What Is SPD Smart Glass?

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) SmartGlass is a technology developed and patented by Research Frontiers Inc. and allows a voltage-driven, reversible and variable change in glass transmittance, benefitting privacy, energy efficiency and security.

Structure

Suspended Particle Devices (SPD) SmartGlass 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). The glass can be tempered or even bullet-proof and can be further treated with anti-reflective (AR) coatings to reduce the overall reflectance from the regular 8% for float glass, down to 3% or even less.

Behaviour

Mechanically, SPD SmartGlass resembles any security-compliant laminated glass panel, acting as a mechanical, acoustic and thermal buffer. A typical security certification might be, for example, BS EN 356 (P4A 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, transitioning from opaque to transparent in the visible light range within 1 second and from transparent to opaque in 1-5 seconds.

The spectral response of SPD film when powered shows a flat characteristic between 400 to 700 nm (the visible light range), proving that no extra colouration is added by the SPD film itself. In the unpowered state, SPD SmartGlass has a blue tint, as can be observed in the image above.

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 stabilizer dissolved in it. The stabiliser reduces the tendency of the particles to agglomerate, thus keeping them dispersed and in suspension.

Electrically, the whole construction acts somewhat 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 SmartGlass 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 SmartGlass is the reduction in Power Factor that results from plugging any reactive load into the mains, which can result in power inefficiencies and cost penalties from your electrical utility. This article explains Power Factor with a very nice analogy to beer.

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 90um in thickness. If the SPD film is 30um 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 SmartGlass is 100v AC, normally a sine wave. For electrical outlets at 240V AC, a step-down transformer is required. For safety reasons, this transformer also serves to isolate the final user from any electrical faults that might occur.

References

  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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