Department of Signal Processing and Acoustics

PhotoLED project (2016-2019)

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


Traditional photometric system relies on incandescent standard lamps, which are quickly becoming obsolete. The aim of the project was 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.

An LED luminous flux standard lamp with E27 base that can serve as a direct replacement of incandescent standard lamps used at calibration laboratories was developed. The spectral power distribution of the lamp was optimized for use in LED lighting applications. The standard lamp is driven with constant voltage and it features an internal temperature controller, which keeps the LED junction temperatures constant. The design can easily be modified to drive LEDs with different electrical and optical parameters. In typical laboratory environment, the controller reduces the temperature dependence of the lamp by more than an order of magnitude. The design effectively keeps electrical power and temperature of the LEDs constant, and, thus, guarantees stable luminous flux and spectral power distribution.

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

Suggested white LED standard spectrum
Suggested white LED spectrum of 4000 K for the standard, standardized sensitivity of the human eye, and CIE standard Illuminant A. [1]

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]. The method was also validated in different integrating spheres of the project partners within the project [3]. 

Contact person: Alexander Kokka

The system is commercially available:

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 [4], 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 [5], 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

Response change of trap detectors with apertures due to nitrogen flow

During the project, measurements and simulations in Aalto were made whether a nitrogen flow used to prevent dust and moisture entering a detector might influence measurements performed with trap detectors in overfilled conditions. Based on preliminary results, the nitrogen flow through the detector forms a nitrogen beam that might act as a waveguide increasing the effective aperture area of the detector and thus the photocurrent. There is also a correlation between the nitrogen flow volume, the measured photocurrent and the standard deviation of the measurements. The resulting shape of the nitrogen beam depends on the size of the limiting aperture of the detector. On the basis of the preliminary results, the nitrogen flow utilized in measurements setups with light overfilling the entrance aperture should be thought carefully.

Contact person: Janne Askola

Simulated nitrogen flow out of the detector aperture
Simulated nitrogen flow with a 4-mm (upper) and a 10-mm (lower) entrance aperture.

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


[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. Kokka, T. Pulli, A. Ferrero, P. Dekker, A. Thorseth, P. Kliment, A. Klej, T. Gerloff, K. Ludwig, T. Poikonen, and E. Ikonen, "Validation of the fisheye camera method for spatial non-uniformity corrections in luminous flux measurements with integrating spheres," Metrologia 56, 045002 (2019).

[4] 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)

[5] 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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