Anti-Reflective Smart Glass

In a Nutshell

If we engineer smart glass with special anti-reflective (or low-reflectance) coatings, it acquires the following benefits:-

• Increases throughput of visible light, which improves sustainable daylighting in buildings, vehicles and smart city infrastructure
• Limits reflected glare and light pollution on adjacent buildings, wildlife and road traffic
• Increases the efficiency of transparent photovoltaic (PV) smart glass in windows, doors, skylights and in PV building cladding
• Improves the clarity and visibility of objects placed behind the smartglass, such as in art museum display cases and retail shopfronts

Other applications that need anti-reflective or low-reflectance glass (including smart glass) include:-

• photochromic or electrochromic eyewear
• electrophoretic smart pricing labels for retail
• augmented / virtual reality headsets
• smart televisions with gesture-recognition
• facial recognition in mobile phones
• vehicle smart dashboards
• heads-up displays in transportation

Anti-reflective windows

Anti-reflective windows are literally the Holy Grail of the smartglass sector.

Something that we at Smartglass World and our parent company, ArtRatio, have been grappling with since 2008.

The inherent issue is the high reflectivity due to the multiple boundaries within any laminated glass. In addition we have internal boundaries introduced by smart films, transparent conductors and low-e coatings.

There is a lot to explain here, so let’s break this down one step at a time.

What is Reflectivity?

Light is reflected at the surface of a window because of the change in refractive index between air (refractive index close to 1) and glass (refractive index of about 1.5).

It’s a bit like walking through a swimming pool: the resistance of the water ‘slows you down’.

Similarly, glass ‘slows down’ light due to the ‘resistive’ interaction between the light wave and the electrons inside the glass.

The refractive index of a material is simply the ratio of the speed of light in a vacuum compared to the speed of light in the material. 

The reflectance at a typical air-glass interface stands at approximately 4%, so in the case of a single pane glass window (with one air-glass interface, and one glass-air interface), the total reflectance is around 8%.

That number is annoyingly noticeable when you are trying to watch television with sunlight reflecting from the screen.

As indicated in the diagram below, some of the light is reflected at the surface, some is refracted into the glass (suffering a deviation due to the change in its speed); and some makes it out to the other side as transmitted light.

Why is reflectance a problem in smartglass?

Smartglass is composed of laminations, coatings and transparent electrical circuits that each have their own refractive index.

As light encounters each of these materials, it is ‘slowed down’ and some light is reflected at each internal boundary, as well as being reflected at the top surface of the smart glass stack.

The following diagram shows a ‘typical’ anti-reflective smart glass stack (representative of a number of smart glass technologies):-

Smart glass can have multiple layers including:-

• switchable films for tinting or privacy
• transparent conductive layers, e.g. indium tin oxide (ITO)
• embedded transparent LEDs
• photovoltaic harvesting of sunlight
• low-emissivity (low-e) thermal rejection coatings
• thermal insulation gaps filled with argon, air or vacuum
• acoustic soundproofing with PVB plastic

Anti-Reflective Glass Coatings

There are two main types of anti-reflective coatings: (a) thick-film, and (b) thin-film.

(a) Thick Film Coatings

As you can see from the above diagram, a thick-film layer sits between the air and the glass. 

The anti-reflective layer has a refractive index between that of glass (1.5) and air (1), typically around 1.3.

Offering a refractive index to the incoming light with a value midway between that of air and glass is what minimises the reflection at the air-glass boundary.

The refractive index of a thick-film coating does not depend on its thickness, as long as it is greater than the wavelength of light at which the coating has been designed to operate.

Thick films are normally deposited as suspensions (e.g. pastes and inks) on glass via processes such as screen printing.

(b) Thin Film Coatings

A thin-film is a nanoscale coating applied to one (or both) surfaces of glass with vapour deposition, which reduces the reflectance at each air-glass boundary down to 1% or smaller.

The light waves reflected from the top and the bottom of the thin-film layer are one-quarter (or one-half) of a wavelength ‘out of sync’ (the correct term is ‘out of phase’). 

When the wavefronts ‘interfere’ with each other on leaving the glass, they cancel each other out. We say that the light wavefronts have ‘interfered destructively’.

On the other hand, transmitted light waves do not suffer this difference in phase and so add together constructively to produce colourations and undesirable effects when looking from various angles of incidence.

3D-nanostructures

Nature is wise. 

Over millions of years, moths’ eyes have evolved into a surface pattern consisting of bumps measuring nanometres in height and width, which are capable of practically eliminating reflected light. 

This relies on the bumps being smaller than the wavelength of visible light, so a photon of light will effectively experience a continuous refractive index gradient at the air interface, which reduces reflectivity drastically.

Artificial nanostructures have been built as 3-dimensional pyramids or 2-dimensional grooves, which exhibit reduced reflectivity, as long as the textures are much smaller than the wavelength of visible light.

Other Techniques

Other techniques used to reduce reflection include lithography, sol-gel, etching and multi-stage deposition methods but often require long processing times or environmentally unfriendly chemicals.

Outlook

The technology behind low-reflectance smartglass is in continuous research and advancement.

At Smartglass World we look forward to seeing important reductions in the high reflectivity of smart glass, from its inherent value of around 8%-10% down to 0.1% or lower.

This would facilitate the use of smart glass in complex applications such as retail displays, smart spectacles and augmented-reality wearables.