EXPERIMENT ON REFLECTANCE MEASUREMENTS CHARLES A. LERMONDLand LOCKHART B. ROGERS Massachusetts Institute of Technology, Cambridge, Massachusetts
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rhemist has long been familiar with the characterization and determination of dissolved substances by means of their absorption spectra. Relatively few, however, have had experience with reflectance measurements unless familiar with the textile, paint, or ceramic industries where reflectance is used routinely t o characterize colored solids. I n these industries it has been recognized that the nature of the surface plays fully as important a part in the appearance of the sample as the chemical composition. For example, a polished section of didymium glass appears moderately pink by transmitted light, the exact color depending upon the angles of illumination and viewing. That same section of glass, after sandblasting or etching, has a distinct whitish cast; powdered didyminm glass is only faintly pink. In each case, however, the sample would have the same chemical composition. The physicist has been concerned with the problem of defining objectively the appearance of colored solids by reflectance, taking into account the physiological and psychological factors that are for a thinks One piece of cloth is "redder than another." An excellent discussion of the subiect can be fouud in the recent book by Judd (.1 1. . ~
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The fact that the nature of the surface makes an important contribution to the appearance of a sample Present address: Fabric Research Laboratories, 665 Boylsmeans that the~ reflectance spectrum will also be gravely ton St,. Rort,on. Ma~~nehnnet,t,s. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ . ., dected. Hence, one would expect the chemist to restrict the a~nlicationof reflectance. as a measnre of comp~sitio&~ to those cases where'the surfaces bf the samples were very nearly alike. An obvious application that fulfills this requirement is the identification and determination of substances on paper chromatograms. An analogous problem that has been handled is the determination of the amount of soil in fabrics (2). Similarly, the amount of carbon impurityin different samples from a large lotof powdered catalyst is amenable to determination by reflectance measurement (3). Thus, because it appears that reflectance will become increasingly useful to the chemist, an experiment was devised two years ago for inclusion in a course on instrumental analysis. The highlights are being reported here as a guide for others who may wish to set up a similar experiment. MEASURING TECHNIQUE
Although reflectance instruments employing filters for isolating different portions of the spectrum are D, Reflecting 45' mirror; E. Expoaed sudace of sample; F. Samsle commercially available, the use of a spectrophotometer drawer; 0.Mirror for collecting Light reflected from the aample a t 35-66': just as it is in making absorbance J~. ~ hd ~ t ~ tube:~ K, ~nousing. ~ t i ~is ~advantageous i ~ ~ H.~ ~ ~ winuser ~ p~hte,.; t Fig,*
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FEBRUARY. 1955
measurements. A standard diffuse-reflectance attachment for the Beckman Model DU spectrophotometer, shown schematically in Figure 1, was used, although the Beckman Model B and its reflectance accessory should be equally satisfactory. In using the Model DU, a photomultiplier attachment was found to be advantageous because only a small fraction of the original light is collected for measurement. The primary reflectauce standard against which absolute measurements should be made is a freshly formed layer of magnesium oxide produced from a burned ribbon of magnesium. However, magnesium carbonate, which can be purchased in the form of 1-in. X 2-in. X 3-in. blocks from many druggists, serves admirably after the surface has been scraped I I ' 4 IM carefully with the flat edge of a spatula. Polished 480 50J 520 510 WAVELENGTH Imp) opal glass is the most practical reference, though as shown in Figure 2, its reflectance is noticeably less F5gu.e 3. Polkh-d Didymium GIthan that of magnesium carbonate. For the sake of Upper ourve: Conventiaml transmittance teohniaue using an air blank. comparison, the reflectance of a sample of pure barium Lower cums: Reflectsnee technique using opal glass as a standard and as a backing for the didymiurn sample. sulfate, pressed flat before measurement, is also shown. I n contrast to the high values for white substances are the reflectances of about 1 per cent for polished paring the curves for a brass sample before and after black glass and 3 per cent for a black cloth. When being highly polished. Relationships between reflectance and concentrameasuring cloths, several thicknesses should be used tion, which are of more direct interest t o an analytical in order to minimize reflectance from the background. chemist, can be shown in a number of ways. Mixtures TYPICAL PROCEDURES of barium sulfate with ferric oxide or cupric oxide The first procedure, designed t o show the similarity in the range up to about 20 per cent (by weight) of in absorbance data and those of reflectance, consists the colored components have been prepared with of measuring the absorption spectrum by regular little difficulty. One should run each sample through transmittance measurements and by means of the a 100-mesh screen twice, to insure complete mixing,reflectance attachment. A simple cuboff filter or Unless the number of screenings is held constant, the didymium glass may be run, the latter being much samples may show larger variations than otherwise. After obtaining the reflectance spectrum for one more time-consuming. It is worth while to note the difference between the transmittance spectra obtained when using an air blank as compared t o one of clear glass. Similarly, the transmittance spectra obtaiued by reflectance may be compared, using either an opal glass byitself or one covered with a clear glass(Figure 3). Owing t o variations between the blanks, spectra obtained by regular transmittance cannot be related in a simple way with those obtained by the reflectance technique. Progressing to "true" reflectance measurements on opaque media, the student is usually quite surprised to discover how low the reflectance of shiny aluminum foil can be. If the foil is then crumpled, more or less flattened by stretching gently, and again measured, its diffuse reflectance is increased many-fold (Figure 2). The resulting increase in reflectance is visible to the eye under an incandescent light as an increase in "whiteness" similar to that observed upon etching the polished face of a glass filter. Figure 2 also shows typical reflectance curves from polished samples of brass and steel. The color of brass, which is readily apparent t o the eye, is seen to be the result of a gentle slope toward slightly less reflectance a t shorter wave leneths. Once aeain. one .Figure 4. Kubdha-Munk Plot of D a t l from 8-mpls. of Cloth Dyed may illustrate the effect of &ace roughnessby 'comdth B ~ O W EN~
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JOURNAL OF CHEMICAL EDUCATION
shows such a plot for a number of cotton cloths printed with different concentrations of dye(Ponso1Brown BN). From work done in the authors' laboratory, it appears that reflectance values of a sample, after subtraction of the reflectance for the pure dye or pigment, will fall on a straight line when plotted as the logarithm against .\/C or against v'C/(l - C), where C is the concentration of the colored substance in the sample
(4).
Fiymra 5. Diffareniial Date for the C o r n p ~ k o nof Cloth. Dyed with Two P r e p a r a t i o ~of P o ~ o lBrown ARD. The Sample Hmd Previously Baen Judged Visually t o Be "Weaker and Duller" than the Standard
Though it has not been included in an experiment, the use of differential reflectance measurements to increase the precision with which small difference~in reflectance can be measured can be performed (4) in a manner strictly analogous to that proposed by Hiskey (5) and by Bastian (6). The resulting differential curves also provide results which can be readily interpreted by persons interested in comparing the '
