Glass that changes
Smart glass, also called switchable or dynamic glass, is glass whose light transmission can be changed on demand. A window might go from clear to tinted, or from clear to frosted. Control may come from an electrical signal, but some versions respond to heat or light instead.
It is included among emerging displays because it sits next to them, and because the line between a window that changes and a window that shows pictures is a topic that people often ask about. The short answer is that most smart glass does not display images. It controls how much light passes through, and sometimes how scattered that light is.
Main approaches
Several mechanisms exist. Each has typical strengths and weaknesses, and the details vary by product.
Liquid crystal privacy glass. A thin liquid-crystal film is sandwiched between clear layers. With power off, the crystals are randomly oriented and scatter light, so the glass looks milky and blocks the view. With power on, the crystals line up and the glass looks clear. This is a close relative of the liquid crystals used in ordinary screens, but used as a whole-sheet shutter instead of pixel by pixel. Such glass usually keeps the privacy state without power and needs energy to stay clear, although designs differ.
Electrochromic glass. A coating changes its color or darkness when a small voltage drives charged particles through layers. The transition can be gradual, taking from seconds to minutes depending on design and size, and it generally needs power only to change state. It is used to reduce glare and solar heat.
Suspended particle devices. Tiny particles float in a film. Without an electric field they are randomly placed and absorb light; with a field they line up and let light through. The tint can usually be adjusted continuously.
Thermochromic and photochromic glass. These respond to temperature or light by themselves, much as some eyeglass lenses darken outdoors. They need no electrical control, which is simple but gives the user little say.
Tinting versus scattering
It helps to separate two effects.
- Tinting reduces the amount of light that passes through. The view stays clear but dimmer. This is useful against sun glare and heat.
- Scattering keeps the light level but diffuses it so the view is blurred. This provides privacy while allowing daylight in.
A single product normally does one of these, though some combine them.
How this differs from a true display
A display forms an image by controlling many small points, each with its own color and brightness. Smart glass typically controls the whole sheet, or a few large segments, as one. It can be dark or light, clear or frosted. It cannot normally show a picture on its own.
Some ideas blur the boundary:
- Projection onto frosted glass. When the glass is in its scattering state, it can work as a rear or front projection surface. The projector forms the image, not the glass. Projector basics apply, including brightness limits in a lit room.
- Transparent displays. A transparent display has pixels that emit or modulate light while still letting the viewer see through. It is a different technology from switchable glass, though the two can be combined, for example by placing a changing backing behind a see-through panel to improve contrast. The topic is covered under transparent displays.
- Segmented shapes. Smart glass can be divided into regions or shapes, giving simple patterns or logos, but not detailed video.
Where it is used
General uses include office partitions, meeting rooms, bathrooms, vehicle windows and roof panels, and shop fronts. In retail settings, window displays can use a surface that switches between clear and image-bearing states, though the practicality depends on brightness and cost. The brightness challenge in direct sun is the same one covered under sunlight readability for outdoor screens.
Building glass may also have a role in managing heat from sunlight, and some designs are marketed as helping reduce energy for cooling or lighting. Actual savings depend on climate, building design and use, so such claims should be checked rather than assumed. They relate to wider questions about display energy and building energy use, which differ in kind.
Practical limits
- Clarity. Some tinted states have a slight color cast, and clear states may not be as clear as ordinary glass.
- Speed. Some types change within a moment; others take noticeably longer, and large panes may change unevenly.
- Power. Some types need continuous power to hold a state, others only to switch. Power use depends on design.
- Lifespan. Lifetime under years of sun and temperature swings varies, and long-term performance is a reasonable thing to ask a supplier about, much like display lifespan for screens.
- Edges and wiring. Electrical connections must be hidden and protected, and damaged edges can fail.
- Cost. Switchable glass is typically more expensive than ordinary glass.
What remains uncertain
How widely smart glass will be used, and whether it will merge with image-forming layers, are open questions. Combining a clear, efficient window with a bright, sharp image is difficult, because the features that make a good window, such as transparency, tend to compete with the features that make a good display, such as contrast and brightness.