Education comes from the screen

 And not from the book, otherwise it would mean booking. At least that's what the almost eighty-year-old German cabaret artist Dieter Hildebrandt says. We are constantly surrounded by screens of all kinds. One can certainly argue about the social and societal implications of such ubiquitous computing (“omnipresent computing”), but the technical side is undoubtedly fascinating. The machines communicate with us via extravagantly illuminated screens, of which there are many different types. However, experts unanimously agree that the future belongs to only one type of screen: the LED screen. Since its functionality is extremely simple and since the marketing apparatus of the electronics companies have constructed a completely impenetrable forest of terms (e.g. capacitive Super AMOLED touchscreen), here is a little overview of the subject of displays and screens.

We used to have screens with a weight of several centimeters using cathode ray tubes (CRT). Nobody likes these medieval “3D screens” anymore. With good reason - not only is the hernia preprogrammed at the next LAN party, the screens also flicker, have a high power consumption (that is to say: eat a lot of electricity) and shine like a medium-sized nuclear waste repository. The rough functional principle was simple (see picture, source: Wikipedia): In a vacuum, electrons are emitted from a glowing cathode (positive electrode) (hence cathode ray tube). These electrons are accelerated by an electric field and deflected by a magnetic field so that they hit a certain point on the screen. On this luminescent screen (the back of the screen you are looking at) a material (e.g. phosphor) is applied that generates visible light when the electron impacts, which reaches our eyes (the more intense the electron beam, the brighter). Three of these electron guns are required for the colored representation and accordingly three phosphors to produce the primary colors red, green and blue. The triple electron beam is focused on exactly one point on the screen at all times and races line by line from left to right and then jumps to the next line.

The era of flat screens began with the buzzword TFT. TFT stands for Thin Film Transistor (thin film transistor). The TFT screen is a form of the LCD screen that is controlled by these thin film transistors. LCD, in turn, stands for Liquid Crystal Display. Liquid crystals are special substances that are both liquids and solids (crystals). And they have an interesting property: if a voltage is applied to them, they preferentially only let light through that have a certain direction of oscillation (called polarization). And that is the secret of LCD screens. The LCD screen is a grid of pixels. These in turn each consist of three sub-pixels (red, green, blue). Each subpixel is controlled by the interplay of thin film transistor (regulates the voltage), liquid crystal (only allows a certain amount of light to pass through depending on the voltage) and color filter (only allows the respective subpixel color to pass). The light generated by backlighting first passes through the liquid crystal from behind. This was aligned by the applied voltage so that it lets the desired amount of light through. The transmitted light passes through the color filter and lets the subpixel glow in its color. Since the subpixels are very close together, the subpixels do not appear as individual basic colors of a certain brightness, but add up to one color - the color of the pixel they represent. This is called additive color mixing. Every single pixel can be controlled in this way.

In order to display an image, the control electronics of the screen only have to generate a signal for each subpixel that determines its brightness, e.g. the signal “red subpixel: light, green subpixel: light, blue subpixel: dark” for a yellow pixel. Since an electron beam no longer wanders over a fluorescent screen, but a constant backlight emits light, there is no longer any flickering. At the moment, LCD screens still dominate the market because they are inexpensive to manufacture, achieve high image quality and can also be manufactured in sizes that allow use in mobile devices.

Plasma screens are very similar to LCD screens. In physics, a gas with a very high temperature in which the electrons have separated from their atomic nuclei is called a plasma (ancient Greek structure). This plasma is generated in plasma screens according to the same principle as with fluorescent tubes: there is a gas (e.g. neon) between two electrodes. If a sufficiently high voltage is applied between the two electrodes, a voltage flashover occurs, i.e. a current flows briefly between the two electrodes - across the gas, which becomes plasma-shaped along the current path. This localized gas discharge produces ultraviolet light. The UV light itself is still invisible, but hits a fluorescent substance, which in turn emits light of a basic color (the fluorescent substance Zn2SiO4: Mn2 + e.g. green). Each subpixel is a small fluorescent tube that can be controlled individually. Now, however, the brightness of the subpixels must also be controlled. A trick is used here: instead of creating a continuous glow in one color, the subpixel is “ignited” at short intervals. The shorter the distance between two ignitions, the lighter the color of the subpixel is perceived.

Although plasma screens are very robust against interference fields, high temperatures and mechanical vibrations, they have not yet caught on. This is due to the fact that they are only practicable from screen sizes of approx. 37 inches, weigh a lot and are prone to burning in images that have been displayed for a long time (the fluorescent material loses its luminosity in the permanently exposed areas).

The most promising candidate among the screen types is the LED screen. An LED (Light Emitting Diode) is a semiconductor component very similar to a transistor. So-called OLEDs (organic LEDs) made of organic (carbon-based) semiconductors seem to be particularly promising, as they are extremely thin and can in principle be vapor-deposited onto almost all materials (e.g. also onto walls or paper). Since an OLED can emit light of a certain color, it is used for the sub-pixels of the screen. An LED is a sandwich: there are two semiconductors (a certain type of crystal) between two electrodes. One semiconductor has an excess of electrons (called n-layer) and the other lacks electrons (p-layer). These electron defects (holes) can migrate through the p-semiconductor. If a voltage is applied to the two surrounding electrodes, electrons and holes migrate towards each other. If they meet in the middle, energy is released, which is emitted in the form of photons (i.e. light). The color of the light can be determined by molecular modifications of the semiconductors.

In a sense, OLED screens take the most direct route; without background lighting and color filters, the light is generated directly at the position where it should appear on the screen. The resulting advantages are a very strong contrast (with LCDs a lot of contrast / brightness is lost due to the filter cascade), a low energy requirement (hardly any heat generation) and very small screen thicknesses (starting at 0.1 mm). So far, however, OLEDs have been very sensitive to contact with water and oxygen and must therefore be well shielded. Most cell phone manufacturers are already successfully offering models with OLED displays (e.g. the Nexus S built by Samsung) and Apple will probably also be offering the IPhone 5 in the future (patent applications).

The illustration shows the OLED display of the Nexus One mobile phone (source: Wikipedia). As you can see very nicely, OLEDs are not point-like but flat light sources, since the organic p- and n-semiconductors are flat as well. The arrangement of the subpixels may seem paradoxical at first, after all, there are twice as many green as blue and red subpixels. However, this is a clever move that makes use of empirical knowledge about the physiology of the human eye: [subjectively perceived overall brightness] = 0.6 * [green value] + 0.3 * [red value] + 0.1 * [blue value]. For this reason, the sensors of digital cameras are also arranged in a similar pattern (so-called Bayer matrix).


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