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I. Soil Color in Perspective
REFLECTANCE PROPERTIES OF SOIL
results of soil color research of the preceding 20 years (Rice et al., 1941).
During this period significant progress was made in two areas: (1) the
selection of an array of soil samples which would cover the range of all
possible soil colors and (2) development of color names or descriptions to
describe the color of a soil. In an experiment in 1939 using the 250 soil
samples from the USDA collection, more than 50 U.S. soil scientists and
several from other countries, all with field experience, were asked to name
and record the colors of these samples. Although the names used generally
followed the nomenclature suggested in the Soil Survey Manual, there was
little agreement on an exact color designation for each sample. In fact, the few
observers who were requested to repeat the exercise were unable to duplicate
completely their original color designations, though there were no great
The next step was to find a system of color names that could be sufficiently
standardized to be acceptable to color scientists, sufficiently useful to satisfy
the needs of soil scientists, and sufficiently commonplace to be understood by
the users of soil information. This search for standardized color names
resulted in the tentative adoption of the names used in the Inter-Society
Color Council/National Bureau of Standards (ISCC-NBS) method (Judd
and Kelly, 1939). The assignment of an ISCC-NBS color designation to each
of the samples in the USDA collection resulted in a total of only 56 color
From this research 56 samples were prepared to represent the central color
of each of the designations in the soil color range. Charts, carrying 56
different color chips, were published as the “Soil Color Name Charts,” which
accompanied the “Preliminary Color Standards and Color Names for Soils”
developed by Rice and his colleagues (Rice et al., 1941). With use of and
experience with color chip matching, it naturally followed that refining and
updating of the method would occur. During the 1940s a committee of the
US. Soil Survey, chaired by E. H. Templin, replaced the earlier charts with a
much wider selection of colors. The Templin committee used 202 regular
Munsell standards instead of the 56 special colors in the early charts. They
also made adjustments in the names (Pendleton and Nickerson, 1951).
Today the standard “Munsell Soil Color Charts” are published on charts
representing 7 different hues and containing 99 different standard color chips
(Munsell Color, 1975).
Great progress was made during the period from the 1920s to the 1950s in
the standardization of methods of measuring and designating soil color.
Today the Munsell soil color notations are widely used throughout the
world. However, the fact remains that the designation of soil color as
normally made in the field or laboratory is subjective and nonquantitative.
MARION F. BAUMGARDNER ET AL.
Remarkable improvements and changes have occurred over the past three
decades in the development of laboratory and field instrumentation for
observing and measuring physical and chemical phenomena. An area of
development as it relates to soil color is an array of new instruments which
scan a wide range of the electromagnetic spectrum and record quantitatively
the intensity of energy radiating from a specific material or scene. In soil
studies it is possible to measure soil reflectance in the laboratory or in situ and
obtain spectral curves which plot intensity of reflectance in the ultraviolet,
visible, and infrared portions of the spectrum.
Although spectrometers have been used by analysts in the laboratory for
many years, new designs of instruments have extended the use of spectroradiometers to many new applications. One of the driving forces of some of these
applications has been the kinds of sensors which have been and are being
designed for earth observation systems, primarily involving sensors on
aerospace platforms. Since atmospheric attenuation severely limits the use of
ultraviolet measurements from such platforms, this article will not discuss the
application of ultraviolet radiation to the study of soils.
On the other hand, the atmosphere is a relatively open window to the
longer visible wavelengths and to infrared reflectance (Gates, 1962, 1963).
For this reason special attention is given to visible and infrared (nonvisible)
With the increasing availability and continuing improvement of these
spectroradiometers during recent years, there has been an expanding interest
among soil scientists in developing techniques to obtain more precise
quantitative reflectance (visible and infrared) measurements of soils (Baumgardner and Stoner, 1982; Cipra et al., 1971b; Condit, 1970, 1972; DaCosta,
1979; Gausman et al., 1977; Karmanov, 1970; Montgomery, 1976; Obukhov
and Orlov, 1964; Stoner, 1979).
c. NEEDFOR QUANTITATIVE
Ever since soil science evolved into an important discipline for study and
research, color has been one of the most useful soil variables in characterizing
and describing a particular soil (Kohnke, 1968; Pendleton and Nickerson,
1951; Soil Survey Staff, 1975). The quantity and quality of soil components
and the variable conditions under which soils are observed affect soil color.
Our commonly used “measurement” of soil reflectance, usually confined to
the visible, is at best semiquantitative. Both in the field and the laboratory the
assignment of a specific soil to a specific Munsell notation or category is
REFLECTANCE PROPERTIES OF SOIL
subjective and is limited to the visible portion of the spectrum and by the
number of Munsell color chips.
Numerous studies in recent years have shown relatively high correlations
between soil reflectance and certain other physical and chemical properties of
soils (Baumgardner and Stoner, 1982; Da Costa, 1979; Montgomery, 1976;
Pazar, 1983; Stoner, 1979; Stoner and Baumgardner, 1981). It has also been
noted that the environmental conditions under which soils have been formed
affect soil reflectance (Montgomery, 1976; Stoner, 1979). If these relationships among soil reflectance and chemical and physical properties can be
established quantitatively and definitively for given environmental conditions, the capacity to extract useful soils information from sensor data
obtained by current and future earth observation satellite systems will be
II. INSTRUMENTATION FOR REFLECTANCE
Reflective optical radiation is defined as propagating electromagnetic
energy with characteristic wavelengths between 0.4 and 3 pm. When +tical
radiation interacts with a surface, a portion of that radiation is either
absorbed in the material below the surface or is transmitted through the bulk
of the material through another surface into another medium. The remainder
of the radiation is said to be reflected from the surface. In general terms, the
ratio of the reflected radiation to the total radiation falling upon the surface is
defined as reflectance. This is contrasted to reflectivity, which is an intrinsic
material property. Reflectance is the result of a measurement concerning the
In order to expedite the discussion of reflectance, it is convenient to
introduce some radiometric terminology. Irradiance is the optical radiative
power falling on a unit area of surface. It has the units of watts per square
meter and is usually denoted by the symbol E . If the distribution of the power
per unit area with respect to wavelength is being described, a related term
called spectral irradiance is used. It has the units of watts/(m* - pm). The
term most frequently used to describe reflected radiation is that of radiance,
denoted by the symbol L. It has the units of watts/(m2 - sr), where sr is the
abbreviation for the unit of solid angle, the steradian. The spectral quantity
associated with radiance is called spectral radiance and carries the units of
watts/(m2 - sr - pm).
MARION F. BAUMGARDNER ET AL.
Normal to sample
FIG. 1. Geometric parameters describing reflection from a surface: 0, zenith angle; 4,
azimuth angle; o,beam solid angle; a prime on a symbol refers to viewing (reflected)conditions.
Figure 1 illustrates the basic geometric relationships between incoming
radiation and outgoing radiation using the previously described terminology.
The reflecting properties of a surface are most precisely described using a
parameter called the bidirectional rejectance distribution function (BRDF).
The defining equation for the BRDF is
The angles are shown in Fig. 1. The BRDF is the ratio of a radiance to an
irradiance; therefore, it has the units of sr-'. If the numerator and denominator of the expression are spectral quantities, then a spectral BRDF has been
defined and is usually denoted by the symbol A. A careful examination of Fig.
1 reveals that the BRDF is the ratio of two differential solid angles. This is a
mathematical abstraction that is closely realized by many physical situations
in which the incident and reflective solid angles are small enough to
approximate the differential case. The physically measured BRDF is therefore an average fr value over the parameter intervals. The incident and
reflected solid angles, however, need to be small to obtain a good estimate of
the true BRDF.
The measurement of the BRDF is, however, a particularly difficult
problem. It would be necessary to place a sensor at the surface to measure the
incoming radiation and then take that sensor, or another sensor, and place it
in the viewing position necessary to measure the reflected radiation. Although this represents a possible approach, an experimentally more convenient method uses a reflectance standard in the measurement procedure. The