Application of LED light source in car lights

With the development of digitalization and intelligence, more and more functions of car lights, and the shape of the opening size is becoming smaller and smaller, have brought different challenges to led light sources. These new challenges drive the development of LED light sources into three dimensions: high brightness, miniaturization and integration. Let's introduce the development direction of these 3 LED light sources respectively.

Section 1 high brightness
In order to cope with the trend of thinner headlights, the size of LED light source also needs to be flat, because the smaller the opening size of headlights is bound to lead to more light loss, thus the optical efficiency of the whole system is lower. In order to ensure the same lighting performance of headlights, the size of the light source needs to be further flat and high brightness.

A linear correspondence between headlamp height and LED light source size. For example, to design a headlamp with 12mm lens height to achieve pixel height with optical expansion of 3°, the height size of LED chip must be controlled within 0.68mm, otherwise the efficiency of the entire optical system will be greatly reduced. 

Even if CCC / ECE / SAE allows near and near light lighting to only meet the minimum regulatory value, for most automakers, the requirements of the end user are considered. Generally speaking, the near light below 600lm is dark for the end user even if it meets the regulatory requirements.The LED near light that meets user requirements now requires lighting of at least 800 lm to 1000 lm of pavement.Assuming an optical efficiency of 40% to 55%, the optical flux of near-optical led sources should be at least 1500 lm to 2000 lm.Assuming that there are one or three near-light lens modules in the headlights, the light source light output of each LED lens module is that of 500 lm to 2,000 l m.

Therefore, through the simple mathematical calculation above, for high-power LED particles with a usual luminous area of ~1 mm², the luminance level should reach 150 Mcd / m² to 500 Mcd / m² to reach the light output of 500 lm and 2000 lm, respectively.The current mainstream LED process technology can achieve brightness levels of around 200 Mcd / m².To achieve higher performance levels, Led suppliers need to develop the next generation of high-brightness led chips.Figure 2 shows an example of a thin lens headlight assumed to be 12 mm high.It consists of two near beam lens modules and two high beam lens modules.If a single-core led of 0.68 mm height is used, when the pixel height of the optical extension is less than 2, the height value of the remote lens module exceeds its optical expansion limit according to the function of FIG. 1.Considering the optical loss of the optical parts and glass lenses, further optical simulations can calculate a module optical efficiency of 38%, while this efficiency is too low.

Section 3 integration also has new full digital headlight technology, such as road projection, higher resolution, even increased to tens of thousands of pixels, led process needs to micro level less than 50 μm x 50μm, and each led chip is independent of each other in optical control, led chip gap is very small.At this point, the circuit interconnection through CMOS integration is required, and the single led particles can be controlled independently, thus controlling the imaging patterns on the road through higher-level protocols.Combining these new digital headlight technology solutions raises the need for developing disruptive and innovative micro-led systems for both high-resolution and microoptical design.The ultimate application of functional integration is the higher-resolution digital headlights.If the resolution and size accuracy of the LED array are high enough, a digital headlight can produce a variety of light types.It includes high-resolution ADB, AFS function, accompanying steering associated with navigation or camera systems, adaptive adjustment of near-light lines, lane markings, obstacles and logo highlighting, etc.The requirements for micro-LED can all be derived from these applications.For the ADB system, at least a luminous angle of + horizontal direction / -12 is required.To create a ently obvious cut-off for the pattern, a minimum of 1:250 contrast is required.For the high-beam ADB partition, the goal is to have a small resolution, and a 0.085 bright spot at 50m is a rectangular bright spot 7.5 cm wide and 4 m long, so we can calculate the resolution required for the corresponding ADB function.For example, for lane projection, assuming a led array board with a 1:3 aspect ratio, we can directly calculate that it takes a minimum of 19,000 pixels to achieve a clear lane projection.A usual high beam requires a light of at least 1 lx illumination (equivalent to 65,000 c d) within 250 m.By converting the optical formula, the illumination = optical flux / stereo angle relationship, we can calculate the required optical flux per chip of 0.14 lm / core.Assuming a ~ 33% optical efficiency of the ADB system, each LED chip is required to provide a ~ ~ 0.43 lm / core.If a reasonable light source luminance of 90 Mcd / m² is used, we can estimate the single-family luminescence area of the micro-led array as 40 μm x 40μm.There are many technical difficulties if a led array architecture is required to meet the needs of the market ADB.First, to create a micro-LED array of about 20,000 dense pixels, it is clear that the micro-led array must be assembled from single blocks or several large subblocks, and that each micro-led is a chip partition.The addressing of each chip must be done by a CMOS drive.

One major challenge for the integration of led chips and CMOS is contrast and overall efficiency.When a single micro-led chip measures 40 m, there is little space between each chip to isolate the string of light.If the spacing between the Micro-led is left by 10 m, this will reduce the led luminous area to 30 μm x 30μm, decreasing by more than 40% compared to the luminous area of-40 μm x 40μm.Reducing the area to achieve the same light output requires a corresponding increase in the current density, which will reduce the overall optical efficiency by 10%.Photoelectric transitions in micro-led and nonradiative reorganization of the edges will further suppress efficiency, the solution by expanding the led luminescence area, but this in turn will rapidly increase the cost of led and the system.So this is a pair of contradictory solutions.In any case, these challenges are challenges for led vendors that can be overcome as the technology evolves.

The Summary and Outlook of the Section 4 LED Light Source

To sum up, the new modeling and new functions are promoting the development of led technology, and the LED new technology is also rapidly promoting the application of new modeling and new functions of automotive headlights.Improve the brightness level of the led to achieve ultra-thin and high-performance headlight design.The miniaturization of Led can significantly reduce the size of the ADB matrix headlights.Further development of the micro-LED will combine the functional integration of the car's headlights with the required compactness.And micro-LED has the inherent advantage of producing light only when needed, which means a significant efficiency advantage over DMD for digital headlights.And in the long run, a fully integrated encapsulated micro-led process will provide significant cost savings, as compared to the discrete LED arrays.So the micro-LED arrays will be the most effective and cost-effective solution for intelligent digital headlamps.It is reported that the digital headlights of megapixel have completed sample trial production, is expected to be assembled to the new car market within two years.

If the led chip exceeds 200 Mcd / m² of brightness, the LED chip structure and the entire optical system need to be re-optimized to achieve a higher energy density.In the led chip field, maximum avoiding led efficiency reduction is the core.The current current density of led chip is generally about 3 A/mm2. If the energy density is 200 Mcd / m², the current density of led chip needs to be increased to 5-8 A/mm². Therefore, the led epitaxial region needs to be redesigned to improve the current density and electric migration in the epitaxial region, and reduce the inhibition of phosphor as much as possible.To achieve an energy density of 500 Mcd / m², we expect a> photoelectric conversion efficiency of 30% required at the junction of the led, at a current density of 8 A/mm².Therefore, it is necessary to optimize the lamp heat dissipation system.As shown in Figure 4, the LED chip will benefit greatly from the optimized lamp heat dissipation system at a higher current density.The current is expected to decrease by nearly 1 A when the junction temperature drops from 110 C to 85 C.Such a reduction has great benefits for efficiency and led longevity.

Section 2 miniaturization also has new headlight features such as matrix ADB, which require each led chip to be powered and controlled separately, and hundreds of led chips are integrated into a led board, which leads to an increase in the size of the ADB module, while the optical efficiency and heat dissipation of the system are also a big problem.The integration of adaptive ADB functions further drives the miniaturization of led light sources.Today, ADB matrix modules based on single LED arrays have been on the market for many years.However, these modules usually require the design of front-facing optical systems that deepen the depth of the lens module (see Figure 5, left).If smaller, miniature, high-brightness led arrays are used, additional front-facing optical systems can be avoided, thereby reducing the depth of the module.

The current automotive-class miniature led arrays are produced by making a led chip array on the plate and then by applying silicon-based sealant fill / side to produce optical isolation.This approach has many drawbacks.First, the opacity of the filling layer decreases with increasing the filling thickness, so it is difficult to reduce the stray light between the led chips and, therefore, the contrast between the pixels of the matrix partitions is very low.Furthermore, the manufacturing and cost challenges are significant when creating led arrays larger than 3 rows.To address the optical performance and cost issues, new led chip structures need to be developed for compact alignment, minimum string, and array configurations

Section 3 integration also has new full digital headlight technology, such as road projection, higher resolution, even increased to tens of thousands of pixels, led process needs to micro level less than 50 μm x 50μm, and each led chip is independent of each other in optical control, led chip gap is very small.At this point, the circuit interconnection through CMOS integration is required, and the single led particles can be controlled independently, thus controlling the imaging patterns on the road through higher-level protocols.Combining these new digital headlight technology solutions raises the need for developing disruptive and innovative micro-led systems for both high-resolution and microoptical design.The ultimate application of functional integration is the higher-resolution digital headlights.If the resolution and size accuracy of the LED array are high enough, a digital headlight can produce a variety of light types.It includes high-resolution ADB, AFS function, accompanying steering associated with navigation or camera systems, adaptive adjustment of near-light lines, lane markings, obstacles and logo highlighting, etc.The requirements for micro-LED can all be derived from these applications.For the ADB system, at least a luminous angle of + horizontal direction / -12 is required.To create a ently obvious cut-off for the pattern, a minimum of 1:250 contrast is required.For the high-beam ADB partition, the goal is to have a small resolution, and a 0.085 bright spot at 50m is a rectangular bright spot 7.5 cm wide and 4 m long, so we can calculate the resolution required for the corresponding ADB function.For example, for lane projection, assuming a led array board with a 1:3 aspect ratio, we can directly calculate that it takes a minimum of 19,000 pixels to achieve a clear lane projection.A usual high beam requires a light of at least 1 lx illumination (equivalent to 65,000 c d) within 250 m.By converting the optical formula, the illumination = optical flux / stereo angle relationship, we can calculate the required optical flux per chip of 0.14 lm / core.Assuming a ~ 33% optical efficiency of the ADB system, each LED chip is required to provide a ~ ~ 0.43 lm / core.If a reasonable light source luminance of 90 Mcd / m² is used, we can estimate the single-family luminescence area of the micro-led array as 40 μm x 40μm.There are many technical difficulties if a led array architecture is required to meet the needs of the market ADB.First, to create a micro-LED array of about 20,000 dense pixels, it is clear that the micro-led array must be assembled from single blocks or several large subblocks, and that each micro-led is a chip partition.The addressing of each chip must be done by a CMOS drive.

One major challenge for the integration of led chips and CMOS is contrast and overall efficiency.When a single micro-led chip measures 40 m, there is little space between each chip to isolate the string of light.If the spacing between the Micro-led is left by 10 m, this will reduce the led luminous area to 30 μm x 30μm, decreasing by more than 40% compared to the luminous area of-40 μm x 40μm.Reducing the area to achieve the same light output requires a corresponding increase in the current density, which will reduce the overall optical efficiency by 10%.Photoelectric transitions in micro-led and nonradiative reorganization of the edges will further suppress efficiency, the solution by expanding the led luminescence area, but this in turn will rapidly increase the cost of led and the system.So this is a pair of contradictory solutions.In any case, these challenges are challenges for led vendors that can be overcome as the technology evolves.

The Summary and Outlook of the Section 4 LED Light Source

To sum up, the new modeling and new functions are promoting the development of led technology, and the LED new technology is also rapidly promoting the application of new modeling and new functions of automotive headlights.Improve the brightness level of the led to achieve ultra-thin and high-performance headlight design.The miniaturization of Led can significantly reduce the size of the ADB matrix headlights.Further development of the micro-LED will combine the functional integration of the car's headlights with the required compactness.And micro-LED has the inherent advantage of producing light only when needed, which means a significant efficiency advantage over DMD for digital headlights.And in the long run, a fully integrated encapsulated micro-led process will provide significant cost savings, as compared to the discrete LED arrays.So the micro-LED arrays will be the most effective and cost-effective solution for intelligent digital headlamps.It is reported that the digital headlights of megapixel have completed sample trial production, is expected to be assembled to the new car market within two years.