Guide to UV Measurement
Infrared Energy
Heat can benefit UV curing when it is used to enhance flow-out (for gloss) or mobility (for cure rate and adhesion). It becomes a problem primarily when, for example, the substrate cannot tolerate it or volatilization of the coating occurs.
All UV lamps produce some Infrared (IR) energy. It is, in fact, emitted by the quartz bulb envelope. It follows that the amount of IR radiation is a function of a bulb’s surface area. The greater a bulb’s diameter, the greater the amount of IR radiation it gives off. This suggests that for heat-sensitive applications, a smaller diameter bulb may be a preferred choice. However, if the radiant output of a bulb is focused onto a surface, then a smaller diameter bulb will generate a smaller area in the focal plane and the localized heating may be comparable or even greater.
The amount of IR energy radiated to the work surface can be controlled somewhat independently from the UV energy. This requires either absorbing the IR or preventing it from being focused onto the surface.
A straightforward way to reduce heat delivered to a surface is to change the reflector shape. Using a parabolic rather than elliptical reflector shape spreads the radiation from the bulb over a larger area resulting in a lower average temperature. However, peak irradiance is sacrificed in doing so.
A second option is to use a reflector that preferentially reflects UV and transmits or absorbs IR radiation resulting in the reflected energy being UV ‘enriched’. A reflector having good reflectance to UV and poor reflectance to IR (intentionally), can reduce the IR irradiance at the surface without sacrificing the UV peak irradiance. Reflectors designed with this different reflectivity to different wavelength ranges are referred to as being dichroic.