Projectors are used for presentations in school classrooms, meeting rooms and the like, as well as in private home theatres. As a light source, such projectors use a high-pressure discharge lamp that is nearly a point light source. In particular, high-pressure mercury lamps are used.
A high-pressure mercury lamp encloses a halogen substance along with mercury, and includes an arc tube in which a pair of tungsten electrodes are disposed in mutual opposition. Light is radiated by the emission of an arc discharge between the electrodes. In a high-pressure mercury lamp, the halogen cycle is used to constrain the blackening effect produced during lighting. The blackening effect refers to the tungsten making up the electrodes evaporating and coming to adhere to the inside of the arc tube. The tungsten so evaporated is made to return to the electrodes through the halogen cycle, and comes to accumulate. As such, given a protrusion formed by such accumulation on the tip portions of both electrodes, the luminescent spots of the arc discharge spanning the inter-electrode distance are the two protrusions.
In recent years, demand for ever-increasing longevity in the high-pressure mercury lamp serving as the light source has grown alongside projector use. As a response to such demand, an effective method has been discovered in preserving the stability of the arc discharge by maintaining an appropriate shape in the protrusions formed on the tip portions of the pair of electrodes.
One proposed method for forming and maintaining an appropriate shape for said protrusions involves a high-pressure discharge lamp lighting device configured to switch between two or more frequencies for the AC current supplied to the high-pressure mercury lamp (see, for example, Patent Literature 1). Patent Literature 1 describes causing growth in the protrusions by applying suitable AC current in at least one frequency, and causing reduction in the protrusions by applying suitable AC current in other frequencies. The shape of the protrusions is thus preserved by repeatedly alternating between the frequencies to cause growth and reduction.