DLP diffuse display advantages show strength, car HUD expands virtual image distance

    Most of the head-up display (HUD) systems on the market currently display redundant information, such as information that can also be obtained by other devices in the car. Unlike existing products, the new generation of head-up display technology HUD2.0 positions the head-up display as a display designed specifically for advanced driver assistance systems (ADAS). By adding in-car sensors, cameras and vehicle-to-vehicle/architecture communication technology (V2X), the information about the surrounding environment that the car can perceive will increase exponentially (Table 1).

HUD2.0 becomes ADAS first-level display platform

    The challenge for this technology is how to effectively communicate to the driver the critical information the car learns, which is bound to grow as semi-autonomous and fully autonomous driving capabilities mature.

    HUD2.0 technology can expand the driver's field of vision in a natural and intuitive way. The displayed content will be adjusted according to the reality outside the car (World-fixed), and the information received by the vehicle will be displayed in conformal graphics. Navigation instructions, lane departure warning (LDW) and active cruise control system (ACC) and other instructions will be displayed at the natural image distance that is most suitable for driving.

    With HUD2.0 technology, the reality in the driver's field of vision can be expanded, providing useful information in real time, and the image is brightly colored and overlapped on the physical object at a natural distance, so that the driver can easily obtain key information and minimize interference. In addition, unlike the current head-up display, which is mostly used to display secondary information in the user interface, HUD2.0 technology can become the focus of the human-machine interface (HMI) strategy and shoulder the function of displaying primary information. Therefore, excellent image quality and readability under different lighting conditions are essential for the new generation of HUD technology.

Expanded virtual image distance HUD2.0 meets high display brightness requirements

    Compared with the design of traditional head-up display, HUD2.0 must meet new requirements in many aspects. As shown in Figure 1, the field of view (FOV) and virtual image distance (VID) play a key role in the size of the virtual image seen by the driver. Traditional head-up display can only cover a small part of the lane, while HUD2.0 allows the driver to see images outside a single lane with a larger field of view and longer virtual image distance. There are several necessary conditions to increase the field of view and extend the virtual image distance - higher display brightness, more saturated colors, more efficient power use, and stronger tolerance to sunlight.

Figure 1 FOV and VID affect the image size of the head-up display

    While meeting these new parameter requirements, HUD2.0 must still meet the existing consistent requirements of all automotive environments. Table 2 compares some parameters of HUD2.0 and traditional head-up displays.

Brightness and energy efficiency

    A wider field of view plus higher brightness can give the driver an easier-to-see image. In addition, to ensure that the information is clearly readable under different light conditions, the head-up display must be able to achieve a virtual image with a brightness between 15,000 and 30,000 candela per square meter.

    However, the absolute power generated when displaying images should still be kept low - in addition to reaching the minimum value for thermal management, the luminous flux must also be maintained within the effective operating range of the light source (LED). However, to achieve a large field of view and high brightness without increasing power consumption, a more efficient imager is required.

    The efficiency of the DLP 0.3-inch WVGA Type A100 digital micromirror device (DMD) on the market exceeds 66%, which can greatly improve system efficiency and thus meet the requirements of the above parameters. The head-up display system that uses DLP technology with RGB LED light source can achieve the necessary brightness and a larger field of view while ensuring efficiency improvement.

    For example, the system design uses a 0.3-inch WVGA digital micromirror device with an OSRAM Q8WP RGB LED, which only requires 6.0 watts (W) of LED energy to achieve a brightness of more than 15,000 cd/m2 and a field of view of 10 degrees. The energy consumption is lower than that of existing small secondary head-up display systems on the market, and the efficiency of the entire system is 10.6 lumens per watt (lm/W).

Color Saturation

    Many traditional TFT/LCD head-up displays use white LEDs and use filters to filter out red, green, and blue colors. In contrast, head-up display systems using DLP technology use red, green, and blue LEDs to present more saturated colors. This approach can improve the readability of images displayed on the head-up display. In addition, key performance metrics can be used to evaluate the color performance of the system, including comparing with the NTSC color gamut to measure the color gamut size, or defining the hue of each color by its dominant wavelength and color saturation.

    Table 3 compares the TFT/LCD white LED architecture with the HUD architecture using DLP technology with RGB LEDs. The results show that using RGB LEDs can greatly improve performance. Not only is the color gamut larger than NTSC, but the red and blue colors are also more deeply saturated.

Heat load from sunlight

    As the field of view of the head-up display system increases, the amount of solar heat collected by the display's optical components also increases; at the same time, in order to allow the driver to view the image adjusted to the real world at the most appropriate angle, the virtual image distance is increased, causing the heat from the sun to be more concentrated on the head-up display's internal imager. As a result, the system absorbs more sunlight, and the heat is concentrated on a smaller internal image area, which may cause damage to the system.

    The head-up display system using DLP technology generates the internal image of the head-up display system by diffusing the screen material; while the traditional head-up display system directly emits the image from the imager (typically a TFT panel).

    The diffuse screen is a passive component that has two major advantages. First, it does not absorb the heat of the sun - it diffuses the light; second, the diffuse screen itself does not generate heat. These properties make head-up display systems using DLP technology more capable of handling large fields of view and long virtual image distances, both of which are necessary conditions for augmented reality head-up display systems.

Polarized Sunglasses

    In addition to having sufficient brightness and being able to be clearly displayed under various natural light conditions, the virtual images presented by the head-up display must also be clearly readable when the driver wears polarized sunglasses. Because DLP technology can emit unpolarized light, relevant manufacturers can optimize the head-up display based on this technology to make it suitable for polarized sunglasses.

Harsh Environment

    Imaging technology used in automotive head-up display systems must operate reliably under harsh environmental conditions, such as high humidity, rapid temperature changes, extreme temperatures, and shock or vibration. Some may question whether a digital micromirror device, a micro-electromechanical system, can withstand the temperature cycles, shocks, and vibrations of an automotive environment.

    In fact, the new DLP digital micromirror device is able to cope with these conditions. Since the vibration frequency of the mirror is far more than 100kHz, the mechanical structure can remain stable even when it encounters sudden shocks or vibrations (within the range of less than 5kHz).

    The introduction of advanced driver assistance systems into automobiles has become a trend, and automotive head-up displays have become one of the important elements of automotive human-machine interface strategies. DLP technology, with its foundation laid by its widespread application in the consumer and commercial markets over the past nearly 20 years, will help create a new generation of automotive head-up displays.