Measuring the Color of Anodized Coatings with a Colorimeter

Q. We use a tri-stimulus colorimeter to check our colored anodized material and to match customer samples. This piece of equipment gives us several color scales to choose from, such as L*a*b* or L*C*h*. They are basically just different ways to represent the same numeric data corresponding to a color. Recently, a company asked us to match its samples so we could do work for them. We matched the samples using our method, but the company rejected them and told us they were out of tolerance. I found out that the company measured the color using a different brand of colorimeter with a different color scale. Is there a set standard in the anodizing industry for measuring and matching colors? –I.F.

A. Ever since they were first marketed for this particular use in the 1970s, there has been confusion in the anodizing industry about how effective colorimeters are for measuring and matching the color of anodized finishes. I have never compared the readings of different machine brands on the same anodized parts, but two different brands might not give the same results. I have found, however (and there are others in the industry who agree), that the tri-stimulus variety of colorimeter, in general, does not give reliable color value readings for anodized coatings. The device is effective for opaque, applied organic coatings (paint), but obtaining reliable color readings for anodized coatings is virtually impossible with a colorimeter because of the metallic, reflective nature of the substrate. This is also true for painted coatings containing metallic or mica flakes.

Even though these machines were advertised to the anodizing industry, there was nothing in the literature stating that they could be used effectively for matching the color of anodized aluminum coatings. Because of this, colorimeters should not be used to gauge colors for anodized coatings. You are not the first to have that problem or ask that question. I believe the use of color range samples and the good old "Mark I Eyeball" (visual inspection) under consistent lighting conditions is the best way to evaluate anodized colors, including clear anodized finishes.