For example, the light from some sodium sources is predominantly emitted in the longer wavelengths, between yellow and red. Light emitted in these wavelengths is less effective than light emitted at lower wavelengths because the human eye is less sensitive to photons in that region of the spectrum. On the positive side, sodium lamps use no phosphors. Their warm, incandescent appearance and wide range of beam spreads make them a good alternative for retail display lighting applications. HID lamps are available with three types of magnetic ballasts or with electronic ballasts.
Power factor is a measure of the efficiency with which input power is converted to useful work.
The ideal power factor is 1.0, and many utilities require power factors of at least 0.9 on products that they recommend. High-reactance autotransformers, which are similar to reactor ballasts, but are capable of providing a voltage boost when line voltage is insufficient to start a lamp. Constant wattage autotransformers, which are the most common type. They regulate lamp power better than the other magnetic ballasts, eliminating flicker and inadvertent shutoffs when line voltage varies. Electronic ballasts are available for HID lamps rated up to 400 watts.