Photochromic smart glass changes tint (i.e. transmittance), becoming dark when exposed to shorter-wavelength UV light and returns to a clear state when no longer exposed to sunlight.
Photochromic smart glass is usually manufactured as a self-adhesive film and does not need 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.
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 IUPAC defines photochromism as 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.
a) 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, glass or polycarbonate.
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, with up to 30 seconds to fully darken in sunlight, and up to 5 minutes to return to a clear state once you return indoors.
b) 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 balloons air conditioning costs.
Photochromic glass can be configured as single panels, or as double- or triple- glazed units.
As we mentioned in our article on Translucent Glass, the “Energy Performance of Buildings Directive” from the European Union requires new buildings to be ‘zero-energy’ by the end of 2020, so photochromic glass would help architects to achieve compliance with this directive.
Photochromic glass can lead to sustainable architecture and green building points for standards such as LEED from the US Green Building Council and BREEAM from it’s UK equivalent, by meeting the minimum daylighting requirements of 300 lux across 50% of building interiors, without the accompanying increase in HVAC costs.
One manufacturer of photochromic film is HOHO Industries in Shanghai (China) who use a high-vacuum magnetron sputtering process (the same as used in semiconductor electronics manufacture) and nano ceramic layers to achieve various levels of transmittance.
The high-clarity, scratch-resistant, self-adhesive plastic film can be applied to interior window surfaces, giving UV and IR rejection.
c) 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, the daily commute may look very different by 2030, consisting mainly of working or resting as an autonomous vehicle drives you to work.
Photochromic glass on the vehicle interior 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 in the vehicle.
Photochromic vs. Thermochromic Glass
As the above 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.
We can thus expect to see some similarities between photochromic glass and thermochromic glass.
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.
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.