FAIC Color Tutorial

Introduction

This tutorial will examine the different systems we use to measure and categorize color in conservation.

Learning Objectives

After completing this tutorial, you will be able to:

Describe the way that the human eye perceives color
Enumerate three systems for describing color, including:

Munsell
CIE XYZ
CIE L*a*b*
Color temperature

List the relative disadvantages of each system
Understand the phenomenon of metamerism

Visible Spectrum

In this section you will learn about:

Why we see color
Types of cones in the retina
Sensitivity of the human eye
Six fundamental color sensations

Visible Spectrum: Definition

The visible spectrum is a very small part of the electro-magnetic spectrum.

Diagram of the electromagnetic spectrum with the visible light range highlighted

Visible Spectrum: Why We See Color

We can see color because our eyes are sensitive to this narrow range of the electromagnetic energy. Our brain translates the energy as color.

Visible Spectrum: Cones in Retina

In the human retina there are specialized cells called cones. We have three kinds of cones and their sensitivity corresponds, loosely, to the short (blue), medium (green), and long (red) wavelengths.

Graph of cone sensitivity range. S cones, often referred to as blue cones, are most sensitive between 400 and 500 nanometers, while M (green) and L (red) have significant overlap, with M cones most sensitive to 450 to 650 and L cones 500 to 700.

Visible Spectrum: Sensitivity of Human Eye

Graph of cone sensitivity. L cones have the highest sensitivity, around 0.6, M cones have about 0.3, and S cones are around 0.6

Of course, like most things in nature, it is a lot more complicated than that in reality. The sensitivity of the three different types of cells depends on the intensity of the light and on the adaptation of the eye to the light source.

The human eye is far more sensitive to color in the middle of the spectrum.

As we will see, this has consequences when we try to measure colors scientifically.

Visible Spectrum: Color Sensations

There are six fundamental color sensations:

Red

Green

Yellow

Blue

White

Black

Many societies also have a range of names for:

Image of a color wheel with primary, secondary, and tertiary colors

Pink

Orange

Purple

Brown

Gray

Take a moment and see if you can think of any color that doesn't fit into these categories:

Turquoise
Vermillion
Viridian
Gamboge

Visible Spectrum: Color Wheel

Goethe's color wheel
"Theory of Colours" 1810

All of these colors can be found as the primary, secondary, and tertiary colors on the color wheel that you used in art classes

We can recall a few dozen color sensation from memory.

We can distinguish millions of colors when seen side by side.

Visible Spectrum Quiz

Visible Spectrum: Summary

In this section, you learned that:

The visible spectrum is a small part of the electromagnetic spectrum.
The human eye contains cones that are sensitive to different wavelengths of light.
Sensitivity is greater to some wavelengths than others.
There are six fundamental color sensations: red, green, yellow, blue, white, and black.

Categorizing Color

In this section you will learn several systems for categorizing color, including:

Chroma, brightness, and hue
Color values in a 3D space
Munsell color system
CIE color system
Color temperature

Categorizing Color: Chroma, Brightness, and Hue

Diagram of a cylinder showing value (or brightness) vertically, chroma (or saturation) extending along the radius, and hue around the circumference.

There have been many attempts to categorize colors. The first step in contemporary theory was to separate their attributes into chroma, brightness, and hue.

Chroma is the saturation, how close it is to grey or full color.

Value is the brightness, how close the color is to white or black.

Hue is the color name, red, green, purple-blue...

Categorizing Color: 3D Space

Diagram of 3d representation of color space, with white at the top and black at the bottom. Green and red, and blue and yellow, are at opposite corners.

Scientists have also arranged color values in three dimensional space.

Red and green are opposing colors. No true reds contain green.

Similarly yellow and blue are also opposing colors.

Yellow is placed nearer to white as we percieve blue as darker than yellow.

Categorizing Color: Munsell Color System

Diagram of the Munsell color space, a cylinder with value in ten steps from black at the bottom to white at the top

The Munsell color system was devised by Albert H. Munsell at the beginning of the 20^th century. This diagram shows part of the system. We'll look at each section over the next few slides.

First of all, notice that the brightness (or lightness) scale runs in ten even steps from black (0) to white (10).

Munsell called these steps values.

Munsell System: Hues and Values

Diagram of the Munsell color space, with chroma in twelve steps running along the radius, and primary, secondary, and tertiary hues around the circumference

The hues (or color names) run in ten steps perpendicular to the values:

Yellow	Purple-blue
Green-yellow	Purple
Green	Red-purple
Blue-green	Red
Blue	Yellow-red

Each hue is further divided into 10 smaller steps, so there are 100 steps going round in a circle.

Munsell System: Chroma

Diagram of the Munsell color space, with chroma in twelve steps running along the radius

The chroma (or saturation) radiate out from the center starting at grey (0) and ending with the saturated hue.

Although the chroma move outwards in even amounts there is no upper limit to the number of steps that can be shown.

The human eye can "see" more saturation in some colors (yellow for instance) than others (light purple).

Munsell System: 3D Space

Cutaway diagram of Munsell color space with all possible combinations represented

This is a cut-away diagram showing how all the colors are arranged in three-dimensional space.

Munsell colors can be identified in this color space by the three unique numbers for their hue, value (brightness), and chroma.

Munsell System: Drawbacks

There are drawbacks to this system.

The most obvious one is that humans do not perceive colors in regular steps because their eyes are not equally sensitive to all wavelengths.

Looking back at slide 21, the hue circle is divided into 100 equal steps but blue is not opposite yellow nor red opposite green.

Number of steps between the pure hues in the Munsell System

From	To	Steps
Red	Yellow	23
Yellow	Green	18
Green	Blue	28
Blue	Red	31

Munsell System: Drawbacks

Three Munsell values, 5P 5/10, a purple, 75YR 7/12, yellow, and 5R 6/14, a bright red.

Because of these uneven steps, the Munsell Color system is not "intuitive". Simply by looking at the three unique color values of a number of different colors, it would be difficult to imagine where they stood in relation to each other.

Categorizing Color: The CIE XYZ Color System

Graph of CIE color space, a tall curve from violet to red with purple along the bottom, and white in the center

The CIE XYZ color space was derived from experiments done by Wright and Guild in the 1920s. The CIE system calculates the value of any color from the sum of its tristimulus values XYZ.

Categorizing Color: The CIE XYZ Color System

Graph of tristimulus values, with wavelength on the x axis and amount of color on the y axis. The greatest relative amount of color, and green and red mostly overlap. There is a dip in the center where our eyes are most sensitive. There is a second small peak for red in the violet region.

$\lambda$ on the $x$ axis represents the wavelength, and the $y$ axis represents the amount of the three primary colors needed to match the color at each wavelength.

The primary colors are red, represented by X at 600 nm, green (Y) at 550 nm, and blue (Z) at 450 nm.

Tristimulus values approximate to the relative sensitivity of the three different kinds of cone in the retina.

Categorizing Color: The CIE XYZ Color System

The calculations behind the CIE system are beyond the scope of this tutorial.

For further reading either go to Wikipedia or read through Color: An Introduction to Practice and Principles, 2^nd Ed. by Rolf G. Kuehni; Wiley-Interscience, Hoboken, New Jersey: 2004.

The CIE Chromaticity Diagram

The CIE chromaticity diagram is a map of human color perception on $x, y$ axis.

The grey figure includes all the colors perceivable by the normal human eye, with the wavelengths of each color around the edge, and a line of purples across the bottom.

The triangle of colors that can be matched by blue, green, and red lights is shown in the middle, with achromatic white light near the center.

CIE Chromaticity Diagram Axis Labels

The $x$ and $y$ axis of the CIE XYZ chromaticity plot are derived from the original X, Y, Z tristimulus values. The final diagram shows a two dimensional representation of the three values given to each color. And, unfortunately this can get a bit complicated as the physicists used X Y Z for the starting values, then designated "lightness" as Y and decided that they would not show that value on the chromaticity plot. They then used X, Y, and Z to calculate $x$ and $y$ .

CIE Chromaticity Diagram Axis Labels

For anyone still following this, here are the equations to calculate $x$ and $y$ .

$x = \frac{X}{X + Y + Z}$

$y = \frac{Y}{X + Y + Z}$

The CIE 1931 Color Space entry on Wikipedia goes into this more fully.

CIE Chromaticity Diagram Problems

One of the problems with the CIE chromaticity diagram is that green takes up far more space than the other colors leaving a much smaller area to categorize the other colors.

Categorizing Color: The CIE L* a* b* Color System

Diagram of a cylindrical color space with value running vertically and hue around the circumference, with arrows marking the L axis from bottom to top, and a and b perpendicular across the top

The CIE L* a* b* color space, which is based on the CIE XYZ color system, was designed to describe, mathematically, all of the colors that can be seen by the human eye, and it allows for the different sensitivity to wavelengths.

The L* axis denotes darkness (0) to lightness (100)
The a* axis runs from green (-a) to red (+a)
The b* axis runs from blue (-b) to yellow (+b)

The CIE L* a* b* Color System

3D representation of L a b color space, an inverted teardrop shape with gray at the bottom point

The image in the previous slide was a much simplified version of the L* a* b* color space. A more accurate version is shown to the right. The human eye can see all of the colors in this shape.

Since all of the colors have three values L* a* b* they can be placed in three dimensional space.

Notice that yellow is closer to white than blue because we perceive yellow as a lighter color.

The CIE L* a* b* Color System Calculations

If the L* a* and b* values of a color are measured before and after treatment, it is possible to calculate $\Delta E$ . This value shows the total amount of change in a color.

Since $\Delta E$ is a dimensionless number, it is not possible to tell where the color change has taken place.

Notice that all the values are squared and the square root is found of the total. This does away with any negative numbers that may have been present.

$\Delta E = \sqrt{(\Delta a)^2 + (\Delta b)^2 + (\Delta L*)^2}$

$\Delta a*$	$= a* \text{(before)} - a* \text{(after)}$	(Red/green axis)
$\Delta b*$	$= b* \text{(before)} - b* \text{(after)}$	(Blue/yellow axis)
$\Delta L*$	$= L* \text{(before)} - L* \text{(after)}$	(Lightness)

The CIE Color System Measurements

Photo of colorimeter with display showing L a b values and color wheel

The larger the value of $\Delta E$ the greater the change in color. Trained color matchers can see a $\Delta E$ value of 2. Most people with normal color vision can see a $\Delta E$ value of about 4.

In order to measure color differences you will need a dedicated piece of lab equipment such as a colorimeter or chroma meter.

The CIE Color System Application

Graph of delta E over exposure duration in weeks, showing increasing color change over time

In this example from Williams et al. (1993) used $\Delta E$ to show changes to watercolors exposed to sulfur dioxide.

Categorizing Color: Color Temperature

Graph of relative spectral power over wevelength, showing north sky light with the highest peak at the high wavelength blue end of the spectrum at more than 20 thousand Kelvin, and the rest with less high and dramatic slopes with noon daylight at 6500 Kelvin, noon sunlight at 5500 Kelvin, nd sunset sky and sunlight at less than 4000 Kelvin.

Unless we are using a mono-chromatic light source, all the light we use, both natural and artificial, is a mixture of different percentages of all the visible wavelengths. One way of describing different lights is by color temperature. This diagram shows how daylight at different times of the day can be expressed as color temperature (in ºKelvin).

Color Temperature Calculation

CIE color space with curved line marking black body radiation beginning from red corner and curving to blue at temperatures above ten thousand Kelvin.

Color temperature is calculated mathematically. It describes an (imaginary) black body that absorbs all the radiation that falls on it—hence the term "black body".

As it is heated it starts to glow and change color. The color of the black body is directly related to its temperature.

Here is the CIE XYZ color space again showing how the color of the black body moves through the spectrum as its temperature rises.

Categorizing Color: Color Temperature Graph

Graph of black body radiation compared with sensitivity of the human eye, with the sun matching the peak of eye sensitivity, and lower temperatures moving into the infrared range.

The light emitted by the black body is not just a single wavelength but can be plotted as a curve.

The color temperature is taken from the maxima of the curve.

Color Temperature Examples

Table of color temperatures with examples, from candlelight at 1500K, 40 watt incandescent bulb at 2680K, sunset and sunrise at 3000K, tungsten bulb at 2400K, one hour before sunset at 3500K, a sunny day at noon at 4500 to 5000K, electronic flash at 5500 to 5600K, overcast sky at 6500 to 7500K, blue sky at 9000 to 12000K, and white fluorescent tube at 15200K

Here are some examples of color temperature. Don't confuse color temperature with "cool" and "hot" colors. Color temperature is calculated mathematically. Cool and hot colors are functions of individual perception.

Categorizing Color: Metamerism

Graphs showing smooth spectral curve for incandescent bulb, and distinct spikes at certain wavelengths for fluorescent bulb

Metamerism happens when two colors (paint samples, dyes, fills) look the same under one light source and totally different under another. One of the problems is that all lights have their own spectral distribution.

These are examples of the spectral distribution of two very common lighting systems. The diagrams show the amount of light emitted at each wavelength through the color spectrum.

Categorizing Color: Another Problem in Color Perception

The other problem is that the human eye has only three color receptors. The three different cones are each, individually, sensitive to the whole spectrum, but in different amounts.

The brain adds the signals from all three kinds of receptors together to produce color information. And the combination of the signals from the three receptors may make certain colors look the same under some lights but not others.

For more information, return to "Cones in the Retina".

Color Systems Quiz

Categorizing Color: Summary

In this section you learned that:

The Munsell color system uses chroma, value, and hue to assign a numerical code to each color.
The CIE XYZ and CIE L*a*b* systems take into account the relative sensitivity of the human eye to different colors.
The color temperature of the light source affects the appearance of a color sample to the human eye.
The phenomenon of metamerism complicates the effort of color matching using the human eye.

References

This article on color can be found at JAIC Online.

Williams, Edwin L.; Grosjean, Eric; Grosjean, Daniel; "Exposure of artists' colorants to sulfur dioxide"; JAIC 1993, 32 (3). 291–310.

Credits

Researched and written by Sheila Fairbrass Siegler

Instructional Design by Cyrelle Gerson of Webucate Us

Project Management by Eric Pourchot

Special thanks to members of the Association of North American Graduate Programs in Conservation (ANAGPIC) and the AIC Board of Directors for reviewing these materials.

This project was conceived at a Directors Retreat organized by the Getty Conservation Institute and was developed with grant funding from the Getty Foundation.

Converted to HTML5 by Avery Bazemore, 2021

Color

Foundation for Advancement in Conservation

© 2008