Selecting a UV sensor: spectral range and measuring range

A UV sensor is not chosen by the labels UVA, UVB or UVC alone. Emission spectrum, expected irradiance, distance, angular distribution and the required weighting function together determine the appropriate spectral and measuring range. This selection guide works from the source and the measuring task to the right sensor type.

Selection in six steps

1. Determine the source and its emission spectrum.
2. Define the measurand and the required spectral weighting.
3. Estimate the minimum and maximum process value.
4. Document measuring geometry, distance and angular distribution.
5. Check ambient conditions and the electrical interface.
6. Define calibration and the permissible measurement uncertainty.

Measuring range by source

The spectral range follows the application or the photoinitiator, the measuring range follows the irradiance to be expected. As guide values:

Particular selection cases

Occupational safety. For risk assessment and workplace safety, DIN EN 14255-1 governs the measurement and assessment of personal exposure to artificial optical radiation; the limit values themselves are set out in Directive 2006/25/EC. For sensor selection this means a small measuring range and an action-weighted responsivity – the design is covered in detail under UV erythema in the workplace, the legal framework under occupational safety and photobiological safety.

Permanent installation. Where measurement is continuous rather than hand-held, inline sensors are available; mounting position, interface and resistance to process conditions then matter as well.

Medical applications. What governs here is a clearly defined measuring task, long-term stability and a traceable calibration. Suitability for a specific medical application requires technical release case by case.

Matching instruments

As a display unit for interchangeable measuring heads, the RMD Pro radiometer has proven itself. Where several spectral ranges have to be captured at once, a spectral measurement is the way – with the UVpad, for instance. How a sensor is built internally and which error sources decide its accuracy is explained under design and error sources of UV sensors.

Which UVC sensor suits low-pressure, medium-pressure or LED emitters?
The sensor has to match the spectrum, not just the spectral range. For low-pressure lamps a narrow-band responsivity around 254 nm is sufficient, because practically all UVC power sits in that line. Medium-pressure lamps emit broadband across the whole UVC range; here a sensor with a wide, flat UVC responsivity is required, calibrated against exactly this lamp type. UVC LEDs (typically 265 to 280 nm) need a calibration to their peak wavelength; the standard parts DIN 19294-5 and -6 for UV-C LED are still being drafted. Without source-specific calibration factors, one and the same sensor does not deliver comparable values on all three sources.

Which UV sensor suits 254 nm?
For the 254 nm line of low-pressure mercury lamps, a UVC sensor with a narrow-band responsivity around 254 nm is suitable, solar-blind against UVA and visible light and calibrated against a low-pressure lamp. In drinking water disinfection, DIN 19294-1 (device radiometers) and DIN 19294-3 (reference radiometers) define the requirements for low-pressure systems; in Austria ÖNORM M 5873-1 applies. The design – inline sensor, pressure-resistant sensor, UV probe or radiometer with interchangeable sensor – follows the mounting position and the measuring task.

Which measuring range does the sensor need?
It follows the irradiance to be expected, not the instrument. UV spot sources reach several W/cm² within the spot – a range of 0–2000 mW/cm² or more is appropriate here. UV low-pressure and UVC amalgam lamps usually stay below 100 mW/cm² in the irradiation plane, for which 0–200 mW/cm² is suitable. Workplace measurements to DIN EN 14255-1, by contrast, call for sensitivity: 0–2 mW/cm² there. For UV LEDs the UVA+ sensor with an extended spectral range was developed – this avoids measuring on the filter edge, where even the smallest temperature and batch variations cause large measurement errors.

Above which irradiance does an installation have to be assessed?
As a rule of thumb: installations that expose the operator continuously to more than 1 to 2 mW/cm² should be assessed. The limit value, a worked example and the requirements on the sensor are set out under UV erythema in the workplace.

Find the right sensor: The UV Sensor Finder proposes the three best-matching UV sensors and meters by application, wavelength and form factor – with key values, system partner and reasons.