The luminous flux of a monochromatic source of 1 W is 450 lumen watt-1. Find the relative luminosity at the wavelength emitted.

Sol. The luminous flux of the given source of 1W is 450 lumen/watt ∴ Relative luminosity = Lu min ous flux of a source of given wavelength/ Lu min ous flux of 555 nm source of same power = 450/685 = 666% [∴ Since, luminous flux of 555nm source of 1W = 685 lumen]