High-intensity-discharge lamp of the mercury-metal halide type which efficiently illuminates objects with excellent color appearance

A high-intensity-discharge lamp which efficiently illuminates objects with excellent color appearance, said lamp comprising:

• a sealed light-transmitting arc tube of predetermined dimensions and enclosing a predetermined volume;
• electrodes operatively disposed within said arc tube and spaced from one another a predetermined distance to define therebetween an arc path of predetermined length, lead-in conductors sealed through said arc tube and electrically connecting to said electrodes;
• a light-transmitting protective envelope surrounding said arc tube and enclosing a predetermined environment, conductor means sealed through said protective envelope, and means electrically connecting said conductor means to said lead-in conductors;
• said arc tube enclosing a small charge of inert ionizable starting gas and a discharge-sustaining filling substantially comprising:
• mercury in predetermined amount as required to provide a predetermined voltage drop between said electrodes when said lamp is normally operating;
• thallous iodide in amount to provide from about 0.02 mg to about 0.2 mg of T1I per cc of volume enclosed by said arc tube;
• stannous iodide in amount to provide from about 0.1 mg to about 0.8 mg of SnI.sub.2 per cc of volume enclosed by said arc tube;
• calcium iodide in amount to provide from about 0.25 mg to about 2.5 mg of CaI.sub.2 per cc of volume enclosed by said arc tube and to provide from about 1.2 mg to about 10 mg of CaI.sub.2 per mg of said stannous iodide;
• and
• said lamp when normally operating having a predetermined power consumption, and means associated with said lamp causing the cold-spot temperature within said arc tube during normal lamp operation to be at least about 750.degree. C.