Learning objectives

When you have completed this session, you should be able to

• define term "color",
• estimate which colors to choose.

Color Thesis, Color Models

The Visible Light

Visible light is electro magnetic energy with wavelengths ranging between 400 and 700 nanometers comprising the spectrum perceivable by human eye.

The spectrum of visible light

Light out of this range is invisible by human eye (ultraviolet and infrared).

In order to have the sensation of color generated three things are needed: a light source, some object to be sensed, and a sensor device. The perceived color of an object depends on the color of the light source. The same object looks different in sunshine and in neon light. Light sources are classified according to spectral emission. The Sunlight ray has approximately even distribution in every wavelength of the light spectrum. Artificial light sources cannot provide this therefore they do not have neutral light. Color shift or the purity of white light is measurable by color temperature (1. in more detail about color temperature). (On better quality monitors the color temperature can be adjusted)

The Color of Objects

Objects can be characterized with their spectral reflection or transparency depending on whether reflected or passing through (e.g. slides) light arrives to the detector.

Neutral colored objects (grey) reflect (or let through) equal amount of radiating energy on each wavelength of the visible spectrum (in each color).

Colorful objects absorb lights of certain wavelengths so the reflected light becomes colorful.

Color Mixing

Different colors are practically created by color mixing. This means that we have a few basic colors and all other colors are mixed from these.

There are two types of color mixing procedures:

• The additive and
• The subtractiv e

Additive Color Mixing

Additive color mixing has three basic colors: red, green, blue. Every color can be produced by adding them together in different ratio. (This fact was observed by Newton in 1730.) After the English mane of the colors this method is called RGB color mixing.

Imagine additive color mixing the following way:

We are sitting in a completely dark room which has white walls. If there is no any source of light we cannot see anything, so the walls are black (this is why in physical terms black is not a color but the absence of light). Let us project red, green and blue colors on the walls. Where all three colors are mixed we see white. The mixture of blue and red results magenta, the mixture of blue and green results cyan, and the mixture of red and green results yellow.

Cathode ray tubes of televisions and monitors also use additive color mixing. If we take a look at the screen with a magnifier we can see that the picture is created from tiny red, green, and blue dots. In our eyes these colors mix and this way the proper sense of colors is created.

Subtractive Color Mixing

The three basic colors of subtractive color mixing are the cyan, the magenta, and the yellow. Subtracting from white color (which contains every colors) these colors all other color can be produced. It is called CMY, according to the first letters of the English names of the colors.

Demonstrating subtractive color mixing

In this case the light source must emit white light which have equally intensive components in all spectrums. With this light different objects can be exposed which absorb certain wavelengths of the light. This way they subtract these wavelengths from the white light. The remains reach the eye of the observer creating the proper color effect. A cyan object absorbs red color, a purple one absorbs green, and a yellow one absorbs blue.

Our eyes perceive colors according to the subtractive model and printers for instance use this model too.

RGB and CMY color mixing methods are the opposites of each other in some interpretations. Mixing two and two base colors from one color mixing methodology results one of the base colors of the other color mixing methodology. Mixing red, green, and blue colors results white while mixing cyan, magenta, and yellow colors results black.

The Characteristics of Colors

Colors have three particular characters: hue, saturation and brightness.

Hue defines the actual color. Saturation is related to the power of color expressing the distance from grey. The less this distance is the more contaminated the color is and the other way around the more the distance is the more saturated the color is. Brightness determines the power of light exposure of the color from black to white.

Colors on the Computer

The computer monitor (and all displaying equipment in general) applies RGB color mixing process to display colors.

The cathode placed in the tube of the monitor emits electron rays. These rays hit the phosphor layer in the inside of the monitor tube. This effect generates red green and blue light emission. The power of light emitted depends on the intensity of electron rays. Observing a monitor from a very short distance we might see that the image is actually made of red, green and blue dots having different intensity.

Why are monitors unable to create every color?

Theoretically every component of the colors (red, green, and blue) can have infinite number of intensity, so infinite number of colors can be created. In the information technology however the intensity of all colors are determined digitally and this number is finite. Since the different components are stored in one bite every one of them can have 256 kind of intensity. The combination 16 777 216 kind of colors the computer able to produce. This is called the RGB color palette. This is a very large number of colors more that human eye can differentiate. However we cannot be satisfied because unfortunately many colors are missing from this palette. It means that there are colors which have numerous shades in the RGB palette and there are some represented only with a few shades. Typically there are a lot of blue and green shade but only a few brown and yellow.

Even more colors are missing from the CMY color palette so fewer colors can be used. This is the reason for editing images usually in RGB process and only at the end of the work they should be converted to the CMY color palette.

Other color palettes have been developed too. LAB color palette can be mentioned because this contains the most colors, this is the most complete one. The application however requires great routine and therefore usually professionals use it exclusively.

The CMYK Color Palette

When using color mixing; only theoretically can every color be produced. In practice it is useless to mix cyan, magenta and yellow to produce black. That will never turn out. This is a result of the fact that the basic colors and the tools used are not perfect. This is the reason for adding black to the three basic colors of CMY model in typography (K comes from the last letter of the word black).

Monitors cannot either create all colors from red, green, blue.

Sometimes we have to switch from one color processing method to the other especially when an image is being edited on computer using RGB technique and later that is intended to be printed out. Due to the fact that printers work according to CMYK color system somehow colors have to be corresponded to one another. Most image editing software help in this. Images created for the net practically should be composed by using RGB model since they are intended for monitor usage operating in this system.

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What do you think?

What is the importance of colors in teaching? What color do you use for correcting and why? What colors are present in the classroom? Which one dominates? What was the color of a board long ago? Reflect these questions in the forums, in your blog!

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Reference Works:

[*] Tay Vaughan: Multimedia, Published by Osborne, Berkeley, California, USA, 1996.

[**] Csánky Lajos: Multimédia PC-s környezetben, LSI Oktatóközpont, Budapest, 2000.