Color has been successfully used for object tracking and recognition. However, the
color of an object changes if the illuminant's color changes.
To see colors, energy in the form of light is required. Color sensation is produced
by physical stimuli associated with the various wavelengths in the visible portion
of the electromagnetic spectrum. To understand color better, we must recognize the
origin of light. Light comes from a wide variety of sources and consists of electromagnetic
radiation, a form of energy that spreads in a wave motion.
Figure 1: Visual color spectrum
All visible light is made up of a mixture of colors, which combine in different
proportions to make up each distinctive light. The way we measure light is by a
Spectral Power Distribution. In Figure 1, the visual color spectrum begins at 400
nm and finishes at 700 nm. Everything below 400 nm is called ultraviolet (UV) and
everything above 700 nm is referred to as infrared (IR). It is not possible for
the naked human eye to see ultraviolet or infrared light.
Average North Sky Daylight (Illuminant D65)
Spectrum of Fluorescent Light
Incandescent Light (Illuminant A)
Figure 2: Sources of light
Note: (Vertical axes: spectral distribution)
White light is composed of a select group of colors; each one characterized by a
specific range of wavelengths, which it absorbs. These are the colors of the spectrum
- red, orange, yellow, green, blue and violet.
Incandescence and luminescence are two main ways of creating light. Incandescence
is light from heat energy. Heating the source of a light bulb to a sufficiently
high temperature will cause it to glow. The stars and sun glow via incandescence.
Luminescence, also known as cold light, is light from other sources of energy independent
of heating. It can be generated at room or even lower temperatures. Quantum physics
explains luminescence as movements of electrons from their ground-state (lowest-energy
level) into a state of high energy. When returning to its ground-state, the electron
gives back the energy in the form of a photon of light. If the time interval between
the two steps is short (few microseconds), the process is called fluorescence;
if the interval is long (some hours), the process is called phosphorescence.
The combination of these wavelengths in light can change according to the light
source. For this reason, colors can look different when compared under the influence
of daylight, fluorescent light or sodium lamps. Natural sunlight varies widely.
It can be very blue, particularly around midday, looking north. Direct sunlight
usually is seen as golden, but, at sunset, it can be bright red. Artificial light
can be yellow, from sodium vapor, blue-green from mercury vapor, or it can be yellow,
from an incandescent light bulb, or varying colors from fluorescent light. The graphs
in Figure 2 show average north sky daylight (Illuminant D65), a cool white fluorescent
light (Illuminant F), and an incandescent light (Illuminant A).
Several phenomena can occur when light hits an object. Transmission occurs
if the light passes through the object, which is the case with transparent colors.
It is referred to as reflection if, for example, a blue object reflects the
part of the color spectrum that represents blue and the remaining light is absorbed.
The reflection curve of white will show roughly equal intensities close to 100%
reflection in all wavelengths of the spectrum. Refraction or scattering is
when light changes direction as it passes from one medium to another, like from
the polymer to a pigment or filler particle in a plastic part. Scattering is influenced
by the difference in refractive index between a particle and its surroundings, particle
size, and wavelength of light. An opaque color provides a high scattering performance.
A translucent color shows a combination of transmission and scattering. Absorption
happens if most wavelengths of the visible spectrum are absorbed. Black surfaces
absorb almost all light.