What Actually Happens Inside a Medium-Pressure UV Lamp?

A medium-pressure UV lamp produces ultraviolet radiation by creating an electrical discharge through mercury vapour inside a quartz tube. The discharge excites mercury atoms, which then release energy across a broad range of wavelengths, including ultraviolet light used for curing inks, coatings and adhesives. This article explains how medium-pressure UV lamps work, why quartz is used, how spectral output can be modified and why the correct lamp specification matters.

From the outside, a medium-pressure UV lamp can look deceptively simple.

A quartz tube.

Two ends.

Electrical connections.

Switch it on and it produces an extremely intense source of radiation.

Inside that tube, however, a controlled electrical and physical process is taking place that makes industrial UV curing possible.

Understanding the basics helps explain why lamp construction, electrical specification, cooling and spectral output all matter.

It starts with an electrical discharge

A medium-pressure mercury lamp is a gas-discharge lamp and operates in several stages.

During start-up, electrons emitted from an electrode collide with atoms of the inert starting gas. The gas becomes ionised, creating a plasma through which electrical current can flow. As the lamp heats, mercury vaporises. High-energy electrons then interact with mercury atoms and excite them into higher energy states. When those excited atoms return to lower energy states, energy is emitted as electromagnetic radiation.

That radiation includes ultraviolet wavelengths.

And that UV light is what the industrial UV curing processes put to work.

Why is the lamp made from quartz?

Ordinary glass isn’t suitable for transmitting many of the ultraviolet wavelengths required in these applications because ordinary glass absorbs ultraviolet radiation. However, quartz and fused silica materials have the optical properties (they allow UV transmittance) that make them suitable for UV lamp construction.

For a UV curing lamp, the envelope therefore isn’t simply a container. It is part of the optical system.

Why ‘medium pressure’?

The terminology relates to the operating conditions of the mercury vapour inside the lamp.

Scientific literature distinguishes low, medium and high-pressure mercury lamps partly according to their internal operating pressure, and these different conditions produce different emission characteristics. [1]

A particularly important difference is spectral output.

Low-pressure mercury lamps are strongly associated with an intense emission around 253.7 nm.

Medium-pressure mercury lamps produce radiation across a much broader spectrum.

For industrial curing, that broad output can be useful because inks, coatings and adhesives can contain photoinitiating systems with different absorption characteristics.

From electricity to UV

It is useful to think of the lamp as an energy-conversion device.

Electrical power enters the system.

The electrical discharge establishes and maintains the plasma.

Excited atoms release radiation.

But not all of the input energy becomes useful UV radiation.

Medium-pressure lamps also produce visible and infrared radiation, and thermal management is consequently an important part of system design. [2]

This explains why cooling, extraction, reflectors and optical components surrounding the lamp are important.

The lamp cannot sensibly be considered in isolation from its fixture.

The spectrum can be modified

Not every UV curing application needs exactly the same spectral output.

One method used to alter mercury lamp output is the addition of metal halides.

Mercury lamps can be doped with different metal halides to produce modified spectral characteristics. Iron-doped lamps, for example, can increase output in longer UVA wavelengths, with published research identifying strong iron-doped output in approximately the 350 to 400 nm region. [1]

This is important because photoinitiators absorb different wavelengths.

The most suitable lamp therefore depends on the chemistry and application.

A lamp that physically fits into a UV system isn’t necessarily the correct lamp for the curing process.

Why does the photoinitiator matter?

UV-curable inks, coatings and adhesives generally rely on photochemical reactions.

The photoinitiating system absorbs suitable radiation and generates the reactive species needed to start polymerisation.

Research literature identifies the photoinitiator as a major factor affecting characteristics such as cure speed, through-cure, hardness and surface properties. [3]

The relationship between lamp spectrum and photoinitiator absorption is therefore fundamental.

If the useful wavelengths from the UV source do not interact effectively with the chemistry, simply increasing electrical power isn’t necessarily the right solution.

This is one reason UV lamp specification requires more information than length and wattage.

Why does the reflector matter?

Once radiation leaves the quartz lamp envelope, the UV system still needs to direct useful radiation towards the substrate.

That’s where reflectors become important.

In many curing systems, the reflector captures radiation travelling away from the substrate and redirects it towards the process.

We’ve found that, depending on the system, reflectors can account for between 50% and 70% of the UV output reaching the substrate.

Reflector geometry can also be used to focus radiation where a high energy density is required.

So even a correctly specified lamp can perform poorly as part of a curing system if its surrounding reflector has become dirty, damaged or distorted.

Power supplies matter too

The lamp also needs the correct electrical operating conditions, which highlights another important point.

A UV lamp, reflector, power supply, cooling system and optical arrangement have been designed to work together.

Replacing one component without considering its compatibility with the others can introduce problems.

Why lamp specification is more complex than it looks

Alpha-Cure manufactures medium-pressure UV lamps for both replacement and OEM proprietary applications.

Its current manufacturing range includes different arc lengths, power specifications, end-cap arrangements and lead configurations, with a database containing more than 20,000 lamp specifications.

Where an existing specification isn’t suitable, bespoke lamps can also be designed for proprietary systems.

That breadth exists because industrial UV equipment varies enormously.

Printing presses, coating lines, bonding systems and specialist manufacturing machinery do not all use the same lamp.

There is a lot happening inside that quartz tube

A medium-pressure UV lamp may look straightforward from the outside.

Inside, however, electrical discharge, plasma formation, mercury excitation and radiation emission are occurring under tightly controlled operating conditions.

Outside the lamp, the process continues.

• Reflectors direct the radiation.
• Quartz components manage its path.
• Cooling manages temperature.
• Power electronics maintain the electrical operating conditions.

And the chemistry determines which wavelengths actually contribute to the curing reaction.

So the next time you look at a UV lamp glowing inside a curing system, remember:

the light you can see is only part of the story.

Need a replacement or bespoke UV lamp?

Alpha-Cure manufactures UV lamps for OEMs, distributors and end users around the world.

With more than 20,000 lamp specifications in our database, plus the ability to manufacture bespoke designs, we can help identify a lamp suited to your UV curing system. Contact Us for more information.

References

[1] To Shed Light on the UV Curable Coating Technology: Current State of the Art and Perspectives, Journal of Composites Science, 2023.

[2] An introduction to continuous fiber UV-cured thermoset composites by in-situ impregnation, Discover Materials, 2026.

[3] Fouassier et al., Photopolymerization reactions under visible lights: principle, mechanisms and examples of applications, Progress in Organic Coatings.