Kasson S. Gibson

Tristimulus Specification of the Munsell Book of Color from Spectrophotometric Measurements

The development of the Inter-Society Color Council-National Bureau of Standards (ISCC-NBS) system of color names, based on the standards in the Munsell Book of Color, made it necessary to specify the master standards of this book in fundamental terms. Accordingly, spectral reflection curves were run for each of the 421 master standards on the General Electric recording spectrophotometer at the National Bureau of Standards, using slit widths of approximately 4 millimicrons. Various corrections were applied to these spectrophotometric data in accordance with methods regularly used for such work at the bureau. Colorimetric computations were then made with these data, resulting in tristimulus specifications according to the 1931 ICI standard observer and coordinate system. Four illuminants were used: ICI Illuminants A and C, representative of incandescent-lamp light and average daylight, respectively, Illuminant D (lightly overcast north sky), and Illuminant S (extremely blue sky). The colorimetric specifications of the Munsell standards for all four illuminants are thus given. The trilinear coordinates for the Munsell standards calculated for ICI Illuminant C have been plotted on large chromaticity (x, y) diagrams and constant Munsell chroma lines drawn in. (Similar values obtained by Glenn and Killian at the Massachusetts Institute of Technology in 1935 for Munsell color standards bearing the same hue-value-chroma designations have also been plotted on the diagram and differences between the two sets of data are discussed.) These diagrams serve as means for determining the Munsell notation and thereby the ISCC-NBS color name for any color whose trilinear coordinates and apparent reflectance are given.

Transmission measurements with the Beckman quartz spectrophotometer.

The Beckman quartz photoelectric spectrophotometer, covering the wave-length range from about 200 to 1200 millimicrons is proving of great utility in spectral transmission and absorption measurements. Essential features of the instrument are described, including a constant-temperature inclosure (for samples) designed and constructed at the Bureau. Various instrumental characteristics are noted and possible errors discussed, particularly those that may be present in the measurement of polarizing samples. Wave-length calibrations are shown for two instruments. A detailed over-all check of the reliability of data obtained on the Beckman spectrophotometer by means of glass standards of spectral transmittance shows it to give results over the wave-length range from 390 to 750 mμ fully as reliable as those obtained with our other spectrophotometers, if various precautions are taken.

Spectral Luminosity Factors

As a result of extensive measurements by the flicker and equality-of-brightness methods, mostly in America, a certain set of average luminosity data was adopted by the International Commission on Illumination in 1924. These I.C.I. luminosity factors were incorporated in the 1931 I.C.I. standard observer and coordinate system for colorimetry and were accepted by the International Committee on Weights and Measures in 1933. In the period from 1935 to 1938, however, the validity of these factors was challenged by several German investigators, who believed as a result of their measurements that a new set of standard values should be adopted based on results by the flicker photometer with pure cone vision (1.2° field and daylight adaptation). The German measurements, however, were not wholly consistent and were in conflict with similar recent measurements in England and America. At the 1939 meetings of the I.C.I., the Germans withdrew their opposition to the 1924 I.C.I. luminosity factors and the Commission formally reaffirmed their validity in the photometry of lights of different colors.

Spectral Energy Distribution of the International Commission on Illumination Light Sources A, B, and C*

Standard light sources A, B, and C were adopted by the International Commission on Illumination (C.I.E.) in 1931 for the colorimetry of materials. Source A is an incandescent lamp operated at 2854°K. Sources B and C, representative of average noon sunlight and average daylight, respectively, are produced by passing radiant flux from source A through specified Davis-Gibson filters described in Bureau of Standards Miscellaneous Publication M114 (1931).The C.I.E. published relative energy data only from 370 to 720 mµ. The present paper gives data for these sources in the ultraviolet and extends the data in the red to 780 mµ. The new data were reported through the U. S. Secretariat Committee to the C.I.E. at its 1951 meeting. The values in the ultraviolet were reported to the Optical Society of America at its March, 1952, meeting.

A Filter for Obtaining Light at Wavelength 560 mμ

A filter transmitting a narrow band of light at 560 mμ is of particular importance in the colorimetry of sugar solutions, in optical pyrometry, in abridged spectrophotometry, and in photometry. A new 4-component glass filter has been designed which isolates and transmits light at 560 mμ more effectively than previous filters. Two components of this filter are of Corning glass and two of Jena glass. The spectral transmissions of the filter and its components are illustrated. The spectral centroid of the light transmitted by the filter is at 560 mμ for both incandescent and daylight illuminants. Various other luminous characteristics of the filter are also defined and values given.

Standardization of the Luminous Transmission Scale Used in the Specification of Railroad Signal Glasses

This is the first of several papers dealing with the development and description of the signal glass specifications formulated by the Signal Section of the Association of American Railroads in 1935 and 1938. The present paper gives the spectral transmissions of the basic standards—red, yellow, green, blue, purple and lunar white glasses—on which the A. A. R. scale of luminous transmission is based, and defines that scale in fundamental, absolute units. Comparison is made with the scales defined in the 1908 and 1918 signal glass specifications.

The Permanence of Glass Standards of Spectral Transmittance

Shortly after the introduction of commercially available photoelectric spectrophotometers, the National Bureau of Standards began issuing calibrated disks for checking the photometric scale of spectrophotometers. At the time these standards were selected no precise information was available regarding their permanence. Recent measurements show that the glass standards kept in the files for 16 years with only occasional use have not changed. Data have also been obtained showing the nature of the changes occurring in glasses similar to the standards when exposed under glass to south skylight (including sunlight) for periods up to nearly three years.

Specification of Railroad Signal Colors and Glasses

This paper is a continuation of RP1209 and describes the cooperative work done by the Association of American Railroads Signal Section, Corning Glass Works, and the National Bureau of Standards, leading to the formulation of the AAR Signal Section specifications for signal colors and glasses. The previous paper defined the luminous transmission scale used by the signal engineers and glass manufacturers. The present paper describes the glasses selected by these engineers to define the limits of acceptable chromaticities afforded by these glasses when combined with kerosene or electric illuminant. The spectral transmissions of the glasses are given, together with the luminous transmissions and chromaticities for the specified illuminants. The photometric and colorimetric parts of the AAR Signal Section three-part specifications are illustrated, and the reasons given for the choice of tolerances both on the acceptable signal colors and on the glasses certified by the National Bureau of Standards as duplicates of the standard limit glasses. Various other data of interest are given, including the expression of the permissible chromaticities of signal colors in a uniform-chromaticity-scale coordinate system.

Note on the spectrophotometric grading of vegetable oilson the N″ Lovibond scale

On the basis of data published by McNicholas, it is shown that there is no significant correlation between the lightness or darkness of an oil and the Lovibond grade assigned to the oil by the practical color grader. The Lovibond color grade of a vegetable oil may therefore be computed from its spectral transmittance by colorimetric methods without taking into account the luminous transmittance of the oil. This method of grading might serve to settle disputes among the oil chemists as to the correct Lovibond grade of an oil.