The radiant flux is the amount of light energy that is given off by an object each second and is measured in ‘joules per second’ (the physical unit of measurement is known as a watt). A 100W lamp therefore, radiates a total of 100 J of light energy each second. However, a 100 W tungsten lamp only radiates approximately 6 W of visible light, the remainder being radiated as non-visible infrared. The more we compress the energy from the light source into the visible spectrum, so we raise the amount of useful watts of light output. Low pressure sodium lamps emit practically all their light at around 590 nm, as this is very close to the peak sensitivity of the eye, it is highly efficient in terms of the number of lumens per watt. Thus, by concentrating the energy into narrow bands, it may be possible to produce light sources with outputs as high as 160 lumens/W and, in fact, it is this type of light generation that is employed in modern high energy light sources such as the HMI, MSR lamps.