. Sci. Instr., heating from one to the other. By this means any mis- 36, 341-346. match of the two strips is compensated. , and G. Szeicz, 1962a: Simple devices for radiation meas- urement and integration. A simple net radiometer for field use was described by Archiv. Meteor., Geophys Ser. B, 11, 491-500. Monteith and Szeicz (1962b). It is a thin, flat glass ring Bioklim , and , 1962b: Radiative temperature in the heat wound with constantan wire which is plated with copper balance of natural surfaces. Quart. J. R. Meteor. Soc., 88, to form a series of junctions along the two surfaces of 496-507. the ring. It is ventilated by vibrating it at the end of a Robitsch, M., 1932: Ueber den Bimetallaktinographen Fuess- strip of metal. The entire assembly is blackened. The Robitsch. Beitr. Geophys., 35, 387. electrical output can be read by means of a potentiome- Schulze, R., 1953: Ueber ein Strahlungsmessgerat mit ultra- ter. The response and calibration check well with more rotdwichlassigar wind schutzhaube am Meteorologischen elaborate and expensive net radiometers. Observatorium Hamburg. Geofis. Pura e Appl., 24, 107. Suomi, V. E., M. Fransila and N. F. Islitzer, 1954: An im- proved net radiation instrument. J. Meteor., 11, 276-282. References , and P. M. Kuhn, 1958: An economical net radiometer. Angstrom, A., 1905: Ueber die Anwendung der elektrischen Tellus, 10, 160-163. Kompensationsmethode zur Bestimmung der nachtlichen Trickett, E. S., L. J. Moulsley and R. I. Edwards, 1957: Meas- Ausstrahlung. Nova Acta Soc. Sci. Uppsala, Ser. 4, 1, No. 2. urement of solar and artificial radiation with particular , 1915: The study of radiation of the atmosphere. Smith- reference to agriculture and horticulture. J. Agr. Eng. Res., sonian Misc. Coll., 65, No. 3, 1-159. 2, 86-110.

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