Metal halide discharge lamp

A discharge lamp ballast for operating a metal halide discharge lamp, said lamp comprising:

• an arc tube filled with at least sodium halide and scandium halide, said arc tube being formed at its opposite ends with electrodes which gives an arc therebetween;
• and
• regulator for keeping a coldest spot temperature of said arc tube at 550° C. or more when operating the lamp at a lamp power which is 50% of rated lamp power of said lamp,
• said lamp having a rated lamp power less than 400 W, and a molar ratio (R) of said sodium halide and said scandium halide filled in said arc tube satisfies a relation that 2.8≦R≦17.0,
• said ballast comprising a dimmer for varying a lamp power to be applied to the lamp from 100% to 50% of a rated lamp power.

A discharge lamp ballast for operating a metal halide discharge lamp, said lamp comprising:

• an arc tube filled with at least sodium halide and scandium halide, said arc tube being formed at its opposite ends with electrodes which gives an arc therebetween;
• and
• regulator for keeping a coldest spot temperature of said arc tube at 550° C. or more when operating the lamp at a lamp power which is 50% of rated lamp power of said lamp,
• said lamp having a rated lamp power is 400 W or more, and a molar ratio (R) of said sodium halide and said scandium halide filled in said arc tube satisfies a relation that 5.7≦R≦22.7,
• said ballast comprising a dimmer for varying a lamp power to be applied to the lamp from 125% to 50% of a rated lamp power.

A metal halide discharge lamp, comprising:

• an arc tube filled with at least sodium halide and scandium halide, said arc tube being formed at its opposite sealed ends with electrodes which gives an arc therebetween;
• and
• a regulator for keeping a coldest spot temperature of said arc tube at 550° C. or more when operating the lamp at a lamp power which is 50% of rated lamp power of said lamp,
• wherein a molar ratio R of said sodium halide and said scandium halide filled in said arc tube satisfies a relation that 2.8≦R≦22.7,
• said arc tube being formed at its opposite sealed ends respectively with foils, each connected to each of said electrodes,
• said regulator including heat insulation layers respectively over said sealed ends in such a manner as to surround said electrodes as well as said foils entirely with respect to an axial length of said arc tube, said heat insulation layer being a metal layer reflecting an infrared radiation,
• said regulator also including a transparent sleeve which surrounds substantially the full axial length of said arc tube, said transparent sleeve being coated on its opposite axial ends with an infrared radiation reflection layer.