LEAF LED headlamps (1): precision light distribution, efficient use of white LED light

Nissan Motor Co., Ltd. launched the electric vehicle (EV) "LEAF" in December 2010. The car will initially produce 50,000 units and the annual output of 200,000 units in 2012 will be mass-produced in an unprecedented scale in EV. This model has a lot of attention, such as guiding the location of the charging facility through linkage with the communication function navigator, and thoroughly managing the lithium ion rechargeable battery for extended battery life. Among them, the standard LED headlamps are also receiving attention.

It is Japan City Light Industry that is responsible for developing LED headlamps for LEAF. In order to achieve "the world's highest power-saving performance" at low cost, Shiguang Industrial has made efforts to suppress the luminescence loss of white LEDs and obtain the desired light distribution characteristics, and also adopted white LEDs. Efficient heat dissipation to ensure high luminous efficiency.

The LED headlamps of LEAF use white LEDs on the low beam light source, and the power consumption of the low beam lights is 23W for the entire headlights. This value is only about 40% of the 55-60W class halogen lamp and about 50% of the 43-45W class HID lamp. Even with the existing LED models of the Toyota Hybrid Prius, the LED headlamps (about 35W when the low beam is on, the value when the new car is released) are less power-hungry.

Use only 2 white LEDs as the light source

In the development of the LED headlamps of the city's light industry, Murakami Katsuhiro (head of the P2P3 1st project team of the Research and Development Department of the City Light Industry Development Division) emphasizes that each headlight of the LEAF uses only two white LEDs. . The price of LEAF is 3.76 million 4,250 yen (including tax), and when the government subsidies are used, it is 2.98 million yen (including tax), and the price of EV is kept at a low level. However, the car is equipped with LED headlights at this popular price. Therefore, it is necessary to control the component cost of the LED headlight to a low level as compared with when it is equipped as an option or on a high-end model. For this reason, it is necessary to "use only two places where three white LEDs are used by other manufacturers" (Shiko Kogyo Co., Ltd.).

However, if you simply reduce the three LEDs to two, the brightness of the headlights will be insufficient. Although the white LED characteristics are continuously improved, it is possible to realize the same brightness as three by two, but the "white brightness of the existing white LED has not yet reached a sufficiently bright level" (village bridge). In general, sufficient brightness can be obtained by increasing the input power to the white LED, but the amount of heat generation tends to increase, causing the temperature of the white LED to rise, further causing the luminous efficiency of the LED to decrease. Moreover, if the power consumption is increased, the charm of the "low power consumption" of the LED headlights will be weakened. Therefore, in order to use the light of the white LED with low loss, the City Light Industry adopts a sophisticated light distribution control technology, and performs a heat dissipation design that can maintain a high luminous efficiency of the white LED.

Realizing the desired light distribution with three reflecting surfaces. In this optical system for light distribution control, a mirror type (reflective type) method is adopted. This method achieves the desired light distribution by reflecting the light emitted by the light source (white LED) with a mirror. This LED headlamp uses a three-reflection surface of a mirror (elliptical mirror), a lampshade, and a secondary mirror (parabolic mirror) for light distribution (Fig. 1).

LED headlights. Each LED headlamp uses 2 white LEDs.

The primary mirror is placed near the vertical direction of the white LED, redirecting the light path of the white LED to the secondary mirror surface. The lampshade has a function of adjusting the shape of the beam emitted by the headlights (occlusion function). Although this part uses the name of the lampshade, in order not to waste the light blocked when adjusting the shape of the beam, a reflection function is added to the lampshade. Thus, those rays that do not travel in the desired direction are turned to the secondary mirror for reflection. The secondary mirror reflects the light reflected from the mirror and the lampshade and some of the light directly from the white LED, and finally radiates as the headlight. At this time, the light of the white LED is reflected by the reflecting surface 1 to 3 times, and then emitted to the outside of the headlight. In the reflective surface, the shape of the secondary mirror is the most complicated. It consists of a plurality of reflecting surfaces, and instead of simply projecting light, it is possible to mix light to a necessary part (Fig. 2).

LED optical system

In the optical system of the LED headlamp, in addition to the mirror type used this time, there is a spotlight type product using a lens. In the past, LED headlamps were mainly used in the spotlight type. According to Shiguang Industrial, although the spotlight type has the advantage of easily creating a light-dark interface by occlusion, it also has a long depth direction of the optical system, which is disadvantageous for the precision light distribution control. Since LEAF's LED headlamps are mirror-type, they can shorten the depth of the optical system. Further, by performing precise control by combining the reflecting surfaces having different functions described above, it is also possible to efficiently produce a light-dark interface. In addition, the design of the headlights is also very characteristic, and the white LED light source cannot be directly seen from the outside.

Secondary mirror consists of multiple reflective surfaces

The idea of mirror-type LED headlamps was born in the process of cooperation between City Light Industry and Valeo. The development of the mirror type was originally carried out in Valeo, but it has not been put into practical use. Later, through the cooperation between City Light Industry and Valeo, the mirror-type LED headlamps eventually became "practical products" (village bridge). Valeo also promoted the development of spotlights and mirrors. At that time, spotlight LED headlamps appeared on the market, and there was no practical precedent for the mirror type. In order to highlight "innovation as a selling point" (Village Bridge), Shiguang Industrial chose to develop mirror-type LED headlamps.

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