How Does a Head-Up Display Work? + Tips for Retrofitting


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In recent years cars have been getting more and more loaded with features. Today it’s no longer enough for a car to just get you from A to B, oh no. It needs to have the full range of safety assistants, digital cockpit features and a Head-Up Display to be taken seriously.

But are these features that we really need? With most of them, its a pretty safe bet that you don’t really know how they work – for example, the Head-Up Display. So we thought we would try and explain How does a Head-Up Display work?

So how does it really work? A Head-Up Display basically projects information onto your windscreen. It usually consists of  image-giving, optical and image-receiving components, and of course your windshield. The imaging components generate an image, video, or animation and projects this onto the windscreen via the optical component.

However, you should know that there are many different technologies at work here. These displays are used in airplanes and in modern car models and work either as independent units or with separate systems – for example with a smartphone.

Did you know that the Head-Up Display, or HUD for short, is not really a new invention? We can basically say that it has now reached almost middle age in technology terms and obviously can prove to be a major upgrade for any car. It is becoming a very common fitment in most new cars today.

Don’t worry though, you don’t need to buy a new car just to get a Head-Up Display, because a Head-Up Display can actually be retrofitted !

In this article, we will learn some more about the functional principle, the difference between Head-Up and Head-Down Displays and other areas of application.

In addition, we’ll learn what variants of Head-Up Displays can be found and how you can retrofit them as well. Since there are numerous possibilities, we have also discussed what the TÜV recommends in this case.

We hope you enjoy reading!

The Beginnings of the Head-Up Display

Strictly speaking, Head-Up Displays have existed since the 1940s. At that time they could be found on aircraft and were known as “reflex visors”. Since then they have been continuously developed and have become an indispensable part of modern aviation technology.

While it was General Motors that first brought HUD 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 in the same way.

The Image Giving and Optical Component

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

The Image Receiving Component

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

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

Safety Considerations

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 Windshield

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

Alternatives to Specially 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.

Difference between Head-Up and Head-Down Displays

Even we aren’t quite whether we had even heard of Head-Down Displays before doing research on this article, but the fact is: they do exist. The standard instrument cluster on a vehicle can be considered a Head-Down Display

Head-Up or Head-Down

The difference between a Head-Up and Head-Down display becomes apparent pretty quickly if you look at the name. With the Head-Up Display (Head-Up Screen) you can keep your head / gaze up in contrast to the Head-Down Display (Head-Down Screen).

Advantages and disadvantages of the Head-Up Display

Of course, there are both advantages and disadvantages to 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 your field of vision on the windshield.

This is the main disadvantage of the Head-Down Display: By lowering your gaze, you take your eyes off the road for a fraction of a second. Unfortunately, that is often enough to recognize a danger too late.

Furthermore, frequently looking 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 and back at the road.

Disadvantages of the Head-Up Display

The disadvantage of bad 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 longer journeys.

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  basic information on the windshield. This is mostly information that is needed to navigate the car. This basic information includes, among other things, the speed, warning messages, characteristic values and navigation arrows.
  • Augmented Reality (AR): The more technically complex 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, all in your line of sight.

Sizes of Head-Up Displays

Most Head-Up Displays known to us only have a relatively small format, usually between 5 and 10 inches. This is the size range mainly used in airplanes or cars.

Over time, however, Head-Up Displays have also found their way into other areas. Today you can find them in sizes up to 80 inches at conventions, events or in museums.

Head-Up Displays in Airplanes 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 at Conventions, 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 Head-Up Displays be Retrofitted?

The answer is very simple: Yes, you can retrofit Head-Up Displays. But keep in mind that there are various options for this.

Complete Systems – so-called HUDs

This can be quite confusing when you consider that HUD is actually just short for Head-Up Display. These – up to 5.5 inches tall Complete Head-Up Systems , like all Head-Up Displays, consist 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 Your Smartphone is 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 fitted by yourself, the placement may not always be optimal. Not everyone knows what details to look out for to achieve an ideal placement.

The HUD must not be placed above the vents or above the street line

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

Furthermore, many users struggle with the power supply. The start-stop function in particular often leads to 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 or at the very least, in tunnels.

How to Install a HUD in Your Car

Naturally, 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 gets 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? Then it also needs to connect 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 windshield 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 get air bubbles out 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 it out!

Head-Up Display via Smartphone App

Sounds too good to be true, doesn’t it? The most interesting thing is Head-Up retrofitting options for smartphones come in many different varieties. And using is as simple as downloading a navigation app with HUD and buying the accessories for it.

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.

It’s worth noting that you can now find really innovative options that have an anti-slip mat and a Qi charging station integrated into the holding 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 a HUD function does exactly that, whereupon the driving information is then also displayed correctly on the projection screen.

Disadvantages of the Smartphone HUD

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

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

What Does the TÜV Say About Retrofitting Head-Up Displays?

Skepticism best describes the attitude of the TÜV towards the independent retrofitting of Head-Up Displays. This is because there is currently no applicable test standard for head-up displays.

Depending on the car model, various points must be taken into account when placing the unit – there is no generally applicable procedure.

Furthermore, it has not yet been conclusively clarified whether permanently connecting the HUD accessories to the OBD interface might not have long-term harmful effects on the car. Finally, the external device has permanent access to the to your Engine Control Unit.

Conclusion: The TÜV is working on a solution to ensure safe installation of head-up displays in the future. They also do not rule out the inclusion of vehicle manufacturers in this.

Further Questions:

It is definitely really helpful if modern assistance packages can also be retrofitted to old cars. We wanted to know more about this and asked ourselves: Which Assistance Systems can we retrofit to our cars?

So which Assistance Systems can we retrofit? You can retrofit Assistance Systems to a car either individually or as a professional multi-assistance system that has been designed and approved according to the manufacturer’s specifications. Such assistance systems are, for example:

  • Collision Warning
  • Distance Assistant
  • Lane Keeping Assist
  • Traffic Sign Recognition

With so many assistance systems, the driver’s reaction in stressful situations is becoming less and less important. All the automation in the event of an approaching collision or inadvertently leaving the lane prevents the driver from behaving incorrectly. This can be considered as both a good and bad thing, depending on your perspective.

We also asked ourselves: What role will the assistance systems play in the future of autonomous driving?

So what role will the assistance systems play in the future of autonomous driving? The simple answer is, with every new driver assistance system, we take a further step towards autonomous driving. Possible incorrect decisions by the driver are intercepted by a preprogrammed function of the assistance system. Therefore the car is already driving autonomously to a certain extent, i.e. independently.

But it is still vitally important that we keep our attention focused when driving a motor vehicle, because as good as these systems can be, they are not quite capable yet of operating entirely by themselves and definitely still need human oversight!

Keep in mind, that every car has different software and modules built-in. Therefore specific features will vary with every model.

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