Recently, the news of the appearance of transparent display screens on the train windows of Shenzhen and Beijing subways is quite eye-catching: this “smart train passenger service system” has appeared on the windows of Shenzhen Metro Lines 6 and 10, and Beijing Metro Line 6. [1][2]. These transparent displays integrated with the windows can not only display weather and site information, but also surf the Internet, watch videos, and visit online stores.

In addition, Xiaomi launched a transparent TV with a price of 50,000 yuan at the recent 10th anniversary conference, which has increased the popularity of “transparent display” technology. Judging from some reports in the domestic media and news on the supply chain, it is basically clear that these transparent display panels come from LG Display. There are not many manufacturers supplying transparent display panels today, and Samsung is said to have decided not to continue producing such panels in 2016[3]. These products are far from widespread.

In fact, the development of transparent display technology has been going on for 10 years at least. It seems that every once in a while, more or less we can always see a transparent TV or display at the exhibition. They come in different shapes, from transparent TVs to transparent cabinet doors to transparent mobile phones.

In 2009, when Sony Ericsson was still around, a phone with a transparent screen called the Xperia Pureness X5 was particularly notable because its transparent screen was so cool for the era—even though the phone wasn’t mass-produced, and It’s just a function machine.

The last time the transparent display was so eye-catching was at the CES show in 2016. Panasonic showed a transparent display in the form of a cabinet door, known as an “invisible TV”; when it is not turned on every day, it looks like a Just an ordinary clear glass cabinet door. And after opening it is a TV that can display dynamic pictures. [4] Last year Panasonic once again demonstrated its transparent OLED TV, but it was still a concept product at the time.

This kind of product is so topical, probably due to its full sense of futuristic technology – after all, in science fiction movies, you can always see various high-end transparent displays. This is very common in Marvel’s superhero movies over the years. We try to briefly talk about the basic principle of transparent display through this article, and how far it is from entering our daily life.

Transparent display? DIY by yourself

There are various directions of transparent display technology, such as LCD and OLED, which can be transparent. Here we put aside some unconventional transparent display technologies, such as transparent display technologies such as Google Glass and Microsoft HoloLens: such augmented reality AR glasses sometimes have 3D display properties.

来源:Geng,Jason.”Three-dimensionaldisplaytechnologies.”Advancesinopticsandphotonics5.4(2013):456-535

For example, the AR augmented reality glasses in the picture above can achieve a 3D image perception through the partial reflection characteristics of the lenses, especially for the left and right eyes, and at the same time ensure that the real world outside the glasses can be seen. This in itself is a type of scheme for see-through display, but it is not the focus of our discussion in this article.

In addition, MIT has been working on developing a passively transparent display system using nanoparticle technology—a scheme that uses a projector as an external light source to project images onto a transparent medium (a transparent medium embedded with a a nanoparticle that can partially display the projected picture). There are also some current transparent display technologies that use similar projection schemes. These solutions cost less, but they are not the mainstream of transparent displays, at least not yet.

In the past two years, we have seen more transparent displays, usually LCD or OLED transparent displays. It feels back to when we discussed flexible screens not long ago, divided into

OLED flexible screenandLCD flexible screen

When discussing in two directions. So how are these two panels transparent?

I won’t spend too much time talking about the structure of LCD and OLED screens here. It is necessary to have a basic concept: no matter what kind of panel it is, they are all multi-layer stack structures. The display screen is divided into many different layers, each of which serves a different purpose – stacking these many together forms the panel and display. The difference between LCD and OLED is that the hierarchical structure that constitutes the two types of panels is very different.

As we mentioned in the flexible screen introduction article, in order to realize the flexibility of the display, in essence, each layer is required to be bendable; then the same is true for the transparent screen. Must be transparent (or have some light transmittance).

Source: TheBackoffice, YouTube[6]

This can be regarded as a semi-natural attribute for display panels. For example, there is a backlight layer at the bottom of the LCD screen, and the backlight layer emits light to illuminate the various stacks on the upper layer – which at least shows that most of the layers of a traditional LCD screen are originally transparent. There are quite a few DIY videos on YouTube showing how to turn your long-used screen into a transparent display[7]—all traditional LCD screens are used, and the backlight layer including the light guide plate is removed, Then use natural light or other light source as a backlight to illuminate the screen, and the screen will naturally become translucent, as shown in the picture above.

It can be seen that the LCD screen originally has the basis of making a transparent screen. As for the OLED screen, the simplest example to prove its transparency is that many smartphones now support off-screen fingerprint recognition (even some have off-screen cameras). These fingerprint recognition solutions are usually optical fingerprint recognition, that is, the screen itself is required. It is transparent to achieve the purpose of still recognizing fingerprints under the screen. These screens are generally OLED screens, which shows that OLED screens naturally have this “transparency” feature. (eg transparent electrode ITO indium tin oxide material; some layers are thin enough to appear translucent)

OLED transparent screen will probably become the future

Obviously, since you can DIY a transparent display screen at home, screen transparency is not a black technology. It’s just that the research on transparent screens has continued over the years, and the core problem should be how to increase the transparency or the transmittance of the screen. After all, DIY transparent displays are still far from transparent compared to today’s Xiaomi transparent TVs.

In theory, this is also an important issue affecting the two transparent routes of LCD and OLED. As mentioned above, the LCD screen needs a backlight, which creates an obstacle for the complete form of the LCD screen to achieve transparency. In the early years, some LCD transparent screens removed the backlight system and used external light sources: whether it was natural light or artificial external backlights, to realize the visualization of the screen. At the same time, the LCD also has two polarizers (polarizers), which are also the components that affect the light transmittance.

Transparent LCD screen from JDI

In 2017, JDI (Japan Display Inc.) introduced a transparent display of its own at the SID2017 conference [8]. This kind of transparent screen is still LCD in the general direction, but JDI has made improvements to the panel light-emitting structure and technology in order to avoid the problems mentioned above. First of all, the backlight is placed on the side of the display screen (hidden in the frame), and the specific light guide scheme is unknown. The introduction of foreign media materials mentioned that the use of “internal reflection of the front wall and rear wall of the liquid crystal unit to achieve LED light diffusion” should be regarded as some kind of side-illuminated backlight scheme. In addition, due to the improvement of backlight, material (“fast-response liquid crystal material” that can support high-speed switching between transparent state and scattering state) and working characteristics, this LCD transparent panel also removes polarizer and CF (color filter) to further improve transparency. light rate.

In the early years, suppliers including Samsung, LG, MMT, etc. have promoted transparent LCD displays, and they have improved the structure of the LCD itself to increase the transparency of the screen. When the structure of OLED is transparent, it naturally does not have these obstacles mentioned above – even ProDisplay, which is promoting transparent LCD and transparent OLED products at the same time, specifically mentioned, “General transparent LCD screens need backlight to achieve The transparent OLED screen is composed of millions of self-illuminating pixels. This creates a whole new field of innovation…[9]” And because the OLED hierarchical structure is simple, it is easier to make it thin.

The screen structure of OLED has been discussed more than once before. The general AMOLED display screen, aside from the TFT backplane, the main hierarchical structure includes the substrate, the cathode layer, the organic molecular layer (including the emission layer, the conductive layer). ), anode layer (anode), as shown above. This is a self-illuminating structure when powered on, and does not require an additional backlight.

As mentioned earlier, these different layers have inherently transparent properties. About contemporary panel makers, all have the technology to make the different layers further transparent. We do not know the specific technical details of this part. However, the Xiaomi transparent TV and the subway window transparent screen mentioned above are all OLED panels. It is not difficult to see which direction the mainstream of transparent display is going.

The left side of the above picture shows what it looks like when the transparent OLED screen displays a light-white to black gradient pattern. Image source: PlanarSystems

On the choice of LCD and OLED transparent screen routes, there is also a detail issue, that is, the display of black and white. Due to the difference in the light-emitting mechanism of the two, the transparent OLED screen is transparent when it displays pure black-because the light-emitting principle of OLED is to stop working when it is displayed in black, so it naturally becomes transparent (or does not emit light).

Some transparent LCD screens can appear black and appear transparent to white – similar to the slide projectors of the early years, and this is also determined by how they work (when the LCD is white, the liquid crystal molecules let all the backlight through, and the pixels are turned off); of course This problem can also be circumvented with more technical assistance.

ProDisplay’s LCD Transparent Screen Demonstration

Transparent displays for black are usually more valuable in terms of picture expression, not only because black backgrounds are easier to integrate into the environment (such as viewing in low-light environments), but also in the presentation of bright colors such as gold and silver. Significantly more advantageous, significantly better than a transparent screen that displays white transparently. In addition, the transparency of the black part also enhances the visual experience. When Panasonic showed an OLED transparent TV last year, it added an extra black panel to the rear to enhance the black presentation [11].

This is more valuable for the inherent usage scenarios of transparent display screens, such as store window item display assistance, and various retail experience enhancements, which are more valuable for black display transparency and enhancement of bright color display. This is also part of the reason why OLED transparent screens are more promising.

Efforts to increase transparency

Although we are not very clear about the specific solutions of different transparent display panel manufacturers in optimizing the transparency of each level of the panel, some data and papers in the past still reveal that when the industry achieves higher light transmittance at some levels of the panel s hard work. For example, in the Panasonic transparent TV mentioned above, there is a layer structure that uses current to control the transparency: it is transparent when energized, and the transparency is reduced when it is not energized (as shown in the figure below) [10]; another example is the TFT layer in the backplane part of the display.

Source: Panasonic via Nikkei

When discussing flexible screen technology, the TFT layer is also a focus of our discussion, after all, this is where semiconductors converge. The AMOLED active array pixel circuit at least includes thin film transistors and storage capacitors. Depending on the desired display brightness, OLED efficiency, and various parameters, the driver circuit can occupy a considerable size of the pixel. TFT has thus become an important factor limiting transparency.

There have been researches on alternative materials or improved technologies for the TFT backplane of the transparent display panel more than ten years ago. Scholars in the field of materials research at the time mentioned transparent transistors using zinc oxide and indium oxide (IZO) nanowires to replace amorphous silicon and polysilicon transistors, because IZO is transparent (the IZO gate electrode is first deposited on the glass, A nanowire solution is applied on the surface and ITO source and drain are deposited on both sides of the nanowire) [12]. However, there were many problems in the implementation of the project at that time, so the progress of the industry in this area is unknown today.

At least the Panasonic transparent TVs of the year, and the Xiaomi transparent TVs today, if you look closely, you can still see a fine grid embedded in the glass.

Source: Xiaomi TV

However, the grid of Xiaomi’s transparent TV has a lot to do with its pixel improvement. This is the effort of modern OLED transparent screens to increase transparency. Xiaomi has written a special article on the official blog to explain the plan to increase transparency: “In addition to the WRGB4 sub-pixels in the conventional OLED screen, the transparent OLED pixels also increase the ‘transparent sub-pixels’ in nearly 50% of the area, the ‘transparent sub-pixel + WRGB sub-pixels’ constitute a single pixel of a transparent OLED screen, and the pixels of this structure have 1920X1080=2073600 pixels on this 55-inch transparent screen.”[13]

In other words, a portion of each pixel is transparent and does not participate in rendering. In this way, the transparency of the screen can be further increased.

Source: LG

This statement is completely consistent with LG’s official explanation [14], LG mentioned that “the key is to add a separate glass substrate to the color filter”, which allows up to 40% of the light to pass through. (The keycomponentistheseparateglasssubstratewhichisaddedtothecolorfiltertoallowupto40percentoflighttopassthrough.)

In addition, the circuit part is stacked behind the RGBW pixels, so that the “transparent sub-pixel” has a higher light transmittance (of course, there is a similar scheme of RGB sub-pixels, plus a transparent area). But it seems that it is also for this reason that the pixel density cannot be too high. Although the Xiaomi Transparent TV has a screen size of 55 inches, the resolution is 1080p.

In addition, theoretically based on this structural scheme, although the picture can be seen from the back of the TV, the color performance should be greatly discounted. PlanarSystem’s content developer document [15] also mentioned that applying a similar scheme, there will be a 25% loss of contrast and brightness when looking at the screen from the back of the TV compared to the front. After all, the circuit parts are in the rear, even if they are dense enough.

Where is the application direction?

Xiaomi controlled the price of this transparent TV within 50,000 yuan, which seems to be the beginning of the transparent screen entering the homes of the common people. Just from a practical point of view, the actual display effect of transparent TVs such as Xiaomi is slightly worse than that of mainstream TVs today: especially when we watch transparent TVs in the showroom of the Xiaomi flagship store in Shanghai, although it is indeed full of futuristic sense, But in an ordinary home with bright lights and a messy background, the TV picture is indeed difficult to see clearly.

It only has a place in low light environments. Unless a rich person buys it purely as a decoration (after all, those who will buy millions of OLED TVs should not care about the mere 50,000 yuan), otherwise it is used as a daily home entertainment device. Restricted.

Judging from LG’s positioning of its transparent panels, such a product was not intended primarily for home entertainment. In addition to what was mentioned at the beginning of this article, as a subway train information interaction tool, it is more used in electronic signboards, auxiliary display of goods/exhibits, etc. Its product name is called “LGTransparentOLEDSignage”. As an auxiliary display of commodities in retail stores, it is quite eye-catching. For example, as a commodity window display, it is integrated into the window glass to create various interesting display scenes, which is very similar to augmented reality.

For example, if a waterproof watch is displayed in the window, the window glass can display water and air bubbles to express the application scene of the product. Another example is the text introduction of a certain product, and even touch interaction with customers. LG mentioned in March this year that it has been applied to the National Palace Museum of Korea to assist the display of historical relics in the form of suspended images.

In addition, one of LG’s preset solutions when promoting its transparent display is to combine this display with “video wall OLEDSignage”. That is, there is a video wall OLED billboard at the back, and a transparent display screen in the front: so that when the two display the screen together, they can create an immersive 3D look and feel.

As transparent display technology becomes more mature, perhaps we will soon be able to see the existence of these transparent displays in many high-end retail stores and public infrastructure, just like the train glass windows in the subway. But it probably won’t enter our home entertainment anytime soon, after all, its practicality is really not very good.

Transparent TV from Samsung in 2015

Finally, it is worth mentioning that this article only discusses the transparent display screen itself, and does not involve more peripheral issues. For example, these transparent screens that LG are pushing can add a touch layer, which may also be included in the transparency scheme. More challenges; and in the system design, as a “transparent TV” and a transparent information interaction interface, in addition to the display itself, there are also compact design issues such as power supply, control, and data processing. These are also issues that need to be considered when building a transparent display system.

Reference source: 11

[1] Beijing subway magic window system upgrade: “Xiaomi Transparent TV” will be installed – Sina Technology

(https://tech.sina.cn/2020-08-22/detail-iivhvpwy2448845.d.html)

[2] Shenzhen Metro uses “transparent TV” – Sina Technology

(https://tech.sina.com.cn/e/2020-08-19/doc-iivhuipn9359049.shtml)

[3]SamsungDisplayreportedlydecidedtohalttransparentOLEDproduction-OLED-info

(https://www.oled-info.com/samsung-display-reportedly-decided-halt-transparent-oled-production)

[4]ThisFuturistic”InvisibleTV”LooksJustLikeAPieceOfGlassWhenTurnedOff!-steemit

(https://steemit.com/news/@sirwinchester/this-futuristic-invisible-tv-looks-just-like-a-piece-of-glass-when-turned-off)

[5]Geng,Jason.”Three-dimensionaldisplaytechnologies.”Advancesinopticsandphotonics5.4(2013):456-535

(https://www.osapublishing.org/viewmedia.cfm?seq=0&uri=aop-5-4-456)

[6]TransparentTFTpanelfrommonitor–TheBackoffice

(https://www.youtube.com/watch?v=fSCK6u0tVgc)

[7]HowimademyOwnTransparenttv/Monitor–Remtech

(https://www.youtube.com/watch?v=iJjQzRekVxw)

[8]JapanDisplayrevealsdetailsonitstransparentliquid-crystaldisplay–LaserFocusWorld

(https://www.laserfocusworld.com/detectors-imaging/article/16569450/japan-display-reveals-details-on-its-transparent-liquidcrystal-display)

[9]TransparentOLEDScreen–ProDisplay

(https://prodisplay.com/products/transparent-oled-screen/)

[10] Panasonic’s transparent screen, the origin is “TV with no presence” – Nikkei

(https://xtech.nikkei.com/dm/atcl/event/15/090600076/100300019/)

[11]Panasonic’sStunningTransparentOLEDTVLooksLikeaPieceofArt–tom’sguide

(https://www.tomsguide.com/news/panasonics-stunning-transparent-oled-tv-looks-like-a-piece-of-art)

[12]See-ThroughTransistors-MITTechnologyReview

(https://www.technologyreview.com/2007/06/04/37147/see-through-transistors/)

[13] How to make a TV transparent? – Mi TV

(https://weibo.com/ttarticle/p/show?id=2309404539221779284220#_0)

[14]TransparentOLED?!MicroLED?!LG’sNextGenerationDigitalSignageonaWholeNewLevel-LGInformationDisplay

(https://www.youtube.com/watch?v=2fPDOP90nwg&t=61s)

[15]PlanarLookThruTransparentOLEDDisplayContentDeveloper’sGuide–PlanarSystem

(https://www.planar.com/media/438041/020-1316-00b_planar_lookthru_transparent_oled_content_developers_guide.pdf)

(This article is reproduced with permission from the EDN sister website “Electronic Engineering Album”

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