Once the required UV dose is fixed and the irradiance of the installation is known, one quantity remains open: how long to expose. This calculator answers exactly that. It takes the target dose in mJ/cm² and the measured irradiance in mW/cm² and returns the required time in seconds and as minutes:seconds.
It is the most common calculation when setting up a UV process. The adhesive supplier specifies a dose, the lamp datasheet or a measurement provides the irradiance – the cycle time is what you are looking for. The same calculation also shows whether a process fits the intended cycle time at all: if irradiance is too low, the time becomes unrealistic and you need a stronger source or a shorter working distance.
The result applies to an irradiance that stays constant over the period at the position of the workpiece. If the part passes the source, what matters is not the cycle time but the dwell time in the irradiated zone. A design only becomes binding through a measurement with calibrated instruments – manufactured and calibrated by Opsytec in its own laboratory.
The reverse calculation – dose from time and irradiance – is in the UV dose calculator; all twelve radiometric calculators are collected in the radiometry calculator.
Fundamentals
Exposure time follows from the dose equation H = E · t by rearranging: t = H / E. Entering the target dose H in mJ/cm² and the irradiance E in mW/cm² gives t directly in seconds — the units cancel because mJ = mW · s. In SI units this is H in J/m² divided by E in W/m². The calculator also gives the time as min:s, because production cycle times are usually stated that way.
A coating requires 500 mJ/cm², the installation delivers a measured 50 mW/cm² at the workpiece. t = 500 mJ/cm² ÷ 50 mW/cm² = 10 s. If a larger working distance halves the irradiance to 25 mW/cm², the time doubles to 20 s.
For moving processes what you need is the dwell time in the irradiated zone, not the cycle time — divide the irradiated length by the belt speed. For pulsed sources, enter the average irradiance, not the peak. And as with the dose calculator: a sufficient time on paper guarantees nothing if the spectrum of the source does not match the photoinitiator or the test task.
Frequently asked questions
Target dose divided by irradiance: t = H / E. At a target dose of 500 mJ/cm² and 50 mW/cm² that is 10 seconds. Both values must refer to the same spectral range.
The measured value at the position of the workpiece. Datasheet values apply to a defined distance and a new lamp; once installed, the actual value is often considerably lower because of distance, contamination and ageing.
Because irradiance varies across the area. The calculated time applies to the value entered; where irradiance is lower, the dose is reached later. For a safe design, use the lowest value within the working area.
Through a maintenance factor: design with the irradiance at the end of the replacement interval, not that of a new lamp. How strongly UV lamps and UV LEDs decline is described in UV sources & lifetime.
Only indirectly. There the dwell time in the irradiated zone governs, that is irradiated length divided by belt speed. The measuring practice is described in UV measurement for conveyor systems.
Three levers: a shorter working distance, a stronger or additional source, or a source with a better matching spectrum. How distance and arrangement take effect can be explored in the UV LED simulation.
Author: Dr. Mark Paravia
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 current focus is on optical radiation measurement technology. He is vice-chair of the DIN Standards Committee FNL 7 “Optical Radiation” and a member of the DVGW Project Group on UV Disinfection.
