Colors are used in the paint industry for safety (e.g., safety yellow), identification (e.g., pipes painted blue to indicate gas flows through them), and branding (e.g., corporate color schemes). The color of the protective coating will change over time; these changes may be caused by aging of the coating and/or exposure to solar radiation (UV). Darker colors are more prone to color cast than lighter colors. Finishes designed for exterior use are often formulated with light stabilizers in order to minimize color cast. Urethane, polysiloxane, and fluorourethane resin systems tend to retain their color longer than vinyl, alkyd, acrylic latex, epoxy, and other resin types, but are also significantly more expensive. Qualitative changes in color can be detected by the human eye once they become apparent, but small changes in color and/or the need to quantify color changes can only be determined using a Colorimeter. This article provides a brief introduction to color measurement technology for measuring color difference in the coatings industry.
There are several ASTM standard methods for measuring color. These include ASTM E1349 Standard Test Method for Spectrophotometric Reflectance and Color Using Bidirectional (45°:0° or 0°:45°) Geometry, ASTM E1331 Spectrophotometric Reflection Factor Using Hemispherical Geometry, and Standard Test Methods for Color, ASTM D2244 Standard Practice for Calculating Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates, and ASTM E1164 Standard Practice for Calculating Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates.
Geometry of Color Measurements – 45/0 or Sphere?
Colorimeters There are basically two types of Colorimeters: 45/0 geometry and sphere geometry. An old joke among color experts is that if you can only buy 2 meters, which 2 meters should you buy? A color expert will return the sphere and...gloss. Got it! You're expecting spheres and 45/0, right? Color measurements obtained using spherical geometries that include a specular component, or "rotation" (as opposed to excluding specular components, or "gauge") do not depend on gloss or texture; whereas color measurements using bidirectional geometries (45/0) do. If you measure the color of two parts, one smooth and one textured, but otherwise identical - made of the same material and painted with the same paint - the spherical color measurements of both parts will be approximately the same, and the 45/ A color measurement of 0 will indicate lighter portions of the texture. Spherical geometries with spin are recommended for aging testing and color matching. Bidirectional geometry provides more appearance measurements than pure color measurements and can be used for quality control of manufactured parts.
Color Difference Units – ΔE*, ΔECMC, ΔE00, or ΔEH?
While the ΔE* blood group has long been popular in the paint industry, of the color difference units listed above, ΔECMC better represents the difference in color seen by humans and is therefore recommended. When calculating color difference in units of ΔE, it is necessary to specify (l:c) or luminance-chromaticity ratio CMC units. A common value is (2:1), which means that differences in lightness are half as important as differences in chroma and hue. ΔEH is frequently used in the coil coating industry.
Color space – CIE 1976 L* a* b* or Hunter LH a H, bH?
To determine the color difference blood type in units of ΔE*, ΔECMC, or ΔE00, units, the color of two samples is measured using L*a*b*. To determine the color difference H in units of ΔE, the color of two samples is measured using LH a H, bH.
Light source – D65 (daylight), A (incandescent), or F2 or F11 (fluorescent)?
The choice of light source depends on the conditions of use. This can be difficult to predict as coated components may be used in a variety of locations including one or more light source conditions. The colors all look different under these lighting conditions.
Observer – 10° or 2°?
The 1964 CIE 10° Standard Observer is superior to the 1931 CIE 2° Standard Observer because it correlates better with human color vision.
Considering only the options described in this article: 2 geometries, 4 color difference units, 4 light sources and 2 observers yield no less than 64 ways to specify color difference measurements!
The choice of color measurement parameters can be predetermined, since only measurements performed under identical conditions can be compared. When determining color differences, make sure to use the same parameters to obtain colorimetric measurements. For example, if one is determining how much the color of a component or structure has faded due to exposure, and a pre-exposure measurement is obtained using a 45/0 geometry, a D65 illuminant, and a 2° observer, the same parameters should be used to obtain a post-exposure measurement. Also, when comparing color differences, be careful to only compare differences in the same units; for example, only compare color differences in ΔE* blood group units to units in ΔE* blood group units and not in ΔE to CMC units.
