Page:Journal of the Optical Society of America, volume 33, number 7.pdf/44

 Munsell value. The reflectances corresponding to the principal value steps are seen to be somewhat different from the Munsell-Sloan-Godlove data (24).

Table I, or Figs. 1-9, together with Table II provide the means of complete specification of the given surface color sample. When the sample falls, as it usually does, between adjacent value levels and off the intersection of hue and chroma loci, interpolation is required. First the particular value (or luminous reflectance) is read from Table II. Then hue and chroma (or x, y) are spotted on both the adjacent value-level charts. Finally the hue and chroma (or x,y) for the sample are found by linear interpolation. For instance, to obtain Munsell equivalents for Y=0.4602, x=0.500, y=0.454:

(1) From Table II it is found that for Y=46.02 percent, Munsell value (V) =7.20/.

(2) Since V=7.20/, Munsell hue and chroma will be found by interpolation between the charts for values 8/ and 7/. On Fig. 8, for x=0.500, y=0.454, the Munsell hue is just redder than 10.0YR. Since the difference is less than +0.25 hue step, it usually will be read as 10.0YR. The chroma lies between /14 and /16 at /14.6. On Fig. 7, for x=0.500, y=0.454, the hue falls at 10.0YR, the chroma between /12 and /14, at /13.1.

(3) Since 7.20 is 0.2 of the distance between 7.00 and 8.00, the interpolated hue will be that of value 7/ plus 0.2 of the difference between the hues read from Figs. 8 and 7. Since the hue on Fig. 8 is 10.0YR, and on Fig. 7 is 10.0YR, the interpolated hue will be 10.0YR+[0.2 (10.0YR—10.0YR) ] =10.0YR. Obviously, in this case, the interpolation formula was unnecessary, for hue could be read by inspection. The interpolated chroma will be the chroma at 7/ plus 0.2 of the difference between the chromas as read from Figs. 8 and 7. Since the chroma on Fig. 8 is /14.6, and on Fig. 7 is /13.1, the interpolated chroma will be 13.1+[0.2 (14.6—13.1)]=13.4.

(4) The complete notation for the sample is 10.0YR 7.2/13.4.

The Munsell hues at the higher values are generally so little different on adjacent value levels that they can be read by inspection. Chroma, however, varies considerably at all value levels, and interpolation usually will be required. The hues at the lower values—particularly in the red and red-purple region—will vary considerably between adjacent value levels. An example for this region is given below.

To obtain Munsell equivalents for Y=0.0428,x =0.550, y=0.280:

(1) From Table II it is found that for Y=4.28 percent, Munsell value (V) =2.40/.

(2) Since V=2.40/, hue and chroma are found by interpolation between charts for values 3/ and 2/.On Fig. 3, for x =0.550, y =0.280, the hue is between 2.5R and 5.0R at 3.25R. The chroma lies between /10 and /12 at /11.2. On Fig. 2, for x =0.550, y=0.280, the hue falls between 5.0R and 7.5R at 6.0R, the chroma between /8 and /10 at /9.0. Since 2.40 is 0.4 of the distance from 2.00 toward 3.00, the interpolated Munsell hue will be that of value 2/ plus 0.4 of the difference resulting when the hue read from Fig. 2 is subtracted from the hue read from Fig. 3. Since the hue on Fig. 3 is 3.25R and that on Fig. 2 is 6.0R, the interpolated hue will be 6.0R+0.4 (3.25R—6.0R)=4.9R. Usually it is sufficient to report hue to the nearest 0.5 hue step. Thus, this figure would be rounded to 5.0R. The interpolated chroma will be the chroma at 2/ plus 0.4 of the difference between the chromas read from Figs. 3 and 2. Since the chroma on Fig. 3 is /11.2 and on Fig. 2 is /9.0, the interpolated chroma will be

9.0+[0.4 (11.2—9.0)]=9.9.

(4) The complete Munsell notation for the sample is 5.0R 2.4/9.9.

The protractor adapted for linear measurement which is illustrated in Fig. 10 is convenient for reading between the hue and chroma loci on Figs. 1-9.

The following example is included to illustrate the reverse conversion, that is, from Munsell notation to I.C.I.

To obtain I.C.I. (Y,x,y) equivalents for Munsell hue (H)=5.0R, value (V)=2.4, and chroma (C)=/9.9: