The luminous efficiency of the radiant energy most efficient in producing the sensation of light (yellow-green) is about 625 lumens per watt.  That is, if energy of this wave-length alone were radiated by a hypothetical light-source, each watt would produce 625 lumens.  The luminous efficiency of the most efficient white light is about 265 lumens per watt; in other words, if a hypothetical light-source radiated energy of only the visible wave-lengths and in proportions to produce the sensation of white, each watt would produce 265 lumens.  If such a white light were obtained by pure temperature radiation—­that is, by a normal radiator at a temperature of 10,000 deg.F., which is impracticable at present—­the luminous efficiency would be about 100 lumens per watt.  The normal radiator which emits energy by virtue of its temperature without selectively radiating more or less energy in any part of the spectrum than indicated by the theoretical radiation laws is called a “black-body” or normal radiator.  Modern illuminants have luminous efficiencies ranging from 5 to 30 lumens per watt, so it is seen that much is to be done before the limiting efficiencies are reached.

The amount of light obtained from various gas-burners for each cubic foot of gas consumed per hour varies for open gas-flames from 5 to 30 lumens; for Argand burners from 35 to 40 lumens; for regenerative lamps from 50 to 75 lumens; and for gas-mantles from 200 to 250 lumens.

In the development of light-sources, of course, any harmful effects of gases formed by burning or chemical action must be avoided.  Some of the fumes from arcs are harmful, but no commercial arc appears to be dangerous when used as it is intended to be used.  Gas-burners rob the atmosphere of oxygen and vitiate it with gases, which, however, are harmless if combustion is complete.  That adequate ventilation is necessary where oxygen is being consumed is evident from the data presented by authorities on hygiene.  A standard candle when burning vitiates the air in a room almost as much as an adult person.  An ordinary kerosene lamp vitiates the atmosphere as much as a half-dozen persons.  An ordinary single mantle burner causes as much vitiation as two or three persons.

In order to obtain a bird’s-eye view of progress in light-production, the following table of relative luminous efficiencies of several light-sources is given in round numbers.  These efficiencies are in terms of the most efficient (yellow-green) light.

Efficiency
in per cent. 
Sperm-candle                                    0.02
Open gas-flame                                   .04
Incandescent gas-mantle                          .19
Carbon filament lamp                             .05
Vacuum Mazda lamp                               1.3
Gas-filled Mazda lamp                           2 to 3
Arc-lamps                                       2 to 7
White light radiated by “black-body”           16
Most efficient white light                     40