A Study on Smart Glass Technology and Applications

Presented by Stanley Varkey Raja Babu Rakesh Kumar

OUTLINE

WHY Smart Glass Technology INTRODUCTION to Liquid Crystals TYPES of Smart Glass Technology

Electrochromic Glass PDLC SPD

Why Smart Glass Technology?

Use of fixed tint gives no control over transmittance according to changing needs.

Increased energy consumption to offset effects of fixed transmittance.

LIQUID CRYSTALS-

The distinguishing characteristic of the liquid crystalline state is the tendency of the molecules (mesogens) to point along a common axis, called the director. 

Initial step towards Smart Glass Technology

LC Phases

Nematic Phases Chiral nematic

Ability to selectively reflect light of wavelengths equal to the pitch length, so that a color will be reflected when the pitch is equal to the corresponding wavelength of light in the visible spectrum.The Pitch can be controlled by temperature.

External Control of LC orientation

LC have property of birefringence(multiple refractive indices.

When an external electric field is applied to the liquid crystal, the dipole molecules tend to orient themselves along the direction of the field.

Light and Polarization

If two polarizers are set up in series so that their optical axes are parallel, light passes through both. However, if the axes are set up 90 degrees apart (crossed), the polarized light from the first is extinguished by the second. 

Effect of Birefringence on Polarisation

Light polarized parallel to the director has a different index of refraction than light polarized perpendicular to the director.

Use of LC for controlling light emission

ApplicationsLiquid Crystal Displays

Liquid Crystal Thermometers Chiral nematic liquid crystals are used. Color reflected is dependent upon temperature. 

Body Imaging and diagnosisSpecial liquid crystal devices can be attached to the skin to show a "map" of temperatures.(Thermograms) Tumors have their own heat signatures which can be identified

Fault detection on circuit boardsLiquid crystal temperature sensors can also be used to find bad connections on a circuit board by detecting the characteristic higher temperature

They are used for non destructive mechanical testing of materials under stress.

Temperature Sensitive

PanesTungsten doped VO2 –reflects infrared in response to high ambient temperature

Electrochromic Glass

 A power source is wired to the two conducting oxide layers, and a voltage drives the ions from the ion storage layer, through the ion conducting layer and into the electrochromic layer.

It is a bistable system.Transparent Conducting Oxide-Indium Tin OxideEC layer-WO3

Electrolyte-Ta2O5

Specifications

Switching Time:~ 1 second at room temperature~ 1,000,000 cycles w/ 2% loss in contrast

Power Requirements:~ 1.5 mW/cm2

Memory:~ 2 days after switching off, best device is 2 months

Photopic Transmission:~ Bleached 70% (Transmission State)~ Colored 28% (Reflection State)

Advantages

Bistable- Power required only for changing state

Tunable transparency

DisadvantagessLarge size panels not possible – Response time decreases

Limited cycle lifetime

Applicationsa) Possible to achieve excellent

viewing properties with better contrast, particularly at off normal angels, than in the conventional liquid crystal based displays.Can be made flexible-allowing for flexible displays.

b) An electrochromic film can be used to produce a mirror with variable reflectance.

Antidazzle rear view mirrors built on electrochromic oxide films

 c). Variable transmittance so that a desired amount of visible light and/or solar energy is introduced. Such windows can lead to energy efficiency as well as comfortable indoor climate. 

d) Crystalline tungsten oxide film at the exposed surface of an electrochromic device. Intercalation/denticulation of ions makes this surface infra reflecting/absorbing, i.e. the thermal emittance is low/high.

Employed for temperature control under such as for space vehicles.

•The main objective in the ECDs research for architectural that can control ,depending on the weather, the interior temperature and luminosity and therefore the energy consumption of the building.

•In Museums, the EC glass displays can be used to control the amount of light/radiation falling on the objects under display.

•EC light filters in specialised imaging systems and cameras.

•Cost of cameras can be brought down by using the an EC filter between the scene and image sensor to control exposure. The EC is connected to a photosensitive element which supplied the control bias.

Polymer Dispersed Liquid Crystal

Devices constructed from thin polymer films (poly vinyl alcohol)with randomly dispersed liquid crystal droplets in it.

STRUCTURE OF PDLC

Working Principle

OFF STATE:

The LC crystal domains –randomly aligned

Light faces varying refractive indices ranging between no and ne , hence scatters.

Refractive Indices:

-Polymer=np

-LC = no,ne

Working Principle(Contd.)

ON STATEWhen Electric Field applied, LC crystals align themselves in the same direction.

Light faces no due to LC .

The refractive index of the polymer,Chosen such that it is very close to no

Uniform refractive index allows for Minimum refraction.

Factors affecting the contrast ratio of PDLC

1.Cell gap and density of the droplets, they are proportional to the contrast ratio.

2.Temperature ,as temperature decreases refractive index seen by the light decreases and the refractive index of the polymer increases, the birefringence( optical property of a material having a refractive index that depends on the polarization and propagation direction of light) becomes larger, so it is the scattering. As a consequence, the off state become darker ,so contrast ratio is higher.

Technical Considerations

•The birefringence of the liquid crystalHigher the birefringence-Greater opacity in

OFF state.Lesser viewing angle in ON state

•Thicker filmIncreased scattering

•Liquid Crystal Diameter For optimum scattering of white light the

droplet diameter should be 1-2μm.

SpecificationsParameter Value

Response Time 2-100ms

Power Consumption 3W/m2

Visible Light(ON) 80%

Visible light (OFF) 40%

UV Block(ON) 98%

IR Block(ON) 40%

Visible angle 140°

Advantages •Do not require polarisers , therefore decreased loss of light intensity.•Wide viewing angles.•Easier fabrication than LC displays.•Large area screens are easily fabricated .•Due to higher transmitivitty , lower power backlit displays are possible.•Blocks UV radiations very well.•Lower response time than conventional LC displays.

Disadvantages •Trade off between ON state viewing angle and OFF state opacity.

•Non bi-stable.

Applications

Low power, large area displays.

As privacy windows without sacrificing illumination.

Use as Spatial Light Modulating Device for holographic data storage.

Colored displays by introduction of dyes in the LC/binder polymer.

Nano-PDLC potentially suitable for wave front correction devices in adaptive optics (used in astronomy, line-of-sight communications and ophthalmics).

Suspended Particle Device

Suspended Particle Devices(light valves) use liquid suspension which droplets of liquid suspension are distributed. Lightabsorbing microscopic particles are dispersed within the liquid suspension.

Working PrincipleOFF STATEIn its unpowered state the particles are randomly oriented and partially block sunlight transmission and view

ON STATETransparent electrical conductors allow an electric field to be applied to the dispersed particle film, aligning the particles and raising the transmittance

SpecificationsThe suspension includes the light control particle-polyperiodideAnd the disperse medium-polysiloxane(Silicone oil)

In transparent state a small electric current required continuously.

Response time depends on viscosity of the suspension medium

Transmission range - 10-75%

Switching time- 50-200ms

AdvantagesVariable transparency possible.

Large panel sizes-Response time not affected.

Principle of absorbtion rather than scaterring-better for thermal regulation

Faster than electrochomic technology

Can be applied to curved surfaces.

DisadvantagesElectricity is required to keep the window transparent

Currently quite costly.

Applications

Proposed use in optical computing because of fast switching times.

Since particles tend to absorb light, and the glass panel looks opaque, it can provide maximum protection to light-sensitive objects.

Allows for gradient of transmission which can be controlled by the bias voltage.

Architectural windows, window coverings, skylights and interior partitions

Can be used for protection against UV raditions.

Controlling the amount of heat entering a building-reduced energy consumption in terms of artificial lighting and air conditioners.

Low power consumption (for portable devices)

Fast switching (for hight frame-rate video applications)

Polarisation independent (for brighter and more efficient displays)

High brightness, contrast and colour resolution

FUTURE ASPECTS