Directional Hemispherical Reflectance (Black Sky Albedo): dhr
The directional hemispherical reflectance is defined as the albedo of a target that is illuminated in a spatially-uniform manner at the top-of-canopy level by a directional light source. i.e., the intensity of the incident radiation is zero in all but one single direction. Based on these illumination conditions, the , the dhr can be said to be:
where the projected solid angle [sr] is given in terms of zenith (θ) and azimuth (ϕ) angle by:
Within RAMI the azimuth angle is counted in an anticlockwise manner from the positive X-axis towards the positive Y-axis (that allow to describe the structural properties of a given RAMI scene). Unless otherwise specified the default illumination azimuth is 0 degree, that is, the illumination direction is parallel to the X-axis and radiation travels from the positive side of the X-axis toward the negative side of the X-axis.
Notice that within RAMI all incident and exiting radiation has to pass through a (virtual) planar reference surface of specified lateral dimension and elevation above the target, and oriented perpendicular to the underlying surface normal.
Within RAMI-IV this measurement applies to all canopy scenes.
Formatting of measurement-file content:
|Content||directional hemispherical reflectance||standard deviation of dhr estimate*|
A real is defined as having 6 digits after the decimal point, e.g., 1.000000.