SPD SmartGlass™ Technical Information
In the area of light control, there are three main technologies: liquid crystal devices (LCD), suspended-particle devices (SPD) and Electrochromic devices (ECD). All three may be used to produce "smart" glass, a glass that can change its light transmission when a voltage is applied. For reasons given elsewhere, SPD is the technology of choice for use in external or solar control windows because of its reliability, price, performance, and manufacturing advantages.
SPD THEORY
SPD uses a special kind of film that contains rod-like particles suspended in billions of encapsulated cells (liquid droplets) that are uniformly distributed throughout the film as shown in Fig. 1. Each of the cells contains many smaller rod-shaped particles. With no applied voltage, the particles are randomly oriented and block light. When a voltage is applied, the particles align with the electric field and let light though. By varying the applied voltage, we can vary the tint of the glass from clear to dark, and any shade in between.
Figure1. Cross section of SPD film
SPD is a film-based technology, meaning that the rotating particles are trapped inside a dual-layer plastic film that consists of two layers of plastic with an emulsion (dielectric) between them. Furthermore, by including conducting layers as shown above, we get a classical parallel-plate capacitor. As discussed in basic electric theory, a parallel-plate capacitor has a uniform electric field. For this reason, SPD film has a uniform response throughout the film. In other words, you do not get the unwanted iris effect, so common in Electrochromic devices.
The physical mechanism behind SPD is similar to what occurs inside the dielectric of a capacitor, where the atoms are polarized by the electric field. In an SPD, rod-shaped particles are polarized, which forces their long axes to align with the electric field. This produces a venetian-blind effect which lets more light pass through the spaces between them. When no voltage is applied, the particles are randomly oriented and block light, similar to closing venetian blinds.
The small blue dots in the above figure are liquid cells trapped inside a solid. Each of these cells contains a liquid that is full of many rod-shaped particles. To understand the venetian blind effect better, look at the Fig. 2:
Figure 2. (a) Dark state (b) Clear state
With the no voltage applied as shown on the left, the particles inside the cell are in total disarray. In this state, no light can pass through the device. On the other hand, when a voltage is applied, it sets up an electric field. This polarizes the SPD particles and forces them to align vertically. As a result, light can now pass through the device. By varying the voltage, we can set up the dark state on the left, the clear state on the right, or any shade in between.
Warning
Do not use a dc voltage with SPD. If you use a dc voltage, the SPD will not function properly. The dc voltage will polarize the particles and initially they will let light through. But gradually, the particles will migrate toward one end of the cell and cluster or pile on top of each other. This leads to a gradual darkening of the glass. The only way to avoid this unwanted darkening is to use an ac voltage. With an ac voltage, the tendency to migrate toward either end of the cell is neutralized.
Electrical Characteristics
Besides requiring ac voltage for normal operation, there are a few additional facts you should know about SPD:
SPD has no memory: If you turn off the applied voltage, the device will go to the dark state. In other words, the dark state is the normally off state. For this reason, if you have an application where the dark state must be avoided during a power failure, consider using an uninterruptible power supply (UPS).
SPD are fast because they are field-effect devices. They rely on an electric-field effect, similar to liquid-crystal devices. An SPD window responds in a fraction of a second. Even faster response times are possible. The cycle lifetime of a smart window is defined as the number of times you can cycle it between its clear and dark states without noticeable degradation of its control over light transmission. SPD have a cycle lifetime greater than 100,000,000, which implies no problems will ever be encountered with cycle lifetime.
The operating temperature range is minus 30 to plus 90 degrees Celsius, equivalent to minus 22 to 194 degrees Fahrenheit.
For external use SmartGlass International recommends that windows be made in double or triple glazed unit format. The use of the following configuration will help to either retain or reduce heat as required.
Safety Double Glazed units (DGU) are comprised of:
All tempered glass panes.
Low-E coated outer lights.
Toughened laminated SPD-Smart™ Glass inner light.
Argon gas fill in chamber.
We recommend this configuration to achieve the highest possible thermal, acoustic, and safety performance possible. The Safety DGU has the following performance characteristics:
Glass State |
U-Value* |
R-Value* |
STC* |
OITC* |
UV Blocking** |
SHGC* |
VT* |
Dividers |
On (Clear) |
0.24 |
4.17 |
34 |
26 |
>96.3% |
0.43 |
0.25 |
N |
0.39 |
0.22 |
<1" |
||||||
0.35 |
0.2 |
>1" |
||||||
Off (Dark) |
0.24 |
4.17 |
34 |
26 |
>99.2% |
0.37 |
0.01 |
N |
0.33 |
0.01 |
<1" |
||||||
0.3 |
0.01 |
>1" |
SPD SmartGlass™ is supplied using toughened laminate glass providing excellent acoustic insulation, ideal for reducing sound pollution in noisy environments.
SPD SmartGlass™ - Applications and Features
SPD SmartGlass™ - Product Benefits
SPD SmartGlass™ - Range of Product
SPD SmartGlass™ - Technical Information
SPD SmartGlass™ - Care And Maintenance
SPD SmartGlass™ - Installation