Describing Color

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Reduced to its most basic definition, color describes a quality of light. Even with that very basic definition, there is no perfect way of describing color.

We can describe light perfectly. That's not the problem per se.

One problem is that our description may not describe what we see.
The other problem is that the description will involve more numbers than we can comfortably comprehend.

What is light?

What we see as light is electromagnetic radiation. It is the same as radio waves, radiated heat and ultraviolet radiation. Any kind of wave has a wave length. Radio waves are long. Ultraviolet waves are very short. Our eyes can't see radio waves because those are too long. They can't see the very short ultraviolet waves. What we see as light has a certain, relatively narrow, band of wavelengths. The longest of these wavelengths is red. The shortest wavelength we can see is violet.

The image below shows how the colors of light correspond to wavelengths:

short
waves,
high
frequency

long
waves,
low
frequency

Color	Wavelength nm	Frequency THz
violet	380-450	668-789
blue	450-495	606-668
green	495-570	526-606
yellow	570-590	508-526
orange	590-620	484-508
red	620-750	400-484

The wavelengths are given in nano meters. A nano meter is a millionth of a millimeter. The frequency is given in TeraHertz. The frequency of visible light is about five million times higher than FM radio. The image and table come from wikipedia http://en.wikipedia.org/wiki/Visible_spectrum

How can we describe light?

We can describe two characteristics of light:

Strength/quantity
Wavelength/frequency/color

So, how would we describe white light? What we call white light is all the colors of light at the same time. The way you can see that is to break the light up into its different colors with a prism.

prism

The result of putting light through a prism and letting it shine on a wall is the spectrum below:

The problem with the spectrum above is that it shows the colors that are present but doesn't tell us how much of each color there is.

We can draw a graph showing how much light there is at different wavelengths/colors.

spectra of various light sources

The graphs above show that different light sources give off different light. For instance, a low pressure sodium lamp gives off just one color. You've probably stood under such a lamp and noticed that you really couldn't tell what colors things were. (Broadly speaking, something that is red won't look red unless there is red light present.)

If we reduce each graph to a set of numbers, we can describe to someone else exactly what kind of light we have.

We can match colors exactly

When we look at a colored object, we are seeing light reflecting off it. If an object looks red, that means it is reflecting red light and our eye sees that. All the other colors are absorbed.

white light reflecting from a red object

white light reflecting from a red object

The diagram above shows white light striking a red object. Only red bounces off the surface and is seen by the eye. All the other colors are absorbed.

We can (as exactly as required) describe the color of an object by measuring how much of each color of light is reflected. When we talk about different colors, we mean that we divide the spectrum into perhaps a couple of hundred slices, each of which is very nearly the same color as its neighbors.

If two objects reflect light the same way, they are the same color.

The technology to measure a spectrum is rather simple: a cd-rom, a webcam, some black plumbing pipe and some software. Amateur astronomers build spectroscopes to measure and describe the light coming from stars. It is easy to build a spectroscope to get the wavelengths emitted by a given light source. Measuring the exact amount of light reaching the spectrometer is the challenge.

The purpose of this project is to develop a spectrometer that is so cheap and easy to use that anyone who needs to measure or describe color can have one.

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