UV process monitoring and controlling, specifications of light measurement systems in UV curing – Part 1

When monitoring a production process with UV curing it is important to validate it to ensure it works with the specified requirements. Too much of a deviation can have a serious impact. To ensure the desired quality standard and minimize downtime, many companies routinely check the UV light power of their process line.
Using a radiometer often requires an accreditation for products and components which are used in medical, automotive and aerospace. In this case you should choose an UV radiometer which is calibrated in an accredited laboratory based on ISO standard 17025. Our calibrated radiometers are NIST traceable!
Curing processes on conveyor belts or in UV chambers require that the measurement device itself is exposed to the UV light (compared with a distant sensor). A measurement system is needed that can resist both the UV light intensity and the temperature increase during UV-exposure. A measurement system with plastic housing and exposed display would not stay intact for a longer period of time under UV exposure.
Furthermore, there are many different designs from both photo initiators and light sources, which complicates the development of a system for universal use across platforms.

So how do you choose the right light measurement system for UV curing if parameters can vary so much? First of all, you must find decide which functions are essential for correct curing.  
Common names for light measurement systems are UV meter, radiometer and optometer. It is a tool that can measure the absolute (calibrated) power of light within a certain range of the electromagnetic spectrum.
Adapting the UV spectrum to a radiometer is challenging as the number of sensor and filter materials which have low absorption and stable physical properties and can withstand long-lasting UV loads is limited. The ideal UV sensor has a flat and even reaction across the specific band that causes the curing and blocks all other wavelengths. Most UV sensors only have low and varying sensitivities in the UV range. This is why selecting the right wavelength range of a measurement system can in itself be challenging.
The first step is to determine which wavelength range is emitted by the lamp/light source. Then it is highly important to know which of these emitted wavelengths actually cause the curing.
The reaction of the photo initiators to UV light is directly related to their absorption and is highly wavelength selective. For effective curing, the wavelength of the light source must comply with the absorption peak of the photo initiator. This can range from 200 and 450 nm, depending on the application. Shorter wavelengths are selected for thinner surface applications, to allow for faster throughput. Longer wavelengths are absorbed into the substrate and provide deeper penetration into thicker materials.
Like the photo initiator, the UV sensor of the measurement system has a set sensitivity. It is important to select a system which corresponds to both the spectral power of the UV source and the reaction of the photo initiator to UV light.
So, if for example you are curing a photo initiator with a specification for UVA (315 - 390 nm), the measurement system should be sensitive in this area.
The UV light measurement system is constantly exposed to the UV light and must therefore be resistive.

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