Question

The unit of spectral radiance is ...................................

• A. $\mathrm{W\ sr^{-1}\ \mu m^{-1}}$
• B. $\mathrm{W\ m^{-2}\ sr^{-1}}$
• C. $\mathrm{W\ m^{-2}}$
• D. $\mathrm{W\ m^{-2}\ sr^{-1}\ \mu m^{-1}}$

Hint:

Radiance $⇒$ area & direction & spectrum all at once:
Irradiance $E$ : $\mathrm{W\,m^{-2}}$ (no sr, no spectral term).
Radiant intensity $I$ : $\mathrm{W\,sr^{-1}}$ (no area, no spectral term).
{Spectral radiance} $L_\lambda$ : $\mathrm{W\,m^{-2}\,sr^{-1}\,\mu m^{-1}}$.

Answer 1

The Correct Option is D

Spectral radiance $L_\lambda$ is the radiant power per unit projected area, per unit solid angle, and per unit wavelength.
Therefore the unit is power divided by area, solid angle, and wavelength interval: $\mathrm{W}/(\mathrm{m^{2}}.\mathrm{sr}.\mu\mathrm{m})=\mathrm{W\ m^{-2}\ sr^{-1}\ \mu m^{-1}}$.