Irradiance Measurement of High-Intensity Gas Discharge Lamps for UV Curing
Controlling the exposure of work pieces to UV energy is essential in the curing processes widely used for coatings, inks, adhesives, encapsulants and potting compounds. To monitor and adjust the UV energy, the irradiance needs to be measured as close as possible to the surface of the irradiated parts. Successful curing requires the correct dose of UV at the wavelengths appropriate for the particular material. Dose, measured in J/cm², is the product of UV intensity and time of exposure (W/cm² x seconds). UV intensity (irradiance) is measured by a UV radiometer in W/cm², but may also be displayed directly as a dose in J/cm². A suitable radiometer must also be able to withstand a high temperature environment.
UV curing is a photochemical process in which polymerization is initiated when photoinitiators, mixed within the material, absorb high energy UV radiation. Medium-pressure mercury lamps, both high voltage arc and microwave-powered, are commonly used for UV curing processes. These gas discharge lamps produce intense UV radiation by vaporizing the mercury into an extremely high-temperature plasma gas within a sealed quartz tube containing an inert gas mixture. The spectral composition can be modified to some degree by adding dopant metals such as iron or gallium. In addition to the UV radiation, gas discharge lamps also produce very high levels of IR radiation from the quartz envelope of the lamp, which may be a help or a hindrance to the successful hardening of materials.
The output intensity of gas discharge lamps degrades over time. Therefore, to ensure the required dose is achieved, irradiance should be monitored enabling the appropriate adjustment to process times. Lamp replacement schedules can also be optimised based on radiometer measurements.
Therefore, the selection of radiometers suitable for UV curing equipment employing high intensity gas discharge lamps requires the following criteria to be met:
- Detector design for minimal ageing and drift when exposed to intense UV and heat radiation;
- Cosine field-of-view detector for correct evaluation of incident radiation;
- Flat-type detector enabling measurement plane to match that of the work piece surface;
- Spectral responsivity of the detectors matched to the spectral sensitivity of the photoinitiators and the emission spectrum of the gas discharge lamp;
- Easy handling of the measuring device;
- Protection of the user from the risk of intensive UV and heat radiation.
The RCH-xxx Series detector heads from Gigahertz-Optik GmbH satisfy all of the above requirements.
The innovative design of these proven measurement heads ensures that the sensitive photodetector is both thermally isolated and protected from the high UV levels present. With this concept, the RCH-Series has been meeting the high demands of radiation curing for many years.