Light conversion (or wavelength conversion) materials such as phosphors are used in a variety of applications, especially in optical devices. One such application is a phosphor wheel, which is an optical device for generating emission light of one or typically multiple different wavelengths from excitation light of a single light source (typically of a narrow range of wavelengths). An example phosphor wheel is described in WO-2014/016574, having common inventorship.
An example, known phosphor wheel structure is shown in FIG. 1. A light converter 101 is provided on a disc substrate 102. Excitation light 103a (source light) causes the generation of emission light 103b, when it is incident on the light converter 101. The light converter 101 converts the light spectrum from excitation light of a first range of spectral wavelength to emission (or re-emission) light of a second, different range of spectral wavelengths Typically, the disc substrate 102 is rotated during use, although this device can be used in a static (non-rotating) configuration, in which case it may not be known as a phosphor wheel. The light converter 101 is conventionally formed as a coating, comprising phosphor particles in a polymer binder (such as a silicone).
Optical devices, especially phosphor wheels, that can cope with higher power incident light are being developed, for example for use in high-power laser projectors where the laser power may be greater than 50 W/mm2. To cope with this high power light, a solid-state light conversion material is being considered, such as a ceramic converter. In addition to wavelength conversion, the ceramic converter can be coated (normally directly) with thin-films for improved optical performance. This is a significant advantage over the previous light converters made of a phosphor in a polymer binder.
However, there are challenges in using such ceramic materials. Firstly, it is desirable to optimise the optical coating so as to enhance the transmission of the excitation light and/or emission light. Anti-reflective coatings are typically used dealing with excitation light, whereas high-reflective coatings can improve emission light output. The ceramic material should be able to handle both, as well as other types of coating, such as metallic coating, which may be used to create a reflective surface. Also, the thermal dissipation of the ceramic material should be considered, to avoid over-heating. Adapting the ceramic coating to all of these issues, so as to enhance the performance of the ceramic light converter, is a significant concern.