Calculation of the Luminous Efficiency of Ionized Cesium Vapor

Theoretical computations are given for the radiation and electrical characteristics of the cesium discharge for pressure and power input above the range of present experimental technique. For a given pressure and temperature the electron concentration is given by Saha’s equation, the resistivity depends on the temperature and electron concentration and for sufficiently high pressures the power input is not much greater than the total power radiated by the vapor. The power radiated is evaluated from values of the emissivity times the radiant flux of a blackbody at the vapor temperature. Graphical integration gives the total radiant and luminous flux. An optimum luminous efficiency of about 180 lumens per watt is estimated for a fairly wide range of conditions centering around tube diameter 1 cm, pressure 176 mm, and temperature 6000°K. This requires a current of about 300 amperes and a gradient of 22 volts per cm. The brightness temperature is 5400°K and the color temperature roughly 3700°K. At 760 mm the brightness and color are nearly those of a blackbody. Values for various conditions are tabulated.