Guide to UV Measurement
Irradiance
Irradiance is the radiant power arriving at a surface per unit area. With UV curing, the surface is most often the substrate and a square centimeter is the unit area. Irradiance is expressed in units of watts or milliwatts per square centimeter (W/cm² or mW/cm² ). In UV curing, the term ‘intensity' has commonly been used to describe irradiance. Irradiance more correctly describes the concept of UV arriving at the cure surface and is the preferred term.
UV irradiance is important in your process because it provides the ‘punch' to:
- Penetrate through opaque and pigmented coatings
- Give depth of cure
- Allow adhesion to the substrate
It is important to determine the minimum irradiance value needed for your process. Staying above the minimum will allow you to get the adhesion and penetration through your coating. Longer wavelengths (UVV) have better chance at penetration than the shorter wavelengths (UVC). More information about UV Basics. Often times the UV system with the higher irradiance will allow you to run at faster belt speeds. Higher irradiance values may also increase cure rate efficiencies.
A radiometer can report the highest irradiance measured (peak irradiance) and/or profile of the irradiance over time (irradiance profile).
In processes where multiple UV lamp systems are used, a profile of the irradiance over time will allow the user to confirm that each lamp system is performing as designed. Measuring Multiple Lamps
Lamp Design Factors Affecting Irradiance
- Power Input (watts)
- Bulb Efficiency
- Bulb Diameter
- Reflector Shape (Elliptical, parabolic, faceted, etc.)
- Reflectivity
Lamp Configuration and Maintenance Factors Affecting Irradiance
- Reflector Shape (Not bent or damaged)
- Bulb Position (Not sagged)
- Distance from the Work Surface (“in-focus or “out of focus)
- Contamination on Reflector or Bulb (Cleanliness)