A see-through display or transparent display is an electronic display that allows the user to see what is shown on the screen while still being able to see through it. The main applications of this type of display are in head-up displays, augmented reality systems, digital signage, and general large-scale spatial light modulation. They should be distinguished from image-combination systems which achieve visually similar effects by optically combining multiple images in the field of view. Transparent displays embed the active matrix of the display in the field of view, which generally allows them to be more compact than combination-based systems. Broadly, there are two types of underlying transparent display technology, absorptive (chiefly LCDs) and emissive (chiefly electroluminescent, including LEDs and "high-field" emitters). Absorptive devices work by selectively reducing the intensity of the light passing through the display, while emissive devices selectively add to the light passing through the display. Some display systems combine both absorptive and emissive devices to overcome the limitations inherent to either one. Emissive display technologies achieve partial transparency either by interspersing invisibly small opaque emitter elements with transparent areas or by being partially transparent.
History The development of practical transparent displays accelerated rapidly around the end of first decade of the 21st century. An early commercial transparent display was the Sony Ericsson Xperia Pureness released in 2009, although it did not succeed in the market due to the screen not being visible outside or in brightly lit rooms. Samsung released their first transparent LCD in late 2011, and Planar published a report on a prototype electroluminescent transparent display in 2012. Not long after, UK-based Crystal Display Systems began to sell transparent LCDs remanufactured from conventional LCD displays. LG demonstrated a transparent LCD in 2015. In the later part of the 2010s, transparent OLEDs started to appear. LG, Prodisplay, and taptl, for example, use conventional LCD technology. LG also uses OLED technology. LUMINEQ transparent displays manufactured by Beneq are Thin Film Electroluminescent Displays enabled by Atomic layer deposition (ALD). This display technology was used by Valtra in 2017 to develop its SmartGlass Head-Up Display on tractors. Samsung and Planar Systems previously made transparent OLED displays but discontinued them in 2016. Prodisplay used both OLED and LCD technology, but no longer makes transparent OLED displays.
How it works There are two major see-through display technologies, LCD and LED. The LED technology is older and emitted a red color, OLED is newer than both using an organic substance. Though OLED see-through displays are becoming more widely available, both technologies are largely derivative from conventional display systems. In see-through displays, the difference between the absorptive nature of the LCD and emissive nature of the OLED gives them very different visual appearances. LCD systems impose a pattern of shading and colours on the background seen through the display, while OLED systems impose a glowing image pattern on the background. TASEL displays are essentially transparent thin-film Electroluminescent Displays with transparent electrodes. Pixel Pitch and Brightness:
Pixel Pitch: Different pixel pitches (the distance between pixels) affect image clarity. Smaller pixel pitches offer higher pixel densities, resulting in sharper images. Brightness: See-through displays have adjustable brightness levels. Higher brightness levels, up to 7500 nits (depending on the technology), ensure visibility in various lighting conditions, including direct sunlight.
Partial Reflection A Partial Reflection Display shows an image by reflecting an image off a smooth transparent surface such as glass or specialty film.
Partial Reflection Displays comparatively simple but are limited by the brightness of the reflected image needing to be considerably brighter than the light sources beyond the display. A common example of partial reflective displays is in vehicular the Head-up display of a car or fighter jet. The Pepper's ghost illusion is a classic example that uses this technique passively.
Head-mounted displays
LCD An LCD panel can be made "see-through" without applied voltage when a twisted nematic LCD is fitted with crossed polarizers. Conventional LCDs have relatively low transmission efficiency due to the use of polarizers so that they tend to appear somewhat dim against natural light. Unlike LED see-through displays, LCD see-throughs do not produce their own light but only modulate incoming light. LCDs intended specifically for see-through displays are usually designed to have improved transmission efficiency. Small scale see-through LCDs have been commercially available for some time, but only recently have vendors begun to offer units with sizes comparable to LCD televisions and displays. Samsung released a specifically see-through designed 22-inch panel in 2011. As of 2016, they were being produced by Samsung, LG, and MMT, with a number of vendors offering products based on OEM systems from these manufacturers. An alternative approach to commercializing this technology is to offer conventional back-lit display systems without the backlight system. LCD displays often also require removing a diffuser layer to adapt them for use as transparent displays. The key limitation to see-through LCD efficiency is its linear polarizing filters. An ideal linear polarizer absorbs half of the incoming unpolarized light. In LCDs, light has to pass two linear polarizers, either in the crossed or parallel-aligned configuration.
LED
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