5.1 The most direct and accessible methods for obtaining the color differences and color coordinates of object colors are by instrumental measurement using colorimeters or spectrophotometers with either hemispherical or bidirectional optical measuring systems. This test method provides procedures for such measurement by use of a colorimeter with either a bidirectional or a hemispherical optical measuring system.

5.2 This test method is suitable for measurement of color differences of nonmetameric, nonparameric pairs of object-color specimens, or color coordinates of most such specimens. A further limitation to the use of colorimeters having hemispherical geometry is the existence of a chromatic integrating-sphere error that prevents accurate measurement of color coordinates when the colorimeter is standardized by use of a white standard.4

5.3 For the measurement of retroreflective specimens by this test method, the use of bidirectional geometry is recommended (see Guide E179 and Practice E805).

Note 3: To ensure inter-instrument agreement in the measurement of retroreflective specimens, significantly tighter tolerances than those given in Practice E1164 in the section on Influx and Efflux Conditions for 45°:Normal (45:0) and Normal:45° (0:45) Reflectance Factor are required for the instrument angles of illumination and viewing. Information on the required tolerances is being developed.

5.4 A requirement for the use of a colorimeter to obtain accurate color coordinates is that the combination of source, filter, and detector characteristics to duplicate accurately the combined characteristics of a CIE standard illuminant and observer. When this requirement is not met, this test method requires the use of local standards for improving accuracy in the measurement of color coordinates (see also 4.2). For the measurement of small color differences between nonmetameric, nonparameric specimens, accuracy in absolute color coordinates is less important and standardization of the colorimeter by use of a white standard is satisfactory. However, accurate color-difference measurement requires that specimen pairs have similar spectral and geometric characteristics.

1.1 This test method covers the instrumental measurement of specimens resulting in color coordinates and color difference values by using a tristimulus colorimeter, also known as a tristimulus filter colorimeter or a color-difference meter.

1.2 Provision is made in this test method for the measurement of color coordinates and color differences by reflected or transmitted light using either a hemispherical optical measuring system, such as an integrating sphere, or a bidirectional optical measuring system, such as annular, circumferential, or uniplanar 45:0 and 0:45 geometry.

1.3 Because of the limited absolute accuracy of tristimulus colorimeters, this test method specifies that, when color coordinates are required, the instrument be standardized by use of a standard having similar spectral (color) and geometric characteristics to those of the specimen. This standard is also known as a product standard. The use of a product standard of suitable stability is highly desirable.

1.4 Because tristimulus colorimeters do not provide any information about the reflectance or transmittance curves of the specimens, they cannot be used to gain any information about metamerism or paramerism.

1.5 Because of the inability of tristimulus (filter) colorimeters to detect metamerism or paramerism of specimens, this test method specifies that, when color differences are required, the two specimens must have similar spectral (color) and geometric characteristics. In this case, the instrument may be standardized for reflectance measurement by use of a white reflectance standard or, for transmittance measurement, with no specimen or standard at the specimen position.

1.6 This test method is generally suitable for any non-fluorescent, planar, object-color specimens of all gloss levels. Users must determine whether an instrument complying with this method yields results that are useful to evaluate and characterize retroreflective specimens, or specimens having optical structures.

1.7 This test method does not apply to the use of a spectrocolorimeter, which is a spectrometer that provides colorimetric data, but not the underlying spectral data. Measurement by using a spectrocolorimeter is covered in Practice E1164 and methods on color measurement by spectrophotometry.

1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.9 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.