SECTION I--CHARACTER AND SOURCE OF LIGHT 



THE NATURE OF LIGHT SOURCES AND TYPES OF TRAPS 



T. E. Hienton 1 



Light is defined in a very recent issue of a collegiate dictionary, in these words: 

 "1 a) that which makes it possible to see: opposed to darkness ; form of radiant energy 

 that acts upon the retina of the eye, optic nerve, etc., making sight possible: this energy 

 is transmitted at a velocity of about 186,000 miles per second by wavelike or vibrational 

 motion b) a form of radiant energy similar to this, but not acting on the normal retina, as 

 ultraviolet and infrared radiation." 



Radiant energy is defined as "any form of energy radiating from a source, as elec- 

 tromagnetic waves, sound, heat, light, X-rays, gamma rays, etc." The chart on p. 3 shows 

 that all of these forms of radiant energy, excepting sound, are included in one continuous 

 electromagnetic spectrum. All of these show a wave nature and have the same speed in 

 a vacuum as that of light, 186,000 miles per second. 



Electromagnetic radiations, being of a wave nature, have the fundamental properties 

 of waves, namely frequency and wavelength. The numerical product of the two is equal 

 to the velocity, 186,000 miles per second, as previously indicated. 



Thus the wavelength of our common electric service, operating at a frequency of 60 

 cycles per second would be 3,100 miles and velocity would be 60 x 3100 = 186,000 miles 

 per second. At the cosmic ray or opposite end of the spectrum the wavelength is four 

 10-trillionths of an inch or 0.0001 Angstrom. It will be noted from the accompanying 

 chart that wavelength is listed in Angstroms (A.), a unit commonly used. It is equal to 

 one 10-millionth of a millimeter, or roughly 4 billionths of an inch. The millimicron 

 (m/i), equal to 10 A., and micron, equal to 10,000 A., are also commonly used units in 

 the visible spectrum band. 



The portion of the spectrum- -ultraviolet, visible, and infrared- -with which we are 

 concerned is relatively small, comprising less than one-third of the entire spectrum. 

 Further, the visible portion- - 3800-7600 A. --is a very small part of the whole radiation 

 spectrum. 



The second section of the chart includes the region we are considering the ultra- 

 violet, visible, and part of the infrared. The remaining infrared (50,000- 1 0,000,000 A.) is 

 shown in the first section. Your attention is called particularly to the curve showing the 

 relative energy of the sun's radiation reaching the earth. I would also invite attention to 

 the mercury and sodium lines shown at the bottom of the third graph as they will be 

 evident in spectral distribution curves of certain lamps which will be shown later. 

 Note the four mercury lines in the visible and seven in the ultraviolet. The single sodium 

 line is at 5893 A. 



The eye sensitivity curve showing the relative response of the human eye to visible 

 radiation may be seen in the center of the bottom graph. This is more readily apparent 

 from a color slide which emphasizes the preponderance of normal eye sensitivity to the 

 green, yellow, and orange with relatively little in the blue and red. Further reference 

 will be made to this curve by Mr. Hollingsworth in his discussion. 



1 Chief, Farm Electrification Research Branch, Agricultural Engineering Research Division, Agricultural Research Service. 



