SPD Smart Glass

SPD smart glass allows a reversible change in transmittance between opaque and transparent states, facilitating net zero strategies.

SPD-SmartGlass panels in the roof of McEwan Hall, Edinburgh University (courtesy of Smart Glass International Ltd). Image copyright: Don Munro Photography


Suspended Particle Devices (SPD) is a smart glass technology developed and patented by US-based Research Frontiers Inc., which offers an instantaneous and reversible change in tint when driven by voltage.

In the deactivated state, SPD smart glass is opaque. When activated by an alternating (AC) voltage, it becomes transparent, reaching typically 49% transmittance (transparency) in less than 1 second.

All smart glass technologies, SPD included, are fully aligned with the goals of the European Climate Law, which aims to make Europe climate-neutral by 2050.

SPD smartglass cabin windows in aircraft - courtesy of Vision Systems (a Gauzy company)

SPD smart glass installation in aircraft, courtesy of Vision Systems (a Gauzy company)

SPD Smart Glass Structure

Suspended Particle Devices (SPD Smart Glass) is composed of the following layers:-

  • An inner layer of SPD plastic film, coated with a transparent, conductive material, normally Indium Tin Oxide (ITO) and sandwiched between:
  • Outer panels of a transparent medium, normally glass or acrylic. The glass can be low in iron content (thus reducing any green colouration, typical in float glass).

SPD smart glass structure

SPD smart glass structure

SPD Smart Glass Behaviour

Mechanically, SPD Smart Glass resembles any security-compliant laminated glass panel, acting as a mechanical, acoustic and thermal buffer.

A typical security certification might be, for example, EN 356 (P4A or P6B security or higher), which is what smart glass laminators such as Smart Glass International offer.

Optically, it behaves like a tunable filter, blocking 99.9% of UV and 50% of infrared at all times, and transitioning from opaque to transparent within 1 second and from transparent to opaque in 1-5 seconds.

SPD smart glass spectral profile

SPD smart glass spectral profile

The above spectral response of SPD film when powered (red line) shows a flat characteristic between 400 to 700 nm (visible light), proving that no extra colouration is added by the SPD film itself. 

In the unpowered state (blue line), SPD SmartGlass has a blue-ish tint, as can be observed in the image at the top of this page.

Chemically, the mechanism causing the optical change is called ‘electrophoresis’, which describes the diffusion of dispersed particles relative to a fluid under the influence of a spatially uniform electric field.

The particles are less than 1 micron in size (i.e. one thousand times smaller than 1 millimetre). In fact, when the particles are based on polyhalides, they can measure less than one-half of the wavelength of blue light, i.e. 200 nanometres, thus reducing light scatter.

The fluid in which the rod-shaped SPD particles are suspended is an organic gel:- a non-aqueous, electrically-resistive liquid with a polymeric stabiliser dissolved in it. The stabiliser reduces the tendency of the particles to agglomerate, thus keeping them dispersed and in suspension.

Electrical Modelling

SPD smart glass electrical tests and chemical composition

SPD smart glass electrical tests and chemical composition (see Ref. 1 below)

Electrically, SPD smart glass acts like a capacitor and can be modelled in a way similar to a Lithium-Ion battery. 

This model is based on a ‘Randles’ circuit, with a Warburg Impedance modelling the diffusion mechanism. The capacitance exhibited by a panel of 1 square metre of SPD smart glass is close to 0.33uF, with a power consumption of about 1W.

One issue that also needs to be taken into account for very large installations of SPD smart glass is the reduction in Power Factor that results from plugging it into the mains, since it is a reactive (capacitive) load. 

This can result in power inefficiencies and possibly even cost penalties from your electrical utility.

This article explains Power Factor with a very nice analogy to beer.

SPD Smart Glass Transmittance

The rod-shaped SPD particles orient themselves randomly (following the laws of Brownian motion) when there is no electrical voltage applied, thus absorbing, reflecting or transmitting any incoming light.

The percentage absorbed, reflected or transmitted depends on the cell structure, the nature and concentration of the particles and the energy content of the light.

Typical transmittance in this opaque state is <1% if the SPD film is 90 microns in thickness. If the SPD film is 30 microns in thickness, the transmittance in this unpowered state is close to 15%.

When a voltage is applied to the SPD smart glass, the rods align themselves in the direction of the electric field to pass light through, giving an overall transmittance of 49% (90um SPD film thickness) or 65% (for 30um PSD film thickness).

Operating Voltage

The typical operating voltage of SPD smart glass is 60-100Vac, preferably a pure sine wave (to avoid harmonics and issues with EMC compliance testing).

Many controllers on the market implement a trapezoidal waveform for simplicity. For electrical outlets having 240V AC (in Europe), a step-down transformer is required to bring the mains voltage down to appropriate voltage levels for the smart glass.

For safety reasons, this is normally an isolating transformer which protects the user from electrical faults.

SPD Smart Glass Manufacturers

If you are looking for SPD smart glass manufacturers, distributors or installers, look no further than our parameterised search.

The screenshot below shows that we have many companies listed already, which can be filtered further if you specify product attributes, such as operating voltage, minimum transmittance or haze:

Smartglass World Marketplace Search for SPD smart glass manufacturers and distributors

Parameterised search for SPD smart glass manufacturers and distributors

If you press the ‘Show Results’ button, this will produce a list of companies, which you can contact by posting a request on the Smartglass World Marketplace.


  1. Electrooptical behaviour and control of a suspended particle device, R. VERGAZ*, J.M.S. PENA, D. BARRIOS, I. PÉREZ, and J.C. TORRES, OPTO-ELECTRONICS REVIEW 15(3), 154–158, URL
  2. SPD films having improved properties and light valves comprising same, Patent WO 01/90797, PCT/US01/16805. URL
  3. BS EN 356:2000, Glass in building. Security glazing. Testing and classification of resistance against manual attack. URL.
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