industrial chemistry for teachers
NORMAN SLAGG' Wertinghowe Corporation
Chemistry and light Generation
Bloomfield, N e w Jersey
Both scientists and laymen express surprise that :I physical chemist is involved in light generation research. This paper was writter. with the idea of showing just how much chemistry is involved. The major problems in the lamp industry, such as finding new light sources, improving standard products and maintaining quality control are, for the most part, chemical in nature. Metallurgy and ceramics with their complementary chemistry are also significant, whereas the electrical and physical problems are not as urgent today as they once were when the light sources were first developed. The three most popular types of light sources arc incandescent, fluorescent, and high pressure lamps. The characteristics of these light sources are given in Table 1. Incandescent Lamps
equal to the fraction emissivity of the emissive power of a black body at the specified temperature. I n the lamp industry, the two most. important characteristics of a light source are its maintenance and its efficiency. Maintenance refers to the change in efficiency with time. The efficiencies are measured in lumens/watt of electrical power. The lumen is a measure of light flux corrected for the seusitivity of the human eye. This visibility factor is taken as 1 at 5540 A, and decreases to nearly zero at 4100 and 7200 A. It is seen in Figure 1 that the higher the temperature, the greater the fraction of emission in the visible region (I). If one corrects for sensitivity of the eye, it becomes apparent that the lumens/watt is increasing. Of course the efficiencyof a non-black body will be less 100 I
Incandescent lamps produce light by using electrical energy to heat a tungsten filament to such a temper* ture that it emits radiation in the visible region (See Fig. 1). If the temperatureis too high the filament can melt or evaporate very rapidly. The filament is an imperfect black body that emits light along the same general lines as a black body, however, not with the same efficiency. The formula for a black body radiator is (1)
where Eh is the emissive power in watts/cm2/sec at wavelength A, fib is the energy per unit volume at wavelength A, h is Planck's constant, k is Boltzmann's constant, cis the velocity of light, and T is the temperature in "IC. For a non-black radiator, the emissive power is Paper presenled a t the 2nd Middle Atlan1.i~Regional Meeting of the ACS in New York. Februarv. " , 1967. ' Present address: Picatinny .4rrenal, Dove]., N. J
EDITOR'S NOTE: ''Chemistry and Light Generat,ion," by Dr. Norman Slagg introduces Industrial Chemistry for Teachers, a feature which we hope will help decrease the inforrnatiou gap between indmtrial and academic chemistry. While this feature is scheduled for only four appearnnces in 1968, we hope to make it a. monthly item in 1969.
Figurs 1. length.
Emission from a black body.
Relative energy versus wove-
Volume 45, Number 2, Februory 1968
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Table 1.
Lamp Incandescent a. Normal b. Halogen containing
Fluorescent
High Pressure a. Hg
h. Additive
Source of Visible Light Emission from hot W filament
... Conversion of uv from low pressure dischssge to visihle by phosphor
Lamp Characteristics
Chemical Components
Electrodes
Efficiency Lumens/ Watt
Important Temperature Features
...
W filament, argon W filament 2300-330O0K. gas, silicate g l w Wall of normal type body -330375' K, ... Above plus haloHalogen type, -525gen containing 1000'K compormd, q~8l'tzbody. W coil with an alka- Hg for discharge, Electron temperature of line earth oxide and phosphor discharge -10,000~K emission material various types
Discharge excitation As above though of Hg vapor to mass is different emit visible Discharge, ExcitaW coil tian of inorganic a additive and Hg
Gas temperature -320°K Wall temperature -320PK Hg, argon, quartz Gas andelectron temperabody alkaline ture -6000°K; Wall earth oxide emistemperature -900°K sian mixture in tungsten coil. Argon, Hg, NaI, Gas & electron temperaTII, In, or other, ture 5000-6000DK quartz body Wall 900-1000°K depend-
.
Life (hr.1
15-20
1000
15-20
>ZOO0
-80
10,000
55
24,000
80-100
-10,000
100-140
-10,000
cerimic body; refractory metal Wall temperature 1000end e m 1'tnnDK Table 2.
Material C W ZrN HfN TaN ZrC H fC NbC Figure 2.
Rate of ernision from o block body at 5500A. the peok of eye
sensitivity.
hut the trend is the same. For tungsten, the emissivity a t 5000 A is about 0.45. In Figure 2 it is seen that the rate of emission at 5500 A (eye sensitivity peak) increases rapidly with temperature, and above 3000°1
