The two minute explainer: Plasma lighting

Nikola Tesla next to the modern take on his plasma light source invention. (Images: Ceravision)

If ever there was a hint of science fiction about a light source it’s the plasma lamp. Well, I say ‘lamp’, but that’s because it’s the first word that comes to my 20th century mind. And it’s difficult to find an appropriate alternative moniker for a light source that requires such futuristic devices as a magnetron, a small glass vial and a column of plasma. In the world of the plasma lighting industry they’ve adopted the phrase ‘light engine’, so I suppose we should go with that.

To continue the futuristic thread, it’s also a technology that evokes the name of its inventor, Nikola Tesla. Depending on your viewpoint, Tesla was either a mad genius or just mad, either way he was ahead of his time.

Tesla died in 1943 and his work was generally ignored until the revolution in electronics design really got underway in the 1990s. At which point a lot of fictional science became startling reality.

Here are the basics of plasma lighting:

If you put an unconnected fluorescent tube into a big enough electrical field, it will glow without a power source being mechanically attached to it. This is because the fluorescent material is activated by the presence of the electrical field.

This is basic science and we have even seen artworks using the technique (Richard Box: ‘Field’, 2004). However, bringing the science up to a practical level is another matter. For the plasma light engine to work it requires two crucial components, the magnetron, or RF (radio frequency) driver, that creates an electrical field and the light emitter itself. These two components are connected by the generated electrical field only. In true Teslerian fashion, there is no wired connection at all, instead it all happens by induction.

Light is created when the radio-frequency signal created by the RF driver is focused via a waveguide into the small quartz glass vial that is the light emitter. This emitter contains a gaseous ‘soup’ and it is this mixture that determines the colour and brightness of the illumination.

The focused energy field ionizes the gas and the resulting atomic interaction raises the energy state of the electrons, forming a column of plasma within the light emitter. And that plasma column radiates visible light.

Benefits of plasma lighting include:

The small size of the light emitter enables efficient control of the light beam.

The mixture of elements within the light emitter provides for very high colour quality, typically in excess of Ra90.

The ability to mix elements also provides for specialist applications such as horticulture and UV treatments (sterilization, etc).

The absence of mechanical components means that the lamp life matches that of LEDs, with some plasma sources being quoted at 50,000 hours at L70.

The light generated is flicker-free.

Does plasma lighting have a future?

Plasma lamps pre-date the introduction of LED and there is no way that plasma lighting has come out unscathed from the LED revolution.

Light output from plasma light engines are high, with the lowest available wattage being around 300W. Light engines have a luminous efficacy ranging from 60lm/W to something above 100lm/W.

A plasma light source. 

These are the kind of light packages useful in sports field, car park and large-area industrial illumination but are difficult to handle in normal commercial applications.

The benefits of plasma over LED are difficult to see. Plasma’s life-span has an advantage over conventional discharge lighting, promising a 25,000 hour-life, but that advantage has disappeared with the development of LEDs. Similarly, the luminous efficacy of plasma doesn’t look so exciting when viewed against LED products offering the same kind of output.

But that doesn’t mean that the plasma light engine doesn’t have a future. Certainly, it will be a niche product but there are features that could guarantee its survival.

The promise of a flicker-free light may be of interest to sports arenas where televised events are held. LEDs are coming in for a lot of criticism over their propensity to flicker.

The bespoke nature of the elemental mixture within the light emitter means that installations can be designed for specific purposes, such as horticulture and intensive farming environments.

It’s considered that the light quality of the plasma column is purer than that of the LED as plasma does not rely on phosphor conversion of the generated light within the chip.

This may be one of the few occasions when UV is spoken of in a positive way, but plasma lighting can be designed to produce a given quantity of UV within its spectrum.

And finally, the plasma light may survive simply because it’s NOT an LED.