What is Photochromic Smart Glass?

Introduction

Photochromic smart glass is usually implemented by applying a self-adhesive photochromic film to existing glass and does not need any electrical power. Hence it is considered a passive smart glass technology. Depending on the manufacturer and model, transmittance values can range from 15%-30% when exposed to sunlight and 60%-75% when there is no incident solar radiation. Switching times are in the region of minutes. Compare this to thermochromic smart glass, which changes tint when exposed to heat. All smart glass technologies are fully aligned with the goals of the European Climate Law, which aims to make Europe climate-neutral by 2050.

Photochromic film from our Samples shop

A Brief History of Photochromism

Photochromism was first discovered in 1867 by Fritsche, but the term was not coined until 1950 by Yehuda Hirschberg of the Weizmann Institute of Science. It derives from the Greek words ‘phos’ (meaning light) and ‘chroma’ (meaning colour). The International Union of Pure and Applied Chemistry (IUPAC) defines photochromism as follows: “the reversible transformation of a chemical species between two forms by the absorption of electromagnetic radiation, where the two forms have different absorption spectra”. This implies that:- (i) photochromism can be triggered not only by visible light but also by other parts of the electromagnetic spectrum, such as ultraviolet and infrared radiation and, (ii) photochromism is not limited to changes in colour but can refer to any change in optical properties as a result of interaction with solar radiation. Photochromism has been observed in organic and inorganic compounds, and also in biological systems.

Applications of Photochromic Glass

Examples of photochromic glass can be found in spectacles, smart buildings and in transportation.

Photochromic Spectacles

Photochromic spectacles were first created in the 1960’s at Corning Glass Works and were originally impregnated with silver halide salts. Later developments are based on organic materials, since they are lighter and thus more comfortable to wear. The lens itself can be made of plastic or glass. Photochromic spectacles block harmful UVA and UVB radiation but are not effective when driving, since the windscreen blocks the very UV which causes photochromic glass to darken. An example of such lenses include the Transitions Signature brand from Essilor. As a result, photochromic lenses have been designed for drivers featuring a green tint in low light conditions and turning dark brown under strong light. Some products also polarise light which reduces glare from reflections. Examples are these are the Transitions Drivewear brand from Essilor and the PhotoFusion brand from Carl Zeiss. A major challenge in photochromic eyewear technology however lies in the relatively long switching times, typically 30-60 seconds to fully darken in sunlight, and 2-3 minutes to clear once you return indoors.

Photochromic Windows for Buildings

Photochromic windows or photochromic film applied to building facades are another excellent application area, since the incoming solar radiation results in glare which lowers productivity. Solar radiation also causes colour fading of furnishings and art collections and increases air conditioning costs. Photochromic glass can be configured as single panels, or as double- or triple- glazed units. Photochromic glass can lead to sustainable architecture and green building compliance with standards such as LEED (US Green Building Council) and BREEAM. This is achieved by helping buildings to meet the minimum daylighting requirements of 300 lux across 50% of building interiors, without the accompanying increase in air conditioning costs. One typical manufacturing technique for photochromic film is a high-vacuum magnetron sputtering process (the same as used in semiconductor electronics manufacture) that achieves various levels of transmittance. The high-clarity, scratch-resistant, self-adhesive plastic film can be applied to interior window surfaces, allowing low-cost retrofits of existing building facades.

Photochromic Glass for Transportation

Photochromic glass and film has obvious benefits for the automotive, marine, railroad and aviation sectors in the form of glare reduction for drivers and improved comfort for passengers. With the real possibility of autonomous electric vehicles and the ‘transport-as-a-service’ business model replacing vehicle ownership in the near future, the daily commute may move to rest and entertainment as the autonomous vehicle drives you to your destination. Photochromic glass on the vehicle would increase comfort and productivity by providing configurable control over the light entering the vehicle, as well as reducing the drain on the electric battery thanks to reduced air conditioning usage.

Photochromic vs. Thermochromic Glass

As the IUPAC definition of photochromism indicates, the ‘back reaction’ which returns the glass to a clear state can occur either thermally or photochemically, implying that there is an inherent relationship between photochromism and thermochromism. In fact, photochromic glass will not turn completely dark under high temperatures and may take longer to change state in cold environments, becoming very dark in colder climes.

Rendering True Colours

When you install photochromic glass in medical facilities (like hospitals, clinics and dental surgeries), you must also consider the colour rendering capacity of the glass. Glass with a high colour rendering index (CRI) allows all colours to be faithfully depicted, which permits above all red tissue to appear a true red. Lower CRI would result in an inaccurate colour rendering, causing tissue to look off-colour. More on this topic in our article about colour rendering.

Manufacturers

If you are looking for photochromic smart film 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 down if you specify product attributes, such as minimum transmittance, U-value or haze:

Smartglass World Marketplace Search for Photochromic film manufacturers, distributors and installers

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.

References

1. “Photochromism Molecules and Systems”, Dürr & Bouas-Laurent (2003), ISBN 9780080538839, URL 2. “Photochromic Coatings”, K. Fries, C. Fink-Straube, M. Mennig, H. Schmidt, URL. 3. “Photochromic Materials: Preparation, Properties and Applications”, He Tian, Junji Zhang (2016), ISBN 9783527683727, URL 4. IUPAC. Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. ISBN 0-9678550-9-8. URL. 5. IUPAC definition of Photochromism URL. 6. All About Vision