The basic idea
A transparent display lets you see the scene behind it while also showing an image. Instead of a solid panel with a backlight behind it, the screen is designed so that light from the background passes through, with the picture added on top. The result looks like graphics floating in a window or a showcase.
That sounds simple, but a normal display is built from layers that block light. Making one that is partly clear requires different choices in each major technology.
Transparent LCD
An ordinary LCD uses a backlight, a layer of liquid crystals that switches light on and off at each pixel, and color filters. The liquid crystal layer is already mostly clear. Much of the opacity comes from the backlight and the polarizing filters on either side.
A transparent LCD removes the usual full-panel backlight. In one common design, the box around the screen is lit by lamps at the edges or around the frame, and the light that reaches the panel illuminates the image. The background is visible through the glass, and pixels that are switched on appear as colored areas. The liquid crystals still do their usual work, which is explained in the article on liquid crystals, but the missing backlight changes everything about brightness.
Transparent LCDs are often used in display cases, where the interior lighting helps the picture show. They tend to be among the more accessible forms of transparent display, but the polarizers and color filters absorb light, so the glass is noticeably dimmer and tinted compared with clear glass.
Transparent OLED
OLED pixels produce their own light and need no backlight. Because the emitting layers are thin, a screen can be built with clear electrodes and a transparent substrate, so that light from behind passes through between and around the lit pixels. The principles are in the article on organic light emitters.
A transparent OLED is attractive because the image comes from the panel itself, with no lamps needed in a cabinet. The transparency is still limited. The layers and electrodes absorb some light, and the display is not as clear as plain glass.
Transparent LED
A third approach uses LEDs mounted on a mesh, film or thin strips, with open space between them. The LED pixels are bright, and the gaps let light through. Such walls are used on glass facades and in stores, where a large area needs to be both a screen and a window.
The pitch is typically coarse, because the openness is what provides the transparency. More LEDs means less clear area. Transparent LED therefore suits large, viewed-from-distance images, much like other LED walls, but not close inspection. The concept of pixel pitch applies in the same way.
Where the limits show up
Contrast and black
On a regular screen, black means dark pixels against a dark panel. On a transparent display, a black pixel is simply transparent or nearly so, so you see whatever is behind it. The display can only add light. It cannot subtract it. This is why contrast is the central limit of the technology. What counts as black is the background. In practical terms, the screen works well with bright graphics and a darker background, and poorly with dim graphics and a bright background.
Ambient light
Light in the room and on the other side of the glass competes with the image. A brightly lit background washes out the image. A dim background makes the picture stand out. The adjustment is similar to the challenge described in the article on ambient light for ordinary screens, but more severe. Windows facing the sun are especially hard, which is why window-facing displays involve high brightness or different techniques.
Reflections and haze
Glass surfaces reflect, and the layers of a transparent panel can scatter light slightly, so the view through may look hazy compared with plain glass. Coatings can help, but they add cost, and the tradeoffs are covered under screen coatings.
Color and what is behind
Colors are mixed with the background, so a bright red graphic in front of a green object shifts in appearance. Content designers usually keep backgrounds simple, or design graphics with bold shapes and heavy outlines. Text is better in white or bright colors than in dark ones, since dark colors may not show.
Viewing angle and parallax
Because the image sits on a layer in front of the background, a viewer who moves sees it shift relative to objects behind, just like parallax. This may be used on purpose, as in museum exhibit screens that place labels beside objects, or it may cause misalignment if graphics are meant to line up with a specific item.
How this differs from related ideas
Transparent displays are not the same as smart glass, which switches between clear and frosted and is often used with a projector or as a privacy tool. They also differ from augmented reality optics, which use waveguides or mirrors near the eye to overlay graphics on the world. A transparent display is a fixed screen that the viewer looks through from some distance.
Where they are used
Common settings include shop windows and showcases, product cases, exhibit labels, counters that show information while customers see the staff, and building facades. They are less common as general-purpose replacements for ordinary screens, because contrast limitations make them a poor fit where dark, detailed images are needed.
Practical advice
Check how a sample behaves in the actual lighting with the actual background. Evaluate with real content, not demo reels. Ask about transparency level, brightness, how visible the framework is, and servicing. Quoted transparency is typically measured under specific conditions, and results in a particular store window may be different.