Anodizing Clinic: Measuring Color of Coatings with a Colorimeter

Q. Recently, a company asked us to match its samples so we could do work for them. According to our method, we matched the samples, but they were rejected when they were sent to the company. We were told the samples were out of tolerance. We discovered the company used a different brand colorimeter with a different color scale to measure the color. Is there a set standard in the anodizing industry for measuring and matching colors?

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 the Lab or LCH scales. They are basically just different ways to represent the same numeric data that corresponds to a color. Recently, a company asked us to match its samples so we could do work for them. According to our method, we matched the samples, but they were rejected when they were sent to the company. We were told the samples were out of tolerance. We discovered the company used a different brand colorimeter with a different color scale to measure the color. Is there a set standard in the anodizing industry for measuring and matching colors?

A. There has been confusion in the anodizing industry about the efficacy of using these devices and about their accuracy in measuring and matching the color of anodized finishes, since the machines were first marketed for that use in the 1970s. I have never compared the readings of different machine brands on the same anodized parts. Two different brand machines might not give the same results. I spent many years in the job shop anodizing business and we used the tri-stimulus colorimeters for a few years. After using the Lab colorimeters to “read” color on several high-profile anodizing projects, we concluded, and there are others in the industry who agree, that the tri-stimulus colorimeters do not give reliable color value readings for anodized coatings. They are fine for opaque, applied organic coatings (paint), but the metallic, reflective nature of the substrate throws off the color value readings for anodized coatings. This makes it virtually impossible to obtain reliable color readings. This is also true for painted coatings with metallic, or mica, flakes.

Even though these machines were advertised to the anodizing industry, there was nothing in the literature that stated they could be used effectively for color matching anodized aluminum coatings.

Colorimeters should not be used to gage colors for anodized coatings. I believe the use of light and dark color range samples and the good old “Mark I Eyeball” under consistent lighting conditions are the best ways to evaluate anodized colors including clear anodized finishes.

Originally published in the December 2016 issue.