The region between two curves, such as circles or rectangles, used to specify the tolerance on component size and geometry. 2. Pertaining to the cross section of an optical fiber, when used to specify the respective diameters and ovalities of, and concentricity error between, the core and cladding; two concentric annular regions which define the core-cladding boundary and the cladding outer boundary. Note: Dimensions are usually expressed in micrometers (m. ) The larger annular region is defined by concentric circles of diameter and , where DC is the nominal diameter of the cladding and DC is the cladding diameter tolerance. The smaller annular region is defined by concentric circles of diameter and , where Dc is the nominal diameter of the core and Dc is the core diameter tolerance. When the core and cladding boundaries of the cross section of the fiber in question fall entirely within their respective defined areas, the fiber meets the specification. 3. Of the cross section of a given optical fiber, when used to characterize the respective diameters and ovalities of the core and cladding, and the concentricity error between the core and cladding; two such pairs of concentric circles, the concentric pairs not necessarily being concentric with one another. Note 1: One pair of concentric circles characterizes the core, and the other pair, the cladding. The cladding ovality is characterized by the smallest circle that circumscribes its cross section, and the largest circle that fits within its cross section. (The cross section is assumed, to a first approximation, to be elliptical in shape, so these defining circles will be concentric. ) The core cross section is characterized by an analogous pair of circles, also concentric with one another, but not necessarily with those defining the cladding cross section. Note 2: The distance between the centers of the two concentric pairs (core pair and cladding pair) defines the offset between the core and cladding (the "core-cladding offset,"also called the "concentricity error". ) The width of the annulus defined by the cladding circles determines the ovality of the cladding, and the width of the annulus defined by the core determines the ovality of the core.