Lyon, John E.

Woosley, Robert D.

English

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Journal of the Arkansas Academy of ScienceVolume 6 Article 221953Apparatus for the Measurement of AtmosphericAttenuation of LightJohn E. LyonUniversity of Arkansas, FayettevilleRobert D. WoosleyUniversity of Arkansas, FayettevilleFollow this and additional works at: http://scholarworks.uark.edu/jaasPart of the Physics CommonsThis article is available for use under the Creative Commons license: Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0). Users are able toread, download, copy, print, distribute, search, link to the full texts of these articles, or use them for any other lawful purpose, without asking priorpermission from the publisher or the author.This Article is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Journal of the Arkansas Academyof Science by an authorized editor of ScholarWorks@UARK. For more information, please contact scholar@uark.edu, ccmiddle@uark.edu.Recommended CitationLyon, John E. and Woosley, Robert D. (1953) "Apparatus for the Measurement of Atmospheric Attenuation of Light," Journal of theArkansas Academy of Science: Vol. 6 , Article 22.Available at: http://scholarworks.uark.edu/jaas/vol6/iss1/22AN APPARATUS FOR THE MEASUREMENTOF ATMOSPHERIC ATTENUATION OF LIGHTJOHN E. LYON and ROBERT D. TOOSLEYUniversity of ArkansasThe attenuation of white light is demonstrated by the masking of streetlamps by fog, by the loss of illumination during overcast weather, and in otherways. Many attenuation studies of white light have been made and transmissionvalues for air having varying degrees of moisture content are available in theliterature (1). Among such values, for example, are 93 per cent transmissionper 1,000 yards in exceptionally clear weather and 0.0005 per cent transmissionper 1,000 yards in heavy fog (1). Less apparent, however, is the variable at-tenuation of certain wave lengths in relation to properties of the atmosphere,such as particle size and concentration. Such phenomena are demonstrated by theapparent blueness of the sky produced by selective scattering of the shorterwave lengths of the sun's radiation and by the redness of the sunrise and thesunset when the greater atmospheric mass through which the light must travelcauses almost total scattering and absorption of the shorter wave lengths. Thedegree of this selective attenuation is subject to investigation and correlationto meteorological conditions.In the present studies (2) it became apparent that more dependable valuesof selective attenuation should be obtained. A light source and a spectropho-tometer, 350 feet apart, constitute the basis of the Ordark system for obtain-ing these values. The short light path provides a homogeneous atmosphere. Therange is free of obstructions and light-reflecting surfaces. A 200-watt incan-descent projector bulb is the source and it receives power from a Sorensonvoltage regulator. The spectrophotometer consists of a lens 4% inches in diam-eter with a long focal length, a Bausch & Lomb constant-deviation spectroscope,a photomultiplier tube and its power' supply, and a General Electric microampererecorder. The slits of the spectroscope are placed at the focal point of thelens, with the arm parallel to the optical axis. The entire assembly is mountedrigidly on a concrete and steel base. The photomultiplier tube is fixed overthe eyepiece and its output is fed directly into the recorder.During each series of determinations of the light reaching the spectropho-tometer, the setting of the spectroscope is changed so that the wave length ofthe light striking the photo-sensitive tube varies from 4,000 A to 7,000 A.Permanent traces of the tube output are made on the recorder, together withwave-length notations by the operator. As stray light is not entirely elimina-ted, a background determination is made for each test. This is done by repeat-ing the spectroscope settings with the light source turned off, the differencebetween the two sets of values being that part of the illumination contributedby the light source. The maximum background observed to date was approximately10 per cent of the response to the light source. Interpretation of the curveis made, expressing the response in terms of luminous flux.The most important component of this apparatus is the photomultiplier tube,which offers appreciable response to very low light levels. The tube chosen forthis work has a photosensitive surface with a so-called S-4 response which ex-tends from 3,000 A to 7,000 A, with peak response near 4,000 A. The tube oper-ates on the secondary-emission principle, with successive stages of multiplica-tion. The latter is accomplished through electron acceleration by increasedpotential in each of 9 stages. Two models of tubes having an S-4 response havebeen tried: the 931-A, which is suitable for moderate light levels, and the1P21, which is best for low levels, because of its extreme sensitivity. The1P21 has been adopted because it allows narrow band-setting of the spectroscope.Instability resulting from the high sensitivity of the 1P21 to stray lightand its susceptibility to fatigue, and the change in tube response caused byfluctuations in the voltage on the stages are primary problems. Sensitivitybecame less bothersome when adequate shielding against random light was provid-ed by placing the slits of the spectroscope at the focus of the lens. Voltagefluctuations were reduced to a minimum by use of an electronically-regulatedpower supply.Research Paper No. 1076 Journal Series. University of Arkansas.*79Journal of the Arkansas Academy of Science, Vol. 6 [1953], Art. 22Published by Arkansas Academy of Science, 195380 ARKANSAS ACADEMY OF SCIENCEThis apparatus functions satisfactorily over the range tested. Absoluteattenuation values could not be obtained from the comparatively few tests madeto date. However, such values can be determined with this spectrophotometerwhen a sufficiently large number of tests have been made during varying weatherconditions.References(1) Illuminating Handbook for 19U7.(2) Ordark Research Project.80Journal of the Arkansas Academy of Science, Vol. 6 [1953], Art. 22http://scholarworks.uark.edu/jaas/vol6/iss1/22

Apparatus for the Measurement of Atmospheric Attenuation of Light

UARK (University of Arkansas )

Journal of the Arkansas Academy of Science Volume 6 Article 22 1953 Apparatus for the Measurement of Atmospheric Attenuation of Light John E. Lyon University of Arkansas, Fayetteville Robert D. Woosley University of Arkansas, Fayetteville Follow this and additional works at: https://scholarworks.uark.edu/jaas  Part of the Physics Commons Recommended Citation Lyon, John E. and Woosley, Robert D. (1953) "Apparatus for the Measurement of Atmospheric Attenuation of Light," Journal of the Arkansas Academy of Science: Vol. 6 , Article 22. Available at: https://scholarworks.uark.edu/jaas/vol6/iss1/22 This article is available for use under the Creative Commons license: Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0). Users are able to read, download, copy, print, distribute, search, link to the full texts of these articles, or use them for any other lawful purpose, without asking prior permission from the publisher or the author. This Article is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Journal of the Arkansas Academy of Science by an authorized editor of ScholarWorks@UARK. For more information, please contact scholar@uark.edu. AN APPARATUS FOR THE MEASUREMENTOF ATMOSPHERIC ATTENUATION OF LIGHTJOHN E. LYON and ROBERT D. TOOSLEYUniversity of ArkansasThe attenuation of white light is demonstrated by the masking of streetlamps by fog, by the loss of illumination during overcast weather, and in otherways. Many attenuation studies of white light have been made and transmissionvalues for air having varying degrees of moisture content are available in theliterature (1). Among such values, for example, are 93 per cent transmissionper 1,000 yards in exceptionally clear weather and 0.0005 per cent transmissionper 1,000 yards in heavy fog (1). Less apparent, however, is the variable at-tenuation of certain wave lengths in relation to properties of the atmosphere,such as particle size and concentration. Such phenomena are demonstrated by theapparent blueness of the sky produced by selective scattering of the shorterwave lengths of the sun's radiation and by the redness of the sunrise and thesunset when the greater atmospheric mass through which the light must travelcauses almost total scattering and absorption of the shorter wave lengths. Thedegree of this selective attenuation is subject to investigation and correlationto meteorological conditions.In the present studies (2) it became apparent that more dependable valuesof selective attenuation should be obtained. A light source and a spectropho-tometer, 350 feet apart, constitute the basis of the Ordark system for obtain-ing these values. The short light path provides a homogeneous atmosphere. Therange is free of obstructions and light-reflecting surfaces. A 200-watt incan-descent projector bulb is the source and it receives power from a Sorensonvoltage regulator. The spectrophotometer consists of a lens 4% inches in diam-eter with a long focal length, a Bausch & Lomb constant-deviation spectroscope,a photomultiplier tube and its power' supply, and a General Electric microampererecorder. The slits of the spectroscope are placed at the focal point of thelens, with the arm parallel to the optical axis. The entire assembly is mountedrigidly on a concrete and steel base. The photomultiplier tube is fixed overthe eyepiece and its output is fed directly into the recorder.During each series of determinations of the light reaching the spectropho-tometer, the setting of the spectroscope is changed so that the wave length ofthe light striking the photo-sensitive tube varies from 4,000 A to 7,000 A.Permanent traces of the tube output are made on the recorder, together withwave-length notations by the operator. As stray light is not entirely elimina-ted, a background determination is made for each test. This is done by repeat-ing the spectroscope settings with the light source turned off, the differencebetween the two sets of values being that part of the illumination contributedby the light source. The maximum background observed to date was approximately10 per cent of the response to the light source. Interpretation of the curveis made, expressing the response in terms of luminous flux.The most important component of this apparatus is the photomultiplier tube,which offers appreciable response to very low light levels. The tube chosen forthis work has a photosensitive surface with a so-called S-4 response which ex-tends from 3,000 A to 7,000 A, with peak response near 4,000 A. The tube oper-ates on the secondary-emission principle, with successive stages of multiplica-tion. The latter is accomplished through electron acceleration by increasedpotential in each of 9 stages. Two models of tubes having an S-4 response havebeen tried: the 931-A, which is suitable for moderate light levels, and the1P21, which is best for low levels, because of its extreme sensitivity. The1P21 has been adopted because it allows narrow band-setting of the spectroscope.Instability resulting from the high sensitivity of the 1P21 to stray lightand its susceptibility to fatigue, and the change in tube response caused byfluctuations in the voltage on the stages are primary problems. Sensitivitybecame less bothersome when adequate shielding against random light was provid-ed by placing the slits of the spectroscope at the focus of the lens. Voltagefluctuations were reduced to a minimum by use of an electronically-regulatedpower supply.Research Paper No. 1076 Journal Series. University of Arkansas.*79Journal of the Arkansas Academy of Science, Vol. 6 [1953], Art. 22Published by Arkansas Academy of Science, 195380 ARKANSAS ACADEMY OF SCIENCEThis apparatus functions satisfactorily over the range tested. Absoluteattenuation values could not be obtained from the comparatively few tests madeto date. However, such values can be determined with this spectrophotometerwhen a sufficiently large number of tests have been made during varying weatherconditions.References(1) Illuminating Handbook for 19U7.(2) Ordark Research Project.80Journal of the Arkansas Academy of Science, Vol. 6 [1953], Art. 22https://scholarworks.uark.edu/jaas/vol6/iss1/22

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