How To Convince Your Customer To Accept Color With Metamerism?
Posted by: admin in Test Method, Paint GuideWhat is metamerism?
If you are color matching a green color from your competitor, let say you are using blue and yellow color to get the required green color while your competitor is using a green pigment colorant to get the green color, then high chances is that when you placed your color panel and competitor panel side by side and view them under different light source in the light box,
you will notice the color is not the same under different light source or it might look the same at one light source but not the same on another light source, this is what we call metamerism.
When check on a color, the followings are required:
A light source - illuminating an object.
An object - reflecting light to an observer.
An observer - sensing/checking the reflected light.
If you check on the color spectrophotometer, you will notice that your panel and competitor panel will have different graph curves as the pigments used in your panels and your competitor panel is not the same, even thought the end result is a green color.Your color panel and competitor color panel that show metameric exhibit the following:
They have different spectral reflectance factors
They do not match under at least one combination of illuminant and observer.
There are two types of metamerism; illuminant and observer. Either one can result in unacceptable colored products.
· Illuminant Metamerism - where metamerism results from changes in illuminants, and where the observer does not change in this case.
· Observer Metamerism - this is where metamerism results from changes in observers, and where the illuminant does not change.
Illuminant metamerism occurs when a pair of objects match under one light source, but do not match under one or more other light sources. This can result in products that match under production light conditions, but do not match under light sources where they may be sold and/or utilized. Observer metamerism occurs when a pair of objects match for one observer, but do not match for another. This situation can occur when production matching is done under one of the CIE observers, and subsequent evaluation done under the other. It also can occur in visual product assessment situations where the color sensitivity and also on how one judge color differ from one another.
How Do We Detect Metamerism?
The followings are the two common test that are use to check on the color metamerism effect.
Visual Test for Metamerism
Please follow the following steps:
1) Confirm that the color match, by viewing (in a light booth) under the agreed upon primary light source.
2) Change the light source to a test source which is different from the reference source.
3) If the objects still match, then it is likely that they will match under any source, and are thus probably not metameric. If the objects do not match under the test source, then they are a metameric pair.
Instrumental Test for Metamerism
Please follow the following steps:
1) Using a spectrophotometer, measure the objects, and record the L, a, b value
2) Compare their reflectance spectral curves. If the curves differ, and cross each other at least three times, then the objects are metameric.
3) Confirm the metamerism and compute its amount, by calculating color differences Delta E under different illuminant/observer combinations.
If your spectrophotometer able to show the Delta E under 3 different light sources in one screen, try to check the L, a, b value is positive or negative value across the 3 different light sources or not, for example, if we compare the "a" value is all show positive value for the 3 light sources respectively, we are quite sure that the "a" value is not causing the metamerism, however if we compare the "b"value for the following example:
light source1 light source 2 light source 3
a value +0.33 +0.48 +0.53
b value +0.5 -0.43 +0.35
You will notice that "b" value is the one that contribute to the metamerism.
How Can We Reducing the Effects of Metamerism ?
Try reduce the metamerism effect by apply the following methods/steps:
1) Use the same pigment colorants in the formulation to color match, in the given example, if we knew competitor is using green pigment colorant to match the green color, try follow the same and do not use yellow and blue colorant to get the green color.
2) Substituting working standards for the original metameric standard, whenever possible in the control and acceptance processes. It is common that your customer may provide you with competitor’s color panel to match, made sure once the color approve by them, your customer should have your new standard for the same color.
3) Do not try to add new colorant to exiting production color batch , in which the standard panel was prepared without the new colorant.
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