| Color Theory for the Desktop |
| The Interaction of Light and Matter |
The nature of light and the visible spectrum are only one part of what's needed for us to see color. The second part of the triad has to do with the interaction of light and matter, for when we see an object as blue or red or purple, what we're really seeing is a partial reflection of light from that object. The color we see is what's left of the spectrum after part of it is absorbed by the object.
First, let's look at the general properties of light interacting with matter. When light strikes an object it will react in one or more of the following ways depending on whether the object is transparent, translucent, opaque, smooth, rough, or glossy:
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It will be wholly or partly transmitted. |
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It will be wholly or partly reflected. |
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It will be wholly or partly absorbed. |
Transmission
Transmission takes place when light passes through an object without being essentially changed; the object, in this case, is said to be transparent:
Some alteration does take place, however, according to the refractive index of the material through which the light is transmitted.
Refractive index (RI) is the ratio of the speed of light in a vacuum (i.e., space) to the speed of light in a given transparent material (e.g., air, glass, water). For example, the RI of air is 1.0003. If light travels through space at 186,000 miles per second, it travels through air at 185,944 miles per second—a very slight difference. By comparison, the RI of water is 1.333 and the RI of glass will vary from 1.5 to 1.96—a considerable slowing of light speed.
The point where two substances of differing RI meet is called the boundary surface. At this point, a beam of transmitted light (the incident beam) changes direction according to the difference in refractive index and also the angle at which it strikes the transparent object. This is called refraction.
Light striking the surface of an object straight on (that is, at normal incidence) will pass through without refraction (as in the illustration above). But light striking at any other angle will be refracted as well as partially reflected:
The RI of a substance is further affected by the wavelength of the light striking it. The RI of a transparent object is higher for shorter wavelengths and lower for longer ones. This is most apparent in the refraction of a light beam through a prism. The red end of the visible spectrum does not refract as much as the violet end. The effect is a visible separation of the wavelengths. The rainbow is another example, where sunlight is refracted through raindrops in a manner similar to the refraction of light through a glass prism.
If light is only partly transmitted by the object (the rest being absorbed), the object is translucent:
The degree of absorption is the only essential difference. Light transmitted through a translucent object reflects and refracts according to the same principles as light transmitted through a transparent object.
Reflection
As we've seen above, light that strikes a transparent object is transmitted in part and reflected in part. But when light strikes an opaque object (that is, an object that does not transmit light), the object's surface plays an important role in determining whether the light is fully reflected, fully diffused, or some of both.
A smooth or glossy surface is one made up of particles of equal, or nearly equal, refractive index. These surfaces reflect light at an intensity and angle equal to the incident beam:
Scattering, or diffusion, is another aspect of reflection. When a substance contains particles of a different refractive index, a light beam striking the substance will be scattered. The amount of light scattered depends on the difference in the two refractive indices and also on the size of the particles.
The most easily observed example of scattering is the color of the sky. Light at the blue-violet end of the spectrum is scattered by particles in the air during periods of average daylight producing blue sky. As the daylight wanes, the shorter blue-violet wavelengths are lost and the longer red-orange wavelengths are scattered, giving the sky the fiery hues of sunset.
Most commonly, light striking an opaque object will be both reflected and scattered. This happens when an object is neither wholly glossy nor wholly rough.
Absorption
Finally, some or all of the light may be absorbed depending on the pigmentation of the object. Pigments are natural colorants that absorb some or all wavelengths of light. What we see as color, are the wavelengths of light that are not absorbed.
However, as mentioned earlier (and as we'll see later when we discuss human vision), the wavelengths of light that concern us most are the red, green, and blue wavelengths. These are the basis for the tristimulus response in human vision, as well as a significant part of color reproduction.
Spectral Reflectance/Transmittance Curve
Just as spectral power distributions are a property of a light source, the spectral reflectance or transmittance curve is a property of a colored object. Spectral reflectance refers to the amount of light at each wavelength reflected from an object as compared to a pure reflection (e.g., from a pure white object that reflects 100% at all wavelengths). Spectral transmittance refers to the amount of light at each wavelength that is transmitted through a transparent colored object as compared to the amount transmitted through a clear medium such as air.
Below are some examples of spectral reflectance curves for objects that appear red, yellow, blue, and purple:
The importance of spectral reflectance or transmittance curves lies in their contribution toward the definition of color. As we've mentioned, seeing color depends on the triad of light source, colored object, and the human eye. The wavelengths reflected or transmitted from or through an object determine the stimulus to the retina that provokes the optical nerve into sending responses to our brains that indicate color.
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