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Delta E: A Key to Understanding Lightfastness Readings

17 thoughts on “Delta E: A Key to Understanding Lightfastness Readings”

  1. I am absolute;y fascinated with information like this. I have been working with JND as a concept for a little while but you have added a completely new dimension. Thank you!

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    • You are SO welcome! We love that you find this information fascinating and valuable as clearly it is part of our passions as well.

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    • Great question. The measure would actually be the same since Delta E is neutral about how the change comes about and simply measures the difference in color between two readings. So, for example, when Cadmiums or Ultramarine Blue are exposed outdoors in an environment with moisture or humidity, chemical changes take place that are not really classic “lightfastness” issues, but which will cause an Ultramarine Blue to bleach out, or a Cadmium Red to get darker. Or we will see cases where a particular pigment will appear lighter after outdoor exposure, but due to chalking of the surface rather than fading, which is a form of erosion different from purely a fugitive color. Ultimately, however, while all of these changes can be described in terms of Delta E, that does not mean all of them are reflected in ASTM’s lightfastness rating even though they can impact how a pigment is used. So, for example, returning to our earlier example, while Ultramarine Blue and Cadmiums are all given a Lightfastness rating of I by ASTM, their use is restricted to indoor applications. As most paintings are exhibited and live their lives indoors few artists notice this, but for those who paint murals they will find them absent from our list of recommended colors for exterior applications. For more information on that aspect, the following are good links to look at:

      Exterior Mural Tech Sheet – Recommended COlors

      Selecting the Best Exterior Mural Pigments

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  2. Technical question. I am shooting a polyurethane powder that is required to be less then 1.0 delta E. For some reason I keep coming out at 2 to 2.5 Delta E. What can I do to get this lowered the manufacturer of the powder came and he couldn’t get it below 1.0 either. It has to be something with the cure schedule or timing, took the same Box of powder to another vendor and he got it below a 1.0. The manufacturer did test my oven, and it has absolutely no issues, the temperatures are right on, no cold spots, everything came back 98.2 % efficient. I have provided a copy of the Cielab results below for the approved piece and the piece I shot.
    Cure schedule is part @ 400F at 10 mins.
    Approved delta E piece: Delta was 0.9 CIELAB – L:78.3 a:2.5 b:-7.3

    Piece I shot.
    2.2 Delta E
    Lab: L:83.2 A:3.5 B:4.1
    You can visually see that the piece I shot is brighter then the approved piece what can I do to get this darker and extract the blues while curing.

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    • Hi Breet –

      I am not going to be able to help in terms of the preparation or formulation of your materials – that is just not something we work with here, being solely an artist paint company specializing in acrylics, watercolors, and oils. However, I do have some questions that might help me understand things better.

      What settings are you using for your spectrophotometer, and what type do you have? Different settings as well as the different equations used to calculate Delta E will give different results. As mentioned in the article, we use CIE 1976, or dE76 for calculating the difference – but that is not the only option and is not the one used by most commercial paint companies, who often rely on CIE 98 or 2000. Our use of de76 has to do with the particular history of lightfastness calculations for artist materials. So you might see what your results are when using these other formulas. And on the reason why these can give different results, I thought this article was helpful:

      https://www.datacolor.com/color-systems-cielab-cie2000/

      and the difference in the equations is decently covered here

      https://en.wikipedia.org/wiki/Color_difference

      In terms of the light source, we use D50, and set the standard observer angle to 10 degrees. Lastly, make sure to choose Specular Component Included (SCI) for your readings to compare actual color divorced from the impact of specular gloss on shiny surfaces. For an explanation of that see the following:

      https://sensing.konicaminolta.us/us/blog/specular-component-included-sci-vs-specular-component-excluded-sce/

      We hope that helps some and if you can let us know what you are using and the settings – for both you and the other people who are getting different readings – that can help if further questions arise.

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