Investigation of Saturation Effects in Ceramic Phosphors for Laser Lighting

Abstract

We report observations of saturation effects in a Ce:LuAG and Eu-doped nitride ceramic phosphor for conversion of blue laser light for white light generation. The luminous flux from the phosphors material increases linearly with the input power until saturation effects limit the conversion. It is shown that the temperature of the phosphor layer influences the saturation power level and the conversion efficiency. It is also shown that the correlated color temperature (CCT), phosphor conversion efficiency and color rendering index (CRI) are dependent both on the incident power and spot size diameter of the illumination. A phosphor conversion efficiency up to 140.8 lm/W with CRI of 89.4 was achieved. The saturation in a ceramic phosphor, when illuminated by high intensity laser diodes, is estimated to play the main role in limiting the available luminance from laser-based lighting systems.

Saturation Mechanisms in Common LED Phosphors

“…Unfortunately, even the best performing phosphors suffer from thermal quenching at high temperatures and “droop” or “saturation quenching” at high light intensities. These unwanted effects are often already observed at LED operating conditions. ,,,− CASN:Eu 2+ , for example, shows a decrease in light output of about 15% when used in high-power LEDs (incident intensity approximately 1 W mm –2 ) . For applications operating at even higher powers, such as automotive head lights and projector devices (incident intensity up to 5–10 W mm –2 ), the quenching issues become even more severe. , Despite rigorous optimization efforts mainly focused on controlling the phosphor temperature, only Ce-doped garnets (e.g., YAG:Ce 3+ ) are known to have acceptable performance in such high-power applications, while no red-emitting material is available. ,,,− These issues have until now limited the commercial success of phosphor-converted solid-state lighting for ultrahigh-power applications, despite impressive improvements in LED and device design. , …”

Design of a Novel La3Si6N11:Ce3+ Phosphor-in-Glass Film for High Power Laser Lighting: Luminous Efficiency toward 200 lmW–1

“…With the increase of the LSN PiF concentration, the higher the phosphor content in the film, the lower conversion efficiency of absorbing blue light will be seen, so the heat accumulated on the surface of the film increases. The accumulated heat will lead to the “escape” effect, and the thermal conductivity and transmittance of the fluorescent glass film will decrease; so the saturation threshold will decrease too . The saturation threshold depends largely on the thickness of the film.…”

Thermally Robust Orange‐Red‐Emitting Color Converters for Laser‐Driven Warm White Light with High Overall Optical Properties

“…The energy losses due to low quantum efficiency will increase the temperature of phosphor converters, yielding a so‐called “thermal‐run‐away effect.” [ 14,15 ] Generally, thermal quenching becomes much more serious as the average temperature of phosphor converters exceeds 200 ℃. [ 16,17 ] Optically induced luminance saturation mainly depends on the fluorescence decay time of phosphors, [ 18,19 ] and a shorter decay time is thus preferred. For instance, the Mn 4+ doped K 2 SiF 6 with a decay time of ≈8 ms only sustains a maximum blue laser power density of 0.23 W mm −2 , [ 20 ] the Eu 2+ doped CaAlSiN 3 having a decay time of ≈0.7 μs bears a maximum of 1.5 W mm −2 , [ 13 ] and the Ce 3+ doped La 3 Si 6 N 11 with a decay time of 41.5 ns withstands a blue laser power density as high as 12.91 W mm −2 .…”