What is the difference between radiance and reflectance?

Answer 1

Radiance is the variable directly measured by remote sensing instruments. Basically, you can think of radiance as how much light the instrument "sees" from the object being observed. When looking through an atmosphere, some light scattered by the atmosphere will be seen by the instrument and included in the observed radiance of the target. An atmosphere will also absorb light, which will decrease the observed radiance. Radiance has units of watt/steradian/square meter. Reflectance is the ratio of the amount of light leaving a target to the amount of light striking the target. It has no units. If all of the light leaving the target is intercepted for the measurement of reflectance, the result is called "hemispherical reflectance." Reflectance (or more specifically hemispherical reflectance) is a property of the material being observed. Radiance, on the other hand, depends on the illumination (both its intensity and direction), the orientation and position of the target and the path of the light through the atmosphere. With effort, many of the atmospheric effects and the solar illumination can be compensated for in digital remote sensing data. This yields something which is called "apparent reflectance," and it differs from true reflectance in that shadows and directional effects on reflectance have not been dealt with. Many people refer to this (rather inaccurately) as "reflectance." For most of the vegetation indices in this FAQ, radiance, reflectance, and apparent reflectance can be used interchangeably. However, since reflectance is a property of the target material itself, you will get the most reliable (and repeatable) vegetation index values using reflectance. Apparent reflectance is adequate in many cases. See the related link.