Low pressure

Low pressure lamps can be distinguished in fluorescent lamps, UV lamps and sodium lamps.

In a fluorescent lamp UVC (mainly 254nm wavelength) is produced by exciting mercury vapor and this UV is subsequently converted to visible light (or UVA and UVB for tanning lamps) by a fluorescent layer at the inner wall.UV lamps have no fluorescent layer, so they emit mainly UV light.

Low pressure sodium lamps produce a monochromatic yellow light generated by excitation of sodium vapor. They have a very high efficiency (200lumen/W) but their yellow color limits their use more and more due to the very limited color rendering.

In general low pressure lamps have filaments on each lamp end. They carry the arc current after ignition and in most cases they are preheated before ignition to maximize lamp life. Also during dimming the filaments must be provided with additional heating to keep the filament temperature at approx. 700-900°C. Outside this range the lamp life will suffer.

As with any discharge lamp the low pressure lamps cannot be directly connected to the mains. The negative resistance of the arc discharge will lead to uncontrolled lamp currents and even to destruction if the lamp. Therefore a ballast in between the lamp and the mains provides a controlled lamp current or power. In addition the ballast provides the preheating of the filaments, an ignition voltage to provide the first discharge between the two filaments and, if required, dimming capabilities with additional filament heating.

To keep the temperature and pressure low, high power low pressure lamps become very large: up to 1.5m for up to several 100W.

In electrodeless lamps the power to the discharge plasma is transferred via an alternating magnetic induction field. The plasma path forms the secondary winding of a high frequency transformer. Since it contains no electrodes that are normally subject to wear out, its life can be much longer than for standard fluorescent lamps.

For lighting in areas with explosion hazard preheating is not allowed. Special lamps are developed that provide sufficient lamp life without preheating.

Nedap Light Controls has the technologies to optimally drive low pressure fluorescent, UV and electrodeless induction lamps, for powers from 10 to 1000W.

High frequency (>1-10KHz) operation of low pressure lamps results in a higher efficiency (10-15% increase) than low frequency operation. The luminous efficacy of fluorescent ramps ranges from 50-100 lm/W depending on type and power rating. Low pressure sodium lamps reach almost 200 lm/W. For UV lamps the luminous efficacy has no meaning, since the luminous efficacy is the amount of visible light divided by the input power.