The relative luminosity of wavelength 600 nm is 0.6. Find the radiant flux of 600 nm needed to produce the same brightness sensation as produced by 120 W of radiant flux at 555 nm.
Relative luminosity = 0.6
Let P be the radiant flux of the source.
∴ Luminous flux = P × 685
Again, relative luminosity
`="Luminous flux of a source of given wavelength"/"Luminous flux of a source of 555nm of same power"`
`therefore0.6="Luminous flux of a source P watt"/(685P)`
So, 200 W radiant flux is needed to produce the same brightness sensation for 600 nm wavelength.