How to distinguish elastically scattered light from Stokes shifted light for solid-state lighting?

Abstract

We have studied the transport of light through phosphor diffuser plates that are used in commercial solid-state lighting modules (Fortimo). These polymer plates contain YAG:Ce þ3 phosphor particles that both elastically scatter and Stokes shift light in the visible wavelength range (400-700 nm). We excite the phosphor with a narrowband light source and measure spectra of the outgoing light. The Stokes shifted light is spectrally separated from the elastically scattered light in the measured spectra, and using this technique, we isolate the elastic transmission of the plates. This result allows us to extract the transport mean free path l tr over the full wavelength range by employing diffusion theory. Simultaneously, we determine the absorption mean free path l abs in the wavelength range 400 to 530 nm where YAG:Ce þ3 absorbs. The diffuse absorptionspectrum is qualitatively similar to the absorption coefficient of YAG:Ce þ3 in powder, with the diffuse spectrum being wider than the absorption coefficient. We propose a design rule for the solid-state lighting diffuser plates. V C 2016 AIP Publishing LLC. [http://dx

Analytical modeling of light transport in scattering materials with strong absorption

“…The absorption cross section tends to zero at the edges of the absorption spectral range (λ i =420 nm and 520 nm) in agreement with Ref. [45].…”

“…The transmission and the reflection spectra reveal a deep trough with a minimum at λ i = 458 nm. The trough matches with the peak of the absorption band of YAG:Ce 3+ [45]. The presence of the trough indicates that a significant fraction of the light in this wavelength range is absorbed by the phosphor.…”

“…The absorption spectrum of the phosphor particles has a peak absorption wavelength λ = 458 nm, and a broad linewidth (full width half maximum, FWHM) of ∆λ a = 55 nm, as reported earlier in Ref. [45]. Based on the absorption spectrum we choose the wavelength λ c = 520 nm to distinguish the absorbing range (λ = 400 nm to 520 nm) from the non-absorbing range (λ = 520 nm to 700 nm).…”

“…The presence of the trough indicates that a significant fraction of the light in this wavelength range is absorbed by the phosphor. In the range 490 nm < λ i < 520 nm where absorption and emission of phosphor overlap [45], the spectra show a steep rise in the transmission due to the reduced absorption. At wavelengths longer than λ i = 520 nm, the transmission spectra are flat.…”

Mapping the energy density of shaped waves in scattering media onto a complete set of diffusion modes

“…In the case of light, the knowledge of the energy density distribution is important for applications such as enhanced energy conversion in white LEDs [32][33][34][35][36], efficient light harvesting in solar cells [37][38][39], and controlled illumination in biomedical imaging [40]. To date, only numerical calculations of scalar waves [41][42][43] and a single-realization elastic wave experiment [44] have addressed the energy density distribution of shaped waves inside twodimensional (2D) scattering media.…”