Automotive Lighting: Laser headlight module uses stable, easy-to-manufacture glass phosphor
A glass-based phosphor converter layer for automotive headlights is much more stable than silicone, less expensive than ceramic.
The future is looking good for semiconductor-based vehicle headlights. Commercially, light-emitting-diodes (LEDs) are already displacing traditional incandescent automotive headlights and intermediate-generation high-intensity-discharge (HID) light sources, working their way from the luxury market on down. And solid-state lighting’s second wave, laser diodes, is set to enter the commercial automotive market in Europe late this year. (Shuji Nakamura, one of the inventors of the blue LED that has made solid-state lighting possible, has formed a company, SLD Laser, to develop laser-diode-based white-light sources for automotive and specialty lighting; in 2018, the company received a Laser Focus World Gold Innovators Award for its LaserLight SMD high-luminance white laser light emitter packaged in a 7 mm surface-mount device.)
A laser headlight populates the visible spectrum with both the blue light from the laser itself and broadband yellowish light from a phosphor converter layer excited by the laser light. As with white LEDs, the laser diode’s blue emission combines with the yellow phosphor emission to create white light. While regulatory hurdles still exist for laser headlights, especially in the U.S., applied R&D efforts around the world are ironing out technological issues, including those related to cost and manufacturability. In one example, a group of researchers from National Chun Hsing University and Taiwan Color Optics (both in Taichung, Taiwan) has been developing phosphors that combine high stability and lower manufacturing costs.
The phosphor (usually powdered cerium-doped YAG, or Ce:YAG) that forms part of a solid-state light source is embedded in a stable and transparent matrix. For white-light LEDs, that matrix has often been silicone, which is low in cost and can be mixed with phosphor and molded into its final form at temperatures no higher than about 150°C. While this works well for LEDs especially as indoor light sources, silicone is not thermally stable enough to withstand the temperature extremes experienced by cars and trucks, say the Taichung researchers. As a result, laser headlights have mostly been fabricated using more stable, but more difficult to manufacture materials such as ceramics, and even single-crystal Ce:YAG. The minimum temperatures required to form the phosphor converters into bulk shapes are high: 1200°C for ceramics and 1500°C for single-crystal YAG.
Glass-based phosphor is highly reliable
The Taichung group has developed a laser headlight module based on a highly reliable glass-based phosphor that allows fabrication of the converter layer into its final shape at a lower temperature of 750°C. The researchers took extra care to reduce the formation of bubbles in the glass, which has been a problem with many glass-based phosphors. The end result was a module consisting of a five-unit linear blue laser array, a dichroic filter to pass the blue light and reflect the yellow phosphor light outward, the glass phosphor converter layer, and an aspheric collimation lens (see figure).
A glass phosphor converter layer for a laser headlight (a) has high thermal stability and can be made at a relatively low temperature of 750°C. In comparison to a silicone-based phosphor converter layer, the glass-based device, when subjected to thermal aging tests, shows much lower lumen loss (b), much less color shift (c), and a much smaller drop in quantum efficiency (d).
The precursor material is created by mixing Ce:YAG powder into melted sodium glass at 1200°C, which is then solidified and ground into a powder. The actual manufacture of the phosphor converter layer consists of sintering the powder at 750°C for one hour, followed by annealing at 350°C for three hours, then cooling to room temperature. The researchers experimented with different concentrations of the Ce:YAG powder, finding that the highest luminous efficiency was reached with a 40% weight concentration. The bulk shape was then cut into disks 25 mm in diameter and 0.35 mm thick, which, when in use in a headlight module, have an external quantum efficiency (EQE) of about 70%.
The thermal stability of the glass-based phosphor layer was compared to that of a silicone-based phosphor layer by subjecting them both to thermal aging tests in which they were both baked at temperatures of 150°, 250°, 350°, and 450°C for 42 days. Results showed a drastic lowering of lumen loss, color shift, and quantum efficiency for the glass-based device.
The resulting laser headlight module built by the researchers had an optical output of 6 W, a luminous flux of 1860 lm, a color temperature of 4100 K, and an extremely high efficiency of more than 310 lm/W (on the order of twice as high as the most efficient white-light LEDs on the market).
1. Y.-P. Chang et al., Opt. Express (2019); https://doi.org/10.1364/oe.27.001808.