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Illuminated LED lamp among unlit incandescent lightbulbs

Developing new light measurement methods and standards to meet the phaseout of incandescent lamps

Background

Spectral irradiance measurements are required to quantify the power of electromagnetic radiation per unit area and wavelength at a specific location, enabling accurate characterisation of light sources. They are essential for evaluating the performance of solar cells, LEDs, and other photonic devices by understanding their spectral distribution. Additionally, such measurements are critical in environmental monitoring, ensuring accurate assessment of solar radiation and its effects on ecosystems.

Accurate measurements of spectral irradiance are challenging as they require appropriate and thoroughly calibrated instruments capable of detecting optical radiation over a wide range of wavelengths. Radiometric properties of the instruments such as stray light, detector nonlinearity, and dependence on ambient conditions (e.g., temperature, humidity) can introduce errors into the measurement process and must be corrected.

Even more difficult is the determination of measurement uncertainties which require rigorous calibration against traceable standards, characterisations of instrument radiometric properties, and statistical analysis of repeatability and reproducibility. Measurement uncertainties and correlations of spectral measurement data may arise from, e.g., wavelength alignment errors, imperfections in optical components, and the finite resolution of spectrometers, necessitating careful uncertainty propagation analysis.

New software under development that enables calibration of spectroradiometers

The calibration of spectroradiometers by National Metrology Institutes (NMIs) or calibration laboratories will involve the characterisation of all the relevant instrumental properties and provide the associated data required for accurate spectral irradiance measurements. This includes spectral responsivities, stray light, nonlinearity, pixel-wavelength assignment, and spectral bandwidth.

To enable the use of this data in end-user applications, new open-source software is under development as part of the work in Metrology Partnership project New calibration standards and methods for radiometry and photometry after phaseout of incandescent lamps (22IEM05 NEWSTAND).

The project aims to provide adequate and affordable replacement transfer standard light sources and alternative procedures for the detector-based transfer of the spectral irradiance unit in the ultraviolet, visible, and near infrared (UV-VIS-NIR) spectral range with measurement uncertainties as low as 0.5 %.

At this stage of the project, a digital model framework for array spectroradiometers based on known key radiometric effects contributing to the uncertainty of the measurements has been developed. The open-source software simulates numerous measurement scenarios, corrects for various radiometric effects in array spectroradiometer measurements, and determines the measurement uncertainty using a Monte-Carlo analysis. As a result, it provides not only the corrected spectral irradiance values but also the corresponding measurement uncertainties and correlations. The open-source software together with the characterisation and calibration data represents the metrological digital twin of the spectroradiometer. The consortium is working to implement further relevant radiometric effects to the digital twin model. 

Additionally, the software is highly versatile and can be easily adapted to accommodate new transfer standards, such as spectroradiometers, following the phase-out of incandescent lamps. This ensures the software remains future proof, supporting the transition to modern calibration methods and maintaining metrological accuracy. This innovation ensures traceable, reliable, and comprehensive spectral irradiance measurements that meet the highest metrological standards.

Project coordinator Dr. Saulius Nevas from PTB said

‘The key feature of the new software is its flexibility and ability to integrate all relevant radiometric effects in array spectroradiometer measurements into a comprehensive Monte Carlo simulation, ensuring a simple and reliable determination of spectral irradiance and its uncertainties.’

This Metrology Partnership project has received funding from the European Partnership on Metrology, co-financed by the European Union Horizon Europe Research and Innovation Programme and from the Participating States.

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