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Colorimetric analysis of hair color.

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COLORIMETRIC ANALYSIS O F H A I R COLOR
B. B. GARDNER
Hurvard Oniversit y
AND
D. L. M A c A D S M
Massachusetts Institute of Technology
SIX FIGURES
The following report is the result of a study of a series of
fifty-three hair samples for the purpose of determining the
colorimetric characteristics of hair, in the hope that such an
analysis might point the way to a metrical classification
capable of statistical manipulations. The work was done
under the auspices of Dr. E. A. Hooton, of the Division of
Anthropology, H a r v a r d University, and was carried out in
the Colorimetric Laboratory of the Massachusetts Institute
of Technology, whose equipment was placed at our disposal
by Prof. A. C. Hardy.
Spectrophotometric curves of the samples were secured by
use of the recording spectrophotometer designed by Professor Hardy, and built under his direction at the Massachusetts
Institute of Technology. This apparatus was described before
the Optical Society of America, meeting in New York,
February 24, 1934. A full description of the optical and
mechanical details will be published in the near future. This
recording photoelectric apparatus f o r measuring the spectral
reflectance of all kinds of samples is a n improvement over
the spectrophotometer described by Professor Hardy ( '29).
The present system consists of a double monochromator, which
is followed by a photometer unit consisting of two Rochon
prisms and a Wollaston prism. The Wollaston prism sepa187
A X E R I C A N J O U R N A L OF PHYSICAL AXTHROPOLOQY, VOL. XIX, N O .
JULY-SEPTEMBER,
1934
2
188
B. B. GARDNER A N D D. L. MAC ADAM
rates the beam leaving the monochromator into two beams,
one of which falls on a standard white magnesium oxide
surface and the otner on the sample. The beams are made
to flicker alternately by passing them through a rapidly rotating Rochon prism which, by its property of transmitting
only light polarized in a plane fixed to it, alternately extinguishes the beams from the Wollaston, which are polarized in
planes perpendicular to each other. The light reflected from
the standard of reflectance and the sample is gathered by an
integrating sphere and acts on a photoelectric cell. The
current from the cell is amplified and acts on the field of a n
alternating current motor, which, through a cam, rotates the
Rochon placed ahead of the Wollaston prism in such a manner
as to alter the relative intensities of the beams falling on the
sample and standard until the amounts of light reflected from
these a r e equal. I t is evident that when this condition is satisfied the reflectance of the sample relative to the standard is
the ratio of the intensities of the beams. This ratio is a
function of the angle of rotation of the cam controlled Rochon ;
therefore, a shaft, driven by a balancing motor, can move a
recording pen along a per cent reflectance axis, while another
motor, which continuously changes the wavelength of the
light used f or the measurement, causes a table containing
the record paper to move along a wavelength axis perpendicular in direction to the reflectance axis. Since both beams
traverse the same optical parts, the effect of selective absorption or selective reflection is completely eliminated; and since
only one photoelectric cell is used, and that one only for detecting inequality in reflected energy coming alternately from
sample and standard at a high rate of alternation, the readings
a r e not affected by variations in the light source, the photoelectric cell, or in the associated amplifier.
A series of fifty-three hair samples were collected which
ranged in color from black to very blond and included a wide
selection of reds. It was impossible a t the time to obtain a
series including all possible shades, but special efforts were
made to include the extreme types as well as the intermediate
189
COLORIMETRIC ANALYSIS OF HAIR COLOR
ones. The result was a series containing a few very blonds,
several ash blonds, a large number of light and dark browns,
and a great many reds and reddish browns. A few samples
of bleached hair were taken to determine the effect of bleaching on the color. A colorimetric analysis was then made for
each sample and the curves carefully studied.
The most striking characteristic of all curves is their uaiformity and simplicity. Figure 1 shows six of the curves for
samples which a r e classed a s black, dark red, red, reddish
brown, light brown, and very blond, respectively. It is seen
that all the curves a r e quite simple; the black and blonds are
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WAVELENGTH
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700
Figure 1
almost straight lines while the reds show a decided upward
curvature in the region of the green. I t is also evident that
differences in the color a r e due rather to changes in the reflectance in the red end of the spectrum than in the violet end,
since the total range of variation is 5.5 per cent a t 400 mp and
31 per cent a t 7OOmp.
Figure 2 shows very strikingly the differences between the
curves for red hair and those for brown hair. Both samples
show a reflectance of 9 per cent at 700mp and only a slight
difference a t 620 mp which is the limit of the red band. F rom
there on they diverge until, at 540mp they reach a maximum
divergence of 1.3 per cent, which is maintained until they reach
190
B. B. G A R D N E R A N D D. L.
M A C ADAiM
the violet band where they draw together again. I n spite of
the slight difference in the curves, the colors a r e definitely
unlike in appearance; no. 23 is a dark red, while no. 28 is
a medium brown. It is evident that the distribution between
the color of red a n d brown hair is due not so much to the
disparity in reflectance in the red bands of the spectrum as
to a difference in the blue and green bands.
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WAVELENGTH - m y
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400
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500
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WAVELENGTH
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700
Figure 3
Figure 3 presents the curves f o r a series of samples ranging
from dark brown (no. 5 ) to very blond (no. 43). When
graded by eye, the samples fall in the same order as that
shown by the curves, with no. 14 a light brown and no. 13 blond.
It is apparent that each of the browns and blonds gives a practically straight line which differs from the other lines either
in slope or in total reflectance. For instance, the distinction
COLORIMETRIC AWALYSIS OF H A I R COLOR
191
between nos. 14 and 13 is purely a difference in slope of the
curves, while the difference between nos. 7 and 14 is principally one of total reflectance, since the curves a r e parallel.
The blonds show a greater reflectance for all the wavelengths
than do either the browns or reds ; the lightest blond studied
had 15 per cent greater reflectance at 700mp than did the
brightest red.
Comparison of all the curves studied shows that they
range from almost straight lines to those with a definite
curvature. Actually, none of them are perfectly straight lines
I
400
600
500
WAVELENGTH
70 0
-my
Figure 4
as is shown by figure 4 where they have been redrawn to a n
enlarged scale. No. 2 is a dark red, no. 38 a dark brown, and
no. 37 is a medium brown ; and although the two browns show
a, slight curvature, the a r e still sharply distinguishable from
the red. A study of different types of curves show that each
curve could be expressed approximately by taking two values,
one for the point of maximum reflectance, 700 mp, and another
for the point of maximum curvature which lies between
540mp and 550mp for all curves. Because 546mp may be
used as a measure of brightness of the color and may be conveniently secured by use of a mercury arc and filters, it was
arbitrarily selected as the point of maximum curvature, and
192
B. B. G l R D N E R A N D D. L. -MACADAM
the height of each curve was taken a t this point and at 700 mp.
These figures were then plotted on a scattergram, as shown
in figure 5, where they grouped themselves with the darker
colors in the lower left-hand corner.
The samples were then carefully sorted by eye into groups
according to color, and these groupings, marked off on the
scattergram, resulted in the eleven groupings labeled A to
K. While the samples in any’group are, in most cases, distinguishable from each other, they do form distinct series with
a gradation of shades ; and each series is distinguishable from
the groups on eit,her side. F o r example, in group R no. 39
is darker than no. 37, but together with-nos. 15 and 22 they
form a series which, a s a whole, is distinguishable from either
group A or group C. Also, no. 39, although only slightly
lighter in shade than no. 41, is still closer to no. 15 than to
41 and fits better in group B than in group A. A comparison
of all the groups shows that they too fall into three graded
series. Group A to F form one series ranging from black t o
d a r k brown in group A, through medium browns in groups B
a n d C, to light browns in D and E, and blond in group F.
This, as a series, was easily separated from a second series
formed by groups G and H, which a re reddish brown, that
is, brown with a definite reddish tinge. The third series is
formed by groups I, J, and K which are all true reds with
a range of dark, medium, and light. After determining the
various groups and series i t is possible to draw lines OX and
OY which seem to delimit the three series. Line OX, with
a slope of 0.47, is the upper limit of reddish shades. That
means that when hair reflects less than 47 per cent as much
light a t 546 mp wavelength as it does a t 700 mp wavelength,
the eye distinguishes some of the red, but when the ratio of the
reflectance a t the two wavelengths is greater than 47 per cent
the resulting impression is brown. Similarly the line OY,
with a slope of 0.40, separates the reds from the reddish
browns. The separation of these two series is quite sharp as
is well shown by nos. 16 and 54. Though these two samples
vary only a t 0.6 per cent a t 546 mil and are together a t 700 mp,
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REFLECTANCE- AT- 546
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COLORIMETRIC ANALYSIS OF HAJX COLOR
197
nevertheless no. 16 is a reddish brown and no. 54 is a bright
red. Of course, these lines are purely tentative and can be
definitely established only through study of a larger number
of samples. Actually, the lines may not be straight or may
vary slightly from the slopes given here. Study of sufficient
samples may, also, reveal the fact that there are no separate
groups in each series, but that there is, rather, a gradual
transition from one extreme to the other.
A calculation of the complete colorimetric specifications of
the color of twenty of the hair samples was carried out following a recent international standardization of the method of
objective color specification (Judd, '33). This is a method
by which a specification of the appearance of the color t o a
normal observer may be calculated from the reflectance values
measured at every visible wavelength. The end result of this
computation from spectrophotometric data is a set of three
quantities. The first of these is the brightness of the sample
relative to the brightness of a perfect reflector. The second
is the dominant wavelength which f o r convenience of visualization may be thought of as the wavelength of the spectrum
color most closely resembling the sample color. The third
is the purity, which indicates the degree of similarity between
the color of the sample and the color of the pure spectrum light
having its wavelength equal to the dominant wavelength of the
sample. Thus colorless grays and whites have zero purity,
browns and other colors closely resembling grays and whites
have low purities up to perhaps 30 per cent. Colors closely
resembling pure spectrum colors have high purity values u p
to 100 per cent.
The twenty samples studied included one or more from each
of the groups ( A to K ) indicated on the scattergram, figure 5.
The computation was performed on the assumption that the
I.C.I. standard illuminant C, equivalent to average daylight,
was the source of illumination at the time of visual examination. Table 1 exhibits several interesting correlations between the visual classifications and the colorimetric specifications. It is immediately noticed that the brightness, comAMERICAN JOURNAL
OF P H Y S I C 1 1 . A N T H R O P O L O Q Y . VOI,. IIX, NO. 2
198
B. B. GBRDNER A N D D. L. LMACADAM
pared to a magnesium oxide surface in the same illumination,
is higher i n almost every case than the reflectance a t 546 mp.
In most cases this reflectance is about 1 per cent lower than
the computed relative brightness. Class A seems to include
all samples having less than 3 per cent relative brightness,
TABLE 1
Colorimetric Ypecifications f o r twenty hair s a m p l e r
-
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A
25
B
B
32
40
C
35
D
14
E
E
E
35
45
13
48
E
580
I
j
580
580
580
583
F
F
36
44
43
580
580
581
G
46
583
H
47
583
F
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J
J
K
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3.8
4.4
5.5
9.1
10.9
11.4
7.3
41
42
43
17.6
15.4
20.8
588
593
587
i
33
11
585
585
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57
58
51
585
I
60
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52
50
55
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42
32
42
45
2
31
32
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7.8
55
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31
45
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4.7
7.3
3.2
3.6
4.4
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5.4
5.4
8.6
with purity of less than 30 per cent. Classes B to F, inclusive,
are marked by possessing a common dominant wavelength
ranging from 580 mp to 583 mp, while they show uniformly increasing relative brightness and colorimetric purity, Groups
G and H a r e characterized by a dominant wavelength of 583
COLORIMETRIC ANALYSIS OF HAIR COLOR
199
mp, almost inappreciably different from the wavelength range
of groups B and F. The groups G and H differ most markedly
from the former groups in having high values of purity
associated with moderately low values of relative brightness.
Group H differs from group G in having higher brightness
and purity. Groups I, J, and K are characterized by having
high purity, 50 per cent to 60 per cent, together with low relative brightness, 3.2 per cent to 8.6 per cent, and a longer dominant wavelength, 585 mp to 593 mp. It is especially interesting
to note that the dominant wavelengths cover a very narrow
range in spite of the wide differences in visual colors, and
also that the range coincides almost exactly with the yellow
band of the spectrum.
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WAVELENGTH
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3igure 6
To determine the effects of bleaching, two samples, taken
from the original sample no. 11,were bleached with peroxide.
The results were lighter reds than the original and showed
typical red curves as seen in figure 6, in which no. 11 is the
original and nos. 51 and 52 are the bleached samples. On the
scattergram the effect of bleaching raised the color from the
original group J to the higher group K, but did not change it
to another series. As shown in table 1, bleaching did not
change the dominant wavelength, but increased both purity
and brightness. Another sample, no. 47, was bleached before
it was received and its original color is not known.
Although the number of samples studied is too few to justify any definite conclusions, the study does suggest certain
interesting probabilities. For an exact metric classification
200
B. B. G A R D N E R A N D D. L. M A C A D A M
of hair color, it would, apparently, be necessary to measure
only the reflectance of the samples at the wavelengths 546 mu
and 700 mp. These two values should be sufficient to identify
the color and to distinguish it from all others, and would a t
the same time be susceptible to statistical analysis. F o r a
descriptive classification there would be three main groups
(red, reddish brown, and brown) and a number of subgroups
corresponding to those in figure 5 or even based on arbitrary
divisions of the main groups. Study of a larger number of
samples would make i t possible to determine whether the subgroups shown a r e normal or whether there is merely a gradual
transition from one end of the series to the other. If the
groups a r e normal, a color chart could be made up with a
color corresponding to that of the midpoint of each subgroup
in each of the three series. Each of these colors could be given
the numerical values a n d colorimetric specifications of an
average of the group, and thus, a sample could be given numerical value merely by comparison to the chart. If further
study reveals, on the other hand, that there a r e no subdivisions, but that each of the main groups shows, rather, a
gradual transition, then a color chart could be devised for
arbitrary intervals within the groups, and the colors thus
selected could be given numerical values accordingly.
SU Y MA RY
1. Prof. A. C. Hardy’s recording spectrophotometer, used
for the analysis of hair color in this study was described.
2. A series of fifty-three hair samples, ranging in color
from black to very blond, and including a wide selection of
reds, was subject to colorimetric analysis.
3. The most striking characteristic of all of the resulting
curves is their uniformity and simplicity. The blacks and
blonds a r e almost straight lines, while the reds show a
decided upward curvature in the region of the green.
4. The curves show that the distinction between the color
of red and brown hair is due not so much to the disparity in
reflectance in the red bands of the spectrum as to a difference
in the blue and green bands.
COLOBIMETFXC ANALYSIS O F HAIR COLOR
201
5. Each of the browns and blonds gives a practically
straight line which differs from the other lines either in
slope or in total reflectance.
6. Each curve can be expressed approximately by taking
two values, one f o r the point of maximum reflectance, 700 mp,
and another for the point of maximum curvature arbitrarily
selected a t 546 mp.
7. The hair samples were sorted by eye into color groups,
and the colorimetric values of the individual samples plotted
on a scattergram. I t was then possible to draw lines delimiting three principal series: blond, red, and brown. Each of
these series showed in its component samples a n approximately continuous gradation.
8. Calculation of complete colorimetric specifications was
carried out in twenty hair samples. The specifications of the
several visually determined groups were discussed.
9. Eleaching of hair did not change the dominant wavelength, but increased both purity and brightness.
10. The samples studied are insufficient to justify definite
conclusions, but the method points the way to a larger investigation which would result in a scientifically graded hair
color series, from which the colorimetric specifications of
matched samples could be read approximately.
LITERATURE CITED
HARDY, A . C. 1929 A recording photoelectric color analyzer.
J.O.S.A.,
XVIII, 96.
JVDD,D. €3. 1933 The 1931 I.C.I. standard observer and coijrdinate system
for colorimetry. J.O.S.A., XXIII, 359.
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