Use monochrome or even full-color dynamic display car windows to create communication between people and cars

In the future, car windows may display a variety of information. New automotive technologies are expected to turn standard car windows into monochrome or even full-color dynamic displays. As this trend develops, it will be mainly used for human-vehicle communication and advertising, as shown in Figures 1 and 2.

 
Figure 1: Example of transparent display for ride-hailing service

 
Figure 2: Example of transparent display for advertising service

Let's first understand the basics. Transparent window displays need to support two display states: fully transparent and dynamic color display states. When not displaying information, the window must be transparent, and ideally, it needs to look the same as other windows in the car. When conveying information or playing videos, the window needs to display bright, colorful graphics that can be easily seen by pedestrians.

Transparent window displays as projectors include small projectors mounted on the inner surface of the roof or other locations, and transparent films that are usually sandwiched or laminated in the side or rear windows of the vehicle, or even the front windshield. Among them, there are several types of films:

• 405nm radioactive fluorescence: This type of film contains fluorescence excited by a 405nm light source. When these transparent, emissive films receive 405nm light, they re-emit energy at different wavelengths, such as blue or green. The light is emitted in all directions, so the image is visible from any viewing angle. Therefore, a color display can be created by using multiple films, each emitting a different color, excited by different wavelengths in the 400nm range. It is essential to illuminate this type of film with a 405nm illumination source.
• Smart Glass: Smart glass films have two states: clear and "frosted." The state transition occurs when a voltage is applied or removed from the glass, similar to the type of film used in electronic coloring applications. When in the frosted state, the image is projected onto the inside of the glass, similar to how rear-projection televisions work. The frosting provides good contrast, improving picture quality. Red, green, and blue (RGB) LED projectors can illuminate this type of film.
• Microlens Array Light Diffuser: This is a diffuser film that is designed to provide a certain amount of screen gain, thereby increasing the brightness of the picture. Screen gain is the result of directional diffusion or focusing of light. For example, the film is designed to determine the viewing angle at which the image is visible, rather than transmitting light in all directions. Outside of the viewing angle, there is no visible image because all the light is focused within the defined viewing angle. RGB LED projectors can illuminate these films.
• Holographic films: These films have some very unique features, such as the ability to create images that are visible from inside the car, rather than from the outside. RGB projectors can illuminate these films. To achieve the highest efficiency and brightness, the LED wavelength needs to be carefully selected to match the holographic film, and true green LEDs (versus converted green LEDs) are used at the appropriate wavelength.

All of these films have their advantages and disadvantages. Depending on the application, one type of film may be more suitable than another. One advantage of TI DLP® technology is that it is light source agnostic and can illuminate all of the different film types. Figure 3 illustrates how a projector can be placed in a car to create a transparent window display.
  
 
Figure 3: DLP projector placement options

Brightness proves to be one of the main challenges in transparent display design. Displaying an image at night is fairly simple, but displaying the same image during the day can be a challenge. It must be bright enough to be visible during the day. In addition, the size of the display is directly proportional to the amount of light required; for example, if you double the size of the display and want to maintain the same brightness level, you need to double the light output power of the projector. Other design considerations include projector size and location, film condition and lamination, and regional automotive window regulations.

Let’s get back to the application. A great example application for a transparent window display is a ride-hailing service like Uber, Lyft, Didi, or Grab. With a transparent window display, passengers can easily identify the taxi they have called by a display on the side window with the name and destination, or perhaps a verification code that corresponds to a code on the ride-hailing application.

Autonomous ride-hailing services are a more suitable application than traditional ride-hailing services. Obviously, autonomous vehicles are not human, so the car needs a way to communicate with other cars and pedestrians. Figure 4 shows the various applications for autonomous vehicle communication. For example, how does an autonomous vehicle indicate that there is a pedestrian ahead and is it possible to pass? When stopped at an intersection, which car goes first? There are many situations where the vehicle needs to signal its intention to pedestrians or other vehicles. A transparent display is a good choice because it is placed high on the car and easily visible to people around.

 
Figure 4: Autonomous vehicle communication requirements

Advertising is another major application direction. The ability of geo-targeted advertising to generate incremental monthly revenue for ride-hailing services is extremely attractive because it can increase profitability of the service ($300 in additional revenue per month) and/or driver wages.

An example of a geo-targeted advertising campaign would be advertising for Starbucks every time the vehicle is within 100 meters of a Starbucks store. Other potential applications include in-car entertainment, such as driver greetings, vehicle diagnostics, and vehicle entry touchscreens as shown in Figure 5.
  
 
Figure 5: In-car entertainment transparent display example

For more information on how DLP automotive products support transparent display designs, see the 405-nm DLP3034-Q1 chipset and the RGB DLP3030-Q1 and DLP5530-Q1 chipsets.