Gilles Vissenberg

Illumination optics of LED luminaires

Illumination optics is a multidisciplinary effort: a good optical engineer should understand the language of physics, mathematics, computer science, materials science, process engineering, cost engineering, psychology, visual perception, marketing, design, lighting standards and regulations. The transition to LED lighting changed the rules of optical design in most of these aspects. The absence of IR and UV light enabled the use of plastic optical components, which allows for a huge variation in optical solutions. This in turn completely changed the appearance of the luminaires and allowed for more precisely trimmed intensity distributions. When these new luminaire types entered the market, people started to question whether the lighting standards, which were defined in the days of fluorescent tubes and incandescent lamps, were still valid for LED-based luminaires. These changes will be illustrated with optical designs of office lighting luminaires. The impact of LED-based luminaire designs on discomfort glare perception and glare standards will be addressed.

Accurate determination of the transport parameters of light in white light emitting diodes

The energy-efficient generation of white light has recently become an important societal issue. The technology of white-light emitting diodes (LEDs) is one of the most promising solutions to efficiently generate white light for home, office, and street locations, and even for remote locations without electric power grid [1]. One of the outstanding in the development of LED technology are understanding the scattering [2], the absorption and emission with analytical physical models [3, 4]. As a first example of the power of the physical understanding of multiple light scattering in LEDs, we describe straightforward tools to extract essential optical parameters.

Analytical modeling of light transport in scattering materials with strong absorption

We have investigated the transport of light through slabs that both scatter and strongly absorb, a situation that occurs in diverse application fields ranging from biomedical optics, powder technology, to solid-state lighting. In particular, we study the transport of light in the visible wavelength range between 420 and 700 nm through silicone plates filled with YAG:Ce3+ phosphor particles, that even re-emit absorbed light at different wavelengths. We measure the total transmission, the total reflection, and the ballistic transmission of light through these plates. We obtain average single particle properties namely the scattering cross-section σs, the absorption cross-section σa, and the anisotropy factor µ using an analytical approach, namely the P3 approximation to the radiative transfer equation. We verify the extracted transport parameters using Monte-Carlo simulations of the light transport. Our approach fully describes the light propagation in phosphor diffuser plates that are used in white LEDs and that reveal a strong absorption (L/la > 1) up to L/la = 4, where L is the slab thickness, la is the absorption mean free path. In contrast, the widely used diffusion theory fails to describe this parameter range. Our approach is a suitable analytical tool for industry, since it provides a fast yet accurate determination of key transport parameters, and since it introduces predictive power into the design process of white light emitting diodes.