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1- Some objects, such as a printed piece of paper are mostly refl ective. Others, such as fi lm or a transparency are transmissive objects: part of the original light goes through the object. This document refers to the basic  

    context of refl ective objects. 
2- This is called 8-bit color because in the binary system, these 256 values can be coded with 8 bits.

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parts of the energy. In effect, the object subtracts part 

of the light originally emitted by the light source

1

The part of the original light that is subtracted 

depends upon the nature of the object’s surface and 

in particular on the pigments, dyes, and inks that 

might be present.

For example, red paint contains pigments that refl ect 

mostly the ‘reddish’ wavelengths situated around 650 

nm, and attenuates (subtracts) other wavelengths.

THE HUMAN EYES AND BRAIN

The light that is refl ected by an object strikes our 

eyes, which contain light sensors called rods and 

cone cells. 

• Rods are mostly sensitive to the intensity of light. 

They enable us to distinguish between light and dark 

under low light conditions. Thanks to rods we can

see at low levels of light and detect different gray 

tones. Under normal lighting conditions our eyes 

only use cones.

• There are three types of cone cells. Some are more 

sensitive to the red areas of the color spectrum, some 

to the green areas, and others to the blue areas. 

Depending on how they are stimulated by light 

striking the eyes, rods and cones send signals to 

the brain, which process the signals to create a 

perception of color.

Exactly which color is perceived depends on the 

composition of wavelengths in the light waves. If the 

sensors detect all visible wavelengths at once, the 

brain perceives white light. When our visual system 

detects a wavelength around 700 nm, we see “red”; 

when a wavelength around 450-500 nm is detected, 

we see “blue”; a 400 nm wavelength looks “violet”; 

and so on. If no light is present, no wavelengths are 

detected and the brain perceives black. 

II. COMPUTER DISPLAYS AND COLORS

Computer monitors display images as pixel 

matrices where each pixel is made up of three 

tiny light sources commonly called dots. A LaCie 

321 Monitor, for instance, displays a matrix of 

1600X1200 pixels. A close-up view of such a 

matrix can be seen in the following illustration.

Each of the three dots that make up a pixel is 

responsible for emitting a shade of red, green or blue 

light. Each dot’s intensity can be adjusted with a value 

from 0 - 255

2

. When the dot’s intensity is set to 0, the 

dot emits no light, and when it is set to 255 it emits its 

maximum intensity. By setting a given intensity 

for each of the three dots, one creates an individual 

color such as: Red=100, Green=100, Blue=100.

A large palette of colors is available, which comprises 

256x256x256 equaling 16.7 million colors.

The following illustration shows a variety of 

combinations of RGB and the resulting colors.

I. INTRODUCTION TO COLOR

Color vision is the capacity of an organism to 

distinguish objects based on the wavelength of the 

light objects refl ect or emit. A ‘blue’ fl ower does not 

emit blue light; it simply absorbs all the frequencies 

of light shining on it except the frequencies we call 

blue, which are refl ected. A fl ower is perceived to 

be blue only because the human eye can distinguish 

between different frequencies. The refl ected light hits 

our eyes and stimulates the visual cells of our retinas. 

Our eyes send signals to our brains, which process 

the signals to create color.

Our impression of color results from interactions 

among three factors: 

• The light source 

• The object that refl ects part of the emitted light

• The eyes and brain

Let’s examine the role that each of these factors plays 

in the creation of the impression of color.

THE LIGHT SOURCE

Light is a wave-like phenomenon. A light source 

emits waves that vibrate at a certain wavelength. 

Among these waves, those with a wavelength 

between 380 - 700 nanometers compose the visible 

spectrum. Waves with higher or lower wavelengths 

are not visible to humans.

A light source can be characterized by its spectral 

distribution. The spectral distribution of a light source 

describes the proportion of the energy it emits in 

various areas of the spectrum. 

A light source that emits most of its energy in 

wavelengths of 570 nm (nanometers) can be 

described as emitting mostly “yellow” light.  A light 

source that has a fl at spectral distribution (equal 

energy emitted across the entire spectrum) will be 

described as gray.

THE OBJECT

When light waves strike an object, its surface absorbs 

some of the waves’ energy, and refl ects some other 

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