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

摘要

Four different optical interference films are formed on the outer surface (13) of an incandescent lamp (11) to achieve good color rendering properties at a high color temperature. A first film formed on the lamp (11) includes high (31) and low (33) 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 of each thereof is 0.12 ∼ 0.7 µm. A second film formed on the first film has the low refractive layer (33) and the optical thickness of 0.132 0.2125 µm, 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 layer (31). 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 µm, which is calculated by the expression (K × d). A fourth film includes the low refractive index layer (33) having the optical thickness of 0.0720 ∼ 0.1275 µm, which is calculated by the expression (K × d/2).