Luminescent light guides based on Dy-doped borate glass

Lithium aluminoborate glass optically activated with the lanthanide ion dysprosium is investigated for its potential as luminescent light guide. For this, ray-tracing simulations are performed on the basis of transmission, photoluminescence, and quantum efficiency measurements. The luminous flux at the end of the light guide depends significantly on its length as well as on the roughness of the output face. The best results are obtained for a light guide length of approximately 70-80 mm with the side faces of the light guide coated with a 100 % reflecting mirror and a rough output face with Lambertian scattering characteristic. The input face is coated with a half-transmitting mirror which is transmissive for the excitation wavelength of 388 nm but reflective for the emission bands in the visible spectral range. For this light guide, a luminance of approximately 20 cd/mm2 is achieved for an excitation power density of 1W/mm2. The geometry of the light guide (cuboid / cylinder) has only a slight effect on the maximum luminance value.