Comparison of Monochromatic Semiconductor Radiation Sources with Tungsten Lamps

The physical efficiency of a light source, ∊, defined as the ratio of radiant power output to electrical power input, which a monochromatic semiconductor radiation source must have to compete with a high temperature radiation source is investigated. Detailed calculations of ∊ are given for infrared and visible illumination. In an infrared system, where an S–1 detector is used, in order for a GaAs diode to be competitive with the infrared produced by a filtered tungsten lamp operated at 3000°K, an ∊ value of 6.4% is required of the diode. For production of visible light a minimum physical efficiency of 4% is required for an optimum monochromatic source which radiates near 555 nm, the peak of the eye sensitivity curve. An efficiency of 100% is required at λ = 445 nm and λ = 642 nm. Monochromatic sources with wavelengths greater than 642 nm and less than 445 nm can never compete with the effective illumination of a 3000°K tungsten lamp for the human eye. Semiconductor light sources are shown to have far higher effective power radiated per unit area of source than tungsten lamps.