GL Optic Luminous Flux Measuring System

How to organize a modern photometric laboratory Part 2 – selection of an integrating sphere system

An integrating spheres are one of the basic instruments for measuring light sources, lamps and luminaires. Traditionally, an integrating sphere is also called an Ulbricht sphere, and its name comes from the name of German engineer Richard Ulbricht, who during the preparation of electrification and lighting of a railway station dealt with photometric measurements in order to find the best illumination method [1]. He proved that the Illuminance measured on the wall of the sphere in which the light source is placed is proportional to the total luminous flux of the light source.

Today, integrating spheres, commonly used in photometry and radiometry, enable a reliable comparison of the luminous flux of different light sources, lamps and luminaires.

 

Fig. 1. Sculpture of an integrating sphere, located on the campus of the Technical University of Dresden. The German inscription reads: “Ulbricht sphere 1990 for checking filament lamps”. © Kay Körner from Dresden-Seevorstadt in Saxony -own work, CC BY-SA 2.5

This part of the article on the organization of a photometric laboratory will focus on the types of integrating spheres and address technical issues that are important for the measurement and evaluation of lighting products. Issues related to the conditions that should prevail in the laboratory will be discussed and practical tips will be given, especially useful in the selection and operation of measuring systems.

 

Why should an integrating sphere be used and what values can be measured in it?

The properties of the integrating sphere allow for a relatively simple and quick comparison of photometric parameters of different lighting products. An object placed in the sphere can be measured within a few seconds (apart from the time needed for stabilisation – see below). This is the fastest way to measure the luminaire’s total luminous flux [lm] and determine the luminaire’s luminous efficacy [lm/W] at a known power supply. The measurement in the integrating sphere is straight forward and allows for direct comparison of the readings with the reference standard.

Contrary to goniometer measurements, which consist of long-term partial measurements integrated to calculate the total luminous flux, the measurement in the sphere is immediate and much easier. A well-described and accepted measuring procedure allows almost every operator to obtain reliable and reproducible measurement results. This is important when comparing many lighting products and measuring different sources. Let’s assume that we have a ready, measured and accepted prototype luminaire. If the power supply unit is replaced by another type or if it is necessary to replace the type of the light-emitting diode or optical elements, when we want to quickly verify the overall flux – the integrating sphere becomes an indispensable tool for everyday work with lighting products.

In addition to the total luminous flux in the integrating sphere combined with the spectroradiometer, we will also measure the correlated color temperature CCT for white light [K], colour rendering index CRI and R f , the D uv parameter determining the position in relation to the Planck curve, chromatic coordinates x, y and many other parameters that characterize the lighting product. Measurement in the sphere will provide all the necessary data for the assessment of the energy class in accordance with the recommendations of the standards defining minimum functional requirements and  ecodesign requirements for energy-related products defined in Directive 2009/125/EC in Europe or
the relevant standards for Energy Star rating in the Americas.

In addition, the Ulbricht sphere system also enables measurements outside the visible spectral range. In the case of products for illumination of plants, it is possible to measure quantities such as the PPF photon flux or the spectraly expanded PBAR photon flux. Integrating spheres are also used for infrared radiation measurements in vision systems and industrial equipment – in this case an integrating sphere appropriately calibrated and equipped with high-quality measuring equipment can be used for rapid verification of the energy efficiency of optical radiation from different spectral ranges.

Also according to the recommendations of the recently published new standard CIE S026 2018, the effective lighting influence on the human day / night cycle (circadian rhythm) can also be evaluated on the basis of data from the measurement of the luminaire’s radiant flux, and then calculated by the ipRGC’s efficiency curves.

Another practical advantage of the integrating sphere is its essential property, namely the lighttight structure. Thanks to this, the integrating sphere can be placed in normal office conditions and illuminated rooms without the risk of the general lighting affecting the measurement result.

What size and type of sphere should be chosen?
Golden principles of photometry described, among others, in the CIE S025/E: 2015 standard and the EN 13032-4:2015 also included in IESNA LM 79 specify that the sphere size must be 10 times greater than the luminaire’s size [2] [3]. This does not apply to linear luminaires with a small total area of the housing. On the other hand, the practical principles applied in many in-house measurement laboratories allow for the measurement of luminaires whose size is up to 30% of the sphere diameter, which is described in more detail in the article: Practical tips for LED measurements in the integrating sphere and on the goniometer according to CIE025 [4]. Let us remember that each element introduced into the integrating sphere interferes with the measurement (and therefore limits the possibility of multiple reflections) and  absorbs a part of the light flux. This effect is compensated for by an additional light source placed in the sphere, allowing for the determination of the absorption coefficient. If we are building a laboratory that is to be accredited in the future, we must take into account the recommendations of the applicable standards. For factory quality control we can adopt our own procedures, but we have to take into account the error, which is caused by the size of the luminaire placed inside the sphere.

When selecting the sphere size for our laboratory, it is necessary to suggest the maximum size of luminaires to be measured in it. In the case of luminaires for general lighting purposes, large integrating spheres with a diameter of 1.5 m or 2.0 m are the most frequently selected. Laboratories that measure LED modules, components or small illuminators (e.g. evacuation lighting) usually buy a sphere with a diameter of 1m. In the R & D departments the most popular are integrating spheres with a diameter of 0.2 and 0.5 m, which make it possible to measure both individual COB diodes and most commonly used LED modules.

On the market there are integrating spheres combined with a photometer, i.e. typical photometric spheres, as well as spheres which use the spectroradiometer as a measuring device – i.e. spectro-radiometric spheres. The advantage of the latter is the ability to measure the luminous flux, including the calculation of additional data such as colour temperature, any effective curves for human centric lighting or lighting for plants, as well as other effective curves for special applications. Moreover, the integrating spheres with the spectroradiometer are not burdened with the so-called spectral missmatch error, which results from a simple photometer measurement system and is dependent on
the class of the applied optical correction filter for matching to V (lambda) curve [5]. The better the photometer class, the smaller the mismatch error. Spectroradiometers used in integrating spheres mathematically calculate the V curve (lambda) hence more accurately measure light-emitting diodes with different spectral power distributions.

Fig. 2. Example of a complete measuring system consisting of a 50 cm diameter integrating sphere, with a Peltier module, a computer with analysis and reporting software and a spectrometer, power supply and programmable temperature controller (placed in a rack cabinet).

The quality of the diffusing coating, which covers the inner part of the sphere and guarantees proper, repeated reflection of the signal, is very important. In the 1970s there was a standard in force indicating the use of a coating with a reflection coefficient of ƿ 80. The current CIE standard from 2015, after many years of various tests and publications, introduced the requirement to use a reflection coating above ƿ 90. The higher the reflection coefficient, the higher the number of possible reflections in the sphere, so that more repeatable measurement results can be obtained. And the impact of the so-called aperture error on the distribution and incident light output is smaller. Various types of paint and barium sulphate mixture (BaSO 4) are used to coat the integrating spheres. A poor quality coating does not have the appropriate reflective properties over the entire spectral range, which may result in larger or smaller errors depending on the type of light source to be measured. In addition, the low-quality coating material will "yellow" over time, which means that the signal from the blue wavelength range is gradually attenuated, and then causes serious measurement errors and difficulties in calibration of the system. As is known, white LED light uses a blue diode and the latest designs use a near UV diodes, so reflection of this part of the radiation when measuring in the sphere is very important in order to obtain accurate results. A good quality coating is sensitive to mechanical damage (so it should be taken care of), but in return it maintains its excellent optical properties for many years. The manufacturer of integrating spheres should instruct the customer on how to maintain the sphere during the system maintenance training.

Fig. 3. GL Opti Sphere 500 integrating sphere with a universal lamp holder.

Modern measuring systems are fully computer controlled. Both the measuring system with additional components, as well as an additional light source to compensate the absorption coefficient with the power supply system, are controlled via a single user interface. At present, for
LED measurements, photometric measurement is automatically combined with colorimetric measurement and power and temperature measurement. All these elements can influence the results and repeatability, so when selecting the equipment, attention should be paid to the details of the additional measuring functions. An important element is the ability to integrate the optical measurement system with programmable and stabilized power sources, power meters or electrical parameter analyzers. Good quality solutions allow for optimal configuration of the measuring system adjusted to the requirements of standards and laboratory needs.

Laboratory conditions and luminaire stabilization before measurement

In accordance with the recommendations of the standard, suitable conditions should be provided in the photometric laboratory that do not interfere with the measurement results. The optimum ambient temperature is 25°C. The room should be free of dust and vibrations and the humidity should be kept constant. The air movement should not exceed 0.25 m/s so as not to cause cooling of the luminaire during measurements. Traditionally, in a photometric laboratory, the walls of the room are black. This has no direct effect on the integrating spheres in the case of measurements made with the sphere completely enclosed. When measuring projecting light and other luminaires introduced into the sphere through the external measuring port, special attention should be paid to the stray light ( ambient light in lab ) that may affect the measurement results. In other cases the integrating sphere can be placed in any room, even a bright one.

Fig. 4: Principle of stabilization before measurement of the luminaire. Where Ø is the luminaire flux and P is the power supply. Source: CIE Expert Sympozium on CIE S025 2015

Fig. 5. Principle of stabilization of LED modules prior to measurement, where tp is the board temperature at the measuring point. Source: CIE Expert Sympozium on CIE S025 2015

Fig. 6. Painting of integrating spheres with barium sulphate.

Fig. 7. Integrating spheres with a diameter of 2 m.

Measurement of luminaires – even if the luminaire is equipped with a power supply unit, must be powered from a stabilized source. Under laboratory measurement conditions, a stabilized power supply system should be provided to protect against fluctuations in power supply from the mains. In addition, it is necessary to remember about the appropriate power cables connected directly to the luminaire, because voltage drops in the cables may affect the parameters of the power supply. The issue of power supply during the measurements has been presented in more detail in the previous issue of “LED lighting”.

Stabilization and measurement of the luminaire should be carried out in the final working position of the luminaire. It is worth noting that in order for this requirement to be met, the integrating sphere should be equipped with brackets or mounting table enabling proper installation of the luminaire inside the sphere in different working positions [Fig. 3]. Professional systems are also equipped with power supply cables and additional circuits allowing to measure power directly near the luminaire.

The luminaire should be stabilised (heated-up) for a minimum of 30 minutes and is considered to be stable if the light output and power supply do not change by more than 0.5% within 15 minutes. If the luminaire does not stabilize during this time, it should be heated for a longer time. There is also a case of technical luminaires, which due to their design are not subject to full stabilization – in such case measurements should be started and the conditions and changes in the flux at individual moments should be noted in the measurement report.

In the case of measurement of LED modules, the stabilisation also lasts a minimum of 30 minutes until the temperature changes are below 1°C. Attention should be paid to the stabilization of the temperature at the point T p measured at a specific point on the board. In practice, the measurement of LED modules takes place in an integrating sphere on a radiator after the module’s operating temperature has been stabilized. Alternatively, TEC temperature stabilization systems with Peltier system can be used, where the module is placed, and then the temperature is stabilized at a preset level, e.g. 25°C or 85°C (it can be similar to the operating temperature of the module placed in the
luminaire). In modern measuring systems the process of temperature adjustment and stabilization of the LED module can be integrated with a programmable system. This enables efficient and comprehensive evaluation and measurement of LED modules.

Logistics, maintenance and technical support
When planning the organisation of the laboratory, attention should be paid to the dimensions of the integrating sphere and the possibility of bringing it into the building and placing it in the target room of the laboratory. In the case of an integrating sphere with a diameter of 2 m appropriate entrance (2.3 m by 2.3 m) must be provided so that the sphere can be transported in its entirety. This applies  to the entire transport route, including gates, entrances, staircases, etc. In some cases it is necessary to partially dismantle the walls or enlarge the door. The sphere itself, placed in the laboratory, does not take up much space – an area of about 10 m is enough. Integrating sphere systems are subject to regular calibrations. Standards require stabilisation of the system between calibrations at the level of 0,5%. To be able to check the conformance a working reference standards may be used. Calibration of a sphere spectroradiometer system should be made with a use of a Total Spectral Radiant Flux (TSRF) standards traceable to National Metrology Institutes (NMIs). Laboratory staff may obtain TSRF standards and perform such calibration on their own or request an onsite service from a supplier or accredited organisation. Laboratory “best
practices” suggest that annual calibrations are recommended however it is up to the laboratory manager to decide.

Summary
A decision to buy an integrating sphere is an investment decision, so it is worth considering the choice of equipment and supplier. As in the case of other measuring devices – it is also necessary to properly train the personnel. If we build a laboratory from scratch and we do not have experience in performing measurements on our own, at the beginning many questions and doubts will arise. Additional training and technical support from the measuring instrument supplier are helpful.

 

References
[1] J.M. Palmer, B.G. Grant, The Art of Radiometry, SPIE Press 2010, ISBN 978-0-8194-7245-8, p. 5.

[2] CIE International Standard, Test Method for LED Lamps, LED Luminaires and LED Modules, CIE S025/E: 2015,2015.

[3] Światło i oświetlenie. Pomiar i prezentacja danych fotometrycznych lamp i opraw oświetleniowych. Część 4: Lampy, moduły i oprawy oświetleniowe LED [Light and lighting. Measurement and presentation of photometric data of lamps and luminaires. Part 4: LED lamps,
modules and luminaire], PN-EN 13032-4:2015.

[4] Oświetlenie LED. Praktyczne wskazówki dotyczące pomiarów LED w kuli całkującej i na goniometrze zgodnie z CIE025 [LED lighting. Practical tips for LED measurements in the integrating sphere and on goniometer according to CIE025.], 3, 2016.

[5] O. Yoshi, Luminous Flux and Spectral Radiant Flux Measurements. NIST Photometry Short Course, 2011.

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