Which of the following is a measure of the amount of electromagnetic radiation leaving a surface?

Radiance is a measure of the amount of electromagnetic radiation leaving a surface. It quantifies the power of radiation emitted, reflected, transmitted, or received per unit solid angle per unit projected area. Radiance takes into account both the intensity of the radiation and the geometry of the radiation's propagation.

This concept is particularly important in fields such as remote sensing, where understanding how different surfaces emit and reflect electromagnetic energy can affect the interpretation of data captured by sensors. Radiance is measured in watts per square meter per steradian (W/m²/sr), providing a comprehensive view of how much energy is radiating from a surface in a specific direction.

The other options do not represent the measure of electromagnetic radiation leaving a surface in the same way. Reflectance refers to the fraction of incident light that is reflected by a surface, but does not describe the total outgoing radiation. Transmittance measures the fraction of incident light that passes through a medium, and absorptance describes the fraction of radiation absorbed by a surface. Each of these terms focuses on different aspects of radiation interaction with surfaces rather than the quantification of radiation emitted into space.