Incandescent lamp having good color rendering properties at a high color temperature

摘要

Four different optical interference films are formed on the outer surface of an incandescent lamp to achieve good color rendering properties at a high color temperature. A first film formed on the lamp includes high and low refractive index layers alternately stacked so that the total number of layers is n (where n is odd number greater than 5) and the optical thickness thereof is 0.12˜0.17 &mgr;m. A second film formed on the first film has the low refractive index layer and the optical thickness of 0.132˜0.2125 &mgr;m, which is calculated by the equation (K+1)×d/2, wherein K is 1.2˜1.5 and d is the optical thickness of each refractive index layer of the first film. A third film formed on the second film has at least a high refractive index layer. The total number of refractive index layers of the third film is an odd number of 1˜(n-4) and the optical thickness of each refractive index layer thereof is 0.144˜0.255 &mgr;m, which is calculated by the expression (K×d). A fourth film includes the low refractive index layer having the optical thickness of 0.0720˜0. 1275 &mgr;m, which is calculated by the expression (K ×d/2).