U N S T A I N E D W O O D 4 0 5

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 38, NUMBER 4 APRIL, 1948

The Color of Unstained Wood PARRY M O O N

Massachusetts Institute of Technology, Cambridge, Massachusetts AND

DOMINA EBERLE SPENCER Brown University, Providence, Rhode Island

(Received October 17, 1947)

Reflectances and colors are given for various natural woods. The results are summarized in a table, which gives the coefficients for empirical equations that fit all the spectral reflectance curves, and a second table which gives integrated values obtained by the analytic method. Clear varnish is found to reduce reflectance to approximately 85 percent of that for the unvar­nished wood.

INTRODUCTION

A NOTEWORTHY modern trend in interior decoration is the use of materials having

high reflectances, including unstained woods, for furniture and wall panels. Recent theoretical research1 shows how this trend correlates with

1 Parry Moon and D. E. Spencer, Lighting Design (Addison-Wesley Press, Cambridge, Massachusetts, 1948); "Light distributions in rooms," J. Frank. Inst. 242, 111 (1946); "Lighting design by the interflection method," J. Frank. Inst. 242, 465 (1946); "A simple criterion for quality in lighting," Ilium. Eng. 42, 325 (1947).

good lighting. The use of high reflectances lowers contrasts and tends to produce better visual conditions, while at the same time more usable light is obtained from a given lighting system.

A previous investigation2 gave the reflectances 2 Parry Moon, "Optical reflection factors of acoustical

materials," J. Opt. Soc. Am. 31, 317 (1941); "Colors of ceramic tiles," J. Opt. Soc. Am. 31, 482 (1941); "Wall materials and lighting," 31, 723 (1941); "Reflection factors of floor materials," J . Opt. Soc. Am. 32, 238 (1942); "Reflection factors of some materials used in school rooms," J. Opt. Soc. Am. 32, 243 (1942); "Colors of furniture," J . Opt. Soc. Am. 32, 293 (1942).

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FIG. 1. Spectral reflectance curves for unvarnished woods: A—Faux satine; B—Mahogany; C—Brazilian rosewood; — Experimental curve; - - - Analytic ap­proximation.

and colors of 600 materials used in rooms, in­cluding some stained and varnished woods. In view of the trend toward unstained woods, it was felt that a study of their reflecting possi­bilities would be of interest. Accordingly, spectral reflectance curves were obtained on 35 kinds of varnished and unvarnished woods. These results should indicate the maximum reflectances that can be obtained from unpigmented woods and should be of value to designers and illuminating engineers in their study of proposed lighting systems.

REFLECTANCE AND COLOR

Reflectance curves were obtained for the seventy samples by means of the Hardy spectro-

FIG. 3. Spectral reflectance curve for white harewood (unstained and unvarnished).

photometer.3 Three typical curves are shown in Fig. 1. They can be represented by the equation4

The heavy curves of Fig. 1 were obtained by experiment, while the dotted curves represent Eq. (1) with the constants given in Table I. These constants K0, K1, and K2 were determined by making the analytical representation coincide with the experimental curve at λ = 0.45, 0.55, and 0.65μ. About two-thirds of the samples were fitted by this simple equation. A few of the highly reflecting woods were best represented by straight lines. An example is shown in Fig. 2. The equa­tion is

Four kinds of wood had reflectance curves that were concave downward. An example is shown in Fig. 3. The equation is

FIG. 2. Spectral reflectance curve for plain maple (unstained and unvarnished): — Experimental curve; - - - Analytic approximation.

As shown previously,5 analytic integration can be used to obtain the total reflectance and the colorimetric specification. Once the analytic rep­resentation of the reflectance curve has been

3 We wish to acknowledge the assistance of the Color Measurements Laboratory of the Massachusetts Institute of Technology and to thank the staff, whose kindness made possible this investigation.

4 Parry Moon and D. E. Spencer, "Polynomial represen­tation of reflectance curves," J. Opt. Soc. Am. 35, 597 (1945).

5 Parry Moon and D. E. Spencer, "Analytical representa­tion of trichromatic data," J. Opt. Soc. Am. 35, 399 (1945); "Analytic expressions in photometry and colorimetry," J. Math. Phys. 25, 111 (1946).

TABLE I.

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TABLE II . Reflectance and color of unvarnished wood.

obtained, the analytic integration takes only about five minutes per sample. The results are listed in Table II. Values of x,y, and ρ/ρ0 are given for incident radiation from a Planckian radiator at 2842°K (distribution A')5 and for Planckian radiation at 7000°K (distribution B'). The samples are arranged in order of decreasing reflectance. The highest reflectance, 66 percent,

is obtained with white harewood, while the lowest value, 12 percent, is for Brazilian rose­wood.

The values of Table II are plotted in Figs. 4 and 5. The points representing the reflected radiation are seen to cluster around the Planckian curve but with more points below it than above it. The most nearly neutral reflector is gray harewood (No. 11), while the reddest wood is sapeli (No. 33).

F I G . 4. Colors of 35 unvarnished woods illuminated by 2842° Planckian radiation. The numbers refer to the samples arranged in order of reflectance (Table I). The curve is the Planckian locus. F I G . 5. Colors of the same woods, 7000° Planckian radiation.

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reflectance with varnish is approximately 85 percent of the reflectance without varnish. If a tinted varnish had been used, the reflectance would have been reduced still further.

FIG. 6. Effect of varnish: A Gray harewood, no varnish; B Same material, varnished.

EFFECT OF VARNISH

The foregoing results are all for unvarnished wood and give the highest reflectances that can be obtained, at least without pigmentation. A sample of each wood was also tested when varnished. A single coat of "water-clear" varnish was used, and this varnish caused an almost constant reduction in reflectance throughout the visible spectrum. An example is shown in Fig. 6. The percentage reduction produced by the var­nish differed somewhat for the different woods. On the average, however, it may be said that the

SUMMARY

The paper describes reflectance measurements made on 35 kinds of unvarnished wood. It is found that all samples can be represented by one of three simple equations and that analytic integrations are easily performed. The reflect­ances range from 0.12 to 0.66, while the colors are in all cases somewhat warmer than that of the incident radiation. Tests on similar samples that had been varnished give approximately 85 per­cent of the reflectance of the unvarnished samples.

The modern trend toward higher reflectances in interior decoration is in agreement with theo­retical studies of room lighting.1 Interflection theory shows that to obtain satisfactory diffusion of light for best visual conditions, one must have walls of at least 50 percent reflectance and floor and furniture of at least 30 percent re­flectance.6 The present study indicates that such reflectances can actually be obtained with un­stained woods.

6 Parry Moon and D. E. Spencer, Lighting Design (Addison-Wesley Press, Cambridge, Massachusetts, 1948).