Sensors for UV and visible light - application notes

Selecting a UV or VIS sensor follows two quantities: the spectral range is determined by the UV application or by the photoinitiator, the measuring range by the irradiance to be expected. As a guide, 0–2000 mW/cm² and above applies to UV spot light sources, 0–200 mW/cm² to UV low-pressure and UVC amalgam lamps, and the extended UVA+ sensor to UV LEDs. For risk assessment and occupational safety, DIN EN 14255-1:2005 applies.

One watt of visible light corresponds to approximately 10¹⁹ photons. This large number of photons is the basis for the application of light and UV radiation as the surface interaction of the photons is usually limited to a few nm².

The scope of our UV sensors is just as varied, ranging from process monitoring, risk assessment, and occupational safety to medical applications of UV lamps and UV LEDs. The following notes should assist in the selection of suitable sensors.

For process monitoring and dose control, the spectral range of the sensor depends primarily on the UV application or usually the photoinitiator. UV spotlight sources such as the HP-120i reach irradiance in the range of a few W/cm². This is lower outside the spots or at a greater distance. The sensor should provide a wide measuring range of 0-2000 mW/cm² or more.

Low-pressure UV lamps and UVC amalgam lamps usually reach irradiances of less than 100 mW/cm² in the irradiation level. We recommend our sensors with a measuring range of 0 - 200 mW/cm².

The emission of UV LEDs occurs, for example, at 365, 385, 395, or 405 nm. A UVA+ sensor has been developed to measure UV LEDs. The latter has a wider spectral range. Measurements of UV LEDs in the area of the filter edge should be avoided since the small temperature changes and load fluctuations can cause high measurement errors.

For risk assessments and occupational or job safety, DIN EN 14255-1:2005 regulates the measurement and assessment of personal exposure to artificial optical radiation. DIN 14255-1 itself contains no limits. Limits are given in directive 2006/25/EC "Artificial Optical Radiation."

The sensors must be sufficiently sensitive for the measurements. To achieve this, select a sensor (e.g., UVA, UVB) with a measuring range 0 - 2 mW/cm². In accordance with 2006/25/EC directive, the UVA radiation limit is 10⁴ J/m² for an eight-hour working day. This corresponds to continuous irradiance of 0.035 mW/cm². The maximum irradiance may be higher for short-term work, for example:
    Scope of work: Daily cleaning
    Duration: 10 min
    Exposure dose: HUVA = 10⁴ J/m²
    Irradiance: 1.68 mW/cm²

Calculate the UV dose based on irradiance and time

Tip: UV systems should be designed so that the irradiance relative to the duration of the activity does not exceed the exposure dose. In general, installations should be checked if they continuously irradiate the worker with more than 1-2 mW/cm².

For medical applications, process safety and calibration are of primary importance. Our sensors are durable and can be individually recalibrated. Repair and spare parts service is available for many years. Take advantage of our many years of experience as a calibration laboratory.

Applications with different UV lamps can be reproducibly measured with our radiometer sensors. A measurement of all spectral ranges at the same time is also possible (e.g., with the UVpad).

A proven display unit for these sensors is the Radiometer RMD Pro.

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 continuously expose the operator to more than 1 to 2 mW/cm² should be assessed. The background is the limit value of Directive 2006/25/EC for UVA radiation of 10⁴ J/m². Over an eight-hour working day that corresponds to a continuous irradiance of only 0.035 mW/cm². For short tasks the irradiance may be correspondingly higher – around 1.68 mW/cm² for ten minutes of daily cleaning. What always counts is the product of irradiance and duration of the activity.