Appendix B: Lamp Performance Data
Rated Average Life
Rated average lamp life is based on tests of large groups of lamps under specified conditions.
All lamps do not last until rated average life. They fail in accordance with a mortality curve. Note that 35%-50% of lamps in a large group can be expected to fail before the rated average life value is reached, and that 50%-65% last longer than the rated average life.
Normal end-of-life is indicated by blackening of the ends of the arc tube, deterioration of the electrodes and/or intermittent starting. Light output will generally be low, but present.
Metal Halide Lamps
Normal end-of-life is caused by sodium loss from the arc tube. The lamp may be hard to start. Color may be blue or pink, depending on operating position. Light output will be low.
High Pressure Sodium Lamps
Normal end-of-life is usually indicated by the lamp cycling on and off. As the lamp accumulated burning hours, its operating voltage increases, eventually becoming higher than the voltage supplied by the ballast. The lamp extinguishes and cools. The ballast restrikes the arc. The lamp voltage increases again and the lamp extinguishes. This process typically blackens the end of the arc tube.
Lamp Lumen Maintenance
The light output of HID lamps depreciates as the lamps accumulate burning hours. The degree depends on the lamp type.
The interval between group relampings is determined by the minimum acceptable light level for the application and economics. The light level will be determined by the lumen maintenance of the lamp and dirt conditions, and the economics by the mortality curve, lamp and labor costs.
Your lamp suppliers have information for determining the best time schedule for your conditions.
In the event of a power interruption, HID lamps require time to cool down before they will restart.
Approximate Restarting Time
Mercury and metal halide: 4 to 10 minutes
High pressure sodium: 1 minute
In tight or enclosed fixtures, mercury and metal halide lamps could take up to 20 minutes to cool down.