266 HANDBOOK OF PHOTOGRAPHY 



off the light entirely (as in the case of motion-picture projection), the highest frequency 

 at which fhcker is just apparent is called the "critical frequency." For a field bright- 

 ness of 1 candle per sq. m., the critical frequency is 30 cycles; it is about 38 cycles at 

 10 candles per sq. m., 22 cycles at 0.1 candles per sq. m., 15 cj'^cles at 0.01 candles per 

 sq. m. and about 12 cycles at 0.001 candles per sq. m. These figures illustrate the 

 importance of taking into consideration the general brightness of the image on the 

 screen in determining optimum shutter speed. 



The eye has a wide adaption to light stimulus and is sensitive over a wide range. 

 When the relative sensation of the eye is plotted against the logarithm of the stimulus 

 (logarithm of field brightness in candles per square meter) as independent ordinate, an 

 s-shaped curve results, covering the tremendously large range of from 0.001 to 10^ 

 candles per sq. m. as shown in Fig. 5. The fact that relatively little change in sensa- 

 tion results from a large change in stimulus (when static conditions are assumed) 

 indicates how unsuited the eye is as a substitute for objective types of instruments in 

 determining light values. 



Production of Light. — Visible light may be produced on a practical scale in two 

 ways: (1) by increasing the temperature of the source until it is incandescent, i.e., 

 until some of the energy radiated falls within the visible region of wavelengths; and (2) 

 by conduction of electricity through gases. The first of these methods is represented 

 by the familiar tungsten fUament lamp. Mercury-vapor lamps, neon signs, and 

 sodium-vapor lamps are among the more common examples of the production of Ught 

 by means of gaseous conduction. 



In incandescent sources, light is produced by passing current through a fine wire 

 contained in a glass enclosure which is either evacuated as completely as possible of 

 the residual gases or is thoroughly evacuated after which an inert gas at low pressure is 

 introduced into the enclosure. The purpose of the inactive gas is to increase the life 

 of the lamp and to prevent the blackening of the enclosing bulb. 



Other incandescent sources include the flames of candles, kerosene lamps, Wels- 

 bach mantles, gas burners, etc. In a popular sense a flame is a gas burning in air. 

 The production of the flame is, however, not confined to a gas burning in air but is a 

 phenomenon observed when any two gases undergo combustion accompanied by the 

 evolution of light and heat, as when hydrogen burns in chlorine to produce hydrogen 

 chloride. The luminosity of flames depends, in a complex manner, on a number of 

 factors. In general, however, it may be said that: (1) the luminositj^ of a flame is 

 dependent upon, or is influenced by, the glowing of solid matter, such as carbon, heated 

 to incandescence; (2) the luminosity is increased by increasing the pressure of the 

 burning gas; and (3) the luminosity is increased by raising the temperature of the 

 burning gas. 



In producing visible light through the use of gaseous conduction, the most common 

 method is to fill, with a gas or vapor, a glass tube having metallic electrodes at its ends 

 and applying a voltage between these electrodes. The voltage must be sufl^cientlj^ 

 high to break down the gas molecules and produce ionization together with radiation 

 in the visible spectrum. Radiations outside of the visible region are also usually 

 produced as well but are of no importance for purposes of illumination. 



Light Spectra. — If the white light from the sun is analyzed, as by passing it through 

 a prism or reflecting it from a finelj'- ruled grating, it will be found to be composed of a 

 continuous band of all the visible colors: red, orange, yellow, green, blue, and violet. 

 Light from an incandescent lamp will also form a continuous band of colors when 

 analyzed, but since this form of light is yellow rather than white, the Colors from an 

 incandescent lamp (operated at temperature which will result in a useful life of the 

 lamp of about 1000 hr.) will be weaker in the green, blue, and violet regions than is 

 the case with the white light from the sun. If we pass the light from a mercury-vapor 



