How does a head-up display work? + Tips for retrofitting

Additional features in cars are always on the rise. Today it is no longer enough that a car can drive, horn and brake. It is too tempting to take the whole range of brake and traffic sign assistants, digital cockpit and head-up display with you.

Head-up display on Ford Escape – (photo by Ford)

But are these features that we really need? With most of them, you don’t really know how they work – like with the head-up display, for example. We asked ourselves: How does a head-up display work?

So how does a head-up display work? A head-up display projects information onto the windscreen. It consists of the image-giving, optical and image-receiving component, the front window. The imaging component generates an image, video or animation and projects this onto the pane via the optical component.

However, there are many different technologies here. It is used in airplanes and in modern car models and works either as an independent unit or with separate systems – for example with the smartphone.

Did you know that the head-up display, or HUD for short, is not really a new invention? One can say that it has now reached the middle of society at the latest and can significantly upgrade a car model.

But don’t worry, you don’t need to buy a new car just because of the head-up display, because the head-up display can be retrofitted!

In the following you will learn more about the functional principle, the difference between head-up and head-down displays and the other areas of application.

In addition, we learn which head-up display variants are available and how you can retrofit them.

The beginnings of the head-up display

Strictly speaking, head-up displays have existed since the 1940s. At that time still known as “reflex visors”, they have been continuously developed and have become an integral part of modern aviation technology.

When General Motors brought head-up technology to the automotive industry in the 1980s and 1990s, European automakers such as BMW and Citroën followed suit in the 2000s.

Head-up display working principle

There are some differences depending on the technology of the head-up display, but in general it always works the same.

The imaging and optical component

The imaging component generates information as an image, video or animation and passes this on to the optical component. Depending on the technology, this can be a tiny projector, laser or the screen of your smartphone.

The image-receiving component

The image-receiving component ultimately shows you the information played out via the optical component. It is important that it is translucent and reflective.

For this reason, glass elements, such as the front pane, or transparent foils are often used in practice.

Head-up display receiving component on Hyundai Kona – (photo by Hyundai)

Security benefits

It is important with this technology that the driver is not distracted by the information. This means that it must be ensured that the information is easily recognizable and displayed at the correct distance so that the driver does not have to focus on the projected information for too long.

Suitability of the front screen

The condition of the windshield also plays a major role. Since it consists of two separate layers of glass, a simple projection would be displayed minimally offset.

While the windshield on new vehicle models with an already integrated head-up display is adapted from the outset to the special requirements, older vehicle models have to be switched to alternatives.

Alternatives to adapted windshields

This alternative consists of OLED or transparent OLED screens (TOLED), which are attached to the windshield and are intended to ensure that the projected information is still legible even in strong sunlight.

Advantages and disadvantages of the head-up display

Of course, there are both advantages and disadvantages of the head-up display.

Advantages of the head-up display

With the head-up display, you can still keep your eyes on the road. The information is projected directly into the field of vision on the windshield.

This also results in the disadvantage of the head-down display (for example, instrument cluster screen in cars): By lowering your gaze, you take your gaze off the road for a fraction of a second. Unfortunately, that is often enough to recognize a danger too late.

Furthermore, taking the view down is very tiring for the eyes in the long run, as they have to adjust to a new distance each time they look at the instrument cluster.

Disadvantages of the head-up display

The disadvantage of poor head-up displays, on the other hand, is that they can hardly be seen in strong sunlight. You spend too much time focusing and this also means you take your eyes off the road.

The head-down display has a clear advantage in this regard, because this display, which is integrated into the cockpit, is usually protected from solar radiation by a small front extension.

What is hardly noticed on short journeys can be of decisive advantage on long journeys.

Head-up display on new Ford Focus with “Fighter jet tech” to minimize solar glare – (photo by Ford)

What types of head-up displays are there?

As mentioned earlier, there are different types of head-up displays. We have summarized the most common types for you below:

  • Simple displays: The simple displays are systems that only show simple basic information on the windscreen. This is mostly information that is needed to navigate the car. This basic information includes speed, warning messages, characteristic values ​​and navigation arrows.
  • Augmented Reality (AR): From a technical point of view, head-up displays belong to augmented reality. In principle, any type of information can be displayed with this type of head-up display. The possibilities range from simple pictures to videos or 3D animations.

What are the sizes of head-up display?

The head-up displays known to us only have a relatively small format, mostly between 5 and 10 inches. In this format they are mainly used in airplanes, private jets or cars.

Over time, the head-up displays have also found their way into other areas. Today you can find them in sizes up to 80 inches at meetings, events or in museums.

Head-up displays in airplanes, private jets and cars

  • Small size, about 5 to 10 inches
  • 2D or 3D animation
  • Low quality up to full HD
  • Mainly shows basic information such as speed, warning messages, characteristic values ​​and navigation arrows.
  • Is projected onto transparent glass elements or foils so that the information is integrated into the background.
  • Possible with and without AR or VR glasses (AR = Augmented Reality, VR = Virtual Reality)

Head-up displays for meetings, events, showrooms or museums

  • Large format, up to about 80 inches
  • Augmented Reality, high-resolution 3D animation
  • Resolution up to UHD (4K)
  • Unlimited display options: projection in so-called “deep frames” or projections in space, eg car models, animals running through space or changing art objects
  • Is projected into the real environment of the user
  • Possible with and without AR or VR glasses (AR = Augmented Reality, VR = Virtual Reality)

Can you retrofit head-up displays?

First of all, a very simple answer: Yes, you can retrofit head-up displays. But there are also various options for this.

5.5 inch OBD II Car HUD Head Up Display Windscreen Projector – (photo by dial2fast)

Complete systems – so-called HUDs

Quite confusing when you consider that HUD is actually just short for head-up display. These complete head-up systems, up to 5.5 inches in size, consist, like all head-up displays, of an image-giving, optical and image-receiving component.

Advantage of complete systems

The advantage: you buy everything together with this complete system. “Everything” in this case includes an image-receiving film that is stuck to the windshield, and a small box in which both the image-giving and the optical components are installed.

Information via GPS or via OBD interface

This box receives information such as speed or the distance to be traveled either via GPS or via the so-called on-board diagnostic interface (OBD).

If the HUD is connected to this interface, it can receive all possible vehicle-related information such as speed, engine speed, engine temperature and other driving information.

Data transfer to the smartphone possible

The OBD interface is usually located on the left in the driver’s footwell and is also used to read error codes. With some devices, this data can also be transferred to your smartphone.

Disadvantages of complete HUD systems

Since the HUDs are usually placed by yourself, the placement is not always optimal. Not everyone knows which details to look out for for a safe placement.

The HUD must not be placed above the ventilation or above the street.

Oncoming vehicles or other road users must not be blinded by the projection.

Furthermore, many users struggle with the power supply. The start-stop function in particular often results in long downtimes, which are rather impractical, especially when the navigation system is running.

If the HUD receives the information via GPS, you have to forego the correct display for a while in rural areas and at the latest in tunnels.

How to install HUD

Of course, we don’t want to withhold the 5 steps you have to take to retrofit the HUD in your car:

  1. Find out whether your HUD receives its information via GPS or via the OBD interface. If necessary, connect the device to this interface.
  2. Does the device need power from the cigarette lighter/power outlet? Then it also connects to this.
  3. Properly tape the cables or route them behind the dashboard. If they fall towards you while driving, it can be pretty dangerous.
  4. If you clean your windscreen from the inside and dust it with a little water, this is the easiest way to attach the film and move it again in an emergency. You can pull out air bubbles by pushing them out with a flat object or by wiping them with a soft cloth from under the foil.
  5. Next you have to install the small box on the dashboard – directly under the windshield. Sit in the driver’s seat and think carefully about where you want to place the small box. If the information is not completely displayed on the slide, you can move it a little.
  6. You have successfully upgraded the HUD. Have fun trying!

If you want to see visual demonstration of installing HUD, then watch this video by dial2fast.

Head-up display via smartphone app

Sounds too practical to be true, doesn’t it? The practical thing is that the head-up retrofit options for smartphones are available in many different versions. And not much more is necessary than downloading a navigation app with HUD and buying the accessories.

An example of smartphone HUD

Accessories for retrofitting the smartphone HUD

This accessory consists of a mat with an attached projection screen. It is advertised en masse in all quality levels, colors and shapes through the usual sales channels.

It must be placed in a suitable place, between the windscreen and the dashboard. The mobile phone is then placed on the mat with the navigation app open, including the HUD function.

There are now really innovative offers that have integrated an anti-slip mat and a Qi charging station into the lying surface for the smartphone.

Why a HUD function is so important

Since the information displayed is mirrored during projection, the driving information must be displayed mirrored on the smartphone. A navigation app with HUD function does exactly that, whereupon the driving information is then displayed on the projection screen in the correct direction.

Disadvantages of the smartphone HUD

Smartphones are not designed for long-term use. In addition to the disappearing battery, they also get hotter and hotter over time. If you don’t attach them properly to the dashboard, they can slip when cornering, which can be quite dangerous.

If you also believe the numerous customer reviews, the quality is not always sufficient enough for the driving information to be displayed legibly even in direct sunlight.

Author: Nabeel K

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