We are often asked to provide LRVs and / or CIE colour-space values with Delta E (ΔE or dE) after accelerated weathering tests to quantify photo stability performance. However, these three sets of numbers are often misunderstood or not understood at all!
We would like to shine some light on the matter (pun intended) and provide a very basic understanding of their differences and how best they can be used.
LAB CIE colour-space readings and Delta E (ΔE)
The CIE LAB colour-space model is a complex mapping system that uses tristimulus (a combination of 3 colour values close to red, green and blue) which are plotted as a 3D space. When these values are combined they can reproduce by providing coordinates for, any colour the human eye can perceive.
Deltas E is defined as the difference or change between two colours within LAB colour space and are used in our laboratory for quantifying photo stability performance after accelerated weathering testing. LABs are best used as raw data for calculating Delta E values to provide a measure of colour shift in any direction after UV exposures and are particularly useful for pigment manufactures when a primary colour may degrade from a secondary colour changing it completely.
For example, if a secondary colour like green is manufactured from a mix of primary colour pigments blue and yellow, the LAB coordinates will show whether it is the blue or yellow pigment (or a combination of both) ‘burning out’. This will identify ‘weak pigments’ should there be even a subtle colour shift.
Please also be aware that although Delta E values provide us with a number to quantify colour shift, the perception of colour change for the same dE value will be strong in a grey scale, but weaker in some colours.
Light Reflectance Values or LRVs
Light Reflectance Values (LRVs) describe the quantity of visible light reflected by a surface in all directions when illuminated by a light source. The LRV is similar to (not the same as) the CIE colour space direction ‘L’ from black to white.
LRV measurements are used to establish how much light a particular colour reflects, and conversely how much light is absorbed. LRVs are reported as percentage values from 0% to 100%. Zero assumed to be absolute black and 100% assumed perfectly reflective white. The instrument is calibrated at 0 and 100% using black and white calibration tiles.
LRVs are best used to measure fade after accelerated weathering tests but are more commonly used to provide interior designers and architects a perception of how much colour will be reflected back into a room when the lights are turned on. You will often see LRVs stated on colour swatches in your local DIY store which is an establishment I avoid like the plaque!
Hope this helps.
Yes I agree, we should both get out more!