Tools for LED Metrology
In photometry of LEDs, the spectroradiometer has displaced the photometer because of its accuracy in the presence of narrow spectral components and because of its power to provide additional information of spectral qualities like correlated color temperature (CCT), color rendering index (CRI), or peak wavelength. In combination with a goniometer, the use of a spectroradiometer is already established in research institutions but will soon reach a broader community when the LED conquers the general lighting market.
Traditional luminaire metrology often employs a large goniometer or relies on the separation of a lamp from a luminaire. This separability does not apply to solid-state lighting (SSL) sources because LEDs cannot be removed from the luminaire without changing their characteristics. The LED assembly performance depends on the complete thermal, electrical, and optical design of the assembly.
The LM-79 standard (Approved Method: Electrical and Photometric Measurements of Solid-State Lighting Products), developed by the Illuminating Engineering Society of North America recommends the measurement of absolute total luminous flux (φv) of the LED assembly either with an integrating sphere or a goniophotometer. Both can be used in combination with a spectroradiometer, which allows all other photometric quantities such as CCT and CRI to be measured at the same time and with the goniometer even spatially resolved.
We performed a goniospectroradiometric study with various LED assemblies and luminaires. The luminous flux is determined by numeric integration from these data and is compared with measurements in an integrating sphere.
Goniophotometers are normally used for measurement of luminous intensity distribution and the total luminous flux by the integration. If a spectroradiometer is used as a detector instead of a photometer, the system is called a goniospectroradiometer. It allows the determination of spatial distributions of all relevant photometric quantities.
Goniometric measurements of the luminous flux take much longer than those taken with an integrating sphere, but they require fewer corrections and are more accurate.
A type C goniometer system can use either a photometer or a spectroradiometer as detector. Various LED assemblies were set up in the goniometer and the distributions of all relevant photometric quantities were measured in combination with a spectroradiometer in our study.
Using this system, it is important to operate the LED assembly for at least 30 minutes prior the measurement to reach a thermal equilibrium, as required by LM-79.
LED devices may have very different radiation patterns. Two luminous intensity distributions of LED assemblies with different beam angles are shown for illustration above.
The beam angle can be calculated as a full width half maximum (FWHM) for each profile.
White LEDs typically have a pronounced spatial structure both in photometric and colorimetric properties. The patterns are characteristic for certain types of LEDs and depend on the quality of the phosphor coating and the radiation pattern of the underlying LED.
This directional dependence can be further enhanced by the use of optical systems in the LED assemblies.
The spatial distribution of CCT and CRI can be obtained only with a goniospectroradiometer.
The example below shows the CCT distribution for an LED assembly with a 3x3 LED array. The distribution is plotted as a function of inclination angle and with the azimuth as a parameter at left. The right-hand figure shows a 3D representation for the same LED assembly.
There is a rich pattern in the CCT for the inclination angle featuring a variation of up to 200K. However, the CCT pattern shows nearly perfect rotational symmetry.
A spectrum and the CRI distribution of another LED assembly are shown below.
This LED assembly has a sealed diffuser, and its spectrum shows additional peaks in the cyan and red region beside the white LED. Therefore, the color rendering index is very high at more than 95.
Probably the position of the LEDs behind the diffuser causes the asymmetric variation of the CCT and therefore CRI. Even at this high level of color rendering, the goniospectroradiometer is able to resolve structures in the spatial distribution of the CRI with variations within a narrow range of 95.6 and 96.7.
For the fast measurement of luminous flux of LED assemblies, an integrating sphere (ISP) with a spectroradiometer is the recommended instrument. It can be easily calibrated for the direct measurement of luminous flux.
The ideal integrating sphere contains no absorbing surfaces and a perfect Lambertian reflecting inner surface, so that the light is distributed uniformly, spatially, and spectrally. For a real sphere, there are inherent technical limitations such as the influence of near-field reflections and self-absorption, the aging of the coating, the geometry of baffles, and sample holder.
Some sources of error in the integrating sphere can be identified using a sphere scanner. The design of Instrument Systems spheres takes account of these results.
A new coating solution for the integrating spheres with an optimum reflection coefficient of ρ=0.94 has been found as a compromise between the sensitivity of the sphere to contamination and aging and the directional dependence during the measurement. Results obtained for diverse LED assemblies with a 1-meter-diameter integrating sphere have been measured as accurate as with a higher reflectance sphere but with reduced aging and contamination problems.
Integrating spheres are preferred for speed of measurements and give accurate results when corrected for self-absorption of the sources.
The results of the luminous flux measurements with the goniospectroradiometer of various LED assemblies are summarized in the table below and compared with measurements from the integrating sphere.
The consistency of the results is very good considering that two different systems and calibrations were used. No systematic dependence is evident from the measurements.
This comparison also indicates the FWHM angle of the radiation pattern as well as the precisely measured electrical power for each LED assembly, and the luminous efficacy.
Goniospectroradiometry is a very powerful tool for characterizing light sources based on LEDs.
Some LED assemblies available on the market show pronounced dependence of colorimetric parameters in the spatial distribution. For example, even minor variations in CRI can be easily resolved with a goniospectroradiometer.
The total luminous flux computed by numeric integration of the goniometric scan results has very good agreement with measurements made with an integrating sphere.
Hence, when the luminous flux is the main interest and averaged colorimetric quantities are sufficient, an integrating sphere will give fast, accurate results. A goniospectroradiometer is the instrument of choice for complete characterization of the spatial distribution of all important colorimetric and photometric properties.
Denan Konjhodzic is an applications engineer at Instrument Systems (Germany). He studied physics at the Universität Duisburg-Essen and has a PhD in physical chemistry from Freie Univesität Berlin for his work at the Max Planck Institut für Kohlenforschung in Mülheim an der Ruhr (Germany).
Related articles:
Optical metrology is supporting innovation - and profits - in the field of solid-state lighting.
Andrew Williams, CEO of Halma (UK), a major supplier of optical sensing systems, reported that profits for the company in the 12 months preceding 2 April 2011 grew by 21% to £104.6 million. Revenues increased by 13% during the same time period.
"Photonics has the second highest sub-sector revenue in Halma," Williams reported, "and benefitted from strong growth in Asia and the fast growing market for LED and low-energy lighting measurement."
Similarly, Instrument Systems closed its fiscal year in June with record results. Company founder and president Richard Distl said key drivers for a 112% growth in sales were the sustained boom in the LED industry, display metrology, and applications in general lighting.
SPIE presents numerous conferences throughout the year on optical metrology. The SPIE Optical Metrology symposium will be held 17-20 June 2013, in Munich, alongside World of Photonics Laser Munich.
Have a question or comment about this article? Write to us at spieprofessional@spie.org.
