Analyzing the technical principles of HUD display

Basic Introduction to HUD

According to monitoring data from Gaogong Intelligent Automobile Research Institute, in 2023, 2.2543 million new passenger cars (excluding imports and exports) with pre-installed standard W/AR HUD were delivered to the Chinese market, a year-on-year increase of 50.26%, and the penetration rate exceeded the 10% mark for the first time.

Among them, the contribution of self-owned brands to delivery accounts for more than 60%, becoming the main contributor to W/AR HUD pre-installed standard configuration. At the same time, the competition pattern of the HUD market segment has gradually become clear. AR-HUD, which can integrate ADAS assisted driving information and realize real-time navigation, has attracted much attention in recent years and has gradually become an important configuration of smart cars in the new era. It has begun to be used more and more. Even some models have begun to use HUD products to replace traditional car instruments, such as Ideal L7, L9, Changan Deep Blue S7, etc.

The above is a brief analysis of the HUD market situation. The following is a brief introduction to HUD.

HUD, the full name of which is Head Up Display, is also called head-up display in Chinese. It uses the principle of optical reflection to project the car's driving assistance information, navigation information, vehicle speed information, check control information, and ADAS information onto the windshield. In this way, the driver only needs to raise his head slightly during driving to quickly obtain this information without having to look down at the dashboard, which greatly improves driving safety and convenience. The figure below shows the display effect of the AR HUD developed by Crystal Optoelectronics. It can be seen that the supported functions and displayable content are very rich and diverse.

History of HUD

First, let's take a look at the history of HUD. HUD technology is not unfamiliar. It was first used in fighter jets. The following figure is a schematic diagram of the HUD system on a fighter jet. Imagine that when the pilot is flying at high speed, he does not need to look down to check the instrument. He can grasp the flight information by just looking straight ahead. How convenient and safe is this!

In the 1980s, some car manufacturers began to apply HUD technology to cars. Initially, only high-end models were equipped with it, and the information displayed was simple and in a single color. With the continuous development of technology, especially in recent years, domestic suppliers represented by Crystal Optoelectronics have been deeply involved in it, which has promoted the further maturity of related technologies and the continuous reduction of costs, accelerating the popularization of HUD. Models such as Changan S7 and Ideal L8 have already achieved HUD standard. I believe that in the next few years, more and more models will be equipped with HUD, making HUD technology truly enter the lives of ordinary people.

HUD Technology Principles

In order to enable everyone to have a deeper understanding of HUD display technology, the following will analyze the technical principles of HUD display from an optical perspective.

HUD technology is similar to a projection system, consisting of two key parts: an image generation unit and an optical projection unit. The image generation unit, or PGU (Picture Generation Unit), contains optical components such as light sources, lenses, and diaphragms, which can generate high-brightness image information. The technologies currently used in PGU include TFT-LCD, DLP, LBS, LCOS, and other technologies. The following briefly introduces the PGU imaging principles of these technologies.

TFT-LCD

The principle of the image generation unit based on TFT-LCD is that the LCD screen controls the rotation of liquid crystal molecules through the thin film transistor behind each pixel under the backlight to adjust the brightness of the light. Then, through the RGB color filter, these lights are converted into colorful images. Because this solution is mature and low-cost, most HUD suppliers currently choose to use it to ensure high-definition, high-brightness and high-contrast image display.

Figure 4 LCD display schematic

DLP

DLP stands for Digital Light Processing, which is a digital light processing technology. The key lies in the digital micromirror chip (DMD). The DMD chip integrates millions of micro lenses, which can accurately control the light emitted by the light source and form an image on the diffuser through the projection lens. DLP technology is favored by luxury brand models due to its high resolution, high brightness and excellent imaging effect. However, its complex optical structure and high design cost also limit its wide application.

Figure 5 DLP schematic diagram

LBS

The LBS solution is a laser scanning projection solution. It uses an RGB laser module to emit red, green, and blue light, which is mixed by a lens and irradiated onto a tiny MEMS micromirror. This micromirror rotates under the drive of the brake to scan and form an image. This technology has a simple structure, small size, and better color performance. However, this technology is still immature, and key components such as laser light sources and MEMS chips still need more verification to ensure that they meet automotive regulations. In addition, there are problems such as brightness and speckle that need to be further resolved.

Figure 6 MEMS schematic diagram

Lcos

LCOS stands for Liquid Crystal On Silicon, which is a silicon-based liquid crystal projection technology. It is an upgraded version of LCD technology. This technology uses a special CMOS integrated circuit chip coated with liquid crystal silicon. By utilizing the optical properties of liquid crystal and the electrical properties of silicon chips, LCOS can accurately control the propagation direction and intensity of light to produce clear and vivid images. The advantages of LCOS technology are high resolution, wider color gamut and excellent contrast.

Figure 7 LCOS schematic diagram

The above introduces several different PGU technologies. Currently, Crystal Optech has successfully independently developed PGU modules based on the above technologies. Among them, the PGU based on TFT-LCD technology has a simple structure, low cost, high luminous efficiency, and good picture uniformity; the PGU independently developed based on DLP and LCOS technology has the advantages of high brightness and wide color gamut.

After talking about the image generation unit, let’s introduce another important system unit of HUD imaging: the optical projection unit.

The optical projection unit includes primary reflectors, secondary reflectors, windshield and other optical components. Its function is to enlarge the image generated by the PGU and display it in front of the front windshield after multiple reflections from the reflective lens, so that the driver can see the image projected in the distance at the eye box. The figure below is a schematic diagram of the HUD projection principle. Crystal Optech has rich technical accumulation in HUD optical design, has a complete optical design development process and specifications, and a short design and development cycle. The HUD projection screen it designs has high clarity, low distortion, high brightness, and good experience, which has been well received by customers.

Figure 8 Schematic diagram of HUD projection principle

Technical Difficulties of HUD

In the era of intelligent automobile development, HUD technology will also face challenges and opportunities. The trade-off between volume and performance, sunlight backflow and heat dissipation, and the design and processing of free-form reflectors will limit the display and layout of HUD. If HUD technology is to become more mature, the above problems must be solved. The application and research of technology will increase the cost of the product, which is also the reason for the continuous update of HUD technology.

Difficulty 1--Tradeoff between size and performance

Field of view, projection distance, and eye box size are important performance parameters of HUD products. The field of view and projection distance determine the visual angle and image size of the projected image. The larger the field of view, the more information can be displayed, and the longer the projection distance, the larger the displayed image. For example, AR-HUD products need to achieve a horizontal field of view of 10 degrees and a projection distance of more than 7.5 meters. The size of the eye box determines the spatial range in which the human eye can see the image. In order to achieve a larger field of view, projection distance, and eye box size, there needs to be enough space and distance for the virtual image to be reflected and magnified, which greatly increases the structural complexity and volume of the HUD, increases the difficulty of arranging the HUD in the vehicle body, and thus increases the overall cost. At present, the trade-off between volume and field of view, projection distance, and eye box size is a major difficulty in HUD design.

Figure 9 Schematic diagram of HUD internal structure

Difficulty 2--Sunlight backflow and heat dissipation

The imaging principle of HUD is that after the PGU generates an image, part of the light enters the human eye after being reflected by the reflector and the windshield, as shown by the red solid arrow in the left figure below, so that the projected virtual image can be observed at the eye box. Since the light path is reversible, the sunlight along the original light path can definitely converge into the PGU unit, as shown by the red dotted arrow in the right figure below, causing the internal temperature of the HUD to rise sharply.

Figure 10 Schematic diagram of sunlight backflow

The problem of sunlight backflow requires the HUD to have good high temperature resistance. If the HUD has poor heat dissipation performance, it is easy to cause damage to optical and electronic components in direct sunlight and high temperature environments, affecting the service life of the HUD. At present, the mainstream PGU technology adopts the TFT solution. In this solution, sunlight backflow is a common problem that needs to be solved urgently. The PGU using DLP and LCOS technology can effectively alleviate the problem of sunlight backflow by adding a relay diffusion film, but due to the high cost, it is rarely used in mass-produced models. Therefore, finding a low-cost solution to the problem of sunlight backflow is another major difficulty in HUD technology.