Tolerancing Part 2: The Role of Light

Posted March 01, 2018 by Tim Mouw

Have you ever bought something at the store, only to return home and realize it doesn’t match anything in your house? It’s not the color that changed. It’s the way you perceived it.

To control color, you need to be able to compare very small differences, determine their impact and understand how to address that impact. In this three-part series, we’re looking at the key components of tolerancing. If you missed part one, The History of Color Analysis, check it out now. Today’s topic explains how light affects the color we see and the importance of controlled lighting for a successful tolerancing program.

Color is light and light is energy

There are many different types of light, and each distributes energy in a different fashion. Our perception of color is strongly affected by the type of light that falls upon an object. But it’s actually much more complex than that. Not all light is the same. Daylight, moonlight, fluorescent, flashlight – they all illuminate objects differently.

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To establish a good tolerancing program, you need to understand the role of light in color.

In 1670, Sir Isaac Newton performed an experiment that put the phenomenon of color into words. He hung a prism in a dark room then introduced sunlight through a small slit. As the light passed through the triangular piece of glass, he observed that it refracted into a series of colors upon the wall: a rainbow. The prism bent the individual components of white light so they could each become visible.

From this experiment, Newton theorized that white light is actually made up of many different types of light: red, orange, yellow, green, blue, indigo, violet. He was right.

Light produces electromagnetic energy with many different wavelengths. At the short end – 400 nanometers – light is violet. As the wavelength becomes longer, up toward 700 nanometers, the light passes from to blue, to green… yellow to orange and red. Newton proved that white light isn’t white at all. It’s actually comprised of all the different types of light… Electromagnetic energy in intervals between 400 and 700 nanometers.

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Light Sources

In Newton’s day, the only light sources were natural… sunlight, moonlight, starlight, or candlelight. Today we have a lot more choices. Incandescent, Fluorescent, LED… each generates energy at different places within the visible spectrum, producing different colors of light. The relative amount of energy at each wavelength differs with each light source.

Illuminant D65 (Daylight)

xrite-tolerancing-part-2-02This is the electromagnetic energy of Illuminant D65… also known as daylight. On the Kelvin scale, its temperature is 6500. As you can see, there’s very little violet energy on the left side of the spectrum. Daylight peaks in the blue portion, then continues to decline down to 700 nanometers, where there’s very little red energy. But daylight changes. D65 describes noon daylight, where the sun is hiding behind a building, and everything appears bluer because the blue canopy of the sky is providing illumination. D75 is a little bluer in shade, and horizon is the reddest of all.

 Illuminant A (Incandescent) 2856 K

xrite-tolerancing-part-2-01This is illuminant A, which we know as incandescent. A tungsten lamp burns at about 2800 Kelvin. It has very little energy in the violet or blue wavelengths, but the energy continues to increase at almost a constant slope to 700 nanometers, which is an abundance of red. While objects that are illuminated by daylight appear bluish, they’ll shift toward red under incandescent lighting.

   Illuminant F2 (Cool White Fluorescent) 4100 K

xrite-tolerancing-part-2-03A fluorescent bulb, Illuminant F2, has a color temperature of 4100 Kelvin. Cool white fluorescent falls between blue daylight and red incandescent with predominance in greenish yellow and weakness in violet, blue, and red. The spikes are mercury vapor emissions, part of the bulb’s design. These spikes cause havoc when judging color under a fluorescent light source.

Common Light Sources

Here’s a visualization of just how much the temperature of light affects what we see. This viewing booth is illuminating the same scene with different light sources.

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In the first image, incandescent light ramps up the red. The middle image is taken under fluorescent lighting, weak in blue AND red with a predominance in green. The daylight in the last image produces blue energy, which causes the objects to take on a bluish shade. 

As an object interacts with light, it can only reflect the light that exists. Objects don’t create light; they reflect the light that comes from the source. So as the source changes, so does the reflection (and color) we see from the object.

Critical Takeaways for Tolerancing

  1. When comparing colors, you must be aware of (and in control of) your light source.
  2. Typically, most industries specify the standard light under which materials should be viewed. Be sure you ask which light source to use, or use the standard lighting for your industry if you’re not sure.
  3. To measure and evaluate color under the same illuminant, you must select the same illuminant in your spectrophotometer, tolerancing software, and light booth for consistency.
  4. Communicate with your suppliers and customers to ensure they are following the same lighting procedures. 

To Read More...

In the third and final part of this series, we define the difference between a color space and a color model, and introduce the most commonly used tolerancing methods - Tolerancing Part: Color Space vs. Color Tolerance. 

Learn more about The Science Behind Visual Evaluation.

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