15SIB07 PhotoLED: Future photometry baed on slid-state lighting products

• Forschungsstelle: METAS
• Projektnummer: F-5116.30074
• Projekttitel Englisch: 15SIB07 PhotoLED: Future photometry baed on slid-state lighting products

Schlüsselwörter (Englisch)

Solid-state lighting (SSL), light-emitting diode (LED), luminous efficacy, photometer, V(λ)-filter, spectral error, Predictable Quantum Efficient Detector (PQED), standard lamp, phase out incandescent

Kurzbeschreibung (Englisch)

The project will address the obsolescence of traditional incandescent standard lamp technology used in photometry and the need to support the introduction and uptake of solid-state lighting (SSL) products. It will develop and validate the basis for a new photometric system based on white LEDs, by developing new LED-based calibration sources and measurement techniques supporting the specific properties of the new light sources and detector technology, including illuminance measurement of the new LED-sources without optical V(λ)-filters in the calibration of photometers and measurement facilities used for determining the energy efficiency of new SSL products coming to market.

 

Classical photometry relies heavily on the use of tungsten filament incandescent standard lamps and V(λ)-filtered photometers as transfer standards in calibrations of luminous intensity, luminous flux and photometer illuminance responsivity. Photometric measurement methods and spectral data of light sources used in colorimetric analysis of lighting were established long before SSL products became available in general lighting. During the last few years, the phasing out of incandescent lamps has changed the lighting market. Test laboratories and other end-users of photometric instruments no longer measure incandescent lighting, but in most cases SSL products that are based on white LEDs. As a result of the phasing out of consumer incandescent lamps, the standard lamps used in calibrations are disappearing from the market as well, leading to an urgent need to develop new standard lamps for photometry that are based on a technology that is widely available.

An illuminant is an agreed published spectral power distribution of a theoretical light source in the wavelength range between 300 nm - 830 nm, defined by the CIE for analysing reflected or transmitted object colours under specified conditions of illumination. There are currently only two illuminants defined as standard illuminants used in photometry and colorimetry and intended for calibration purposes; the Standard Illuminant A describes typical incandescent (tungsten filament) lighting and Standard Illuminant D65 describes typical daylight. Due to the phasing out of incandescent transfer standard lamps from the market, and the introduction and uptake of LED-based lighting, there is a need to develop candidate standard illuminants that more closely represent current lighting eg. LED light sources.

Luminous flux (lm) and active electrical power consumption (W) of new SSL products coming to market are measured at test laboratories in order to determine the luminous efficacy (lm/W) and energy class of the products. Due to the fact that spectral responsivities of V(λ)-filtered photometers differ from the defined CIE V(λ) curve, all photometers are typically calibrated using incandescent light with correlated colour temperature (CCT) of 2856 K. This causes unwanted spectral errors in the measurements of SSL products at test laboratories and also in the field due to the large differences in the spectra of calibration sources and the light being measured, and in many cases cannot be corrected for. New LED-based standard lamps with well-defined spectral power distributions (SPDs) and supporting detector technology need to be developed for photometric calibrations at national metrology institutes (NMIs), to provide convenient and reliable traceability for measurements of SSL products and calibration of photometers at test laboratories to pave the way to lower uncertainties.

This is a joint research project carried out in the framework of the European Metrology Programme for Innovation and Research (EMPIR) (see:http://www.euramet.org/research-innovation/empir/). The EMPIR initiative is co-funded by the European Unions's Horizon 2020 research and innovation programme and the participating states. METAS is one of the project partners in the Project.

Projektziele (Englisch)

The specific objectives of the project are:

1. To develop LED-based standard illuminants that can complement or replace the CIE Standard Illuminant A in photometric calibrations and in analysis of colorimetric parameters and to evaluate the consequences of the defined spectra of the new candidate standard illuminants.
2. To develop new LED-based photometric standard lamps for dissemination and maintenance of the units of luminous intensity, illuminance and luminous flux triggered by the ban on incandescent lamps. The new LED-based standard lamps will be optimised for compatibility with existing calibration facilities, spectral properties close to the defined candidate standard illuminants, well-defined angular uniformity, long lifetime and temporal stability of electrical (DC- or AC-operation), photometric and colorimetric characteristics to enable low uncertainties in measurements of their photometric and radiometric properties.
3. To develop new photometers and photometric measurement methods that enable illuminance measurement of the new LED-based standard lamps with uncertainties as low as 0.2 % (k = 2) in the primary realisation of photometric units, or in calibrations of photometer illuminance or luminous flux responsivities at NMIs, accompanied by high-end spectral irradiance measurement of the new standard lamps with uncertainties as low as 0.4 %.
4. To reduce the uncertainties of luminous flux and luminous efficacy measurement of solid-state lighting (SSL) products at national metrology institutes to 0.5 % (k = 2) and to demonstrate that uncertainties as low as 1 % (k = 2) can be achieved in a test laboratory.
5. To facilitate the uptake of the measurement methods developed by the project by the measurement supply chain, ensuring traceability of measurement results to the end users (test laboratories, lighting manufacturers) and contribute to the development of standards by the international standardisation committees (CIE) concerning solid state lighting.

Abstract (Englisch)

METAS was mainly involved in the following tasks:

1. Contribution to the definition of a white LED reference spectrum (WP1 Photometric standards based on white LED sources).

METAS provided more than 750 anonymized customer LED spectra that were measured during the last years. Out of all spectra provided by the consortium typical spectral power distributions (SPDs) were obtained and grouped/binned into eight different CCTs according to an ANSI standard. This SPDs were submitted to CIE Division 1 (TC 1-85) and five of them with their precise CCT of 2733 K, 2998 K, 4103 K, 5109 K and 6598 K were selected and published in the technical report (CIE 015:2018).

By investigating the best LED reference spectrum for calibration of photometers, it was found that the spectrum with CCT 4103 K was providing the smallest mean absolute spectral mismatch error. This was discovered by comparing measurements of different standardized spectra (HPx, FLx, …) with photometers calibrated on one hand with CIE standard illuminat A and on the other hand calibrated with the selected LED calibration spectrum.

Due to this excellent results the consortium agreed on starting a technical committee at CIE (TC2-90 LED Reference Spectrum for Photometer Calibration) to create a technical report about this spectrum. The spectrum is therefore recommended for calibration purposes of photometers.

2. Realization of a portable Fabry-Perot etalon for wavelength calibration of spectroradiometers.

This activity was aiming on the development of a Fabry-Perot (FP) etalon for a wavelength range of 360 nm and 830 nm. A FP etalon consist of a cavity formed by two partially transparent mirrors placed plane-parallel at a given separation/gap between the two mirrors. With that air space gap and the optical properties of the mirror coating (index of refraction and coating thickness) a specific resonance pattern with given wave-length peaks can be generated. This pattern is created by illuminating the FP with a parallel beam of a spectrally broadband source and measuring the transmitted light with a spectroradiometer. The measured positions of the resonance wavelengths depend on the pixel-to-wavelength assignment of the spectrometer. Furthermore, the spectral lines of an HgAr lamp with known peak positions in terms of wavelength is also measured with the spectroradiometer. One peak of the HgAr measurements is used to find the precise spacing between the two mirrors by an optimization algorithm.

3. Electrical measurements of a newly developed mains operated LED luminous flux standard.

During the project a new LED based transfer standard for luminous flux was designed and developed by VSL. After seasoning the lamp for 800 hours, METAS was responsible for the electrical characterization of that lamp. In a first step, the stability of the lamp was investigated for the volt-age, current, power and power factor (according to the standard CIE S025). After 30 min of operation all measured electrical parameters stabilized. Electrical tests on the transfer source show that the input power variation can be restricted to 0.1 % (the goal was 0.5 %) after 2 minutes of stabilization time and the RMS current fluctuation can be limited to 0.1 % instantly (the goals was 2 %). The archived performances are much better than the goals. The transfer source shows high power factor (PF) of 0.971 and low current THD of 3.80 % if the impedance stabilization network ISN (developed in a past EMRP project) is used, that facilitates reaching good lamp stability with different types of AC voltage sources.

To gain further information, visit the webpage www.photoled.aalto.fi.

Umsetzung und Anwendungen (Englisch)

METAS will offer calibration service with the white LED reference spectrum for illuminance meter calibrations, as soon as this will be published by CIE. METAS has developed a fully automated setup with different LED arrays (different colors and different CCT's) on a rotating table for comparison of the response of a DUT with a reference photometer.

Publikationen / Ergebnisse (Englisch)

• A. Kokka, T. Poikonen, P. Blattner, S. Jost, A. Ferrero, T. Pulli, M. Ngo, A. Thorseth, T. Gerloff, P. Dekker, F. Stuker, A. Klej, K. Ludwig, M. Schneider, T. Reiners and E. Ikonen; Development of white LED illuminants for colorimetry and recommendation of white LED reference spectrum for photometry; Metrologia 55 (2018) 526-534.
• F. Stuker and P. Blattner Auf der Suche nach dem neuen Lichtmass der Dinge, METinfo 2/2019.
• P. Dekker, M. Ali, E. Houtzanger, Y. Zhu, D. Zhao, T. Poikonnen, A. Klej, F. Stuker, B. Imhof, S. Källberg; Mains operated LED based transfer source for luminous flux scale realisation and dissemination, DIE 2019 29th Quadrennial Session, Washington, USA.