LEAF's LED headlamps (1): Precision light distribution, efficient use of white LED light source The white LED is purchased from Philips Lumileds Lighting. City Light Industry has been developing LED headlamps with Nissan and Philips Lumen since 2005. The LED headlamps use Philips' white LEDs and are equipped on Nissan vehicles, which is one of the joint development results. In addition, Shiguang Industrial has been developing LED headlamps before joint development, and has exhibited LED headlamps using more than 20 LED chips in exhibitions and other occasions. 
Figure 1: White LED
The white LEDs are multi-chip type, and four 1mm square white LEDs are arranged in a row in a package (Figure 1). By using 4 chips, sufficient brightness is available for the headlamps.
According to Shiguang Industrial, in the development of white LEDs, in order to easily equip the headlights, the company put forward a lot of requirements for Philips Lumens in terms of package shape and power connector. The requirements even relate to the mounting position accuracy of the white LED chip within the package. The detailed values ​​are not disclosed, and it is said that the reason is that the mounting position of the white LED chip is slightly deviated, which affects the optical system based on the reflecting surface.
Figure 2: "Thin Film Flip Chip" used by Philips Lumen on white LEDs
The white LED chip is formed by attaching a phosphor to the blue LED chip. The blue LED chip was flip-chip mounted on a ceramic substrate using Philips Lumen's technology called "Thin Film Flip Chip" (TFFC), and then a "Lumiramic" fluorescent plate was attached thereon (Fig. 2). The plane containing the four white LED chips was sealed with silicone resin. The sealing portion is a planar shape rather than a lens shape that is common in white LEDs.
When the LED headlights are lit, approximately 10W of power is input to each white LED. When the white LED is lit, the luminous flux is 550 lm, so the luminous efficiency at the time of lighting is about 55 lm/W. Among white LEDs, varieties with luminous efficiencies claiming to exceed 100 lm/W have increased in recent years. Compared with these varieties, the white LED of the LED headlamps appears to be somewhat low in luminous efficiency, but this view is actually wrong. Those luminous efficiencies exceeding 100 lm/W actually refer to the values ​​when the junction temperature of the LED chip approaches the room temperature in the pulsed state. The white LED used by LEAF, its performance is the value in the actual use environment. The junction temperature in the actual use environment is much higher than room temperature. The higher the junction temperature, the lower the luminous efficiency. Therefore, if the junction temperature is tested at room temperature, the luminous efficiency is actually much higher than at 55 lm/W.
City Light Industry has not announced the luminous efficiency when the junction temperature is close to room temperature. In general, the luminous efficiency in the actual use environment is usually several tens of percent lower than when the junction temperature is close to room temperature.
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