Februarv 1, 1921 



THE INDIA RUBBER WORLD 



331 



produces a colored light. All arlificial lights are more or less 

 colored. 



THE TWO DIFFERENT KINDS OF LIGHT 

 By the use of glass prisms, or other devices, it is possible to 

 separate the colors contained in any given kind of light, i.e., to 

 produce the spectrum of that light. The rainbow is a natural 

 spectrum of sunlight, and is called the "solar spectrum." The 

 noticeable thing about this band of colors is that each color blends 

 by the most perfect gradations into adjacent colors; the spectrum 

 is continuous, there are no gaps, nor abrupt changes. This is 

 true of all light produced from glowing splids or liquids. If, 

 however, a vapor be heated to the point of incandescence or lumi- 

 nosity, and a spectrum formed of its light, it will be found that 

 this spectrum is not continuous, but consists of certain lines of 

 color separated by gaps or bands of darkness. This is called a 

 line spectrum; and each kind of vapor has its own particular 

 arrangement of color lines. One of the forms of electric lamps 

 produces its light from glowing vapor (of mercury) and its light 

 therefore differs from all other kinds of light in common use in 

 that it has a line spectrum. 



MEASUREMENT OF LIGHT 



Light in the sense of energy can be measured. The most 

 familiar measurement is the one expressed in candle-power. To 

 speak of a 10-candle-power light is like speaking of a 40-horse- 

 power motor. In each case the power developed from one source 

 is compared to the power generated by another source taken as a 

 unit; both are relics of the crude beginnings of scientitic meas- 

 urement. Candles and horses are .themselves extremely variable 

 in their powers, while the sole virtue of a unit is its constancy. 

 However, very definite values have been evolved for these units 

 so that their names are now only metaphorical. It is important 

 to keep in mind that candle power does not signify quantity of 

 light; it ret'er^ (inly to the intensity (if the radiation in one direc- 

 tion, the horizontal. 



The instrument for measuring candle-power is called a pho- 

 tometer. It consists of an arrangement by which two surfaces, 

 one illuminated by a standard light (theoretically a candle, but 

 actually an electric lamp), and the other by the light to be meas- 

 ured, can be seen side by side, and the brightness of one or the 

 other varied until the two surfaces appear equally bright. The 

 means generally used to vary the brightness is by changing the 

 relative distances of the two lights from the observed surfaces, 

 which are called the screen. 



One of the fundamental laws of light is what is called "the law 

 of inverse squares." and states that "the intensity of light varies 

 inversely as the square of the distance from the source." Thus, 

 if the intensity of light in a given direction at 1 foot from a lamp 

 is I c.-p. (candle-power), at 2 feet from the lamp it will be 

 %-c.p., at 3 feet, 1/9-c.p., etc. So, if the light to be measured 

 has to be removed 4 times as far from the photometer screen as 

 the standard lamp to bring the surfaces to the same brightness, 

 then the light is 16 times as intense as the standard. 

 MEASUREMENT OF ILLUMINATION 

 A surface is illuminated when light falls upon it, and the 

 intensity of illumination varies inversely as the square of the 

 distance of the surface from the light source. To express the 

 degree of intensity of illumination, therefore, a unit of distance 

 must be used in connection with a unit of light. The units com- 

 monly used are the font and candle, thus forming the foot-candle 

 which is the intensity of illumination of a surface placed one foot 

 from a light of one candle-power. 



An instrument for measuring illumination is properly called an 

 illuminometer. Such instruments operate on the same principle 

 as photometers. A very small and compact instrument of this 

 kind has been put on the market under the name of "Foot-candle- 

 meter," which is not as accurate as the larger instruments, but is 

 quite satisfactory for measurements of actual illumination as dis- 

 tinguished from laboratory conditions. 



Measurements of intensity, whether of light or illumination, are 

 not measurements of quantities, but of degrees of difference, and 

 are similar to the measurements of intensities of heat as degrees 

 of temperature. In order to form a unit quantity in either case 

 it is necessary to introduce an additional unit. Thus, the unit 

 quantity of heat, the calorie, is the amount of heat necessary to 

 raise the temperature of a cubic centimeter of water one degree C, 

 the definite quantity of water being the additional unit. In form- 

 ing the unit quantity of light, a unit of surface (1 square foot) 

 is taken, and when illuminated with an intensity of one foot-candle, 

 it receives a definite quantity of light, which is called the lumen. 

 Intensity in fool-candles multiplied by the number of square feet 

 gives the total quantity of light in lumens falling on a given 

 surface. 



NATURE OF REFLECTION 

 There is one more measurement of light which is useful in the 

 study of illumination, but before considering it we would better 

 examine briefly the subject of reflection. When light falls on a 

 surface, more or less of it is turned back, or reflected. What is 

 not reflected is absorbed, i.e., changed from light into heat. Re- 

 llection is never complete; that is, a surface never reflects as much 

 light as it receives. (There is one exception, but it applies only 

 to certain optical apparatus.) This fact should be carefully 

 noted. The fakes that have been based upon the assumption that 

 light can be increased by reflection are many and varied, and date 

 back for more than a century. If light could be increased by re- 

 flection, perpetual motion would be possible. The proportion of 

 light which a surface reflects, expressed in per cent, is called the 

 coeflicient of reflection of that surface. 



Besides differing in quantity, reflection also differs in quality, 

 according to the character of the surface from which it is re- 

 flected. Thus, if the surface is perfectly smooth, like still water, 

 or plate glass, or burnished silver, the reflection gives us images 

 of objects, or, as we commonly say, we can "see things by its 

 reflection." This is called regular, or specular reflection. If the 

 surface is rough, the reflected light is scattered in all directions, 

 which prevents its forming images, and gives only an effect of 

 brit,'htness to the surface. This is called irregular, or diffuse re- 

 flection. Most surfaces reflect the different colors of light with dif- 

 ferent degrees of completeness, or what is the same thing, absorb 

 the different colors in different amounts, that is, they have "selec- 

 tive absorption." .\ surface which reflects only red light is called a 

 red surface, and so for the other colors. Polished, or shiny sur- 

 faces are tlinse which give a predominant amount of specular 

 reflection ; dull, or mat surfaces are those which give a predomi- 

 nant amount of diffuse reflection. .MI surfaces give some of each 

 kind. Mat surfaces which reflect all the colors equally are white. 



SURFACE BRIGHTNESS 



We are now prepared to consider the other measurement of 

 light, which is called surface brightness, and is the intensity of 

 light reflected or emitted from a surface. The unit used for meas- 

 uring this is called the lambert, and is the brightness of a surface, 

 one square foot of which emits or reflects a light of 2 c.-p. in- 

 tensity. The mille-lanibert is .001 of this unit. 



The (physical) efiiciency of a lighting system is the ratio of 

 the amount of light received on "the working plane," an imagi- 

 nary surface parallel to and 30 inches above the floor, to the 

 total amount of light generated. This is generally expressed as 

 a ratio of watts per lumen. This "efliciency" is much talked of 

 by illuminating "engineers," but is of trifling practical value, for 

 reasons that will appear later. 



So much for the physical measurements of light. While the 

 lighting manager should be familiar with all the terms used, and 

 the general principles upon which they depend, he will have com- 

 paratively little use for them in practice. Let us now give our 

 attention to the mechanical side of the question, that is, to the 

 lamps which generate the light, and the reflectors and other de- 

 vices by which the light is distributed and diffused. 



