Question

A screen recieves $3$ watt of radiant flux of wavelength $6000$ $\mathring{A}$. One lumen is equivalent to $1.5\times {10}^{-3}$ watt of monochromatic light of wavelength $5500$ $\mathring{A}$ is $1.00$, then the luminous flux of the source is ___

• A. $4\times {10}^3$ $lm$
• B. $3\times {10}^3$ $lm$
• C. $2\times {10}^3$ $lm$
• D. $1.37\times {10}^3$ $lm$

Answer 1

The correct option is D $1.37\times {10}^3$ $lm$

radiant flux in lumens = $\frac{3}{1.5\times {10}^{-3}}=2000lm$

relative luminosity at 6000 angstorm is 0.685

relative luminosity at 5500 angstorm is 1

luminous efficiency = $\frac{0.685}{1}$

therefore luminous flux = luminous efficiency $\times$ radiant flux = 0.685 $\times$2000 = $1.37\times {10}^{3}lm$