1.1 The purpose of this standard is to describe the procedure for performing instrumental measurements on pairs of opaque, non-fluorescent plastic test tubes, or on transparent test tubes, of similar colors, and transforming the results into color difference values, which approximately correlate with the visual color differences estimated by a normal observer in midday light and sheltered from direct sunlight or in the light of another luminous source, previously defined. 1.2 The procedure is also suitable for pairs of translucent plastic test tubes. Depending on the material application, it may be necessary to evaluate the color difference with transmitted light or reflected light, or both. To make measurements by reflection it is necessary to place a background with well-defined optical properties. For each test tube, the same background is used unless it is desired to determine the influence that different backgrounds have on the color of the reflected light. When comparing pairs of translucent plastic specimens using transmitted light, it is important that the orientation of the specimen with respect to the beam of transmitted light is the same for all specimens.