EMPIR project’s new photometry methods draw attention from the IOP’s Metrologia

LED standard lamp LIS-A for photometric calibrations

Using a standard lamp and fisheye camera method for the testing and energy efficiency classification of LED lighting products

The project

In recent years, a newer and greener technology has taken the place of traditional, incandescent lighting. Solid-state lighting uses light-emitting diodes (LEDs) as the illuminating source instead of the traditional basis of electrical filaments, leading to much sought-after qualities like lower power consumption and greater durability.

However, current methods for testing the performance of lighting products are still based on instruments that have been calibrated using electrical filament lamps. In fact, the light emission spectra and the directional spread of light (i.e. the angular intensity distribution) from electrical filament lamps differ greatly from those of LED products – a factor that can introduce measurement errors.

To address these inconsistencies, EMPIR project ‘Future photometry based on solid-state lighting products’ (15SIB07, PhotoLED) has developed new LED-based standard lamps for instrument calibrations, in addition to other techniques that can better support this new form of lighting. Project outcomes have been instrumental in boosting consumer confidence in solid-state lighting, a critical step to advancing this innovative, energy-efficient technology.

Improved methods for measuring luminous flux

Working under the EMPIR project, researchers have developed new methods for measuring the luminous efficacy (lm/W) of solid-state lighting products, supporting their energy classification and helping to establish the new technology within existing markets.

In addition to electrical power (W) consumed by a light source, the luminous flux (lm) – the total visible light emitted from a source – is another key measure for lighting products. To develop new standard lamps for luminous intensity and luminous flux calibrations at National Metrology Institutes (NMIs) and test laboratories, the project consortium has collaborated with several partners from the lighting industry and photometry community. Researchers have brought together their expertise to reduce spectral errors in the measurements of LED products and improve the accuracy of photometer calibrations – specifically, for integrating sphere photometers.

As directional light sources, LEDs emit their maximum power perpendicular to the object’s surface. Therefore, it is common for a typical LED lamp to consist of several elements to control the spread of light, such as a reflector or diffusing bulb. Uniquely, project-developed methods have strived to correct for the spatial non-uniformity of integrating sphere photometers, utilizing fisheye cameras to capture the sphere’s interior as illuminated by the LED products under assessment. This correction importantly will enable photometers that are responsible for measuring luminous flux to account for the large variety of angular intensity distributions found in solid-state lamps and luminaires.

By incorporating the new LED standard lamps and fisheye camera method with integrating sphere photometers, project researchers have ultimately come up with an efficient approach for reducing spectral and angular errors of luminous flux measurements – providing experts with reliable information to classify the energy efficiencies of solid-state lighting products.

Photometry highlights in the IOP’s Metrologia

Project researchers have submitted a paper describing LED reference spectrum analysis for photometer calibration to the IOP’s leading metrology journal, Metrologia (Issue 55). A key proposal in this paper – that has been downloaded over 3000 times to date – involves the use of one of the project-analysed LED spectra as a reference spectrum in photometer calibrations. As one of the journal’s most downloaded articles, the paper has also been selected as one of the ‘2018 Highlights of IOP Metrologia’.

The project consortium has additionally published their findings relating to their validated fisheye camera method in two other papers for Metrologia (Issue 54 and 56). The technique has since been adopted into several European test and calibration laboratories that measure solid-state lighting products on a daily basis.

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

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