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 displays (Twisted Nematic) such as in pocket calculators or e-paper.The display can then hide individual segments (e.g. TFT) or add content (transparent OLED or transparent LED module).Thin film transistor displays are based on semiconductor technology.Electric fields influence the position of liquid crystals enclosed between two glass plates.Polarizing filters are laminated to this cell, which only let light through in one direction of polarization.The polarization direction of transmitted light is changed pixel by pixel by the field and it can reach the viewer's eye.The light source is behind the display, the display itself only acts as a light valve.Due to the many layers and filter foils, the transmission of a TFT is significantly less than 10 percent.Therefore, the background has to be very well lit in order to achieve an acceptably bright image.Typical applications are therefore backlit showcases and vending machines, i.e. devices where the manufacturer can set the brightness of the lighting himself.This technical article is part of a series on transparent displays.Also read part 1 and part 2 on invidis.de.Sharp is taking a different approach: the newly developed transparent TFT only changes between the display of a color and the transparent state.However, the special display technology allows a high transparency of 60 percent.This is ideal for applications where the main purpose of the display is to switch between transparent and opaque.There are two variants: one that is transparent with no energy, and one that is opaque.It is not bistable.When a current flows through the materials of an OLED, electrons and holes recombine.The energy released in the process escapes as visible light.Different material pairings result in different wavelengths, which are perceived as different colors.OLEDs are not transparent, in the normal design the pixels emit the light through the substrate, which reduces the brightness.With a transparent OLED, the challenge is to design the lines on the display in such a way that they are transparent and electrically conductive at the same time.Compared to TFTs, they also have to be able to transmit a current flow in order for the OLED elements to light up.Unlike TFTs, which filter the light that passes through, OLEDs emit light themselves.Rudolf Sosnowsky is Head of Technology at Hy-Line Computer Components.As a member of the Hy-Line Group, the company is a specialist in complete system solutions in the field of display and touch technology as well as embedded computing at chip and board level.On the other hand, Hy-Line Computer Components supplies solutions for the management and transmission of high-speed signals such as DVI, HDMI, Displayport, USB, LVDS and V-by-One.The new technology from LG Display does not have much in common with that known from OLED TVs: the so-called stack was developed from scratch and the materials for the lines were made from indium gallium zinc oxide (IGZO), which compared to the otherwise used indium tin oxide (Indium Tin Oxide ITO) offers some advantages.This made it possible to produce a transparency of 40 percent with brilliant colors at the same time.Transparent LED modules (TLM) consist of foils coated with conductor tracks made of transparent material such as ITO.RGB LEDs with integrated drivers are mounted there at intervals of 10 to 20 millimeters.The film is flexible and has an adhesive layer on the back.As a result, it can be easily laminated onto a carrier glass, for example a shop window pane.The dimensions of individual modules are so compact that displays of almost any size can be created by arranging them.The foil can be cut so that non-rectangular displays in the grid of the LED pitch are also possible.The table shows the presented technologies for transparent displays in comparison.A normal TFT is listed for reference.