The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.

Characterization of Diffusers and Light-Emitting Diodes Using Radiometric Measurements and Mathematical Modeling

Pasi Manninen

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Faculty of Electronics, Communications and Automation for public examination and debate in Auditorium S4 at Helsinki University of Technology (Espoo, Finland) on the 21st of November, 2008, at 12 noon.

Overview in PDF format (ISBN 978-951-22-9556-2)   [2020 KB]
Dissertation is also available in print (ISBN 978-951-22-9555-5)


Entrance diffusers enable good cosinusoidal angular responsivities of optical detectors. However, they also affect the position of the apparent detector receiving plane (reference plane). In this thesis, the reference planes of four spectroradiometer diffusers have been investigated and found to be located, in most cases, inside the diffuser. The reference plane shift should be taken into account in the calibration of the spectroradiometer equipped with such a diffuser. If the reference plane shifts are omitted, the measurement results may become systematically erroneous. The reference plane distance offsets correlate with their angular responsivities. Thus, with available angular responsivity data of a diffuser, the need to measure accurately the reference plane offset of the diffuser can be evaluated. The results obtained can be utilized in solar ultraviolet irradiance measurements.

In the thesis, new kinds of measurement methods for characterizing photometric properties of light-emitting diodes (LEDs) have also been developed. The modified inverse-square law method has been developed for analyzing LED illuminance at varying distances. The method has been tested for several LEDs with different packages, colors and power levels. The method gives information about the LED luminous intensity, the LED directivity and the size and location of the LED virtual image source. The new method developed is useful in designing LED-based luminaires when photometric characteristics of LEDs become more reliably predicted.

Pulse-width modulation (PWM) is often used to dim LEDs. An LED is driven at pulsed current and by changing the duty cycle of the current the LED brightness can be controlled. The influence of the PWM on spectral, thermal, and colorimetric properties of low-power, epoxy-encapsulated LEDs have also been investigated. In the thesis, a measurement system, with which the LED temperature at different duty cycles could be determined, was constructed. Using the experimental set-up, the peak-wavelengths and bandwidths of LEDs under the PWM control were evaluated. A blueshift of the peak wavelength and bandwidth narrowing of the LEDs were found. The changes in the emission spectra of the LEDs were concluded to be caused by the changes in the LED temperature. Apparent color shifts, which can be perceived by the human eye, were observed in emitted colors of two LEDs. The results indicate that it would be useful to include information on optical and thermal properties of pulsed LEDs more in detail in the datasheets by the LED manufacturers.

This thesis consists of an overview and of the following 5 publications:

  1. Pasi Manninen, Jari Hovila, Lauri Seppälä, Petri Kärhä, Lasse Ylianttila, and Erkki Ikonen. 2006. Determination of distance offsets of diffusers for accurate radiometric measurements. Metrologia, volume 43, pages S120-S124. © 2006 Institute of Physics Publishing. By permission.
  2. Pasi Manninen, Petri Kärhä, and Erkki Ikonen. 2008. Determining the irradiance signal from an asymmetric source with directional detectors: application to calibrations of radiometers with diffusers. Applied Optics, volume 47, number 26, pages 4714-4722. © 2008 Optical Society of America (OSA). By permission.
  3. P. Manninen, J. Hovila, P. Kärhä, and E. Ikonen. 2007. Method for analysing luminous intensity of light-emitting diodes. Measurement Science and Technology, volume 18, pages 223-229. © 2007 Institute of Physics Publishing. By permission.
  4. Erkki Ikonen, Pasi Manninen, and Petri Kärhä. 2007. Modeling distance dependence of LED illuminance. Light & Engineering, volume 15, number 4, pages 57-61. © 2007 Znack Publishing House. By permission.
  5. Pasi Manninen and Pasi Orreveteläinen. 2007. On spectral and thermal behaviors of AlGaInP light-emitting diodes under pulse-width modulation. Applied Physics Letters, volume 91, 181121. © 2007 American Institute of Physics. By permission.

Keywords: irradiance, solar ultraviolet, diffuser, light-emitting diode, luminous intensity, pulse-width modulation

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© 2008 Helsinki University of Technology

Last update 2011-05-26