Energy efficiency and colour quality improvement and subjective preference of LED office lighting

Rupak Raj Baniya

Research output: ThesisDoctoral ThesisCollection of Articles


The objectives of this thesis were to explore different approaches to improve energy efficiency and colour quality, and to find out the subjective preference of LED office lighting. Four approaches (optimization of LED spectra, lighting retrofitting, hue-heat hypothesis, and smart indoor lighting control) were studied for energy efficiency and colour quality improvement. Two experiments, one in lighting booths and the other in an office room, were conducted to examine subjective preferences. In the optimization of LED spectra, simulation work and subjective preference studies conducted in lighting booths suggested that preferred complex LED spectra (realized with 9 to 11 different types of LEDs) found in previous user acceptance studies can be optimized both for efficiency and cost without sacrificing the colour quality of the light. The lighting retrofitting case study showed that by replacing T8 fluorescent luminaires with LED luminaries, and the LED luminaries with inbuilt control systems, energy savings of up to 38% and 68% respectively, can be achieved with improvement of lighting quality and user satisfaction. The experiment conducted to test the hue-heat hypothesis using different correlated colour temperatures (CCTs) of white light did not provide support for the existence of a general correlation between CCT and thermal sensation or thermal comfort and indicated that people felt thermally more comfortable in an indoor workplace at the CCT of 4000 K than at the CCT of 2700 K or 6200 K. A methodology to incorporate data from lighting studies relating colour quality to comfort and productivity to the building automation system has been proposed and demonstrated. The subjective preferences in terms of illuminance and CCT and the CCT preference of three ethnic groups (Asian, European, and African) for LED office lighting were investigated in an office room. Nine different lighting situations (combining 300 lx, 500 lx, and 750 lx with 3000 K, 4000 K, and 5000 K) were compared. The combination of 750 lx with 4000 K was statistically significantly preferred over the other combinations for LED office lighting. It was also found that the CCT of 4000 K was preferred over the CCT of 3000 K for office lighting at 300 lx, 500 lx, and 750 lx. The results indicated that the impression of brightness, the feeling of stimulation, and the impression of spaciousness in office room increases with increased CCT. The study also showed that for office lighting the European group prefer the CCT of 4000 K, and with the Asian and African groups the preference between 4000 K and 5000 K depends upon illuminance levels.
Translated title of the contributionEnergy efficiency and colour quality improvement and subjective preference of LED office lighting
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
  • Halonen, Liisa, Supervising Professor
  • Tetri, Eino, Thesis Advisor
  • Bhusal, Pramod, Thesis Advisor
Print ISBNs978-952-60-7062-9
Electronic ISBNs978-952-60-7061-2
Publication statusPublished - 2016
MoE publication typeG5 Doctoral dissertation (article)


  • light emitting diode
  • energy efficiency
  • colour quality
  • lighting preference
  • correlated colour temperature
  • office lighting


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