IV 



MEASUREMENT AND APPLICATION OF VISIBLE AND 

 NEAR-VISIBLE RADIATION 



F. S. Brackett 



Smithsonian Institution, Washington, D.C. 



Introduction: Electromagnetic spectrum — Systems of measureynenl — Units and 

 definitions — General concepts — Black body. Measurement of total radiation: Absolute 

 detector — Comparison of black-body emission — Thermocouple technique — Radiometers. 

 Comparison of detectors. Monochromatic measurements. Filters and reflectors: 

 Filters — Reflectors. Sources of continuous emission: Sun — Tungsten filament — Carbon 

 arc. Selective detectors: Black body plus filter — Photocell — Photochemical. Application 

 of selective detectors. References. 



INTRODUCTION 



In the first paper, the duahty of our conception of radiation and 

 matter has been indicated. The fact that both matter and radiation 

 behave on the one hand as though made up of particles and on the other 

 hand as though conditioned by wave motion seems inescapable. In 

 dealing with the problems of measurement, we are chiefly concerned 

 with the propagation of radiation, i.e., a type of transfer of energy from 

 one point to another. Here the wave-motion aspect of radiation domi- 

 nates our picture. In considering the interaction of radiation and matter, 

 we must resort to the particle conception. This conception has particu- 

 lar applicability to the problems of emission and absorption of radia- 

 tion. Regarding radiation as a type of transfer of energy from one point 

 to another, we recognize two essential characteristics to be measured- — 

 quantity and quality. In the absolute measurement of quantity, we 

 shall base our observations upon the fundamental unit of the c.g.s. system, 

 the erg, or some convenient multiple of that unit. In the evaluation of 

 quality, we shall appeal to our wave picture and base our measurements 

 upon wave-lengths, the units being a convenient submultiple of the 

 centimeter, either the Angstrom (10~^ cm.), the millimicron (10"^ cm.), 

 or the micron (10"'* cm.), depending upon the magnitude of the wave- 

 length to be dealt with. 



Since the velocity of wave propagation is constant for free space, 

 the relation of wave-length to frequency is given by the simple relation 

 Xv = C, where C = 2.998 X 10.^" In propagation through matter, 



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