• Head up: information in the driver’s field of view.

  • Head up: information in the driver’s field of view.

    • 1. April 2014
    • Advanced Engineering
    • Text: Jochen Kruse
    • Illustration: Realgestalt

    The head-up display projects information directly into the driver’s field of vision, thus improving safety and comfort.

    The instrument panel and its displays are almost as old as the automobile itself. They have always provided drivers with information about road speed and engine speed and other important data such as coolant temperature and how much fuel remains in the tank. Over the years, the volume of information available has increased immensely, with the introduction of navigation and road sign recognition systems, for example. An intelligent information management system is becoming increasingly important in order to avoid ‘information overload’ which could distract the driver. Vehicle design engineers are prioritizing the information displayed according to its importance at any given time. Their task is helped by the fact that, in the latest instrument panels, (electro-)mechanical dial-type gages have long since been replaced by monitors which allow information of all kinds to be presented with less clutter.

    The head-up display, which Mercedes-Benz is introducing into its first vehicle model as of 2014, takes this to a higher level. The system provides data on vehicle speed and speed limits, and issues navigation instructions and warnings from assistance systems. The ‘head-up display’ supplements the information in the instrument cluster. As in modern day aircraft, important information is reflected onto the windshield, directly in the driver’s field of vision, ensuring drivers are less distracted away from what is happening on the road ahead. Furthermore, the eyes do not have to adjust between distance vision and short-range vision, because the information seems to hover above the engine hood some two meters in front where drivers can absorb it without having to avert their gaze or make optical corrections. One consequence is that drivers do not become tired so quickly. Not only does the head-up display offer considerable safety benefits, but displaying the information directly in the field of vision is felt to be comfortable.

    In view of the wealth of information provided in today’s vehicles, the head-up display certainly has an important task to fulfill; a task which could be expanded still further in future vehicles, for example, via ‘augmented reality’, the computer-assisted expanded perception of reality. Or in driver-less vehicles, where the vehicle interior could be re-designed and the cockpit integrated in a different way.

    HOW THE DRIVER OPERATES THE HEAD-UP DISPLAY

    The driver can adjust the height of the virtual image – similar to adjusting the mirror. In vehicles equipped with a seat memory function, this setting is saved, allowing the systems to be accurately adjusted as soon as the driver is seated in the vehicle and has called up the relevant saved driving position. Furthermore, different display elements can be activated or deactivated and the brightness level of the display can be individually adjusted.

    HOW THE HEAD-UP DISPLAY WORKS

    The head-up display is based on the principles of mirror optics plus a full-color display module with a 480 x 240 pixel resolution. High power LEDs illuminate the display module. If a sheet of paper is placed on the cockpit upper shelf, then the image of the paper reflected onto the windshield is seen by the driver on the other side of the windshield. The mirror optics used in the head-up display generate an apparent image source which seems to lie considerably below the cockpit. The driver sees both the reflection of this apparent image source hovering some two meters away above the engine hood and, at the same time, the real world beyond the windshield. The size of the reflection is approximately 21 x 7 centimeters. The image resolution, at over 60 pixels per degree of viewing angle, ensures a high-definition representation.

    In order to prevent drivers from seeing double images, caused by the reflections at the outer and the inner windshield interface surface, a wedge-shaped composite film is affixed to the glass. This tilts the outer interface surface towards the inner interface causing the secondary image generated on the outer surface to be brought into line with the primary image. This compensatory action depends on the angle of view and has been optimized for a standard driving position.

    AUTOMATIC ADJUSTMENT TO ALL LIGHTING CONDITIONS

    A light sensor near the vehicle roof upper edge automatically adjusts the head-up display brightness to the background brightness, i.e. to that of the road surface. On sunny days, a luminance of 10,000 and more candela per square meter is achievable. Since the contrast figure is better than 1.000:1, the system offers a high-quality display even at night. The luminance delivers detailed data on the locational and directional dependence of the light flux given off by a source of light. It is the photometric measure of that which the human eye perceives as the brightness of any surface. Luminance describes the brightness from extensive, two-dimensional light sources. The SI unit for luminance is candela per square meter (cd/m²). THE HISTORY OF THE HEAD-UP DISPLAY Many engineering developments were kick-started by defense technology before being taken on further – and the head-up display is no exception. It originated with the reflex sight, the prototype of which was demonstrated by the Irish optician, Howard Grubb, in 1901. The reflex sight reflects an illuminating aiming point into the gunner’s eye via a semi-transparent mirror. An optical lens, the collimator, ensures that the aiming mark appears to the gunner in infinity. Looking through the semi-transparent mirror, the gunner sees the target and, simultaneously, the aiming mark reflected via the mirror. Since the light beam of the reticle falls precisely from the line of sight into the eye, the aiming mark always appears in the right place independently of the position of the eye relative to the sighting device. From the 1940s, reflex sights were installed as aiming devices in aircraft. Shortly afterwards, additional information was passed to pilots via a front screen projector. Today, the head-up display, still reflected into the pilot’s field of view via a second screen, remains the most important display even in many civilian aircraft. Also in service are so-called ‘head-mounted’ displays which project the data onto a display attached to the head, a helmet display or video spectacles, for example. Camera bright-frame viewfinders also work on the head-up display principle. HEAD-UP DISPLAY Category: safety, comfort Function: In addition to seeing the actual road, a virtual image containing vehicle information is reflected into the driver’s field of vision via the windshield. Specifications: Head-up display, consisting of: full-color display module, illuminated by high-power LEDs, lens and mirror system, special head-up windshield with wedge-shaped composite film, light sensor near the roof upper edge for adjusting brightness, settings in the instrument cluster menu, pushbutton in the driver’s assistance bar for activating or de-activating the system

    Mercedes-Benz head-up display: a virtual image containing vehicle information improves safety and comfort.

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