Lecture 5
Spectral Reflectance Curves
Reflectance
• When specular reflection occurs, the surface
from which the radiation is reflected is
essentially smooth (i.e. the average surface
profile is several times smaller than the
wavelength of radiation striking the surface).
Specular
reflection
• If the surface is rough, the reflected rays go in
many directions, depending on the orientation of
the smaller reflecting surfaces. This diffuse
reflection does not yield a mirror image, but
instead produces diffused radiation. White
paper, white powders and other materials reflect
visible light in this diffuse manner.
• If the surface is so rough that there are no
individual reflecting surfaces, then scattering
may occur. Lambert defined a perfectly diffuse
surface; hence the commonly designated
Lambertian surface is one for which the radiant
flux leaving the surface is constant for any angle
of reflectance to the surface normal.
Lambertian
reflection
Reflectance
• There are no perfect specular and Lambertian surfaces in nature
– most bodies approximate these two end members.
• Diffuse reflections contain spectral information about the
“color” of reflecting surfaces, while specular reflections generally
lack this information.
• Hence in remote sensing, we are mostly interested in measuring
the diffuse reflection properties of terrestrial objects.
Radiation budget equation
The total amount of radiant flux in a specific wavelength (l) incident to the terrain (  i ) must
be accounted for by evaluating the amount of radiant flux reflected from the surface ( reflectedl ),
the amount of radiant flux absorbed by the surface (  absorbed l ), and the amount of radiant flux
transmitted through the surface (transmitted l ):
l
il  reflectedl  absorbed l  transmitted l
Or,
reflected  il  [absorbed l  transmitted l ]
More terminology : Hemispherical reflectance, absorption, and
transmittance
The Hemispherical reflectance (rl) is defined as
the dimensionless ratio of the radiant flux reflected
from a surface to the radiant flux incident to it:
rl 
 reflected
il
Hemispherical transmittance (tl) is defined as the
dimensionless ratio of the radiant flux transmitted
through a surface to the radiant flux incident to it:
tl 
 transmitte d
 il
Hemispherical absorption (al) is defined by the
dimensionless relationship:
 absorbed
al 
 il
Percent reflectance, absorption, and transmittance
These radiometric quantities are useful for producing general statements about the spectral
reflectance, absorption, and transmittance characteristics of terrain features.
If we take the simple hemispherical reflectance (or absorption/transmittance) equation and
multiply it by 100, we obtain an expression for percent reflectance ( r l ):
%
rl 
%
 reflectedl
 il
 100
Spectral reflectance curves
• Spectral reflectance curve is a graph of the spectral reflectance of an
object as a function of wavelength.
• Configuration of the spectral reflectance curves gives us insights into the
spectral characteristics of an object.
• Spectral reflectance curves guide us in selecting wavelengths region(s) in
which remote sensing data should be acquired for the given science goal.
Spectral reflectance curves: Vegetation
Spectral reflectance (%)
Spectral reflectance curves
Red=0.65μm, G=0.55 μm, B=0.45 μm
50
40
30
20
10
Red=0.9μm, G=0.55 μm, B=0.45 μm
0
0.4
0.5 0.6 0.7 0.8 0.9 1.0
Wave length (μm)
Spectral reflectance curves
Bigleaf
Maple
B -1, G -2, R - 3
Coniferous – dark
Deciduous - bright
Douglas
Fir
2090-2350 nm Band 7
LANDSAT ETM+
1550-1750 nm Band 5
775-900 nm Band 4
450-515 nm Band 1
525-605 nm Band 2
630-690 nm Band 3
B -2, G -3, R - 4
BANDS
Dr. Nicholas Short, Remote Sensing Tutorial
Spectral reflectance curves: Soil
a: Organic dominated
b: Fresh soil (unaltered)
c: iron altered
d: organic affected
e: Iron rich
Spectral reflectance curves: Water
A: water
a: Ocean
b: Turbid water
c: Water in leaf
Spectral reflectance curves
B -1, G -2, R - 4
Red shades : vegetation (forest,
valley, crops)
Medium greyish-browns - barren
soil + dry grass
Deep blues/black - ocean –
(lighter where sediments are
present)
Blue and white patterns – breakers
Bluish tones + streets - Towns
Spectral signatures and response
• Spectral responses measured by remote sensors is often diagnostic of the type and/or
condition of objects – hence these are called spectral signatures
• However, a preferable term could be spectral response patterns, because signature is a
pattern that is absolute and unique.
• Spectral response patterns are influenced by:
• Atmospheric effects
• Temporal effects
• Spatial effects
• Geometric effects
• Pixel purity