<P> Of the power consumed by typical incandescent light bulbs, 95% or more is converted into heat rather than visible light . Other electrical light sources are more effective . </P> <P> Luminous efficacy of a light source may be defined in two ways . The radiant luminous efficacy (LER) is the ratio of the visible light flux emitted (the luminous flux) to the total power radiated over all wavelengths . The source luminous efficacy (LES) is the ratio of the visible light flux emitted (the luminous flux) to the total power input to the source, such as a lamp . Visible light is measured in lumens, a unit which is defined in part by the differing sensitivity of the human eye to different wavelengths of light . Not all wavelengths of visible electromagnetic energy are equally effective at stimulating the human eye; the luminous efficacy of radiant energy (LER) is a measure of how well the distribution of energy matches the perception of the eye . The units of luminous efficacy are "lumens per watt" (lpw). The maximum LER possible is 683 lm / W for monochromatic green light at 555 nanometers wavelength, the peak sensitivity of the human eye . </P> <P> The luminous efficiency is defined as the ratio of the luminous efficacy to the theoretical maximum luminous efficacy of 683 lpw, and, as for luminous efficacy, is of two types, radiant luminous efficiency (LFR) and source luminous efficacy (LFS). </P> <P> The chart below lists values of overall luminous efficacy and efficiency for several types of general service, 120 - volt, 1000 - hour lifespan incandescent bulb, and several idealized light sources . The values for the incandescent bulbs are source efficiencies and efficacies . The values for the ideal sources are radiant efficiencies and efficacies . A similar chart in the article on luminous efficacy compares a broader array of light sources to one another . </P>

What is the mass of the light bulb