A properly designed professional dimmer provides voltage regulation to prevent changes in output when the line voltage changes. Another critical aspect of dimmer regulation allows the dimmer to regulate in the presence of a non-sinusoidal or harmonic-rich voltage waveform on its input. Such harmonics can be typical in a large phase-control dimmer installation. A simple test can determine dimmer regulation performance in the presence of harmonics: 1. Set a single lightly loaded dimmer at 15%. 2. On a single submaster, simultaneously fade all other available loads in the system slowly from 0 to 100% and back. Observe or measure the output of the dimmer at 15%. Does the level change? If so, either you have a very "soft" power service, or your dimmers do not regulate in the presence of non-sinusoidal voltage input.