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Low-pressure mercury lamps are lamps with emission at 254 nm, which are often used in UV applications such as the disinfection of water, air and surfaces. Coated with phosphors, these lamps also emit UVA and UVB radiation.

Low-pressure mercury lamps are efficient and cost-effective and are therefore widely used. These lamps contain a small amount of mercury (Hg), which changes to a gaseous state during operation and is excited as a plasma by the electrical discharge and emits UV radiation at 254 nm and 185 nm.


Physical principles of UV lamps

The vapor pressure of mercury in a low-pressure mercury lamp is a decisive factor for UV emission. The vapor pressure describes the pressure exerted by the mercury in its gaseous state inside the lamp. It is strongly temperature-dependent: At higher temperatures the vapor pressure increases, at lower temperatures it decreases. The efficiency of the UV emission depends directly on the vapor pressure of the mercury. 

The Hg pressure increases as the ambient temperature rises. This means that more Hg atoms are emitted, but the reabsorption of resonance radiation also increases. There is therefore a maximum radiation yield at the optimum temperature of approx. 40 °C - 50 °C or the optimum Hg pressure of 0.8 Pa. The optimum depends, among other things, on the tube diameter.


Temperature ranges of low-pressure mercury lamps

The performance of a low-pressure mercury lamp is highly temperature-dependent. There is a narrow temperature range in which the lamp functions optimally:

At an optimum temperature of around 40°C to 50°C (for low-pressure lamps), the lamp reaches the ideal vapor pressure of mercury at around 0.8 Pa (Pascal). Efficiency is optimal in this range.

  • If the lamp becomes too hot, the vapor pressure of the mercury rises above the optimum level. This leads to excessive mercury vapor pressure and the efficiency of the lamp decreases. As a result, the UV radiation flux also decreases.
  • If the temperature is too low, the vapor pressure falls below the optimum level, which means that there are not enough mercury atoms in the gas phase. In simple terms, the exciting electrons are too fast or do not hit the mercury atoms. This also leads to reduced UV emission.


Coldest point on the UV lamps

The coldest point in a low-pressure mercury lamp is the point inside the lamp where the temperature is lowest. This point is crucial for regulating the mercury vapor pressure, as this is where the mercury droplets collect and evaporate. The position and temperature of this point directly influence the amount of mercury vapor present in the bulb.


Practical application

As a rule, low-pressure mercury lamps reach their optimum operating temperature regardless of the ambient temperature when sufficient heat is generated in the lamp. When the lamp is ignited and the burn-in phase (self-heating) begins, the mercury vaporizes. A higher mercury vapor is produced and the lamp voltage increases. 

The optimum point is reached when the radiant flux drops again. From this point onwards, the lamp is too hot. Typically, a thermally stable state then develops (later). This means that the lamp burns stably at one level.

The position of this stable level is now dependent on the cooling at the coldest point.

Well-cooled low-pressure mercury lamps achieve stable operation approximately 10-15% below the maximum reached at the optimum temperature of around 40°C to 50°C.

Undercooled low-pressure mercury lamps sometimes only reach 30-70% of maximum output.

Low-pressure mercury lamps that are cooled too much may not reach the optimum temperature at all.

Controlling the temperature in low-pressure mercury lamps is not a major challenge in practice. Common problems and their solutions include

  • Problems:
    • Overheating: This leads to excessive vapor pressure and thus to reduced UV emission.
    • Undercooling: Can occur due to low ambient temperatures or insufficient heating of the lamp. This leads to too low a vapor pressure and also to reduced UV emission.
  • Solutions:

The efficiency and performance of low-pressure mercury lamps can be maximized by carefully parameterizing the cooling and measuring the run-up after switching on.


Notes for amalgam lamps and medium pressure lamps

UV amalgam lamps and medium-pressure lamps differ significantly from low-pressure mercury lamps in terms of their temperature behavior and the vaporization of their dopants.

Amalgam lamps achieve higher outputs and temperatures. For this purpose, UV amalgam lamps use an amalgam mixture that enables a more stable UV output at higher temperatures of approx. 100 °C - 120 °C. The amalgam, an alloy of mercury and another metal such as indium or gallium, ensures that the mercury vapor pressure remains low even at these temperatures. This allows the lamp to work at higher operating temperatures without losing the efficiency of the UV emission.

Medium-pressure lamps, on the other hand, operate at much higher temperatures and pressures than low-pressure lamps. They produce a broad spectrum of UV radiation, including UV-A, UV-B and UV-C, and are suitable for applications with higher power requirements. In these lamps, the mercury and any doping from additional metals evaporates completely. The cold point for these lamps should reach approx. 600°C - 800 °C.

Cooling is critical here, as overheating can destroy the lamps.

To strong cooling can severely impair the efficiency of the radiation and individual dopants can condense or sublimate. As a result, these are missing in the discharge and consequently the lines in the spectrum are "missing"


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