Abstract

Measurements have been made of the spectral energy distribution of daylight (sunlight plus skylight) and skylight in the near-uv and visible region of the spectrum as a function of solar altitude for various atmospheric conditions as measured on planes of different orientation. A characteristic vector analysis was made of the digitized data. From these data, a variance-covariance matrix was computed for the daylight energy data and another for the skylight energy data. The four vectors were capable of accounting for 98.2% of the trace of the variance-covariance matrix for daylight and 99.4% for the skylight data. Spectral reflectances extending from 320 nm to 1000 nm have been obtained for 160 soil samples collected from thirty-six states. Measurements were made of both wet and dry samples, which vary widely in color and reflectance. An examination of the 160 sets of curves indicates that they can be classified into three general types with respect to their curve shapes. A characteristic vector analysis was made of the spectral reflectance data; it showed that by linear combinations of four vectors and the mean curve, each set of data could be reconstituted to a high degree of accuracy (99.9% of the trace). Empirical regression equations have been derived that relate spectral reflectance data at thirty-five wavelengths spaced at 20-nm increments to measurements made at only five specially selected wavelengths. To the extent that soils may be identified by their reflectance characteristics, this abridged technique seems to have sufficient accuracy for the 160 samples that have been measured.

© 1972 Optical Society of America

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References

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  1. J. L. Simonds, J. Opt. Soc. Am. 53, 968 (1963).
    [CrossRef]
  2. H. R. Condit, F. Grum, J. Opt. Soc. Am. 54, 937 (1964).
    [CrossRef]
  3. H. R. Condit, Photogrammetric Engineering 36, 955 (1970).
  4. The atmospheric conditions given in the captions for Figs. 4–14 were indicative of the entire sky.

1970 (1)

H. R. Condit, Photogrammetric Engineering 36, 955 (1970).

1964 (1)

1963 (1)

J. Opt. Soc. Am. (2)

Photogrammetric Engineering (1)

H. R. Condit, Photogrammetric Engineering 36, 955 (1970).

Other (1)

The atmospheric conditions given in the captions for Figs. 4–14 were indicative of the entire sky.

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Figures (43)

Fig. 1
Fig. 1

Diagram illustrating the three principal planes of orientation on which spectroradiometric measurements were made. They include: perpendicular (P–0°) plane, 15° (15–0°) plane, and normal (N) plane. Altitude h, sun S azimuth A.

Fig. 2
Fig. 2

Plot of the daylight mean curve and four characteristic vectors.

Fig. 3
Fig. 3

Plot of skylight mean curve and four characteristic vectors.

Fig. 4
Fig. 4

Spectral distribution of energy on a 15–0° plane from daylight at a solar altitude of 40°. Hazy H, clear C, light clouds LC, overcast O.

Fig. 5
Fig. 5

Spectral distribution of energy on a 15–0° plane obtained on 26 June 1962 from daylight with clear sky, for solar altitudes of 70°, 40°, 20°, and 8°.

Fig. 6
Fig. 6

Spectral distribution of energy on a 15–0° plane obtained on 3 July 1962 from daylight with clear sky, for solar altitudes of 70°, 40°, 20°, and 8°.

Fig. 7
Fig. 7

Spectral distribution of energy on a 15–0° plane obtained on 15 June 1962 from daylight with hazy sky for solar altitudes of 70°, 40°, 20°, and 8°.

Fig. 8
Fig. 8

Spectral distribution of energy on a 15–0° plane obtained on 20 June 1962 from daylight with heavy overcast sky for solar altitudes of 70°, 40°, and 20°.

Fig. 9
Fig. 9

Spectral distribution of energy on a 15–0° plane obtained on 20 June 1962 from daylight with heavy overcast sky for solar altitudes of 15°, 10°, and 8°.

Fig. 10
Fig. 10

Spectral distribution of energy on a P–0°, 15–0°, and N plane obtained on 3 July 1962 from daylight with clear sky for a solar altitude of 60°.

Fig. 11
Fig. 11

Spectral distribution of energy on a 15–180° plane obtained on 15 June 1962 and 27 June 1962 from clear skylight for solar altitudes of 70°, 60°, and 50°.

Fig. 12
Fig. 12

Spectral distribution of energy on a 15–180° plane obtained on 15 June 1962 and 27 June 1962 from clear skylight for solar altitudes of 40°, 30°, and 20°.

Fig. 13
Fig. 13

Spectral distribution of energy on a 15–180° plane obtained on 15 June 1962 and 27 June 1962 from clear skylight for solar altitudes of 15°, 10°, and 8°.

Fig. 14
Fig. 14

Correlated color temperature of skylight and daylight for different atmospheric conditions as a function of solar altitude. Curve 1: clear skylight, 15 June 1962 a.m. and 27 June 1962 a.m. Curve 2: daylight, heavy overcast sky, 20 June 1962. Curve 3: daylight, clear sky, 26 June 1962. Curve 4: daylight, clear sky, 3 July 1962. Curve 5: daylight, hazy sky, 15 June 1962 p.m.

Fig. 15
Fig. 15

Map of the United States showing locations where soil samples were collected.

Fig. 16
Fig. 16

Type 1 curves for a sample of chernozem-type soil.

Fig. 17
Fig. 17

Type 2 curves for a sample of pedalfer-type silt.

Fig. 18
Fig. 18

Type 3 curves for a sample of red quartz and calcite sand.

Fig. 19
Fig. 19

Type 3 curves for a sample of laterite-type soil.

Fig. 20
Fig. 20

Plot of the mean curve and four characteristic vectors.

Fig. 21
Fig. 21

Location of the five selected wavelengths with respect to the curve shape of one of the soil samples.

Fig. 22
Fig. 22

Type 1 curves for a sample of chernozem-type soil. Measured values are shown by lines, predicted values by a series of circles.

Fig. 23
Fig. 23

Type 2 curves for a sample of pedalfer-type silt. Measured values are shown by lines, predicted values by a series of circles.

Fig. 24
Fig. 24

Type 3 curves for a sample of red quartz and calcite sand. Measured values are shown by lines, predicted values by a series of circles.

Fig. 25
Fig. 25

Type 3 curves for a sample of laterite-type soil. Measured values the shown by lines, predicted values by a series of circles.

Fig. 26
Fig. 26

Type 1 curves for a sample of chernozem-type soil. Measured values are shown by lines, predicted values by a series of circles.

Fig. 27
Fig. 27

Type 1 curves for a sample of chernozem-type soil.

Fig. 28
Fig. 28

Type 2 curves for a sand sample containing quartz, rock fragments, and shell fragments. The unusual feature of these curves is that they are flat over such a long wavelength span: 600–1000 nm.

Fig. 29
Fig. 29

Type 2 curves for a sample of quartz and rock-fragment sand.

Fig. 30
Fig. 30

Type 2 curves for a sample of quartz and carbonate sand.

Fig. 31
Fig. 31

Type 2 curves for a pedocal-type soil sample.

Fig. 32
Fig. 32

Type 2 curves for a pedocal-type soil sample. Note the small difference in reflectance between the dry and wet curves.

Fig. 33
Fig. 33

Type 2 curves for a sample of quartz sand.

Fig. 34
Fig. 34

Type 2 curves for a carbonate sand sample.

Fig. 35
Fig. 35

Type 2 curves for a pedalfer-type soil sample.

Fig. 36
Fig. 36

Type 2 curves for a pedocal-type soil sample of very high reflectance.

Fig. 37
Fig. 37

Type 2 curves for a sample of quartz sand collected at Ft. Walton Beach, Fla. This sample was found to have the highest spectral reflectance of any thus far measured, reaching a value of 79% at 1000 nm.

Fig. 38
Fig. 38

Type 3 curves for a sample of a very low reflectance, pedalfer-type soil. Note the very small difference in reflectance in the wet and dry curves.

Fig. 39
Fig. 39

Type 3 curves for a sample of a pedalfer-type soil.

Fig. 40
Fig. 40

Type 3 curves for a sample of a laterite-type soil.

Fig. 41
Fig. 41

Type 3 curves for a sample of a pedalfer-type soil.

Fig. 42
Fig. 42

Type 3 curves for a sample of a pedalfer-type soil.

Fig. 43
Fig. 43

Type 3 curves for a sample of quartz sand with a hematite stain.

Tables (4)

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Table I Correlated Color Temperature Values

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Table II Wavelength-Dependent Additive Term and Regression Coefficients for Soils

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Table III Standard Error of Estimate

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Table IV Evaluation of the Match Between Predicted and Measured Spectral Reflectance Data

Equations (1)

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R = a 0 , λ + a i , λ R 440 + a 2 , λ R 540 + a 3 , λ R 640 + a 4 , λ R 740 + a 5 , λ R 860 .

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