Clear induction lamp

The special compact induction lamp presented here is a demonstration model made without a fluorescent coating in order to show the nonthermal plasma excited around the central open-core antenna. The lamp is built using the HPV platform of compact self-ballasted induction lamps developed during the second half of the 1990s at Philips, in the Netherlands. Interestingly, the lamp was also made without its mercury dose, which normally comes in the form of an amalgam located at the end of the central exhaust tube, placed inside the induction antenna for a proper control of the cold-spot temperature. Mercury was not added certainly in order to prevent the glass bulb from darkening via solarization (i.e., reaction between the glass material, short-wave UVs, and mercury ions). As a result the discharge vessel is only filled with krypton at a pressure of a few Pa, which is the normal buffer gas in standard HPV fluorescent lamps.

Interestingly, the light emission spectrum of this lamp shows the presence of oxygen in the discharge. This is most likely a consequence of some air pollution in the lamp fill that resulted from the degassing of the glass bulb. The lack of any metal and phosphor in the discharge vessel makes the gettering of impurities not as effective as in actual fluorescent lamps. Although the discharge dissociates most gaseous molecules into highly reactive species (atoms and radicals), only nitrogen is cleaned up effectively by reacting with the glass bulb. Oxygen do not bind nearly as effectively with glass, which is already an oxide material, and as a result this impurity remains in the lamp atmosphere. Despite this contamination, the plasma remains very stable and there is no sign that oxygen is disappearing during operation.

The lack of mercury and the presence of oxygen has a strong impact on the discharge characteristics compared to that of a conventional induction fluorescent lamp. These changes effectively reduce the fraction of energy transferred in the gas volume by way of resonant optical emission-absorption processes, while the gas ionization is not as effective due krypton’s higher ionization energy and the negative influence of oxygen (i.e., strong electronegativity and electron energy quenching). As a result, the plasma is more compact, it doesn’t fill the whole bulb volume, and it has a higher electric field. Moreover, the resulting impedance mismatch with the RF power generator effectively lowers the system efficiency. However, since this is only a demo lamp designed to show its working principles, none of the aforementioned issues really matter. Despite its presence in very small quantity, oxygen has also a very strong impact on the emitted light color. While electrical discharges in low-pressure krypton emits a cool-white light color, the light from this S-HPV has a clearly noticeable greenish tint, a sort of light cool-green white color.