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

TL;DR

This study develops a spectral separation method to distinguish elastic scattering from Stokes shifted light in phosphor plates, enabling precise measurement of light transport properties for improved solid-state lighting design.

Contribution

The paper introduces a technique to isolate elastic scattering in phosphor plates, allowing accurate determination of transport and absorption mean free paths across visible wavelengths.

Findings

Successfully separated elastic and Stokes shifted light spectra.

Measured transport mean free path across the visible spectrum.

Provided a design rule for solid-state lighting diffuser plates.

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 $\mathrm{YAG:Ce}^{+3}$ phosphor particles that elastically scatter light and Stokes shifts it 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 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 $l_{\mathrm{tr}}$ over the full wavelength range by employing diffusion theory. Simultaneously, we determine the absorption mean free path $l_{\mathrm{abs}}$ in the wavelength range 400 to 530 nm where $\mathrm{YAG:Ce}^{+3}$ absorbs. The diffuse absorption $\mu_{\mathrm{a}} =\frac{1}{l_{\mathrm{abs}}}$ spectrum is qualitative similar to the absorption coefficient of $\mathrm{YAG:Ce}^{+3}$ in powder, with the $\mu_{\mathrm{a}}$ spectrum being wider than the absorption coefficient. We propose a design rule for the solid-state lighting diffuser plates.