The light-emitting element of an LED radiates light and heat according to the input power. However, the surface area of an LED package is quite small, and the package itself is expected to release little heat to the atmosphere. An external radiator, such as heat sinks, is thus required. The heat release configuration for the connection portion of the external radiator mainly uses heat conduction. Regarding LED packages, to control the junction temperature of the light-emitting element Tj is important. The Tj must be kept from exceeding the absolute maximum rating in the specifications under any conditions. Because direct measurement of the junction temperature of a light-emitting element inside a package is seldom possible, the temperature of a particular part on the package outer shell (the case temperature) Tc [deg C] is normally measured. Tj [deg C] is calculated from the thermal resistance between the junction and the case Rj-c [deg C/W] and the amount of emitted heat, which is nearly equal to the input power Pd [W]. The package structure of the CL-L251 series minimizes the thermal resistance Rj-c, and the heat generated at the light-emitting element can be conducted to the external radiator efficiently. This document describes the detailed heat release configuration of the CL-L251 series and provides necessary data for thermal design of lighting apparatus, which leads to optimal utilization of LED performance.