Displays impress with their luminosity, high resolution and brilliant colors. They have one thing in common: when they are turned off, a black hole remains. You can't see through them. Transparent displays, on the other hand, are available in various technologies. Similar to a pair of glasses for augmented reality, it can allow you to see through to the workpiece on a machine or to the exhibits in a shop window, providing additional information about them.

The basic requirement for transparency is that the display technology allows light to pass through the panel. This is not the case with reflective TN such as in calculators or ePaper. The display can then hide individual segments (e.g. TFT) or add content (transparent OLED or transparent LED module). 

Special attention must be paid to the design of the content. The color "black" does not exist - there the display is simply transparent and shows the background. To show off the transparency, the displayed content must consist of slim lines rather than large areas.

OLED displays have their place where brilliant, bright colors and high contrast are required. They outperform LCD (TFT) in several ways: The colors are generated by actively light-emitting pixels, and not from the interaction of backlight and color filter as with TFT. If they are switched off, the pixel is black, whereas in TFTs a dark gray spot remains, which results from the transmission of the liquid crystal when the backlight is fully switched on. Only with local area dimming, where the brightness of the backlight is controlled depending on the image content, does the TFT catch up.

The new transparent OLED from LG Display brings another factor into play: transparency. This means that not only can bright images and information be displayed, but the background can also be included.

The illustration shows a simulation for a museum installation: while the object in the background is optimally illuminated and strongly protected against theft and vandalism, information can be displayed in the foreground.

The transparency of the OLED is about 40%. As a comparison, a conventional TFT is well below 10%.

The self-luminous display offers perfect picture quality. With Full HD resolution and a wide color gamut and simultaneous transparency for the world behind the display, it impresses every viewer. The electronics almost completely disappear: The surrounding frame is very slim and the control board is connected with a 3m long cable and can be mounted remotely. This also simplifies the assembly of several displays to a large screen.

Displays are usually permanently installed in devices, where they provide an interactive interface to the user in conjunction with a touchscreen. What matters here is a suitable diagonal that is as large as possible and a clear, full-color interface.

Low-complexity displays can be found in simpler devices, where the display of a measured value with a 7-segment display is sufficient. Examples of this are medical instruments such as devices for determining blood pressure or blood oxygen saturation.

For some applications where you would like to have both hands free, displays that are mounted close to the eye are suitable. Large glasses for stereo visualization of augmented or virtual reality come to mind first, but there are also micro-displays that show specific information in the beam path of the eye. Examples of this are optical measuring devices that display auxiliary lines, dimensions and measured values, or target markers with distances and inclination angles.

OLED technology is particularly suitable for this as the aperture, the ratio of active to inactive area, can be sufficiently large. Passive matrix technology offers an advantage over AM OLED because it dispenses with active semiconductor components within the field of view that restrict transparency. Optical refinement can drastically reduce reflection at the interfaces between air and glass. There are virtually no limits to the design, whether rectangular or round. Parts of the display can be finished in a different color, e.g. red and green.

Technical data: Transparency >95%, brightness 3,000cd, pixel dimensions 55µm
Applications: Rangefinder for sports and geodesy, spotting scope for animal observation, telescope, riflescope for shooting sports

In contrast to Asia, transparent displays have not yet gained widespread acceptance in Europe. Ideas for applications already exist:

• Functional discs: Partition panes to maintain hygienic distance (public authority, hotel, information desk, cab), partition panes of meeting rooms with controllable trans¬parency (for confidentiality), shading (e.g. skylight)
• Augmented reality, e.g. on the machine for visualization of problems, or in commercial vehicles
• Front sides of vending machines and freezers for viewing the product and simultaneously displaying information about it
• Showcases with explanation of the exhibits, e.g. luxury goods or museums
• Infotainment at product presentations, e.g. vehicle showroom
• Transport: information at bus and train stops: timetable and map; timetables and advertising in public transport vehicles
• Building technology: elevators with glass fronts, flanks of escalators, window fronts, transparent doors
• Digital signage: display windows of fast-food restaurants, shopping centers
• Security: guidance of flows of people where visibility must be guaranteed