APPENDIX B RADIANT QUANTITIES AND UNITS There are two unit systems for measuring light quantities. The radiant quantities are derived from a measurement of energy, i.e., that which is measured by a thermal detector. Photometric quantities are based on visible radiation that is measured by the response of the eye. Table B.l lists the radiant quantities with their definitions, units, and the usual symbol by which it is known. Table B.2 is the parallel chart for luminous quantities and is arranged such that the quantities as numbered correspond to the radiant counterpart under the same number in Table B.l. This study will use the radiant system. To avoid possible confusion a brief description of each of the listed quantities is provided to clarify what is intended when a quantity is used in this study. The radiant energy is the basic unit from which the system is derived. This study will use the MKS system of units which makes that energy unit the joule. Note that the term radiation is often a synonym for radiant energy. The radiant energy density is, as implied, the energy per unit volume. This unit is seldom used and is included here only for comparison. The radiant flux is the time rate of energy transfer to or from a surface. A common synonomous phrase is radiant power, or just power. The radiant flux density is the radiant flux leaving or entering a surface per unit area of the surface normal to the directions of the flux. When the flux leaves the surface either by reflection or emittance, it is called radiant emittance. When it is incident on the surface, it is called irradiance. Radiance is the radiant emittance per unit solid angle. Radiant intensity is radiance integrated over its surface area so that for small surface areas, the radiant flux density measured some distance away from the area will conveniently include the energy emitted from the entire source.
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