There are many misconceptions about smart glass. We are going to try to clarify them here.
What is Glass?
By the general term ‘glass’, we refer to amorphous materials (Greek: ‘without shape’), lacking long-range internal order, as opposed to crystalline materials which have a long-range internal order, i.e. they are organized in a definite lattice pattern.
Glass, Crystals and Polycrystals
A polycrystal (or polycrystalline structure) comprises many such microscopic crystals. All crystals and polycrystals are solids, whereas amorphous materials can be solids, liquids or gasses. In fact, glass (which is amorphous) is considered neither solid nor liquid.!
In nature, examples of crystals include snowflakes, diamonds, and table salt. Most inorganic solids are actually polycrystalline, with examples including metals, rocks, ceramics, and ice. Other examples of amorphous solids include waxes and some plastics.
Most glass, crystals and glazing are transparent in at least the visible part of the electromagnetic (EM) spectrum and may also be transparent in other wavebands, such as the infrared. Examples of crystals which are not transparent in the visible range include silicon, germanium and gallium arsenide.
By ‘smart glass’, we refer to modifications or technologies which augment the normal functionality or behaviour of any glass- or polymer-based product. This can be via surface coatings, internal lamination structures or electronic circuits embedded in or on the glass (think: ‘Google Glass’).
Strictly speaking all materials which change their optical properties as a function of an electrical input stimulus can be classified as electro-optic. Unfortunately, such materials are often referred to as ‘electrochromic‘, which implies only a change in colour (Greek: ‘khroma’).
Examples of electro-optic glass technologies would include, by that definition, SPD, PDLC and electrochromic technologies.
In this case, the input stimulus are photons and the effect is a change in the material’s optical properties. The term ‘photochromic‘ is also widely used. Examples include lenses and spectacles which change their transmittance when struck by sunlight.
The thermo-optic effect deals with a variation in the refractive index of a material due to a variation in temperature. Such materials are sometimes referred to as thermochromic, again implying only a change in colour.
As we embark on this wideband investigation into the whole smart glass industry, both in terms of its constituent sciences, technologies and applications, we should pay heed to using the terminology consistently, both within the technology itself as well as in the marketing of these technologies.