A see-through display is an electronic display that allows the user to see what is shown on the glass screen while still being able to see through it.
It is a technology that has been around for a decade or two, but only as of 2019 was it being incorporated by companies such as LG and taptl into consumer products like handheld devices, televisions, and other technology as well as building materials such as glass. Most companies use OLED technology, but not all. LG, Prodisplay, and taptl, for example, use conventional LCD technology. LG also uses OLED technology.  Samsung and Planar Systems previously made transparent OLED displays but discontinued them in 2016. LG and Prodisplay are the only current manufacturers of transparent displays. Prodisplay used both OLED and LCD technology, but no longer makes transparent OLED displays.
These screens can be used for augmented reality, and other applications such as shopping displays and more sophisticated computer screens. See-through displays based on OLED or microLED technology may display black through the addition of an LCD, as they cannot do it on their own. This is because in OLED and micro LED, the OFF state corresponds to black (or in this case, transparent since there is no black background) and the ON state corresponds to white; this is because OLED and microLED pixels emit their own light. See-through LCDs cannot display whites because LCD pixels do not emit their own light, rather they selectively block light from a white backlight, although this could be theoretically be fixed though the addition of a transparent monochrome microLED or OLED display. In LCDs, this is because in the OFF state, the pixels turn off, allowing light from a backlight to pass through, while in the ON state, the pixels turn on, blocking light. 
How it works
There are two major see-through display technologies, LCD and LED. The LCD technology is older, though OLED see-through displays are becoming more widely available. Both technologies are largely derivative from conventional display systems, but 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 colors on the background seen through the display, while OLED systems impose a glowing image pattern on the background.
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 back light 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 are its lineral polarizing filters. An ideal linear polarizer absorbs half of the incoming unpolarized light. In LCDs, light has to pass two linear polarizers, either in crossed or parallel-aligned configuration.
LED screens have two layers of glass on both sides of a set of addressable LEDs. Both inorganic and organic (OLED) LEDs have been used for this purpose. The more flexible (literally and figuratively) OLEDs have generated more interest for this application, though as of July 2016 the only commercial manufacturer Samsung announced that the product would be discontinued. OLEDs consist of an emissive and conductive layer. Electrical impulses travel through the conductive layer and produce light at the emissive layer. This is different from LCDs in that OLEDs produce their own light, which produces a markedly different visual effect with a see-through display. The narrow gap between the pixels of the screen as well as the clear cathodes within allow the screens to be transparent. These types of screen have been notoriously difficult and expensive to produce in the past, but are now becoming more common as the method of manufacturing them is advancing. OLED transparent displays generate their own light, but can not show black; this can be solved by the addition of a special LCD layer.
Passive transparent displays
MIT Researchers developed an inexpensive and passive transparent display system that uses nano-particles.
Unlike transparent LCDs and OLEDs that requires integrated electronic modules to process visual signals or emit their own light, a passive transparent display uses a projector as the external light source to project images and videos onto a transparent medium embedded with resonance nanoparticles that selectively scatter the projected light. This approach improves the deficiencies observed with transparent LCDs and OLEDs, such as high cost, difficulty of scaling in size, and delicate maintenance.
See-through screens are an emerging market that have several potential uses. Cell phones, tablets and other devices are starting to use this technology. It has an appealing appearance but more importantly it is also effective for augmented reality applications. The device can add its own twist to what is behind the screen. For example, if you look through a tablet with a see-through display at a street, the device could overlay the name of the street onto the screen. It could be similar to Google street view, except in real time. For example, Google Translate has a feature that allows the user to point the camera at a sign or writing in another language and it automatically displays the same view, but with the writing in the language of your choosing. This could be possible with see-through displays as well.
A device using a transparent display will have much higher resolution and will display much more realistic augmented reality than video augmented reality, which takes video, adds its own supplement to it, and then displays that onto the screen. It could be simpler to display the addition onto the see-through screen instead. The Microsoft HoloLens is an application of this idea.
These displays are also used in shopping windows. The shopping windows show the product on the inside as well as show text or advertisements on the glass. This type of showcase is becoming more popular as see-through screens are becoming cheaper and more available.
A transparent LED display can be used by stage designers and event producers to realize creative holographic-like visual effects.
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