By Behlol Nawaz
Transparent displays (or see-through displays) are devices that can display images while the other side of the screen is also visible (just as the name suggests). Most display devices widely used today usually form images on opaque screens.
The most obvious application is that digital information can be over-layed on the physical world around us. This has many potential uses in different fields like education, graphics designing, engineering design, military hardware and advertising.
There are different technologies that allow transparent displays. Some are relatively old and have been around for some time. Most of these are large systems and have limitations in image quality. While other newer ones offer smaller size and better imagery, but have certain challenges that need to be overcome.
If you have played any flight simulation games with military aircraft or watched any videos showing the cockpits of fighter jets, you would have seen a transparent glass screen in front of the pilot’s seat, called the Heads Up Display (HUD). It displays basic images, which are usually monochromatic (single colour). This sort of transparent display has been in wide use for HUDs since 1950s and its underlying technology is even older. But it has been improving with time. Now, even some cars have windshields as HUDs. This removes the need to look away from the road to look at the GPS, speedometer etc. Similarly, images from infrared cameras on the vehicle are “laid” over the windscreen. This allows drivers to see things that they normally wouldn’t in fog. It can be a life-saver.
Such display systems usually consist of a projector or laser that projects an image through a system of lenses onto a glass screen. The glass screen is partially reflective. It reflects light of only a certain colour or light that comes onto it from a particular direction, which appears as an image to the observer. The glass screen is made partially reflective either by the material used in making the glass, its orientation to the viewer or both.
Another more recent technology that uses a system of lenses is called a virtual retinal display. In virtual retinal displays, the image is formed directly on the retina, as opposed to a transparent screen seen by a retina. It also works through lasers/projector and a system of lenses. Due to how close the whole contraption has to be to the person watching and its size, this is being used and developed for head-mounted displays. It can be designed to be transparent or otherwise.
Different varieties of the above techniques are used by some augmented reality devices like AR glasses from Meta, Optinvent ORA, Epson Moveria BT-200, Laforge Optical’s Shima, Google Glass and the Hololens announced by Microsoft recently.
Another relatively new technology is OLED based displays. OLEDs (Organic Light Emitting Diodes) are light emitting diodes in which the semi-conductor is usually a polymer. Passing current through certain semi conductors releases light of different colours. This is the basic principle of operation of all LEDs, including the OLED.
When displays are made, the polymer semi-conductor is practically a layer between two glass sheets. Tiny electrodes are embedded in the glass sheets which pass current through the polymer layer. So by controlling the current in the polymer sheet through the electrodes, different colours and so, images are displayed. The electrodes in the glass sheets as well as the polymer layer can be made transparent. And as there is no need of back lights OLED displays can be made completely transparent.
Up till now, OLED based displays have been used for screens of hand-held devices and have been mostly opaque. But due the quality of its images and low power consumption full sized TVs and transparent screens for Augmented Reality and hand-held devices are also being developed. They are entering the market but for the time being, their initial costs are high. OLEDs also offer flexibility, which might give us flexible, transparent displays in the future.
LCDs (Liquid Crystal Displays), a widely used display type at the moment, requires back-lights to the liquid crystal film, which is manipulated through electric currents to create images. These back lights are in different forms. If there is no need of built-in back lights, LCDs can also be made transparent. In settings where there is an abundant ambient light behind the screen, it can be used. For example display cases, store windows or billboards which can utilize sun-light. Samsung’s Smart Window, is a development of this concept.