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I. Soil Color in Perspective

I. Soil Color in Perspective

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REFLECTANCE PROPERTIES OF SOIL



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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

discrepancies.

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

names.

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.



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MARION F. BAUMGARDNER ET AL.



B. NONVISIBLE

REFLECTANCE

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)

reflectance.

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

REFLECTANCEMEASUREMENTS

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



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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

greatly enhanced.



II. INSTRUMENTATION FOR REFLECTANCE

MEASUREMENTS

A. NOMENCLATURE

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

aforementioned ratio.

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).



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MARION F. BAUMGARDNER ET AL.

Normal to sample

Incident flux



I



Viewed flux



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



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