HID lamps include high pressure sodium lamps, metal halide lamps and mercury vapour lamps. They are typically used in areas such as sports stadiums, warehouses and large public areas, where high levels of light over large areas are required. HID lamps tend to have relatively high power ratings, for example above 150 Watts. They tend to operate under higher pressures and temperatures than fluorescent lamps.
Generally, a HID lamp system will use an inductive ballast that is designed to make the lamp operate at its approximate design power. When operated this way, the luminous flux output from the lamp is not constant and deteriorates over time. A lamp may lose up to one half of its light producing capacity by the end of its operating life.
In situations where a specified constant level of light is required, for example in sports stadiums, this means that either 1) the lamp needs to be replaced once its luminous flux output decreases below the specified level, or 2) additional lamps need to be installed to ensure that the particular level of light is achieved, despite deterioration in the luminous flux output from the lamp. Both options result in a higher cost of the lighting system. Also, in the first case, some of the operating life of the lamp may be wasted, and in the second case, higher energy use may be required.
It would be desirable to provide a method of operating HID lamps, and apparatus for performing the method, that ameliorates the effects of lamp deterioration.