UV radiation

 

UV radiation is widely used in many industrial processes, primarily for curing, purification and sterilization. The measurement of high-energy ultraviolet radiation places particular demands on the design and calibration of suitable instrumentation. Gigahertz-Optik GmbH offers a comprehensive product range of instruments and measurement accessories for measuring high energy ultraviolet radiation, a selection of which are presented in the application examples on this page.

Additionally, particular UV measurement solutions are also presented in the following application categories:

Our sales team will be pleased to support you regarding your particular application requirements. Please contact us via +49 (0) 8193 93700-0 or info@gigahertz-optik.de. 

App. 019

Irradiance measurement of high-intensity gas discharge lamps for UV curing

Controlling the exposure of work pieces to UV energy is essential in the curing processes widely used for coatings, inks, adhesives, encapsulants and potting compounds. To monitor and adjust the UV energy, the irradiance needs to be measured as close as possible to the surface of the irradiated parts. Successful curing requires the correct dose of UV at the wavelengths appropriate for the particular material. Dose, measured in J/cm², is the product of UV intensity and time of exposure (W/cm² x seconds). UV intensity (irradiance) is measured by a UV radiometer in W/cm², but may also be displayed directly as a dose in J/cm². A suitable radiometer must also be able to withstand a high temperature environment. 

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UV curing is a photochemical process in which polymerization is initiated when photoinitiators, mixed within the material, absorb high energy UV radiation. Medium-pressure mercury lamps, both high voltage arc and microwave-powered, are commonly used for UV curing processes. These gas discharge lamps produce intense UV radiation by vaporizing the mercury into an extremely high-temperature plasma gas within a sealed quartz tube containing an inert gas mixture. The spectral composition can be modified to some degree by adding dopant metals such as iron or gallium. In addition to the UV radiation, gas discharge lamps also produce very high levels of IR radiation from the quartz envelope of the lamp, which may be a help or a hindrance to the successful hardening of materials. 

The output intensity of gas discharge lamps degrades over time. Therefore, to ensure the required dose is achieved, irradiance should be monitored enabling the appropriate adjustment to process times. Lamp replacement schedules can also be optimised based on radiometer measurements.

Therefore, the selection of radiometers suitable for UV curing equipment employing high intensity gas discharge lamps requires the following criteria to be met:

  • Detector design for minimal ageing and drift when exposed to intense UV and heat radiation;
  • Cosine field-of-view detector for correct evaluation of incident radiation;
  • Flat-type detector enabling measurement plane to match that of the work piece surface;
  • Spectral responsivity of the detectors matched to the spectral sensitivity of the photoinitiators and the emission spectrum of the gas discharge lamp;
  • Easy handling of the measuring device;
  • Protection of the user from the risk of intensive UV and heat radiation. 

The RCH-Series detector heads from Gigahertz-Optik GmbH satisfy all of the above requirements.

The innovative design of these proven measurement heads ensures that the sensitive photodetector is both thermally isolated and protected from the high UV levels present. With this concept, the RCH-Series has been meeting the high demands of radiation curing for many years. 

App. 020

Reduced measurement uncertainty for UV radiometers

UV radiometers do not come in ‘one size fits all’ format. They may be required to have a flat spectral response over specific spectral bands (e.g. UV-A, UV-B and UV-C) or to match particular actinic spectral functions (e.g. ICNIRP or erythema). UV radiometers are typically constructed from a photo-detector and filter combination, often with an entrance optic such as a cosine diffuser.

What is common to all filter-corrected radiometer designs is that their spectral responsivity function will never perfectly match the target specification and further production-related tolerances will also arise. These inevitable spectral mismatches introduce measurement uncertainties. Their magnitude depends not only on the deviations of the spectral responsivities, but also on the relative spectral power distribution of the radiation source being measured. The technical report CIE 220: 2016 [1] presents a methodology for determining the expected measurement uncertainty due to spectral mismatch.

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Spectral mismatch errors arise from imperfections in the spectral response functions of radiometers. However, these errors have negligible effect on the measurement result if the relative spectral power distribution, i.e. ‘spectral shape’, of the reference lamp used for calibrating the radiometer matches the spectral shape of the UV source in the application. 

But what happens if the emission spectrum differs significantly from that of the calibration lamp? The technical report CIE 220: 2016 [1] provides useful information for determining the measurement uncertainty of UV radiometers resulting from the spectral mismatch. The values ​​determined can be used as correction factors for the adjustment of the UV radiometer for known emission spectra.

In addition to knowledge of the spectral power distribution of the source to be measured, the application of CIE 220: 2016 [2] requires the following information to be provided by the manufacturer of the UV radiometer:

  • Spectral power distribution of the reference lamp used for calibration
  • The spectral responsivity measured for each individual UV radiometer

Gigahertz-Optik GmbH individually calibrates every UV radiometer it produces with regard to its relative spectral responsivity. This is carried out in our own calibration laboratory accredited for optical radiation measurements. The spectral data for the calibration lamp and the radiometer’s responsivity is available upon request. Knowledge of the relative spectral distribution of the source under test enables the spectral mismatch error to be corrected for, thereby reducing the overall measurement uncertainty. Optionally, Gigahertz-Optik GmbH offers correction values for specified sources to be incorporated within regular calibration. This is of particular interest when the UV radiometer is used in applications with known UV source types.


References

[1] CIE 220:2016, Characterization and Calibration Method of UV Radiometers

[2] How to apply CIE 220:2016 – Characterization and Calibration Method of UV Radiometers

App. 021

Irradiance measurement of UV LED curing equipment

UV LED lamps are being enthusiastically developed and adopted as alternatives to the medium pressure mercury lamps traditionally used for UV curing processes. UV LED curing offers several potential advantages including reduced power consumption, less heat generation, instant switching, longer lifetimes, as well as the environmental benefit of being mercury-free. UV radiometers are widely used to monitor and control the UV exposure (or ‘dose’) from high-intensity gas discharge lamps on work surfaces. However, UV LED devices emit a narrow spectrum of radiation (typically ±10 nm), whereas mercury lamps have a much broader spectral distribution. This has significant implications for the design and calibration of radiometers if they are to be suitable for measuring the irradiance of UV LED curing equipment.

The UV radiometers produced by Gigahertz-Optik GmbH generally have a broadband spectral responsivity function resulting from the combined characteristics of the photo-detector and filter.

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This technology does not permit the design of a detector that has a perfectly ‘flat’ radiometric function over its entire spectral range. Variations in responsivity at different LED wavelengths would result in deviations in the measurement values ​​of irradiance for LEDs of different wavelengths.

UV LED manufacturers specify devices based on their peak wavelength. The most commonly supplied LEDs incorporated into curing lamps have the following peak wavelengths: 365nm, 375nm, 385nm, 395nm, 405nm and 430nm. Gigahertz-Optik GmbH offers a single UV radiometer solution incorporating calibration at each of these specific wavelengths.

UV Radiation Figure1

Our accredited calibration laboratory for optical radiation measurements calibrates the radiometers for UV LEDs at these typical LED wavelengths using CIE 220: 2016 [1] recommendations. The UV radiometer, consisting of the RCH-116 measuring head and the X1-1 optometer, has proven itself over an ever increasing range of industrial applications.


References

[1] CIE 220:2016, Characterization and Calibration Method of UV Radiometers