LED-based lighting products are the most rapidly evolving light sources on the market for general lighting in urban areas or domestic dwellings. These include not only white light sources, but also coloured and multi‑coloured light sources, whose colour depends on internal or external control mechanisms. Due to spectral and geometrical peculiarities, SI traceable measurements of LED-based light sources are much more difficult than those of traditional tungsten filament lamps or fluorescent light sources. This makes it difficult for test laboratories to evaluate application-relevant properties using existing measurement methods and standards developed for incandescent light sources. Standards for traceable test methods exist but important metrological aspects for their practical application have not yet been fully considered. For example, the assessment of uncertainty, and input and output tolerances for imaging luminance measurements, intensity distributions of luminaires as well as spectral measurements and coloured sources, have yet to be resolved. This project will identify essential test parameters, develop validated procedures and good practice guidelines for test laboratories and help shape the forthcoming revisions of CIE S 025:2015, ISO/CIE 19476:2014 and EN 13032-4 standards.
Reliability and validity of product specification are of great importance in the general lighting market, which is unsettled by unfulfilled performance promises of cheap lighting products. Trust can only be rebuilt if test laboratories can deliver reliable SI-traceable test results of photometric quantities, which meet the demands of industry and customers, for a large range of diverse LED-based light sources. Statements on uncertainty assessment to improve the evaluation of LED products became mandatory in the first worldwide accepted standards CIE S 025:2015 and EN 13032-4 on test procedures for LED based lamps, luminaires and modules published by the International Commission on Illumination (CIE) and the European Committee for Standardization (CEN). However, these standards are incomplete, as there is still no validated procedure available to assign measurement uncertainties, at test laboratory level, for some listed measurement quantities and metrological procedures.
Requirements and boundary conditions for precise image-based luminance measurements and concepts for evaluating the uncertainty of typically correlated spectral measurement data are of high importance for test laboratories to determine the real performance of LED-based sources. Although prerequisites for the application of the standards, respective guidelines are not available. Moreover, a suitable harmonised metric including associated tolerances for a target/actual performance comparison of luminous intensity distributions of LED-modules or luminaires is also missing.
Photometers i.e. illuminance and luminance meters are ubiquitous metrological devices in lighting, being used by a wide range of lighting professionals from designers to electricians, for testing installations, performance, compliance and utility. To estimate the spectral mismatch of the response of the photometers (in relation to standardised efficiency of the human eye) within a measurement setup, the so called f1’ index defined in ISO/CIE 19476:2014 can be used according to EN 13032-4. However, the metric i.e. the mathematical equation for determining f1’ is not the most appropriate for LED light sources. Therefore, a new mismatch index for recently defined LED reference spectra and its applicability for coloured LEDs needs to be developed for the revision of ISO/CIE 19476:2014, CIE S 025:2015 and EN 13032-4.
The CIE has identified these issues as one of their research priorities.
The overall goal of the project is to deliver metrics and procedures as well as guidance on metrology issues, and to make existing CIE and CEN test standards for LED-based light sources applicable to testing laboratories as a whole. The specific objectives are:
- To develop a strategy for the evaluation, validation and traceability of spatially and angularly resolved luminance and luminous intensity distributions of LED-based lamps, luminaires, and modules. This should be based on measurements using imaging luminance measurement devices (ILMDs). Additionally, to develop guidelines on the determination of uncertainty and tolerance intervals required in the revision of CIE S 025:2015.
- To develop guidelines on the estimation and uncertainty of i) the spectral mismatch of integral (filtered) measurements for sources emitting coloured light, and ii) integral quantities derived from spectral measurements. Additionally, to propose an extension of CIE S 025:2015 and EN 13032‑4 for an alternative spectral mismatch quality index, based on the new LED reference spectrum published in CIE 15:2018 for white LEDs.
- To propose a harmonised metric to compare luminous intensity distributions, including the definition of the associated tolerance intervals and uncertainties, with a focus on test methods that require the declaration of measurement uncertainties.
- To contribute to the revision of CIE S 025:2015 / EN 13032-4 through CIE Division 2, CEN/TC 169 and IEC TC 34. Outputs should be in a form that can be incorporated into the standards at the earliest opportunity and communicated through a variety of media to the standards community and to end users. Additionally, to promote the take up of the results by end users e.g. manufacturers of LED-based sources.
Progress beyond the state of the art and results
Evaluation, validation and traceability of spatially and angularly resolved luminance and luminous intensity distributions
The spatial and angular luminance, and luminous intensity distribution of LED sources are of great importance to lamp manufacturers, as they precisely describe the performance of their products. The ability to indicate light distributions, tolerances and measurement uncertainties, is a prerequisite for the comparison of different lighting products and for users to select the best possible products for their applications. The strategy developed in this project for the evaluation, validation and traceability of spatially and angularly resolved luminance and luminous intensity distributions will provide input to the revision of the standards CIE S 025:2015 / EN 13032-4.
Guidelines on the estimation and uncertainty of the spectral mismatch and integral quantities derived from spectral measurements
Currently, it is not technically possible to produce photometers with a perfect spectral match to the spectral responsivity of the so-called standard observer defined in the standard, which reflects the spectral efficiency of the human eye. This leads to large unknown mismatch errors and high uncertainties when measuring coloured LED sources. The determination of photometric quantities by mathematical integration of spectral measurements is a solution, but often fails due to unknown correlations in the measurement chain, when uncertainties of a few percent are required. Guidelines and methods for testing laboratories will be developed in this project to improve spectral measurement, considering the correlations along the measurement chain. This is of special importance to determine reliable colour coordinates of coloured sources and the spectral mismatch correction factor including its uncertainty. The outcomes of this project will provide input to the revision of CIE S 025:2015 / EN 13032-4.
For white light, an alternative spectral mismatch quality index, based on the new LED reference spectrum published in CIE 15:2018 for white LEDs will be developed, which will enable the user to characterise the performance of photometers used for measurements of non-tungsten-filament lamps. The outcome of this work will be directly fed into the standardisation work of CIE for the revision of the ISO/CIE19476:2014 standard where f1’ is defined. In this context, it will also be evaluated whether new mismatch indices would provide more benefits to coloured sources.
Harmonised metric for luminous intensity distribution
Traceable measurements of luminance and luminous intensities including uncertainty evaluation are prerequisite to describe the performance of lighting products. The ability to compare light distributions is of high importance for customers and lighting designers to choose the best suitable product for their installations. Additionally, a reliable standardised metric i.e. method to describe distributions of light is crucial to enable the comparison of the luminous intensity distributions of different light sources.
This project will develop a generic metric from a set of defined parameters describing the symmetry, envelope and size of the light distribution created by LED lamps, modules and luminaires. The parameters, their associated uncertainties, sensitivity regarding the metric and possible tolerance levels will be proposed and forwarded to CIE and CEN to be included in the revision of CIE S 025:2015 and EN 13032-4, respectively.