It is essential to provide suitable cooling for the lamp to prevent the inside wall of the envelope from overheating.
The cooling requirement for lamps can be easily calculated by dividing the average input power by the surface area of the internal wall of the arc region of the lamp – this figure is expressed in W/cm2.
To determine the recommended method of cooling for your particular application, please refer to the table below:
|Wall Loading||Recommended Cooling Requirement|
|<15 W/cm2||Convection Air Cooling|
|15 – 30 W/cm2||Forced Air Cooling|
|20 – 200 W/cm2||Fluid Cooling|
Convection Air Cooling
For low average power applications below 15 W/cm2 it is possible to effectively not cool the lamp and to allow the air movement around the lamp to provide adequate cooling. This method of cooling is rarely used in commercial systems.
Forced Air Cooling
For applications where the average lamp power calculates to between 15 W/cm2 and 30 W/cm2 it is recommended to use forced air cooling. This can be achieved by blowing air across the lamp using a fan. Try to cool the complete lamp including the connectors if possible to ensure even cooling. It is also recommended to filter the air to prevent contamination.
For higher power pulsed lamp applications and for all continuous lamp applications it is essential to fluid cool the lamps.
Most customers use filtered deionised water (1Mohm or better) pumped at high pressure along the length of the lamp. The anode in the lamp operates at a higher temperature than the cathode and consequently it is recommended that the water flow direction is from the Anode to the Cathode.
To ensure turbulent water flow and to prevent hot –spots on the lamp, it is essential to restrict the water flow around the lamp. This can be done using either a flowtube or a flowplate - both of which can be purchased from First Light Lamps.
The annulus between the outer wall of the lamp and the inner wall of the envelope should be 1-2mm with a minimum flowrate of 1.5 litres per minute per kW of input power. The velocity of the water should be around 4m/s.
All of the figures quoted here are based on a 1mm wall thickness of Clear Fused Quartz. If Cerium Doped Quartz is used then the figures must be derated by 15%. If the wall thickness is reduced to 0.5mm then the figures can be increased by 60%.
First Light offers a simple downloadable program for calculating the maximum average lamp power and the recommended cooling requirement. Please go to the Downloads section of the website for this and several other usefull programmes.