Practical Guidelines for Sphere and Goniometer Measurements in View of CIE S 025/E:2015

The latest CIE S 025/ E:2015 standard provides the conditions required to perform reproducible photometric and colorimetric measurements on LED lamps, LED modules, and LED luminaires (LED devices).  The main goal here is to obtain reliable and accurate photometric data for LED devices and ensure that the data is consistent, meaning that the results are the same (within the limits of the declared measurement uncertainty), no matter which laboratory conducts the measurement.

Published by Miko Przybyla  LED professional Symposium +Expo 2016 | September 20-22, 2016 | Bregenz 

Creating such a common foundation i.e. the first globally recognized LED measurement standard, in turn facilitates the comparison of different products. This paper was an introduction to the workshop organized during the LpS2016 in Bregenz. We were presenting participants with a set of ready-to-apply tools that any laboratory, LED manufacturer or light designer can apply to ensure compliance with the standard. We also provided tips on the optimal laboratory and environmental conditions and explain the importance of stabilizing LED modules or LED luminaires before measurement. We were sharing our experiences in measuring key parameters like total luminous flux  or color quantities, how to present them and to treat measurement uncertainty. Finally, we presented our best practices on the preparation and operation of measurement instruments, as well as issues associated with instrument aging.

Methods for measuring LED products described in the CIE S025 Standard put additional requirements on optimal laboratory and environmental conditions to perform reliable and comparable measurements. These requirements concern not only optical measurement systems but also electrical parameters, thermal considerations and all other peripheral devices used in the laboratory environment. This comprehensive approach becomes a new challenge for laboratories and manufacturers who would like to follow recommendations and be in line with the global standard.

1.     Introduction

The aim of the paper is to present practical consideration of the integrating sphere measurement system as well as the setup of the gonio-spectrometer system. Starting from choosing the proper equipment, setting up the system through the lamp positioning and stabilization stage to the final results presentation. We will also share our experiences in measuring key parameters like total luminous flux or color quantities, how to present them and to treat measurement uncertainty. Finally, we will present how to determine the most important quantities in LED product characterization like CCT, CRI, D u,v, luminous efficacy, angular color uniformity and other international norms relating to the minimum performance requirements for LED products.

2.     New Global Regulation
2.1.      International standard

CIE S 025/E:2015 published in March 2015 is the first internationally recognized standard describing measurement method for LED Lamps, LED Luminaires and LED Modules. Before this standard was published an American standard LM 79 was used in many regions of the world as a reference document to perform measurements of SSL products. However laboratories and manufactures were in the need for regulation which would be approved by local and national standards and at the same time globally recognized. This new document introduced by CIE which is international standards organisation is the beginning for global harmonization and recognition of test results presented by different laboratories around the world.

2.1.       European standard

The preparation of the new standard in Europe was a joint work between CIE TC2-71 CIE Standard on test methods for LED Lamps, luminaires and modules, Chaired by Yoshi Ohno (US) and CEN TC169 WG7 Photometry, Chaired by Guy Vandermeersch (BE) the new standard was adopted quickly as a new European standard EN 13032: Lighting Applications — Measurement and Presentation of Photometric Data of Lamps and Luminaires— Part 4: LED Lamps, Modules and Luminaires. The preparation of CIE S025 was globally representative and it is expected that national and regional standardisation organization should start to adopt to this new regulation for LED measurement in all continents.

3.     New comprehensive approach

LED measurement is not only about the optical testing anymore. CIE S 025 is using experience of LM-79 but it is providing more detailed information not only about photometric and colorimetric measurements but also electrical parameters and thermal consideration. This document also refers to many other CIE standards and publications which are describing specific measurement requirements. There are also suggestion regarding the use of proper equipment for each type of the LED product and data presentation.


Figure 1: New comprehensive approach of the CIE S025 standard covers broad range of measurements and ambient conditions.

 

The standard describes following measurements

  • Total luminous flux
  • Partial luminous flux (useful lumens)
  • Electrical measurements
  • Luminous efficacy [ lm/W ]
  • Chromaticity coordinates [ x, y ]
  • Correlated colour temperature [ K ] ( for white LEDs only )
  • Distance from Planckian locus D u,v
  • Colour rendering indices CRI ( for white LEDs only )
  • Luminous intensity distribution LID
  • Centre beam and beam angles
  • Angular colour uniformity
3.1.      Integrating spheres

The rigorous requirement is put for maximum DUT (device under test) size to be measured in an integrating sphere. The size of the DUT should not exceed 10% of the sphere diameter. This requirement will have a major impact on the choice of the equipment for laboratories. This also leads to the conclusion that only LED modules, LED lamps and only very small luminaires can be measured in an integrating sphere. There are several types of integrating spheres approved by the standard.

  • Sphere-photometer including integrating sphere connected with photometer with V lambda correction. The minimum correction class f1` of the filter must be blow 3%. It is recommended to apply spectral mismatch correction.
  • Sphere – spectroradiometer which combines integrating sphere system calibrated with spectral device. Such system will not only allow to avoid mismatch correction requirement but also will allow for measurements of photometric quantities with high accuracy and colorimetric values at the same time.

Another important specification regarding the type of an integrating sphere is minimum sphere reflectance coating recommended at 90%. Traditional systems using photodetectors were coated with reflectance material reaching 80% only. Studies show that the sphere geometry errors like baffle error and coating non-uniformity is dropping with the increasing number of reflections in the sphere. Especially high differences in readings for directional lamps were noticed in integrating sphere with lower reflectance coating. Higher reflectance increases where throughput and the number of reflections which helps with better integration of the signal.

The standards describes other important procedures for accurate sphere measurements like:

  • Stability between the calibrations below 0,5%
  • Minimum required resolution of the spectroradiometer FWHM 5nm and data point 5nm
  • Spectral range 380 – 780nm
  • Self-absorption compensation – in full visual range
  • Temperature measured in near surroundings of the DUT
3.2.      Goniometer systems

Goniometer system for LED luminaires measurements should combine photometric and colorimetric measurements. Such system is required to obtain LID ( Light Intensity Distribution ), luminous flux and also all colorimetric data like CCT, CRI, D u,v and Angular Color Uniformity which is specific requirement for SSL products testing to check if the product conforms the minimum performance requirements for directional lamps.

Despite the fact that there are several goniometer systems described by the standard it is clear that for this kind of combined photometric and colorimetric measurements the easiest way would be to use goniometer system with spectral device. Similarly to integrating sphere systems, using spectral device in combination with goniometer will help to avoid additional corrections and will allow to measure photometric and colorimetric quantities at the same time. It is important to mention again that for absolute measurements it is necessary to use spectrally calibrated instrumentation.

Due to the fact that the goniometric measurements are made in the dark room which must provide  sufficient space it is important to check and choose the right distances and sizes.

Minimum photometric distances depends on the DUT type and are as follows:

  • Near cosine distribution ( with beam angle ≥ 90 ⁰ ) distance: 5 x Diameter
  • Broad angle (beam angle ≥60 ⁰ ) distance: 10 x Diameter
  • Narrow angle ( directional lamps ) distance: 15 x Diameter
4.     Environmental conditions

Factors like operating position, warm up time, air movement and temperature may have significant influence on the measurement results therefore specific requirements are described in the standard.

Unlike the LM79 where operating position during the measurements had to be the same as operating position of LED product, the new standard opens the door for other types of goniometers with  turning luminaire system. However, if the operating position during the measurement is different than the one described by the applicant, an auxiliary photometer correction should be applied.

It is also possible that the applicant ( manufacturer or responsible reseller ) will describe alternative operating positions of its LED product, this would allow to test it using more straightforward goniometer systems which are also more reasonably priced. This would help to promote reliable goniometer testing among larger number of manufacturers and importers and in turn the data put in LED products catalogues and informative materials for end-users will include more reliable information. Nevertheless the operating position during the warm-up time and during the measurement must be the same and it should be described in the test report.

Recommendation for stabilization before the measurements are as follows:

  • LED modules should be thermally stabilized before measurements (TEC) or the Tp temperature must be measured and described in test report.
  • LED lamps and luminaries should operating for minimum 30 minutes before the test and monitored light output ad power should be within 0,5% stability to perform measurement. If the DUT is not stabilizing in 45 min ( 150 for luminaires ) measurements can be performed and recorder fluctuations should be put in the test report.

Recommendation regarding ambient conditions are as follows:

  • Ambient temperature 25⁰C
  • Air movement up to 0,25 m/s
  • No dust, vibrations etc.
5.     Summary

The CIE S025 standard describing test method for LED Lamps, LED Luminaires and LED Modules is the first globally recognized document which will help to recognize measurement results between laboratories around the world. This will also help with harmonization process of the global market of SSL products.

The standard is much more comprehensive than previously available local standards and it puts obligation on accredited laboratories to use the latest techniques and reporting uncertainties to improve reproducibility of the measurements. The demand for high standards of the equipment and measurements procedure may be difficult to reach for medium-sized companies making their steps to build its own laboratories but is definitely a reference point for all parties involved in SSL market and will help to control minimum performance requirements of LED products which are introduced to the market around the world.

6.     References

[1]     CIE S 025/E:2015 Test method for LED Lamps, LED Luminaires and LED Modules, Vienna 2015

[2]     Ohno, Y, Integrating Sphere Measurements for CIE S025. CIE Tutorial and Expert Symposium on CIE S025, Brauschweig, Nov 24-26, 2015.

[3]     Bergen, T, General Introduction to CIE S025. CIE Tutorial and Expert Symposium on CIE S025, Brauschweig, Nov 24-26, 2015.

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