The invention relates to a discharge lamp having an oval sectional shape, and more particularly to a circular fluorescent lamp.
Research and studies for developing a circular fluorescent lamp having a non-circular sectional started many years ago for the purpose of increasing the illuminance of the lamp on a plane beneath its installed position, as disclosed in Japanese Patent Publications Nos. 50-32785 (1975) and 51-11876 (1976). Also, Japanese Utility Model Publication No. 37-22455 (1962) proposes a straight fluorescent lamp in which the ratio between the larger and smaller tube diameters is selected to be 4:3 or 4:2, and the thickness of its phosphor film is made non-uniform, so as to improve its illuminance in a specific direction relative to its installation.
Although a discharge tube having an oval sectional shape has been proposed for years and is well known in the art, as disclosed in the prior art publications, the mechanical strength of the discharge tube decreases inevitably due its oval sectional shape. However, no proposal has been made hitherto for solving the problem of an undesirable decrease in the mechanical strength of such a discharge tube.