TGIF: Metamerism Tuesday, Nov. 21, 2017

TGIF Metamerism

Metamerism, n, In colorimetry, a perceived matching of the colors that, based on differences in spectral power distribution, do not actually match. Colors that match this way are called metamers.  In color science, such sensations are numerically represented by color matching functions.

Say “Whaaaa?”

In wood finishing, metamerism simply means “the same thing looks different.”   There are several instances of this:

Geometric metamerism

This is where the finished wood looks different based on viewing angle. (this is sort of the derivative of last week’s topic – chatoyance).   There are a couple of common instances of this.

Bookmatched plywood or veneer.  

The usual description of this is called “barber poling.”  In plywood, the two adjacent pieces of wood  are opened like a book, so there is an “up” side and a “down” side.  Adding to the effect is every other panel has blade checks from the veneering cutting blade.  From certain viewing angles one can appear darker than the other.

Diamond veneering. 

On a table top the veneer is in a spoke type pattern, often just quartered, but sometimes more spokes.  In viewing the table from one angle, two sides can appear light in color, the other two darker.   Walk around 90 degrees and they all appear the same.   Walk another 90 degrees and now the light sides appear dark and dark sides appear light.  This is sort of the rotary effect of barber poling.  If you are touching up or staining any part, you need to be careful of this and don’t just observe from one place.

Illumination metamerism

If you finish a piece in your shop and then move to an area with natural light, fluorescent light, incandescent light, or LED light, it can have a different color.   What used to color match no longer does, or it has a distinctively different hue.  Or even if you put it next to a beige wall, red wall, or green wall, all of a sudden it looks different. The best approach is to try to color match in its final destination.  If you can’t do that north-facing natural light is good, or 5000K fluorescent lights (daylight) is a good choice.  But just beware the differences.

Same piece (photo credit, Bob Flexner)

Daylight

Cool white fluorescent

Full spectrum bulb

Incandescent bulb

I once had to touch up a girl’s white desk that had a lot of marker stains.   I could only get about 80% of the markers out of the finish, so I picked out a close match white lacquer and applied a coat to the whole top.  Let it dry and the back ¼ was a different shade.   Ok, waited and applied a second coat all over.  Same result.  Puzzled, I pulled it a little further away from the wall and all of a sudden it all looked alike.  It was sitting in front of a bedroom window on a cloudy day.  Just the difference between the direct, though diffuse, light and that slightly below the window sill made a huge difference.   I’ve also discovered that there are a lot of whites.   I usually have to use two or three different shades to get a good match.  They have varying degrees of other colors in them.   Also, due to human physiology, you can detect more different shades of green than any other color.

And for some really weird effect of colors: https://www.washingtonpost.com/news/wonk/wp/2015/02/27/12-fascinating-optical-illusions-show-how-color-can-trick-the-eye/

Observer metamerism

We all know about color blindness.   I read a book once on color theory by an art teacher.   He did experiments in his classes showing students different combinations of colors.  Some found the combinations pleasingly coordinated and some found them discordant.   So what may look great to you, may not to your spouse or customer.  Each person may see an object or set of objects with slightly different subjective coloration.  https://www.washingtonpost.com/news/morning-mix/wp/2015/02/27/the-inside-story-of-the-white-dress-blue-dress-drama-that-divided-a-nation/?tid=hybrid_collaborative_3_na

You all have a safe and happy Thanksgiving.  Don't eat too much, but if you do, don't nap too much.

Edited November 20, 20178 yr by kmealy