ABSTRACT
Although the range of human vision is small compared with the total energy spectrum, human discrimination-the ability to detect differences in intensity or quality-is excellent.
5.2 Sources of Illumination Light reaching an observer usually has been reflected from some object. The original source of such energy typically is radiation from molecules or atoms resulting from internal (atomic) changes. The exact type of emission is determined by:
• The ways in which the atoms or molecules are supplied with energy to replace what they radiate
• The physical state of the substance, whether solid, liquid, or gaseous
The most common source of radiant energy is the thermal excitation of atoms in the solid or gaseous state.
