SPECTROPHOTOMETRY FOR CHEMISTS1 ROBERT S. CASEY . W. A. Sheaffer Pen Company, Fort Madison, Iowa
ALLCHEMISTS deal with color, and we suggest that more students of chemistry be offered courses in colorimetry based on spectrophotometry. Students are taught to measure and record their observations and experimental results in terms of fundamental units. In the case of color, spectrophotometry provides the means of accomplishing this. PSYCHOLOGICAL ATTRIBUTES OF COLOR
For the discussion and calculations which follow, it is necessary to understand the three attributes of color. These are hue, value, and chroma. Colors may differ from one another in any one or in any combination of these three independent dimensions. The attribute hue, which we commonly call "color," is designated by such familiar terms as "red," "blue," and "green." The value indicates whether the sample reflects much or little light. It is the attribute ordinarily described as "light" or "dark." The attribute chroma is a little less commonly understood. It expresses whether there is much or little neutral gray mixed with the color under consideration. It is somet'mes designated in common language as "brilliant" or "dull." Zero chroma is represented by a neutral gray. The Munsell system (4) of color samples and notation is arranged according to these attdbutes. An inspection of the Munsell charts gives one a clear comprehension of the three dimensions of color. SPECTROPHOTOMETRY
Figure 1 shows a collimated beam of light, A, being dispersed by a prism, B, in the familiar fashion and casting a continuous spectmm upon the screen. Most of the light sources under which we view the objects around us are incandescent solids which give continuous spectra. Such "light" is a combination of all visible wave lengths. Now let us cut in the screen a narrow, vertical slit,
D,which passes light in a restricted wave-length range. This spectrum light illuminates an object, E. The light which is reflected, F, will be of the same wave length as the incident radiation. Regardless of what color we associate with that object when it isviewed under ordinary conditions of illumination, it reflects only the spectral color which illuminates i t under the conditions illustrated in Figure 1. The only diierence between different colored objects when illuminated in this way lies in the per cent of the incident spectral radiation which they reflect a t a given wave length. The percentage reflected (or transmitted) is independent of the intensity of the incident beam. To study the color of a given object, we scan the visible spectmm with such a slit, recording and plotting the per cent of the incident light that is reflected (or transmitted) a t the various wave lengths. Plottimg reflection factor as a function of wave length gives the spectrophotometric curve. The data represented by this curve give the basic physical definition of that attribute of the substance which we call the "color." Figure 2 shows spectrophotometric curves for blue ( B ) and green (G) ink marks on paper, 0.002 ml. of ink per sq. cm. of paper surface. The curves were made on a G. E. automatic recording uhotoelectric suectro~hotomPresented before the Division of Chemical Education at the 111th meeting of the American Chemical Society in Atlantic City, April 14-18,1947. ~
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eter by the Electrical Testing Laboratories, Inc., New York City. LIGHT SOURCES
In order to evaluate the appearance of a substance, it is necessary to take into consideration the quality of the illuminant which shines on the object at the time of observation. The characteristics of light sources are evaluated by scanning the spectrum from such source and measuring the energy a t each xave-length interval. The absolute or relative energy may then be plotted as a function of wave length. The curve in Figure 3 is a plot of relative energy as a function of wave length for illuminant C, adopted in 1931 by the International Commission on Illumination as closely approximat,ing average daylight. The stimulus which reaches the eye of an observer is defined physically by the spectral distribution of energy in the light reflected from the object. The latter function is determined by multiplying each ordinate of the curve for distribution of energy in the light source by the corresponding reflection factor from the spectro~hotometriccurve for the sample. These products may then be plotted as a function of wave length. To evaluate such a stimulus, an equivalent stimulus must be defined. A sensation cannot be described.
amounts of light from these hypothetical primaries which would match the color under consideration hen it is illuminated by the standard illuminant. Thc values of X, Y, and Z are simply numbers; they give no direct idea of the color, except in the case of the Y function which was chosen to correspond with the curve shown in Figure 4. This curve shows the relative brightness as perceived by the average human eye for 'equal amounts of energy a t each wave length through the visible spect,rum. Relative brightness values are plotted as ordinates, wave lengthsas abscissas.
TRISTIMULUS VALUES
It is well known that a color can be matched by mixing light from three properly chosen primaries. At the 1931 Meeting of the International Commission on Illumination various research data on color matching were recalculated and a set of convenient primaries was adopted-X, Y, 2-called the tristimulus values. There are tables for calculating the numerical values of X, Y, and 2 from the spectrophotometric curve of any color when illuminated by one of the standard illuminants. Values of X, Y, and Z might be considered the
400
600 800 700 wave Length (Millimicrons) Piwre 4. Relative Bdphtnu. of Epual Energg at Each ~ a v e ' ~ e n s t h As Psrc.i..d by th. N0.mslEy. (I). (R.pr0d"c.d through the courttty of The T.chn.logg P..B. M-chusette Institute of Technologl.. C.mbridg., Mass.)
As a result of this choice in calculating the primaries, the value of Y represents directly the relative brightness of the sample under consideration. CHROMATICITY DIAGRAM
In order to lot the chromaticity of colors and light sources, the additional functions x, y, and z, called trichromatic coefficients, are defined:
400
600
800
700
Wave Lensth (Millimicrons) Dhtribution of E n e w for 1.0.1. IllumiPigum 3. Rdatlv. Sp.c-1 of The T.chnolow nant C (I). (Raproducad through the courtPre,. Mus.chulutt~ Institute of Techndogg. Cmmbrid~.Mu..)
The numerical values of x and y are calculated for various colors and illurninants; x values may be plotted as abscissas and y values as ordinates. This procedure
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gives a chromaticity diagram as shown in Figure 5. The horseshoe-shaped curve is the locus of the spectrum colors from 400 to 700 mp. The point marked C shows the position of illuminant C . The loci of the ink marks mentioned above are B and G. This gives another means of designating color. We may specify x, y, and Y.
z Y
Dominant wave length Brightness Purity
0.2080 0.1828 472 mp 17.2% 54%
0.1940 0.3250 492 mp 22.5% 43 %
The figures given apply when the samples are illuminated by standard illuminant C and viewed under conditions stated in reports of the International Commission on Illumination. Another simple application, illustrated in Figure 6, concerns commercial blue-black inks. Writing fluids of this type should be permanent, for the writing must remain legible even though the records have been subjected to deteriorating influences, such as water soaking incident to fire or flood. Colorimetry based on spectrophotometry provides a means for quantitative measurement of such effects. Figure 6 shows the spectrophotometric curves for a blue-black ink mark on paper, before and after soaking in water, and for the paper on which the marks were made. Blueblack ink spot Dominant wrwe length Brightness Purity
Paner 564mp 71.6% 1.0%
Olia'nal 474"~ 13.6% 42.2%
Soaked 440 mp 39.6% 2.8%
The paper and the spaked mark have very low purity, indicating that they have no pronounced hue hut are of
DOMINANT WAVE LENGTH, BRIGHTNESS, PURITY
The chromaticity diagram is also used to calculate color specifications corresponding to the psychological attributes mentioned above. If a line is drawn from the illuminant point through the sample point G, it intersects the spectrum locus at wave length 492 mr. This is called the "dominant wave length" for that color and corresponds to the psychological attribute of "hue". The "brightness," corresponding to the psychological attribute "value," is given from the value of Y referred to previously. Purity is calculated from the relative position of the sample point between the illuminant point and the spectrum locus. A definite percentage value may be obtained for the purity. Thus, we have a third method of specifying color and ohtainine three numerical values-"dominant wave length," lLbrightness,"and L'purity"-khich correspond v e v closely to the ps.ychologica1 dimensions or attributes .. of "hue," "vahe,,uk,i' and "chroma," respectively. The result ofthe above calculations for the ink marks Band . .. G follows.: :, .
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,
400
rjpun e.
500 600 (Wsve Length Millimicrons) sp.~trophotornqtri.curr... I ~ spot.. L and pap..
700
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neutral shade. In such cases the dominant mave length is not of great significance, but merely indicates the direction of the slight departure from neutrality. A measure of contrast between ink mark and paper in such a case may be estimated from the difference in brightncss. Such differrncc could be considered a measure of the efficiencyof a permanent writing ink in resisting obliteration and could provide definite figures for specifications.
diffcrentilluminants, consideration of light sources and filters for accelerated fading tests, and comprehension of light and color mixtures in general. References useful to those who wish to pursue this subject are given ( I , t,5, 5, 6). Hardy's "Handbook" (I) is particularly recommended as an introduction.
CONCLUSION
(1) HARDY,A. C., AND STAFFOF TEE COLORMEASUREMENT LABORATORY, M. I. T., "Handbook of Colorimetry," The Technology Press, Cambridge, 1936. , (2) "Three Monographs on Color." International Printing Ink Corporation, New York, 1935. (3) MELIaN, M, G,, iiColoIimetry for Chemists,8, G.Frederick Smith'Chernical Company, Columbus, 1945. A. H., "Book of Color," Munsell Color Company, (4) MUNSELL, Inc., Baltimore, 1929. D., "Selected References Relating to the Field (5) NICKERSON, of Color Science," Textile Research, 16, 74 (1946). (6) -colorimetry committee ~ ~J. optical ~ sot. ~ A ~ . 33, , ~ 533 (1943); 34, 183, 245, 633 (1944); 35, 1 (1945).
The study of colorimetry based on spectrophotometry provides a of establishing engineeringspecifications and tolerances for colors. It is also necessary for analytical work involving applications of Beer's and Bouger's laws and for comprehensive study of additive and subtractive color mixtures. Furthermore, spectrophotometric concepts help greatly to orient one's thinking about qualitative aspeots of color. Examples are color matching under
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