What Does Lightfastness Mean?

Lightfast is a measurement and rating system used for understanding how the human eye perceives colour differences.

A Lightfast # ranking is used to specify the Lightfast qualities of a given material. This testing and ranking system makes it possible to tell which product is more lightfast than others.  New products and technologies are developing very quickly and often based on research that is out of date or yet to be written.

Lightfastness is based on a measurement of change.  

The formula is ∆E

stands for ‘Delta’.  

Detla refers to the change between the original measurement and the measurement after a period of time.

In the case of lightfastness, the chart below explains how the rating system works.

= original value

E = total change

∆E* means the difference between the original and the new…

These are the lightfastness categories based on ASTM Standard D 4303:


How is it?

Total change based on ASTM D4303

Lightfast I


∆E* 0 - 4

Lightfast II

Very Good

∆E* 4.1 - 8

Lightfast III


∆E* 8.1 - 16

Lightfast IV


∆E* 16.1 - 24

Lightfast V


Fugitive (complete crap…)


Not Rated


* = based on ASTM D4303

Does that make sense?

More test data soon!  ArtResin is kicking butt... ;)


Interesting new fact:

Some samples from cheaply manufactured epoxy resins will even yellow in the dark!

Materials not only have to remain 'lightfast', but they must also be “darkfast’.

'Photochemical reactions' can continue in the absence of light from things like temperature fluxuation.  Unstable materials can continue to discolour in the dark! 

Who would have thought...



Update 1 on ResinTint Lightfast tests! (April 2018):

Suppliers of the dispersions we use to produce ResinTints really have very little data on how they all perform longterm.

Because of the lack of information on long term stability of our raw materials, we have our technical lab investigating all of this further.

Our own tests based on the Blue Wool Scale.

Blue Wool Scale

"The Blue Wool Scale measures and calibrates the permanence/lightfastness of dyes and pigments

Two identical dye/pigment samples are coated out then one is placed in the dark as the control and the other is placed in the equivalent of sunlight for 3 months. A standard Blue Wool textile fading test card is also placed

In the same light conditions as the sample under test. The amount of fading of the sample is then assessed by comparison to the original colour."


After the first 3 months there were no visible signs of fading.

This is very good!

When we run the ASTM test for lightfastness ratings, we will then be able to apply the impartial findings to our marketing materials :)  If any color does not get a Lightfast Rating of: 'Lightfast I', then we will fix it!



Thanks! :)

 Learn more about the types of epoxy resin and their uses and read our guides about coloring epoxy resin:

About the author: Dave Zak

Hi, I'm Dave, a Canadian-based entrepreneur. I was born an artist and I love to tinker and create things. After studying art and working in marketing, I founded ArtResin in 2008 in response to the toxic and yellowing resin products that were giving me headaches and ruining my artwork.