Measuring UV dose and irradiance correctly

Irradiance and UV dose describe different properties of a UV process. Irradiance is the instantaneous radiant power per area; the dose – in standards terminology the radiant exposure – is its integral over time. Reliable results require the spectral range, the measuring range, the calibration source, the measuring distance and the measurement geometry to match the application. For moving processes the time profile of the irradiation is added.

This page sorts out the quantities, names the influences on the measurement result and shows which class of instrument solves which task. The selection of specific instruments is covered in the category UV meters and radiometers.

Irradiance E is the radiant power arriving at a surface, given in W/m² or – commonly in UV engineering – in mW/cm². Radiant exposure H is its integral over time, given in J/m² or mJ/cm². Where irradiance is constant over time, H = E · t; where it varies, the actual profile has to be integrated.

In practice, radiant exposure is usually called “dose”. The term is established but imprecise: in water disinfection, fluence describes the radiation arriving at a volume element from all directions, while in photobiology the dose is additionally weighted with an action spectrum. Anyone comparing values between plants, suppliers or test reports should therefore always state which quantity, which spectral range and which measurement geometry are meant.

A reading is only as reliable as the conditions under which it was taken. Five influences decide:

• Spectral responsivity. A broadband radiometer evaluates the source with the responsivity curve of its sensor. If the source spectrum differs from the calibration source, spectral mismatch occurs – systematically the largest error with changing sources.
• Measurement geometry. Irradiance from an extended or area source requires cosine-corrected input optics. Without cosine correction, obliquely incident radiation is underestimated.
• Measuring distance and plane. Measure where the process takes place – at the component or sample plane, not in the free beam. For point and spot sources, irradiance changes strongly with distance.
• Calibration. The calibration must match the spectral range, the measuring range and, as far as possible, the type of source used, and must be documented traceably.
• Measuring range, linearity and temperature. Very high irradiance, pulsed sources and elevated temperatures at the measuring point limit the sensors that can be used.

Calibrations for radiometers and UV sensors are carried out by the accredited calibration laboratory; the procedure is described under UV calibration.

With a stationary source and a stationary part, a static measurement of irradiance at the component plane is sufficient; the dose follows from irradiance and exposure time. As soon as the part or the source moves, irradiance at the part becomes time dependent. Peak irradiance, the time profile and the dose integrated from it then have to be recorded – a single static reading under the lamp does not describe the process.

How such a measurement is set up in conveyor, printing and pass-through lines is covered on the page UV measurement for conveyor systems.

• Stable source with a known spectrum, spot measurement: a calibrated broadband radiometer such as RMD Pro or RM-12 with the sensor matching the spectral range.
• Changing sources, LED wavelengths, ageing studies: a spectrally measuring instrument such as UVpad, UVpad E or SR900 in the laboratory.
• Dose in the running process: flat loggers such as curelog or tinyTracker that travel through the line with the workpiece.
• Permanent plant monitoring: permanently installed UV sensors with suitable evaluation electronics.

The same applies to every one of these chains: sensor, evaluation and calibration must match the process value to be demonstrated. A selection aid by measuring task is provided in the category UV meters, and the distinction between the instrument classes on the page spectroradiometer or broadband radiometer.

Which quantity should be monitored on a UV system?
Primarily the irradiance in W/m² at the accessible point closest to the process, and from it the dose as a time integral in J/m². For conversion: 10 W/m² corresponds to 1 mW/cm². Sensor and source temperature belong alongside them. A process window has to be specified in two dimensions, with a lower and an upper limit for both quantities. The electrical lamp power is not a measured quantity of the process.

Where in the process is the measurement taken?
At the level of the part or sample, where the process actually takes place – not in the free beam and not at the lamp housing. Extended and area sources require cosine-corrected input optics, otherwise obliquely incident radiation is under-weighted. With point and spot sources the irradiance changes strongly with distance; as a rule of thumb the measuring distance should be at least 10 times the largest dimension of the emitting area.

How high is the irradiance of a UV system compared with the sun?
The reference is the sun at ground level: under optimal conditions about 1,000 W/m² of global radiation, of which some 62 W/m² is UVA and 5.6 W/m² UVB. A UVA test chamber with 80 W/m² is therefore at 1.3 times, a UVB chamber with 50 W/m² at nine times that value. UV-LED systems reach 8,000 W/m² and more, roughly 130 times the solar UVA. In the UVC no comparison is possible, because radiation below 280 nm does not reach the surface of the earth. Every such factor comes with two further statements: the spectral range it refers to and the measuring plane it was measured in – without both, the number cannot be verified.

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.