Fred W. Billmeyer

Particle-size determination by low-angle light scattering: New instrumentation and a rapid method of interpreting data☆

Abstract New instrumentation is available for measuring the intensity of light scattered from a system down to very small angles, allowing the light-scattering technique to be extended to the evaluation of particle sizes in the range 0.2–100 μ. A rapid method deduced by Sloan and Arrington for interpreting the scattering data, using graphical approximations to the Mie theory, is reviewed. Results are independent within fairly wide limits of the size and concentration of the disperse phase and of the relative refractive index. Thus measurements are valid even when the suspended phase cannot readily be resolved by the optical microscope, either because of insufficient refractive index differential or because of motion within the system. This combination of instrument and interpretation of results is particularly useful not only in the study of normal particle suspensions but also in the comparison of biological and inert systems with respect to size distribution and type and length of applied treatment, the characterization of inhomogeneities in gel systems, the effect of particle size on the optical clarity of films, and the study of such processes as precipitation, aerosol formation, emulsification, mixing, grinding, dispersion, and sedimentation.

Method for deriving color-difference-perceptibility ellipses for surface-color samples

We have developed a method for determining the parameters of color-difference-perceptibility ellipses for surface colors and have evaluated it by use of new visual data. A new theory was devised, based on the assumption of a normal distribution of color differences in local areas of CIE 1931 chromaticity space. A likelihood function that combines the observer characteristic with the probability of a certain chromaticity point being seen as a match to a color center provides a mathematical description of color-difference perception in x, y space. Best estimates of the color-difference-perception-ellipse parameters and the value of a false-alarm-rate parameter are found by maximizing this likelihood function. Use of this function leads to predicted data that are not significantly different from collected visual data. The unit color-difference-perception ellipse thus calculated describes the locus of chromaticities that have approximately a 60% chance of being seen as matches to the color represented by the center of the ellipse.

Comparative Performance of Color-Measuring Instruments

The comparative performance of fifteen different color-measuring instruments was studied for precision (short-term repeatability) and accuracy of color measurement and of color difference measurement. For estimates of accuracy, a GE spectrophotometer was considered the referee instrument. The instruments tested included two integrating sphere spectrophotometers, six integrating sphere colorimeters (four of which were individually calibrated for close conformance to CIE coordinates), and seven 45 degrees / normal calorimeters (four of which were individually calibrated for close conformance to CIE coordinates). Up to fifty-three samples were measured, most of them several times, on each instrument. Paint panels, plastics, porcelain enamels, and ceramic tiles were among the samples used. Overall, the well-established IDL D-1 Signature Color-Eye colorimeter-abridged spectrophotometer and the Hunter D25 Color and Color Difference Meter demonstrated the best and next best performance, respectively, in all categories. Several other instruments, both well established and new, were outstanding in one or more respects. All production instruments tested gave generally satisfactory results.

Current American Practice in Color Measurement

Current Aimerienn practice in color measurement is reviewed from the standpoint of instrumentation practice, measurement concepts, and computational methods. Instrumentation practice is described for spectrophotometers, abridged spectrophotometers, and tristimulus colorimeters. Measurement variables discussed include photometric and wavelength scales, standards and standardization, illuminating and viewing geometry, and instrument sources simulating standard illuminants. Computation-methods practiced for obtaining color coordinates and color differences are discussed. Topics indirectly related to the measurement step, such as the basis of colorimetry, color mixing laws, and computer color matching, are specifically excluded from this paper.

Statistical Study of Color-Measurement Instrumentation

In a statistical study of the variability of instrumental color-measurement data, two instruments (a Kollmorgen KCS-40 colorimeter-abridged spectrophotometer and a General Electric Recording Spectrophotometer equipped with a Davidson and Hemmendinger digital tristimulus integrator) provided three modes of measurement. Ten samples were measured 48 times in each mode. Frequency distributions were constructed for several colorimetric quantities, including tristimulus values, chromaticity coordinates, and color differences from the mean. To allow study of the error involved in the measurement of color-difference pairs, three such pairs were included in the ten samples. The beneficial effects of averaging were quantified.

Comparative Performance of Color-Measuring Instruments; Second Report

The comparative performance of twelve different late-model color-measuring instruments was studied for precision (short-term repeatability) and conformance of color measurement and color-difference measurement.The instruments tested included four true and two abridged spectrophotometers and six colorimeters, all being integrating-sphere instruments except three 45 degrees /0 degrees colorimeters. All the colorimeters were individually calibrated for close conformance to CIE coordinates. Forty-four samples taken from the group studied in the previous paper were measured three times on each instrument, by three different well-trained operators. Again, a GE spectrophotometer was considered the reference instrument for conformance studies. Overall, the GE spectrophotometer, the Hunterlab D25D colorimeter, and the Kollmorgen KCS-18 and KCS-40 abridged spectrophotometers demonstrated the best performance. Statistical treatment of the data has been improved vastly since the earlier paper, and some recalculated results are included.

Precision of Color Measurement with the GE Spectrophotometer. I. Routine Industrial Performance

A round-robin study of color measurement on the General Electric recording spectrophotometer was carried out by 15 participating laboratories, using transparent glass filters and opaque plastic and glass specimens. In terms of CIE Y and x, y for Illuminant C (daylight), 95% confidence limits for individual measurements averaged ±1.5% for Y and ±0.0165 for x and y if all the data were included. Elimination of results known or suspected to contain systematic errors reduced these values to ±0.57% for Y and ±0.0119 for x and y. The confidence limits for x and y are much greater for low-luminance than for high-luminance specimens, but those for Y showed relatively little dependence on luminance. The short-time repeatability of the spectrophotometer averaged ±0.09% for Y and ±0.0007 for x and y, while its reproducibility over a 14-month period averaged ±0.62% for Y and ±0.0028 for x and y.The results of this round-robin study clearly show, as in the repeatability and reproducibility studies, that the GE spectrophotometer, when suitably calibrated and operated, can still be considered the referee instrument for accurate color measurement. Serious doubt is cast, however, on the suitability of the procedures generally practiced for the calibration and operation of this instrument.

Psychometric Scaling of Gloss

Visual observations on gloss have identified approximately six types of gloss. To study the interrelations among these a method for the multidimensional scaling of gloss was devised. Two sets of painted specimens were examined. The first set covered a wide range of gloss using specimens that were a single grey color. The second set covered a more restricted range of gloss but consisted of black, grey, and white specimens. The results for these particular specimens under the particular viewing and illuminating conditions described yielded unidimensional interval scales. These interval scales were then correlated with the instrumental measurements obtained from the specimens. Equations relating the visual data to the instrumental data were derived.

Effect of Illuminating and Viewing Geometry on the Color Coordinates of Samples with Various Surface Textures

Color measurements with several different illuminating/viewing geometries were carried out for samples with four different surface textures in four different colors: matte papers, glossy papers, ceramic or porcelain enamel tiles, and polished opaque glasses, with ISCC-NBS color designations moderate pink, pale orange-yellow, dark bluish-green, and dark gray. On a single instrument (Cary 14 spectrophotometer), three geometries were used: normal/diffuse (N/D), diffuse/normal (D/N) and normal/45 degrees (N/45). For comparison, measurements were also made on a GE spectrophotometer (GERS) using near-normal/diffuse geometry. All integrating sphere (diffuse) measurements were made with specular component both included and excluded. Specular gloss and goniophotometric reflectance measurements were made. For these samples, the Cary 14 N/D and GERS results are in good agreement, and the results with N/D and D/N geometries are essentially equivalent, but there is strong evidence of the serious problem of incomplete exclusion of the specular component with all of the integrating sphere geometries when operated in the specular-excluded mode, even with samples normally considered to be highly glossy or highly matte.

The refraction correction in light scattering

Abstract The n2 form of the refraction correction in light scattering is confirmed experimentally for a photometer with a cylindrical cell and a detector which does not see past the edges of the beam. The experiments involved intercomparisons among the turbidity of Ludox silica sols and of the Debye Standard Polystyrene, the molecular weight of 12-tungstosilicic acid, and the Rayleigh ratios of acetone, benzene, carbon tetrachloride, chloroform, ethanol, methanol, and toluene. The prefactor to the n2 correction and the volume correction as a function of both angle and refractive index are also discussed.