ASD FieldSpec 3 User Manual page 67

www.asdi.com Chapter 7 Field Measurements
Figure 7-1 The major sources of illumination.
Note: It is possible to have several sources of light scattered off of surrounding
objects, each with its own unique spectral distribution.
Parameters such as solar elevation angle and atmospheric conditions will
effect the overall intensity and spectral characteristics of direct solar
illumination. Diffuse skylight illumination can contribute as much as 5-10%
of the total illumination reaching a surface. At shorter wavelengths, diffuse
skylight can contribute as much as 20-25% of the total.
The spectral characteristics of the illumination scattered off of surrounding
objects is determined by their reflectance characteristics. In the case of a
forest clearing, as much as 20% of the illumination in the 750-1200 nm
wavelength range can be attributed to sunlight scattered off the surrounding
forest canopy.
One important source of surrounding scattered light is the person and the
instrumentation making the measurement. Objects in the surroundings also
effect the overall illumination of the target surface by obscuring a portion of
the diffuse skylight and, possibly, shading the target from direct solar
illumination.
The magnitude of both the diffuse skylight and scattered from surrounding
illumination components is determined by the solid angle subtended by these
sources when viewed from the reference frame of the target surface.
The surface texture of the material being measured also affects the relative
portion of the various sources of illumination. When compared to a smooth
surface, a surface with a rough texture will tend to have a higher portion of
illumination from the diffuse and scattered-from-surroundings sources
relative to the direct solar illumination.
ASD Document 600540 Rev. J 57 FieldSpec® 3 User Manual