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) temperature. COLOR   RELATIONSHIPS 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 the color. 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 brilliant. 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. REFLECTION 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. 1-4