What is a head-up display? And how does it work?

3 min read
A head-up display (HUD) is a transparent or semi-transparent display that presents data without requiring users to look away from their usual viewpoints.

The term "head-up" comes from the military aviation sector, where it was crucial for pilots to keep their "heads up" and maintain situational awareness while accessing important information.

In modern contexts, HUDs are commonly found in cars, where they project information such as speed, navigation, and other critical data onto the windshield, allowing drivers to stay focused on the road.
Head-up display in a 2020 Toyota Camry.
Image source: ToyotaJeff Reviews YouTube channel.
The basic principle of a HUD is to project light from a projector/display onto a transparent surface within the user's field of view. Here’s a step-by-step explanation of how it works:

  1. Data generation: The system's computer collects data from various sensors and inputs, such as vehicle speed, GPS for navigation, and engine status.
  2. Image creation: This data is processed and transformed into visual elements (text, symbols, graphics) by the video generation unit.
  3. Projection: The processed image is sent to the projector, which emits light rays carrying the image.
  4. Focusing: The optical system ensures the projected image is sharp and clear. Advanced HUDs use collimation, which makes the light rays parallel, helping the image appear at a virtual distance (typically a few meters ahead), reducing the need for the user’s eyes to refocus between the display and the external view.
  5. Reflection and display: The light rays hit the combiner, which reflects them towards the user's eyes. The combiner is designed to reflect the light from the projector while remaining transparent, so the user can still see the real-world view beyond it.

Main components of a head-up display

A typical HUD system consists of the following components:

  1. Video Generation Unit: The hardware and software responsible for creating the images or data to be displayed.
  2. Projector/display: The core component that projects the image or data. In vehicles, this is usually built into the dashboard.
  3. Optical System: Lenses and mirrors that ensure the projected image is focused and correctly aligned on the combiner.
  4. Combiner: A transparent screen or a specially treated section of the windshield that reflects the projected image toward the user.
Below are schematic images of the most popular car HUD technologies.
Components and operation of automotive head-up displays.
Image source: Lumineq.
Most commonly, the video generation unit and optics are located in the dashboard and the image is projected onto the windshield. However, there are other configurations as well. For example, automakers such as Mazda, Ford, Peugeot, Mini, and Mitsubishi project the image onto a separate lens combiner in some models.

Most aftermarket HUDs, such as our HUDWAY Drive, also project the image onto a separate combiner. But all the electronics and the projection module are in the body of the device.
HUDWAY Drive aftermarket projected head-up display.
In addition to projection technologies, there are other variations on the market, such as in-glass laminated displays and mini/micro-LED arrays. Such HUDs do not reflect light but emit it themselves. These displays are not very common yet, as they also have limitations. One of them is the inability to adjust the focal point, which makes some drivers uncomfortable with emissive displays.
GReddy Boost Gauge based on Lumineq emissive in-glass laminated display.
Image source: Lumineq.


So, what is a head-up display? It's a complex system that projects information onto transparent or translucent surfaces. The basis of the image is usually the light reflected from the windshield or a special lens seen by the driver.

The evolution of HUDs will depend heavily on the integration of AR technologies, which will require the use of the most advanced solutions to increase the focal distance, image area, and brightness.

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