Wm2sr1

Wm2sr1 is the SI unit for radiance, commonly written as W·m^-2·sr^-1. Radiance measures the amount of radiant energy emitted, reflected, or transmitted from a surface per unit area in a given direction per unit solid angle. It is defined as L = d^2Φ/(dA dΩ), where Φ is radiant flux, dA is differential area, and dΩ is differential solid angle.

Radiance is distinct from irradiance (W m^-2), which is energy arriving at a surface per unit area, and from exitance (W m^-2), the total emitted or reflected energy per unit area. It is also related to luminance in photometry through spectral weighting, but radiance uses radiometric units.

In optics and radiative transfer, radiance is important because it is conserved along a ray in lossless media (the etendue invariant), making radiance a robust descriptor of brightness through optical systems.

Applications include remote sensing, astronomy, climate science, computer graphics, and optical engineering. Radiance can be measured with calibrated imaging radiometers, spectroradiometers, or radiance meters, and is often specified per wavelength or integrated over ranges to yield spectral radiance or total radiance.

Note: Wm2sr1 is a non-normative textual representation of W m^-2 sr^-1; the standard notation uses superscripts and the dot times sign. In coding contexts, the string may appear as W/m^2/sr or Wm^-2 sr^-1 depending on formatting.