Guide to UV Measurement
Measuring UV Systems with Multiple Lamps
When a single lamp UV system is measured with a radiometer that reports watts and joules, the peak irradiance and energy density values can be easily obtained for that single UV system. Changes in the amount of UV arriving at the cure surface are easy to track and identify with the system.
Many UV processes and production layouts utilize multiple rows or “banks” of UV systems. The substrate often passes under multiple UV systems as it moves down or through the production line. A good example of applications using multiple lamps is a wood line where UV cured stains, fillers, sealers and top coats are applied as the wood moves down the production line. Multiple lamps are generally used with each type of coating that is applied. The number of UV systems and their power rating, their focus setting and the type of bulb in the system varies. view a Sample Wood line layout The properties in the finished product, formulation, type of application method and desired production speed determine the production set-up when the process is first designed and tested. Some companies also elect to build in extra UV stations to allow flexibility for different products or to provide backup/redundancy for maintenance without stopping production.
When a multiple lamp UV system is measured with a radiometer that reports watts and joules, a single irradiance value is reported. The value reported is the peak irradiance from the most intense UV system that the radiometer passed under. There can be wide variations in the output in each of the other systems in the line. In some cases, individual lamps may actually be set at the wrong power level, off or malfunctioning. The reported energy density is a sum of all UV systems that the radiometer passed under. The joules from the first system are added to the joules from all the other systems that the radiometer passed under and a single value is reported. As with the irradiance values, there can be wide variation in the energy density output of each of the individual systems. view a chart of Multiple Lamps Profile / view a chart of Multiple Lamps Profile Left Middle Right. If the number of joules expected drops, how do you identify which of the many UV systems need maintenance and corrective action?
Options for measurement of UV Systems with multiple lamps
1. Joule/Watt Reading Radiometer: Measuring the entire UV line at once only gives you a very general idea of what is happening on the production line. Several parameters can change within each individual system and contribute to lack of or a poor cure. If your process has a wide window and lots of latitude, you may be able to perform a general check on the system with this type of radiometer. At the first sign of any small change, a more thorough examination needs to be done with your radiometer by breaking the entire line down into smaller sections, including individual UV systems. Checking a specific section (first sealer section for example) is better that trying to check the entire line. The best option if you have a radiometer that only reports joules and watts is to break the production line down to the individual UV systems and then measure each UV system individually. This option allows you to measure and track each individual system. Maintaining the performance and UV output of each system can be targeted and effective. The biggest drawback to measuring each system individually is that it can be very time consuming, especially if you have several UV systems joined together in ‘banks' that cannot be separated. For arc lamp systems in ‘banks', time must also be budgeted for each UV system to warm up and stabilize before measurements can be taken. Consult your equipment supplier to determine the recommended warm-up/stabilization time for your system. Often several hours or an entire morning are needed to verify the UV conditions on a production line with multiple UV systems. Sometimes, the large amount of time needed to adequately measure each production line can be budgeted between production runs. Time is not always available when there are changes or equipment malfunctions during a production run.
2. Profiling Radiometer: In addition to measuring joules and watts, a profiling radiometer generates an irradiance profile, which is a plot of the irradiance as a function of time. Instruments are currently available that provide this information on either a small LCD display on the instrument or transfer the information to a computer for viewing and analysis. Earlier models that are still supported provided the irradiance profiles on thermal paper. Profiling radiometers offer the following advantages when measuring and maintaining lines with multiple lamps:
- Valuable production time does not have to be wasted waiting for individual systems to warm-up when you use a profiling radiometer The peaks from each UV system on the irradiance profile can be scanned very quickly to identify the system(s) that are not performing as expected. Maintenance can be targeted and specific to individual systems that fall below the threshold that you set.
- The way the UV is delivered (focus) to the cure surface can be checked and adjusted for each UV system. The profiling radiometer identifies when a system or bulb has changed position in relationship to the cure surface or reflector. Changes in the focus can happen after maintenance or with changes in the system over time. In some applications, the specified process parameters (focused, defocused) can be verified and set as needed.
- Some profiling radiometers allow you to save the irradiance profiles for comparison of the line over time. An irradiance profile saved when the line is performing well can be compared to an irradiance profile when the line is having problems. This feature allows you to quickly determine if the UV output has changed or if there is something else in your process to target to get your line back up and running.
- Some profiling radiometers allow you to break down with your computer the individual joule and watt output from each system. This individual breakdown can be quickly done in an office setting instead or waiting for lamps to warm up out on the production floor. view a chart of Multiple Lamps Profile Individual Breakdown.
3. Multi-Band Profiling Radiometer: Multi-band profiling radiometers have all the advantages of the profiling radiometers that are described above. Instead of having just one spectral response, multi-band radiometers measure in multiple areas of the UV spectrum. Up to four spectral response areas (UVA, UVB, UVC, UVV-near visible) are available on current multi-band profiling radiometers. Comparing the four different response areas allows the user to easily and quickly identify different types of bulbs. In a large production line with many UV systems and different bulb types, this check can confirm that the proper bulb has been installed in the correct location on the line. view a chart of Mercury to Mercury-Gallium Comparison UVA-UVV
Wide Systems
UV systems can be configured to cover wide substrates. The technique used to span wide substrates differs based on the technology of the UV equipment. Arc lamp manufacturers claim that they can make systems that can span seven feet or more. Microwave powered lamps can expand in width to cure wide substrates by placing 6” (15 cm) or 10” (25 cm) modules next to each other and perpendicular to the direction that the substrate travels. With either type system (arc or microwave) it is important to measure in several places across the width of your substrate to make sure the UV reaching the substrate is uniform.