UV measurement for conveyor systems and continuous processes

In conveyor systems the irradiance at the workpiece changes continuously as it passes through. A static reading under the lamp is therefore often not sufficient to assess the process. What matters are the peak irradiance, the time profile and the dose integrated from it at the real workpiece plane.

This page describes how these quantities are recorded in the running process and what to look for when selecting the measuring equipment. The quantities themselves are defined on the page measuring UV dose and irradiance correctly.

A workpiece passes through the irradiation zone in a finite time. It enters the radiation field, reaches its maximum under the source and leaves the field again. The dose acting on the part is the integral of this profile, not the product of the peak value and the dwell time. Two lines with the same peak irradiance can deliver clearly different doses depending on reflector geometry, number of lamps and belt speed.

In addition, the measuring location has to represent the component plane. A measurement taken close under the lamp, or at a different height than the workpiece, yields values that cannot be transferred to the process.

A pass-through instrument records irradiance over time and derives three quantities from it:

• Peak irradiance – the maximum value of the run. It limits the achievable reaction rate and is a measure of load for temperature-sensitive substrates.
• Time profile – the shape and width of the profile reveal the effect of reflectors, lamp spacing and shadowing. Several lamps produce several maxima.
• Integrated dose – the time integral over the entire run. This quantity is usually documented for process release.

The sampling rate of the instrument has to match the belt speed: in fast lines with short irradiation zones, sampling that is too slow can miss the maximum and underestimate the dose.

• Overall height. The instrument must fit through the available gap without distorting the distance to the source.
• Temperature resistance. In lines with mercury lamps the thermal load is considerable; the permissible housing and sensor temperature limits the number of runs.
• Spectral range. The sensor has to cover the range that is actually relevant to the process. With UV LED lines the peak wavelength is decisive, with mercury lamps the line spectrum.
• Sampling rate and memory. Both determine how finely the profile is resolved and how many runs can be stored.
• Evaluation and documentation. For process release what counts is a traceable, exportable data record with time stamp and calibration reference.

• curelog and curelog Base – flat radiometers and dosimeters for pass-through use, autonomous or connected to the line.
• tinyTracker – compact dose logger for confined irradiation zones.
• UVpad – flat, spectrally measuring handheld instrument for lines with changing sources.
• XT sensors – sensors for elevated temperatures in curing lines.
• Inline sensors – permanently installed monitoring at the defined measuring position.

The reference chain for process release is defined separately from continuous monitoring and calibrated traceably in the accredited calibration laboratory.

This page deals with the measurement problem itself. Its sector-specific form is described on the application pages: UV measurement in industrial printing for printing processes, UV curing and photopolymerisation for process design, and automation and process integration for the link to control systems and documentation.

How is the UV dose controlled in an automated process?
In two steps. For release, a dose logger travels through the system with the workpiece and yields peak irradiance, the time profile and the integrated dose; that run is documented. After that, a permanently installed sensor at a defined position keeps the condition between two releases traceable. The sampling rate has to match the belt speed: at 120 m/min with a 25 mm irradiation zone around 1,600 Hz are required. Too slow a sampling misses the maximum and underestimates the dose.

Why is a single reading at standstill not enough?
Because it describes only the irradiance at one position with the system stationary. The process quantity is the dose that a surface element receives while passing through; it arises as a time integral over the entire pass. Reflectors, lamp spacing and shadowing shape the profile, and several lamps produce several maxima. At a belt speed of 20 m/min and a 100 mm irradiation zone the pass lasts about 0.3 s – too short to read off by hand.

Dr.-Ing. Mark Paravia is the managing director of Opsytec Dr. Gröbel GmbH in Ettlingen and heads the accredited calibration laboratory. Following his research on pulsed xenon excimer discharges at the Institute of Lighting Technology at KIT, his focus today is optical radiation measurement. He is vice-chair of the DIN standards committee FNL 7 “Optical radiation” and a member of the DVGW project group on UV disinfection.