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"Color Tolerancing using a Spectrophotometer"


I am a quality engineer working with Siemens Transportation Systems, who makes Light rail vehicles.
We are implementing a greater paint color control using a spectrophotometer, to make sure that our suppliers are consistent.
I am trying to determine the best tolerance system and was wondering what the standards for other industries are.
What color space do you use? CIELAB? CMC? CIE94?
If CIELAB, what is your delta e* tolerance value?
If CMC, what is your CMC commercial factor?

What else should I know about tolerancing?

Eugene Oh
Siemens Transportation Systems - Sacramento, California, USA


You don't have the spectrophotometer yet I take it. I've only used the CIELAB method and have been very satisfied with it. None of this info is kept a secret from anybody, just search CIELAB, or any of the other color spacing methods that you mentioned and through the magic of the www, it pops up right before your eyes. Then you can decide on which will work the best for your situation. In theory the naked eye can detect a visible difference in color at a .5-.7 delta e(CIELAB). The tighter you hold the delta e, the more that it could cost you though. I was given a delta e tolerance of 1.5 on some telecom equipment, but our company told them we could, and would, hold a .5-.7 range.
Make sure that you're suppliers are going to be using the same equipment and method so as to save yourself headaches down the road, and you'll probably come up against some resistance by trying to add a control like this if they haven't been color checking up to this point.
Come to think of it, you're at the top of the food chain, tell you're suppliers that you want them to control their color using method(X) and then send you proof that they are doing it, that way they buy all of the equipment and do all of the research.

Sheldon Taylor
Sheldon Taylor

supply chain electronics
Wake Forest, North Carolina



Here are my recommendations:

Illuminate D65
10 degree observer
CMC tolerancing with ratio of 2 to 1 for L and c
Commercial factor of 0.5

If you really want to make your system effective you need to control both film thickness and bake on the lab panels you supply the paint supplier treated on your treatment system. Substrate, gauge and treatment type will all effect colour.

We supply our coating suppliers with lab panels treated on our line in 50 lb lots which they are instructed to use. No substitutions allowed. They are given a specification for each colour and a wet film requirement which is referenced to a specified wire wound drawdown bar. If they use the wrong bar the paint is rejected even if it is on colour. It is not possible to measure dry films
accurately do not let them convince you otherwise. Our lab oven is calibrated and a temperature profile obtained for each substrate. Then as a condition of using their product, if it is colour critical, their oven must be profiled to match ours. This is done for them on request using an appropriate panel with a thermocouple attached. Bake times to be used are specified for each product. Using this method no off colour product has ever got on our line now for over four years and we have never had a colour complaint from our many customers across a variety of industries over this time.

My experience dealing with colour control goes back almost thirty years and includes everything from appliance, automotive, coil over steel, aluminum, galvanized (e.g., and HDG) tin plate, plastics. This is on post painted and pre-painted product.

Ronald Zeeman
Coil Coating - Brampton, ON, Canada


Good afternoon Mr. Zeeman,
I'd like to ask you a couple of questions about your response to Mr. Oh, namely that you stated that dry films can't be measured accurately. Are you talking about getting a color reading from a dry film can't be done accurately? I'm not a wet painter so a lot of your answer went over my head. I've been a powder coater for 16 years, but I have on occasion had to get a wet paint match(automotive)for touch up, and my wet paint supplier always matches the color I need to my powder painted color chips using the same spectrophotometer that I use.
If you could submit a picture of a draw down bar that would be cool, I'd like to see what one looks like. I didn't know that a liquid color match could be made wet.
Very good information!

Sheldon Taylor
Sheldon Taylor

supply chain electronics
Wake Forest, North Carolina



Powder coating is one area I do not have experience in. My guess would be that it probably would not work with powder but unless you experiment you will never know.

Drawdown bars are special bars you can purchase that are basically wire wound tightly around a bar. They come in different sizes depending on what film thickness you want. Using the volume solids of the liquid paint, a wet film was determined for each size. This information is then referenced on the spec the paint supplier is to use when matching the colour. My most commonly used bar is a #24 which gives 1.6 mils wet. Some of the higher bars have groves milled in them which may work with powder but you will have to experiment. I just recently started using bars on ink and found a drawdown machine was required as the smoothness required was not possible at such a low film thickness when done manually. Now once the ink has been made no more tinting is required. Three colour prints one test strip off and running.

The main obstacle is the various technicians at the paint companies, but I'm stuborn, we are a big customer and change is not easy.

I wish I could give you a picture but I am not computer literate enough to do so.

Ronald Zeeman
Coil coating - Brampton, ON, Canada

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