Is Geometric Metamerism Wrecking Your Color Program?

Posted October 17, 2017 by Mike Huda

The World Series starts next week. While players and fans are gearing up for the big event, stadium groundskeepers are preparing, too. You’ve surely seen those meticulous patterns in the grass – crisscross, spiral, plaid – but do you know how the groundskeepers create them?

Thanks to a phenomenon called geometric metamerism (aka gonio-appearance), the grass really is greener on the other side. Read on to learn more about this optical illusion that can trick your eyes and wreak havoc on your color control program.

This creative use of geometric metamerism tricks the eye into thinking the flat field is concave. Image courtesy of https://growinggreengrass.net

Let’s start with metamerism.

Metamerism is a phenomenon that occurs when two colors appear to match under one lighting condition, but not when that light changes. If you’ve ever put on socks in your dark closet, only to realize once you hit daylight that one is black and the other dark brown, you’ve been a victim.

The science behind it is interesting. White light is actually made up of all of the colors of the visible color spectrum, a rainbow that includes red, orange, green, blue, indigo and violet. Objects have properties like dyes, pigments and texture that determine which wavelengths of the visible spectrum are absorbed, and which are reflected back to us. It is this mixture of reflected light that enters our eyes and gives us the perception of color.

When the type of light changes, so does the mix of light that reflects off the surface. These graphs show the reflectance curves for incandescent (illuminant A) and daylight (illuminant D65). Notice how incandescent has a lot of energy in the red area, but not much in blue? Because of the increased energy in red, objects illuminated by incandescent lighting appear redder than those under daylight.

Geometric metamerism is a little different.

The science is similar in geometric metamerism, and that baseball diamond is a great visual example. By changing the direction of the grass against the ground, the groundskeepers manipulate the way the light reflects, giving us a unique perception of color. When it’s laying one way, we see more of the flat part of the blade, but when forced in another direction, we see more of the ends. Since the flat side of the grass is wider, it reflects more green light, while the narrower sides appear darker, causing us to perceive the grass as different colors of green.

Whenever the color of an object seems to change when the light reflects at a different spatial alignment, you’re experiencing geometric metamerism.

This eye trickery doesn’t just happen on the baseball field. Geometric metamerism can strike in just about every industry. You can see it in carpets and fabrics like corduroy and velvet, when the shade changes as you brush the nap in the opposite direction. It also occurs in nail polish and automotive coatings, which use metallic flecks to create the illusion of different colors depending on your angle of vision.

Vacuumed carpet is an excellent example of gonio-appearance. How many colors do you see?

Geometric metamerism is becoming more of an issue with the zero gap trend we are seeing in the automotive industry. While manufacturers used to include gaps in plastic and painted items, often covered by metal or plastics to distract the eye, modern designs place assembled pieces as closely together as possible – without a gap. This makes the color match much more important.

The same holds true for assembled fabric pieces, such as the sleeves and body of a garment. If the weave is not running exactly the same way through the sewn pieces, they might not match when rotated or brushed to change the nap.

Here are some tips for controlling geometric metamerism.

1 – Use a Multi-Angle Spectrophotometer

Spectrophotometers are engineered with three different geometries – 0°/45°, sphere, and multi-angle – for the most accurate measurements. To evaluate textured surfaces for metamerism, you must select the right tool for the job.

  • The most common, a 0°/45°, measures light reflected at a fixed angle to the sample. While this works great for smooth or matte surfaces, it cannot capture and evaluate the details found in shiny and textured surfaces.
  • Sphere instruments can measure light reflected at all angles to calculate color measurements that closely match what a human eye would see. They are commonly used for measuring textured surfaces like textiles, carpets and plastics, as well as shiny or mirror-like surfaces, including metallic inks, printing over foil, and other highly glossy surfaces.
  • Multi-Angle devices view the color of a sample as if it is being moved back and forth and measure it at various angles. Today’s multi-angle instruments are used for specially coated pigments and special effect colors with additives such as mica and pearlescents, such as nail polish and automotive coatings.

The new MA-T multi-angle spectrophotometer family combines color imaging with multi-angle measurement to deliver the most complete characterization of color, sparkle, and coarseness. See it in action here.

2 – Visually Evaluate Under a Light Booth

Proper evaluation reveals how the product will look in each possible position, under multiple types of lighting. It can simulate colors in lighting that might represent their final environment. For instance, carpet manufacturers can evaluate how their products will look in the showroom as well as under daylight and bulbs found in homes. Using a light booth, they can brush the pile in multiple directions to ensure the color is acceptable.

When a finished good is comprised of several materials, a light booth can ensure that the harmony among components remains constant under all lighting conditions.

This Judge QC light booth offers 5 light sources for accurate evaluation of color under controlled lighting conditions. In this example, you can see how the fabric color appears to change with the light source.

Keep in mind, it’s not uncommon for two different people to look at the exact same two items and disagree about whether they match. We call this “observer metamerism.” Although testing for color acuity can help minimize this issue, our eyes are only human. You may even disagree with yourself if you compare evaluations from Monday morning at 10 am and Friday at 3:30.

Don’t be a victim of metamerism!

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