Wai Qian Tham

Wai Qian Tham

Ph.D Student

Project Title: Detailed modelling and characterisation of luminescent materials for daylighting purposes

Keywords: Daylighting, light guide, luminescent solar concentrator, circadian rhythm, sleep, fabrication, material optimisation.

Figure 1: Daylighting using active and passive daylighting methods and examples.

Circadian rhythm is the periodic behavioural changes throughout the day. Such effect is mainly regulated by light response, as the light would cause melatonin suppression resulting in alertness. For a healthy circadian rhythm, light response should happen in the daytime while it should be reduced at night, which matches with the natural daylight cycle. A disrupted circadian rhythm can lead to insomnia or lower quality of sleep. This can occur through lack of light for circadian regulation during the daytime or too much light at night through artificial lighting.

Daylighting refers to the practice of the controlled use of daylight in the indoor environment (Fig. 1). The trivial method of daylighting is through the installation of windows. However, with increasing building size, certain areas within buildings have limited to no daylight penetration. Therefore, certain daylighting systems are suggested to transport daylight deep into the building interior for illumination. One of such methods is by using luminescent solar concentrators (LSCs).

Figure 2: A comparison between the melanopic response (circadian rhythm) to the photopic response visual).

Photoluminescence is the spontaneous emission of light after absorbing certain wavelengths of light, based on their characteristic spectra. Using this property, luminescent materials can absorb a part of the solar spectrum and shifting it to a new wavelength through down-conversion or up-conversion. This meant the spectrum of light could be manipulated to the desired output.  Besides, LSCs can work as a waveguide to channel high intensity light deep into the building interior, through total internal reflection.

Most research has shown that circadian light response has a peak sensitivity of  (Fig. 2). With LSCs, the solar spectrum could be shifted to increase the amount of blue light to enhance the light response, increasing circadian regulation. The aim is to provide sufficient lighting for indoor activities, while also providing sufficient blue light for circadian regulation during the day. This could be beneficial for people stuck indoors and lacking sufficient daylight, especially during short daytime periods in winter where seasonal affective disorder is common. Besides, the lack of natural light at night would also mean the absence of light channelled through the waveguide, which naturally turns the system off at night in preparation for sleep.

Thus, the objective of the project is to determine optimal sets of materials for desired spectral shifting, with good daylighting purposes. The system should be able to efficiently transport daylight into indoor spaces, providing sufficient lighting for indoor activities while supplying sufficient light for circadian regulation. The project itself will look towards luminescent materials suitable for daylighting, testing the output spectrum and intensity.

Funding: Velux Stiftung

Supervisor: Professor Sarah McCormack