Low-pressure mercury discharge lamp with color temperature adjustment

A low pressure mercury vapor discharge lamp emitting white light of controllable color temperatures, said lamp comprising:

• a. a discharge vessel sealed in a gas-tight manner and including a filling of mercury and a rare gas, said discharge vessel having a first light-emitting portion with a first luminescent layer which radiates white light at a first color temperature and a second light-emitting portion with a second luminescent layer which radiates white light at a second color temperature different from said first color temperature, said discharge vessel having a first discharge electrode for said first portion, a second discharge electrode for said second portion, and a third electrode common to said first and second discharge electrodes, a gas discharge being maintainable between said first discharge electrode and said common electrode to emit light from said first portion and between said second discharge electrode and said common electrode to emit light from said second portion;
• and
• b. a ballast circuit connectable to said electrodes for energizing said discharge vessel to emit white light with a color temperature determined by the light output from said first and second portions, said ballast circuit including means for generating a first AC current in said first portion between said first discharge electrode and said common electrode and a second AC current in said second portion between said second discharge electrode and said common electrode, and control means for controlling (i) the ratio of the effective value of the first AC current and the effective value of the second AC current to obtain a blended light with a controlled color temperature and (ii) for adjusting the sum of the effective values of the first and second currents to adjust the total light output of the lamp.

A discharge lamp apparatus which comprises:

• a gas-tight discharge vessel containing an ionizable filling and having first and second sections in communication with one another and each with a luminescent layer which radiates white light at respective first and second color temperatures different from one another, first and second heatable electrodes in said first and second sections, respectively, and a third electrode spaced apart from and common to the first and second electrodes,
• means for simultaneously supplying operating voltages to said first and second electrodes for generating first and second currents of alternating polarity through the first and second sections between said first and second electrodes and said common electrode, respectively wherein said first and second currents are high frequency currents
• means for controlling the ratio of the effective value of the first AC current and the effective value of the second AC current to obtain a light with a controlled color temperature;
• and
• means for periodically interrupting the flow of said first and second current at a low frequency.