THE NATURE OF PERCEPTION

Color Perception

Color Biology

Color Psychology

Guidelines for Effective Use of Color

Persistence of Vision

Frequency and Pitch

Properties of Visible Light

All light sources, except lasers, emit waves uniformly in all directions. The amplitude of these waves is percieved by the eye as brightness, or luminance. Light waves have extremely high frequencies. They are measured in nanometers (visible light ranges from 380 to 760 nanometers) or millimicrons or angstroms.

The wavelength of light determines its color, with red on the long end, and violet on the other. The range in between is the color spectrum. Pure spectral color can be seen in a rainbow or a prism. Most light sources emit a range of colors, and are called polychromatic. Some light sources, like neon, emit only a very narrow band of frequencies.

Visible Light

Color Perception
Human visible perception is based on the trichromatic theory of color. All color can be blended using three primary colors. Presence of the three colors at full strength produces white light, their complete absence produces black. People perceive direct light in an additive color process, and reflected light in a subtractive color process. The subtractive process means that different surfaces absorb cetain wavelengths of light and reflect others. That is what gives an object its color.

There are three ways we percieve color -

• Brightness is simply how light or dark something is.
• Hue refers to the spectral colors of red, orange, yellow, green, blue, and purple.
• Saturation (chroma) refers to the strength of a hue added to its brightness.

Frequency

• Measured in nanometers (nm)
• Humans can see wavelengths of 250-780 nm

  Red = 700 nm

  Green = 546 nm

  Blue = 436 nm

Psychology of Color

Different colors evoke different reactions in viewers. Be aware that some of these reactions will be culturally specific. For example, in the U.S., brides often wear white as a symbol of purity and widows wear black as a symbol of mourning. However, in many Asian cultures, brides wear black.

Red is a color not commonly seen in nature, and when it is (fire, blood), it usually means danger
— warning, prohibition (stop sign), negative (a budget in the red), excitement, hot

Dark Blue is the color of the ocean
— stable, calming, trustworthy, masculine (often the color of police uniforms)

Light Blue is the color of the sky
— cool, transparent (grid lines on grid paper)

Green is the color of the landscape
— growth, positive, organic, go (traffic light), comforting

White is the color of a blank sheet of paper
— pure, clean, honest

Black is the color of night
— serious, heavy, death

Gray is the color of fog
— neutral (icons for unavailable features in software are "grayed-out")

Brown is the color of earth
— wholesome, unpretentious, dependable (United Parcel Service ad campaign)

Yellow and Orange are the colors of sunshine
— active, positive (color schemes of fast food restaraunts)

Purple is traditionally the color of royalty

Pink is the color of flesh
— feminine

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Color is never seen in isolation

In the example below, one color looks like two different colors:

In the next example, two different colors look similar:

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Color Biology

The iris of the eye determines how much light strikes the retina. The retina contains about 120 million rods and cones. Cones are sensitive to bright light, and rods primarily are employed in night vision, and are less sensitve to color. Chemical composition of the rods and cones is altered depending on the wavelength (color) striking them. This chemical change is converted to voltages that send the messages along the optic nerve to the brain for deciphering.

Physiological Principles

Color results from the interaction of light with the nervous system. There are several components that affect color perception, including the eye lens, the retina, and a color processing unit along the optic nerve.

Lens
The function of the lens is to focus the incoming light on the retina, which contains the photo receptors. Different wavelengths of light have different focal lengths so, for pure hues, the lens must change its shape so that the light is focused correctly.
 
For a given lens curvature, longer wavelengths have a longer focal length, (i.e., red is the longest focal length and blue is the shortest). To have an image focused on the retina the lens curvature must change with wavelength. This means that if pure blue and pure red hues are intermixed, the lens is constantly changing shape and the eye becomes tired.

A related effect is called chromostereopsis, which is that pure colors located at the same distance from the eye appear to be at different distances, e.g. reds appear closer and blues more distant.

The lens also absorbs light, it absorbs about twice as much in the blue region as in the red region. As we age the lens yellows, which means it absorbs more in the shorter wavelengths. So the result is that people are more sensitive to longer wavelengths (yellows and oranges) than they are to shorter wavelengths (cyan to blue) and this increases with age. The fluid between the lens and the retina also absorb light and this increases as we age, so older people become less sensitive to light in general, and especially the sensitivity to blue decreases.

Retina
The retina contains the photo receptors that absorb photons and transmit chemical signals to the brain. There are two types: rods which are night-vision receptors and have no color dependency, and cones, which have color sensitivity and require a higher level of light intensity than the rods.

There are three types of photopigments in the cones; "blue" with a maximum sensitivity at 430 nm, "green" with a maximum sensitivity at 530 nm, and "red" at 560 nm (this wavelength actually corresponds to yellow). The percentage of cones is not equal but is as follows: blue (4%), green (32%), and red (64%). In addition, the cones are differentially distributed in the retina. The center of the retina has a dense concentration of cones but no rods while the periphery has many rods but few cones. The color distribution is also asymmetrical. The center of the retina is primarily green cones, surrounded by red-yellow cones, with the blue cones being mainly on the periphery. The center of the retina has no blue cones.

We see objects by edge detection, where an edge can be created by a difference in color or brightness or both. Edges formed by color differences alone, with no brightness differences, appear fuzzy and unfocused.

Photoreceptors adjust their sensitivity to the overall light level, e.g. going into or out of a dark room requires some adjustment time. There is also a required minimum intensity level for the photoreceptors to respond. Blues and reds must have a higher intensity than greens or yellows in order to be perceived.

Brain
From the retina the optic nerve (actually a collection of nerves) goes to the brain but before it reaches the brain there is a color processing unit, called the lateral geniculate body. This recombines the RGB color information into three new channels as follows:

R-G gives red or green color perception
R+G gives the perception of brightness and also yields yellow (Y)
Y-B gives yellow or blue color perception

Thus, blue plays no part in brightness perception so that colors differing only in amount of blue don't produce sharp edges. Also, note that since blue & yellow and red & green are linked together it is impossible to experience combinations such as reddish green or bluish yellow.

Color Blindness

About nine percent of the population has some kind of color perception problem. The most common is red-green deficiency, which can arise from a deficiency of either the red or the green photopigments. These people have difficulty distinguishing any color that is dependent upon the red:green ratio.

Click the link below to take a color blindness test:

http://www.toledo-bend.com/colorblind/Ishihara.html

Guidelines for the effective use of color:

1. Avoid the simultaneous display of highly saturated, spectrally extreme colors. Remember that this causes the lens to rapidly change shape and so tires the eyes. Desaturate the colors or else use colors that are closer together in the spectrum.
2. Pure blue should be avoided for text, thin lines, and small shapes. Since there are no blue cones in the center of the retina, these are difficult to see. But blue makes an excellent background color, e.g. for a computer display it tends to blur the raster lines.
3. Avoid adjacent colors that differ only in the amount of blue. Since blue does not contribute to brightness, this creates fuzzy edges.
4. Older operators need higher brightness levels to distinguish colors.
5. Colors change in appearance as the ambient light level changes.
6. The magnitude of a detectable change in color varies across the spectrum.
7. It is difficult to focus upon edges created by color alone.
8. Avoid red and green in the periphery of large displays.
9. Opponent colors go well together.
10. For color-deficient observers, avoid single color distinctions.

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Persistence of Vision:

Your eye and brain retain a visual impression for about 1/30th of a second. (The exact time depends on the brightness of the image.) This ability to retain an image is known as persistence of vision. Persistence of vision accounts for our failure to notice that a motion picture screen is dark about half the time, or that a television image is just one bright, fast, little dot sweeping across the screen. Motion pictures show one new frame every 1/24th of a second, and a computer monitor is scanned 30 times per second. The image of each frame is retained long enough to provide the illusion of continuous smooth motion.

Joseph Antoine Ferdinand Plateau was the first to recognize that the eye and brain required a resting time between images, and also realized that there existed an optimal number of frames per second to produce a moving image (he determined it was 16, which was the standard until the sound era of motion pictures).

Afterimage:

Prolonged stimulation by a bright object desensitizes part of the retina. This area appears as a negative afterimage, a dark area or complimentary color that matches the original shape. The afterimage may remain for 30 seconds or longer.

Stare at the black dot in the center of the image above. When the image changes, you should see a yellow afterimage in the negative (white) space.

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Visual Memory

Visual memory is how many images can be remembered in a given time span, and how long those images are retained.

Click the links below to test your visual memory:

http://www.gluck.edu/learning-memory/mem_games/visualtest1.html

http://people.brandeis.edu/~sekuler/MemoryDemo/

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Frequency and Pitch, Amplitude and Volume

• Sounds are oscillations of air pressure that stimulate the eardrum to send a message (voltages) to the brain via the auditory nerve.
  
• The frequency of the wave is interpreted as pitch, and the amplitude of the wave is perceived as loudness.
  
• The hearing range of the average person is in a range between 20Hz to 20KHz.
  
• Frequency is a quantifiable measurement, but pitch is a subjective interpretation by the brain.
  
• Shorter wavelenghts have a higher pitch. On a piano the shorter strings correspond to the higher notes. On an organ the shorter pipe lengths produce the higher notes.
  
• 0 dB has been established as the threshold of human hearing- the softest sound a person can percieve. A change of 1 dB represents the smallest difference the ear can perceive at a frequency of 1000 Hz. The intensity of sound must be increased tenfold for it to be perceived as having doubled in loudness. Two violins are not twice as loud as one.

Click the link below to test your hearing:

http://www.freehearingtest.com/test.shtml

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References

G. Scott Owen, Hypergraph (http://www.siggraph.org/education/materials/HyperGraph/hypergraph.htm)

"Physiological Principles for the Effective Use of Color", G. Murch, IEEE CG&A, Nov. 1984