Richard S. Hunter

Photoelectric Color Difference Meter

The color difference meter has three photodetectors, each with a separate tristimulus filter, and each receiving some of the light reflected from the specimen. Signals from the photodetectors are measured by analog circuits that give rectangular coordinates for surface colors in close correspondence to their positions in uniform color space. The first model described in 1948 uses barrier-layer photocells and three tristimulus filters. Recently, a model employing vacuum phototubes and four filters has been built. Use of vacuum phototubes makes it possible to substitute a dc amplifier and pivot meter for the suspension galvanometer necessary with barrier-layer photocells. By thermostatting the phototube chamber, excellent stability is obtained. A light pipe in the viewing beam provides a more stable and efficient mixer of light to the different photodetectors than the white-lined sphere used previously.

Photoelectric Tristimulus Colorimetry with Three Filters

The term, photoelectric colorimetry, is commonly employed to designate both photoelectric tristimulus colorimetry, used to evaluate the appearance of materials, and abridged spectrophotometry, often used to assist in chemical analyses. This paper is devoted to the first type of measurement. For a photoelectric tristimulus colorimeter, it is desired to find three or more source-filter photo-cell combinations of such spectral character that they duplicate the standard I.C.I. observer for colorimetry. With an instrument having these combinations, tristimulus values would be obtained by direct measurement. Although no one has duplicated the I.C.I. observer perfectly, several investigators have obtained source-filter photo-cell combinations suitable for the measurement of color differences between spectrally similar samples. To measure color differences as small as those which the trained inspectors of paint, textile, plastic, paper, and ceramic products can see, an instrument must have high precision. If the needed precision is available, a photoelectric tristimulus colorimeter may be used to measure: (1) I.C.I. colorimetric values, x, y, and Y, relative to those of a spectrally similar, calibrated standard; (2) relative values of α and β, components of the chromaticity departure from neutral in a new uniform-chromaticness-scale mixture diagram for representing surface colors; (3) amounts of color difference between pairs of spectrally similar samples; (4) amounts of color change accompanying fading; and (5) whiteness of white and near-white surfaces. In giving examples of the measurement of some of these different properties and in describing the errors of color measurement to which the tristimulus method is subject, reference is made to operations with the author’s recently developed multipurpose photoelectric reflectometer.

New Reflectometer and Its Use for Whiteness Measurement

A new 0°45° blue- and green-light reflectometer has been built with S-4 vacuum phototubes in a ratio-measuring circuit. Where whiteness is of interest, materials are usually yellowish in hue. In these cases, precise reflectance measurements with just the blue and green tristimulus filters are adequate for whiteness determination. Investigators have found that, in general, yellowness detracts from perceived whiteness much more than does grayness. For best correlation with visual rankings, the green-minus-blue reflectance difference corresponding to yellowness should receive four-to-five times the weight of luminous (green) reflectance alone. Because inadequate blue reflectance detracts so strongly from perceived whiteness, widespread use is now made of blue fluorescing dyes for the whiteness enhancement of textiles, papers and plastics. These “fluorescent brighteners” absorb in the near ultraviolet and fluoresce in the blue. An ultraviolet-absorbing filter in the new instrument may be alternated between the sample-viewing and incident light beams to include, and then exclude the near-ultraviolet which excites fluorescence. It is thereby possible to obtain a measure of the contribution of these fluorescent brighteners to blue reflectance, and thence to whiteness.

Description and Measurement of White Surfaces

This is the report of Inter-Society Color Council Subcommittee on Problem 19 formed in 1953 to study the color technology of white surfaces. Physically, surfaces which appear white reflect strongly and diffusely throughout the visible spectrum. Psychophysically, whites occupy a volume without sharply defined boundaries in the top center of the color solid. Whiteness is the attribute of white surfaces which corresponds to their visual proximity to preferred white. Preferred white varies somewhat with changes of either observer or observing situation. Measurements and intercomparisons of the colors of whites are made to determine adequacies of match to standard and to determine compliance with color specifications. One-number reflectance measurements of whites are widely used as partial determinations of whiteness. There have been a number of investigations to find which formulas yield the most reliable measurements of whiteness from tristimulus values, but the whiteness scales which have resulted from these investigations have not enjoyed widespread commercial use.

A Multipurpose Photoelectric Reflectometer

The multipurpose reflectometer was developed primarily to measure apparent reflectance, specular gloss and trichromatic coefficients. These measurements are useful in the ceramic, paint, textile, paper and chemical industries to indicate lightness, gloss and color of finished articles. In the reflectometer, two light beams from a single source are directed along separate paths to two barrier-layer photo-cells. Various types of these photo-cells were studied to find which could be used most advantageously. The reflectometer employs a substitution null method and requires a galvanometer to indicate equality of the currents generated by the two photo-cells. For each sample tested, there is a photometric adjustment to restore equality of the currents. The amounts of photometric adjustment are measured on the direct-reading scales; one of which is used for apparent reflectance and the other for specular gloss. Because of its high precision, the instrument is well suited for measuring small differences in apparent reflectance, gloss or color of nearly identical samples. However, for greatest accuracy, it is necessary to correct the scale readings by calibration.

Direct-Reading Tomato Colorimeter*

Color is a factor of primary importance in the market value of raw tomatoes. J. N. Yeatman A. P. Sidwell K. N. Norris [ Food Technol.14, 16 ( 1960)] have developed an equation for color quality of raw tomatoes using purees of fresh fruit as specimens. The U. S. Department of Agriculture asked Hunter Associates Laboratory to design a photoelectric tristimulus instrument to measure this quality index directly. This has been done by modifying the Ohm’s law analog scales of the Hunter color difference meter [see R. S. Hunter , J. Opt. Soc. Am.48, 985 ( 1958)] to solve the tomato-color equation in four steps.

New Automatic Colorimeter for Cotton

This new instrument for measuring the color of cotton is based on a satisfactory application of the Hunter Color and Color-Difference Meter to problems of raw cotton measurement. It is designed to be fully automatic and self-standardizing, and graphically to show values for reflectance (Rd) and yellowness (+b) on a two-dimensional chart. It is self-contained in a movable cabinet, with a minimum of exposed parts. Although this particular model is limited to measurements in the range of cotton color, the principles on which it is designed are adaptable to other limited ranges of color, in either two or three dimensions, thus providing an automatic, self-standardizing small-difference colorimeter for other limited ranges of color.

Gloss Investigations Using Reflected Images of a Target Pattern

A target pattern of concentric rings varying from fine lines to broad bands has been placed in the open face of a desk lamp. This luminous target is useful in the study of the gloss characteristics of the more glossy surfaces. The lines and bands of various sizes in the target provide means for studying surfaces of a wide range of “distinctness-of-reflected-image” gloss. Records may be made of which lines and bands are visible by reflection from different surfaces. Such records serve as permanent gloss values for the different surfaces studied. The dark areas of the target immediately adjacent to the luminous areas provide ideal conditions for the identification of surface “bloom.” The best gloss differentiations are made when the lamp is used in a darkened room so that the luminous pattern is the only source of light illuminating the surfaces inspected. Photographic records of gloss and unusual gloss effects are discussed.

A Glossmeter for Smoothness Comparisons of Machine-Finished Surfaces*

Since shininess is one indication of surface smoothness, a photoelectric glossmeter was developed for possible use as a production-inspection device for determining the roughness of machine-finished surfaces between 100 and 500 micro-inches root-mean-square deviation from mean surface. Because these roughnesses are about one thousand times the smallest roughnesses that affect gloss, it is necessary to coat each machined surface tested with a thin film of light mineral oil which fills microscopic cracks and pores, but conforms to the larger surface irregularities. A near-grazing angle, 75°, was chosen for the measurement of gloss so that surfaces on which there are ridges of metal between adjacent tool cuts are rated low in gloss and, therefore, rough. Although the glossmeter essentially measures the fraction of the unshadowed surface which is nearly parallel to mean surface, the instrument resulting from the present development has proved to be a simple and useful device for making rapid comparisons of the roughnesses of surfaces machined with about the same tool feeds.

Tests for the Uniformity of 45° 0° Reflectometers*

Tristimulus 45° 0° reflectometers are widely used for color-control measurements of a variety of products. These instruments often differ in the results they give because of spectral, geometric, or photometer-scale disparities. A series of seven test panels has been designed to evaluate the most frequent sources of disparity. For the photometric scale, medium and dark grays are measured against white. For spectral centers of gravity, a selective cream and a tan are each measured against a nonselective gray. For geometric disparities, a geometrically selective aluminum panel and a translucent plastic foam block are each measured against the same nonselective gray.