Solar principal plane plot
The BRDF of a uniform surface generally shows most variability in the solar principal plane and so one way to capture this is to plot the directional reflectance in this plane as a function of view angle. View zenith angles increase to 90° either side of a central axis which represents nadir view. Sometimes view angles up-Sun and down-Sun are shown as positive and negative, but there is no agreed convention for which is which. This type of visualisation is especially useful for investigating the size and shape of the hot-spot. In the diagram below, which is based on field measurements, the data from the hot-spot region had to be estimated, as the shadow from the sensor prevented direct measurements. This problem commonly affects field measurements of the hot-spot region.
Solar principal plane plot of the directional reflectance of a grass lawn. The zenith angle of the Sun is approximately 35 degrees (from Sandmeier et. al., 1998, Remote Sensing of Environment, 64, 176-191.
