Transparent Screens: The Basics | individually

Screens impress with their brightness, high resolution and brilliant colors. But they have one thing in common: when they are turned off, a black hole remains – you can’t see through them. Transparent screens, on the other hand, are available in different technologies. Similar to glasses for augmented reality, they can allow to see the workpiece on a machine or from the displays behind in a shop window and provide additional information about them.

Building a screen

A color display consists of individual pixels. Each pixel usually consists of three sub-pixels in the primary colors red, green and blue. Color gradations are represented when running at less than 100 percent current or voltage. The image elements are connected as a matrix and controlled line by line. In addition to the control information for the image content, the wiring level also contains the power supply lines, depending on the display type. The more current a picture element needs to operate, the lower the resistance and therefore more massive they need to be. With passive matrix LCDs, applying a voltage is enough to make the liquid crystal molecules switch, with technologies like LEDs, a current must flow to make the LED light up.

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This technical article is part of a series on transparent displays. Parts two and three will follow shortly here at invidis.de.

First, a distinction must be made between emissive and modulating display technologies. This means whether the screen itself emits light or modulates the light from a light source that shines from behind. Figure 1 (not to scale) shows the structure of an emissive transparent display. The active pixel only occupies a small area, since it is not transparent in itself. The rest of the subpixel is accessible to the pixel’s drive and supply lines. The conductivity increases with the thickness of the material, but the transparency decreases.

Figure 1: Structure of an emissive display (Image: HY-LINE)

Figure 2 shows the pixel structure of a modulating display. To use the light source efficiently, most of the sub-pixel consists of the modulating part. The color filter is located here on liquid crystal displays. Each sub-pixel is controlled by a transistor that charges a capacitor with the desired voltage level depending on the gray level. The control lines are placed between the subpixels. For example, the dimensions of a 55-inch screen with FHD resolution (1920 x 1080) are 630 microns for a pixel and 210 microns for a subpixel, minus the holes.

Figure 2: Structure of a modulating screen (Image: HY-LINE)
Figure 2: Structure of a modulating screen (Image: HY-LINE)

Depending on the application and viewing distance, different display technologies are used. While Apple tries to achieve the highest possible pixel density (dot pitch) with the smallest distance between pixels (the gap) with the “Retina” screens, it is exactly the opposite with transparent screens: the luminous pixels are not transparent, but the spaces between them are , that they can be made transparent.

aperture

In displays, the term aperture is the area from which light emerges relative to the total size of an area used for a pixel. The emissive display in Figure 1 has a small active area from which the light stream emerges. This design is favorable for a transparent display, where the active area is opaque, as the largest part lets light through from behind. The opposite is the case for the modulating display in figure 2, where the large areas block or allow the light to pass through. Here in each pixel the area used for the transistor and capacitor is opaque. The areas required for the wiring either shine through permanently (in the case of a transparent display) or are made opaque in the case of a normal TFT with black print (black mask).

viewing distance and pixel pitch

Outdoor digital signage displays must be large to be read from a distance. They must be scalable because the image formats are not fixed there and do not adhere to the usual image formats, but are intended to make optimal use of the available space. In some applications, such as bus stop signs, the dimensions are dictated by the information to be displayed. Displays such as directional signs through shopping malls, on the other hand, need a low pixel pitch because they are viewed from up close.

Figure 3 shows a relationship between the viewer’s distance and the required distance between pixels on a screen. The graphics assume that an optimal ratio is that which a TV set offers the viewer, namely a distance of 3.5 meters with a 55-inch diagonal with Full HD resolution. The y-axis shows the corresponding distance given the pitch given in the x-axis. Two conclusions can be drawn from this: To achieve the same impression as on the TV set, the pitch should not be higher, i.e. not below the right line. However, to achieve the same impression, the pitch does not need to be finer, i.e. it does not have to be above the straight line.

Figure 3: Display distance and pixel pitch (Image: HY-LINE)
Figure 3: Display distance and pixel pitch (Image: HY-LINE)

For transparent displays, this means that the smaller the active picture element in terms of pixel pitch, the better the transparency. The more free space left for the transparent area between pixels, which is also used as a wire level. Contrary to this trend is to achieve the greatest possible brightness in relation to the total area. If the picture element is only small, it must be very permeable – in the case of transparent pixels as in TFT. In active displays such as LED or OLED, the picture element must convert high power per area from electricity to light, which accelerates aging.

scalability

A single display module is too small, especially when used for digital signage. Placing multiple screens in a matrix arrangement increases the viewing area. To enhance the impression, the seams must be invisible, i.e. the edges of the individual modules must be as slim as possible. Unlike opaque displays, the control electronics and supply lines cannot be hidden behind the display.

Figure 4: Arrangement of several screens (Image: HY-LINE)
Figure 4: Arrangement of several screens (Image: HY-LINE)

Figure 4 shows possible configurations to increase the active area using T-OLED as an example. The control electronics, which are located on one of the long sides of the display, are designed so that they are out of sight. It can also be hidden behind structural elements such as window frames. Assuming a screen with an aspect ratio of 16:9, the aspect ratios indicated in red can be displayed.

About the author

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 within display and touch technology as well as embedded computing at chip and board level. On the other hand, Hy-Line Computer Components provides solutions for the management and transmission of high-speed signals such as DVI, HDMI, Displayport, USB, LVDS and V-by-One.

Rudolf Sosnowsky (Photo: HY-LINE)
Rudolf Sosnowsky (Photo: HY-LINE)

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