CHAPTER 3 



Generation, Control, and Measurement of Visible 

 and Near-visible Radiant Energy 



Robert B. Withrow and Alice P. Withrow 

 Smithsonian Institution, Washington, D.C. 



Fundamental concepts: Nomenclature — Electromagnetic spectrum — Quantitative units 

 of energy — Propagation of radiant energy — Absorption of radiant energy — Reflection — 

 References. Sources of radiant energy: Thermal sources — Gaseous discharge lamps — 

 Fluorescent lamps — Short-duration sources — References. Control of radiant energy: 

 Optical properties of materials — Spectral control of radiant flux — Control of irradiance — 

 References. Measurement of radiant energy: Detectors — Measuring instruments — 

 Standards of radiant energy — -Voltage regulators — References. 



In the investigation of the biological role of radiant energy, one must 

 consider both the complexity of photochemical processes in biological 

 systems and the proper selection and application of appropriate technics 

 from the great diversity of modern physical methods of radiation pro- 

 duction, control, and measurement. 



Rapid technological development has markedly increased the experi- 

 mental opportunities in the field of photochemical biology. ]\Iany diver- 

 sified sources of radiant energy are available. Detectors such as the 

 photomultiplier make it possible to measure extremely low levels of visi- 

 ble and ultraviolet energy, and photoconductive cells such as the lead 

 sulfide cell extend the spectral range of photocell detectors well into the 

 infrared. Great strides have been made in electrical measurements, prob- 

 ably the most striking of which are inverse feedback circuitry, servo- 

 control, and the various modulation systems for converting low-level 

 direct currents to alternating currents capable of being amplified readily 

 by electronic means. Fast thermal detectors, such as thermocouples, 

 bolometers, and pneumatic cells, have also made it possible to modulate 

 the radiant-energy beam itself. Such systems of modulation permit the 

 substitution of a-c vacuum-tube amplifiers and strip-chart recorders for 

 galvanometers and photographic methods of recording. 



The present chapter represents an effort to assemble the most pertinent 

 information on modern developments, together with the most important 

 of the fundamental principles and technics of application of visible and 

 near-visible radiant energy to biological problems. 



125 



