With each wavelength, there is a variation
in the amount of energy.
This variation of energy is called spectral energy
distribution. The spectral energy of a light source is
represented by color temperature. These terms are used
in photography to describe and define the sources of
light being used.
Color temperature describes the color quality of a
light source in terms of the amounts of red light and blue
light. Color temperature is based on what is called a
Planckian radiator, or simply a black body. As the
temperature of the metal of the black body is raised, it
goes from a dull black through red and orange to blue
and finally to white heat. The quality of the light emitted
is a function of the temperature of the metal. When the
object is red-hot, the color temperature is low since red
is at the low end of the scale; and when it is blue-white,
the color temperature is high. However, the temperature
at which a light source is burned does not control color
temperature; for example, a fluorescent tube burns at a
low 125°F, yet it has a high color temperature. Color
temperature then is raised or lowered relatively by the
amount of visible white light radiated from the source.
Be careful not to get confused. Traditionally reddish
light is known as warm and bluish light as cold; in
actuality, the color temperatures is the other way around.
The most convenient way to describe the color
temperature of a light source is by its Kelvin
temperature. From a practical point of view, this term
refers to the degree of whiteness of the light. Color
temperature is measured on the Kelvin scale and is
stated as Kelvin temperature. On the temperature scale,
0 K is the same as -273°C. Therefore, degrees Kelvin
(K) are always 273 degrees higher than the same
temperature on the Celsius scale. Thus a red-hot piece
of iron with an approximate temperature of 2000°C has
a color temperature of 2273 K. As the Celsius
temperature of an object is raised, it emits a whiter light
and produces a relatively higher color (Kelvin)
Many ways have been devised to classify the colors
we see. Though terminology may differ, it is generally
agreed that color can be defined by three qualities: hue,
brightness, and saturation.
Hue-Hue is the actual color or wavelength
reflected by an object-red, yellow, green, and so forth.
For example, it could be said that the color of an object
is blue. Blue identifies the hue. There are seven hues in
the visible spectrum. These seven hues are as follows:
blue, green, red, cyan, magenta, yellow, and white. Hue,
however, is an inadequate description of a cola. To be
more specific, we should say that an object is dark blue
or light blue. Now we have described the brightness of
Brightness-The brightness of a color is
independent of the hue. Two colors may have the same
hue but different brightness. Thus, to describe a color or
brightness, we say that it is dull, bright, vivid, or
Saturation-The saturation of a color is the degree
to which the color departs from neutral gray of the same
brightness. You can think of it as mixing black, gray, or
white paint with a colored paint, thus diluting the color.
In other words, saturation is a measure of color purity.
BEHAVIOR OF LIGHT
Light waves travel in straight lines. When light
waves encounter an object or new medium, they act in
one or more of the following ways:
They may be reflected.
They may be absorbed.
They may be transmitted.
When light is reflected, it acts in a certain way.
When the reflecting surface is smooth and polished, the
reflection is orderly, or specular. Specular light is
reflected at the same angle to the surface as the light
incident to the surface; that is, the path of the light
reflected from the surface forms an angle exactly equal
to the one formed by its path in reaching the surface.
Thus the angle of reflection is equal to the angle of
incidence, which is a characteristic of specular light
(fig. 1-6, view A). However, when the object surface is
not smooth and polished but irregular, light is reflected
irregularly or diffused (fig. 1-6, view B); that is, the light
is reflected in more than one direction.
Practically all surfaces reflect both specular and
diffused light; smooth surfaces reflect more specular
light, and rough surfaces more diffused light. Since
diffused light is more common than specular light, it is
of greatest value in photography. Objects that are not
light sources are visible and therefore photographic.