PhotoLED project (2016-2019)

Metrology Research Institute is involved in European Metrology Programme for Innovation and Research (EMPIR) Project PhotoLED - "Future Photometry Based on Solid-State Lighting Products".

Background

Traditional photometric system relies on incandescent standard lamps, which are quickly becoming obsolete. The aim of the project is to develop new LED-based standard sources and compatible measurement methods that increase the accuracy of the energy efficiency measurements of new LED products coming to market.

The key activities of Metrology Research Institute in this project are listed below.

Development of LED-based calibration spectra and sources

The development of white-LED-based calibration spectra for photometers [1] aims to reduce spectral mismatch error arising from the differences in spectra between the incandescent standard lamps and LEDs. In addition to addressing the increased measurement uncertainty due to spectral mismatch between the sources, this task will also prevent the shortage of calibration sources as the manufacturers will likely phase out all products based on incandescent technologies.

After the calibration spectra have been developed, LED-based calibration sources will be developed for different photometric applications. Both AC- and DC-operated sources will be built to ensure the compatibility with existing instrumentation in test laboratories.

Contact persons: Alexander Kokka, Tomi Pulli and Timo Dönsberg.

Fisheye camera method for improving measurement accuracy in integrating spheres

The total light output of lamps – their luminous flux – is often measured using an integrating sphere, which is a hollow sphere whose inner surface is coated in matte white paint. Ideally, the signal measured with a photometer attached to the sphere would be directly proportional to the luminous flux of the lamp inside the sphere. Unfortunately, the coatings of the spheres are never totally uniform, and to decrease the uncertainty of luminous flux measurement, this spatial nonuniformity needs to be corrected. The correction requires knowledge of both the spatial responsivity of the sphere and the angular light distribution of the lamp.

Fisheye camera for integrating sphere
Fisheye-lens camera for spatial non-uniformity corrections.

Traditionally, the angular distribution of light sources has been measured with goniophotometers, which are large, expensive and slow instruments. An early outcome of Metrology Research Institute in the PhotoLED project is the fisheye camera method developed as a convenient way for applying spatial nonuniformity corrections in luminous flux measurements with integrating spheres. This method relies on taking a fisheye photo of the inner surface of the sphere and extracting the angular light distribution of the lamp from the photo using image processing. The method and comparison results have been published in [2].

Contact person: Alexander Kokka

The system is commercially available:

https://hehkulab.fi/

Processed fisheye camera image
Processed fisheye image of the inner surface of the integrating sphere, as it is illuminated by a lamp under test. The lamp itself is covered by the circular baffle in the center of the image.

New optical method to determine aperture area

In various photometric and radiometric applications, a detector is used together with a precision aperture. In some cases, the area of the aperture is one of the most critical factors affecting the measurement uncertainty. Several methods for determining the area of an aperture have been developed, which utilize either mechanical contact or optical techniques. Recently, an upgrade to an previously developed optical method for direct determination of aperture area was introduced. In the existing method [3], superposition of equally spaced Gaussian laser beams is used to form a uniform irradiance distribution. In practice, this is accomplished by scanning the aperture in front of an intensity-stabilized laser beam. In the upgraded method [4], the aperture is attached to the detector and the whole package is transversely scanned relative to the laser beam. Main advantage of the method is that it does not require removing the aperture from the detector. Such dismantling and separate measurement produces a significant risk of either dust contamination of the detector or damaging of the sharp aperture edge. Furthermore, the uncertainty due to aperture alignment is decreased as the geometry is identical in both the laser scanning of the aperture area and in the actual measurement.

Contact person: Timo Dönsberg

 

More information about the project can be found at the project website:

http://photoled.aalto.fi/

References

[1] 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, 526–534 (2018).

[2] A. Kokka, T. Pulli, T. Poikonen, J. Askola, and Erkki Ikonen, "Fisheye camera method for spatial non-uniformity corrections in luminous flux measurements with integrating spheres," Metrologia 54, 577–583 (2017).

[3] A. Lassila, P. Toivanen, and E. Ikonen, E, "An optical method for direct determination of the radiometric aperture area at high accuracy," Meas. Sci. Technol. 8, 973–977 (1997)

[4] T. Dönsberg, H. Mäntynen, and E. Ikonen, ”Optical aperture area determination for accurate illuminance and luminous efficacy measurements of LED lamps,” Opt. Rev. 23 510–521 (2016)

 

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