Question

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.

Answer 1

Given,
Relative luminosity = 0.6
Let P be the radiant flux of the source.
∴ Luminous flux = P$\times$685
Again, relative luminosity$=\frac{\mathrm{Luminous}\mathrm{flux}\mathrm{of}\mathrm{a}\mathrm{source}\mathrm{of}\mathrm{given}\mathrm{wavelength}}{\mathrm{Luminous}\mathrm{flux}\mathrm{of}\mathrm{a}\mathrm{source}\mathrm{of}555\mathrm{nm}\mathrm{of}\mathrm{same}\mathrm{power}}$

$\therefore 0.6=\frac{\mathrm{Luminous}\mathrm{flux}\mathrm{of}\mathrm{a}\mathrm{source}P\mathrm{watt}}{685P}$
$\Rightarrow 685\times P\times 0.6=120\times 685$
$\Rightarrow \mathrm{P}=\frac{120}{0.6}=200\mathrm{W}$
So, 200 W radiant flux is needed to produce the same brightness sensation for 600 nm wavelength.