Abstract
A photoluminescence spectrum depends on an emission angle due to self-absorption in a phosphor material. Assuming isotropic initial emission and Lambert-Beer’s law, we have derived simple expressions for the angle-resolved spectra emerging from the top and bottom surfaces of a uniform phosphor layer. The transmittance of an excitation light through the phosphor layer can be regarded as a design parameter. For a strongly-absorbing phosphor layer, the forward flux is less intense and more red-shifted than the backward flux. The red-shift is enhanced as the emission direction deviates away from the plane normal. When we increase the transmittance, the backward flux decreases monotonically. The forward flux peaks at a certain transmittance value. The two fluxes become similar to each other for a weakly-absorbing phosphor layer. We have observed these behaviors in experiment. In a practical application, self-absorption decreases the efficiency of conversion and results in angle-dependent variations in chromaticity coordinates. A patterned phosphor layer with a secondary optical element such as a remote reflector alleviates these problems.
Highlights
A laser-pumped phosphor has been investigated for lighting applications such as indoor/outdoor luminaires,[1,2,3] automotive headlamps,[4,5] and spotlight illuminators.[6]
We have proposed a backlight for a liquid crystal display[11] and an energy-harvesting laser phosphor display (LPD).[12,13]
It has been reported that combination of de-speckling means, such as the use of multiple laser diode (LD) emitting independently and a remote phosphor introducing multiple scattering events, reduces speckle contrast to the level of light emitting diodes (LEDs).[1,2,4]
Summary
A laser-pumped phosphor has been investigated for lighting applications such as indoor/outdoor luminaires,[1,2,3] automotive headlamps,[4,5] and spotlight illuminators.[6]. The phosphor material surrounding the emission point can absorb these fluxes Some PL photons are trapped in a planar structure by total internal reflection (TIR) at its interface with the air This problem has been recognized for LEDs since the early days of development and various measures such as semiconductor shaping and surface texturing have been investigated.[20]. It has been recognized that self-absorption limits its performance from early on.[22,23] Efforts for optimizing its design and performance continue.[24] Expertise in this field might be applicable for phosphor-based light sources. The idea of patterning a phosphor layer has been studied for reducing self-absorption.[35] Waveguiding is important in a different context: an LSC utilizes the PL photons trapped inside.
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