Spectroscopic and electrical study of rare-gas-based, hollow cathode luminescent discharges: Application to the lifetime and efficiency enhancement of mercury-free signs

This work deals with the potentialities of mercury-free mixtures for the efficient operation of luminous signs used for publicity lighting. The experimental study of rare-gas-based, cold hollow cathode, low-pressure luminescent discharges is reported. Electrical and spectroscopic measurements are presented with a special emphasis on the radiation properties of the positive column. The best mixtures for phosphor excitation are described in terms of composition and pressure together with their sensitivity to potential fluctuations of the operating conditions during sign lifetime. Aging studies are presented, showing evidence of a strong trade-off between the output power and the sign lifetime. The selective trapping of the heavier gas present in the gas mixture is presented and proven to be at present the main reason for the sign lifetime limitation. The limitation of commercial (sinusoidal or square waveform) ballasts for the excitation of such mercury-free discharge is presented. Finally, preliminary measurements, performed using a lab-developed short rise time and low duty-cycle driver, are reported, showing a significant increase of the illuminance and efficiency (up to a factor three) of mercury-free signs as compared to the values obtained with conventional excitation.