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Flight Deck Windshield Colours

Images by Eric Guicherit at Zürich airport. He noticed that parked aircraft had interference type colours on their windshields.

Images ©Eric Guicherit, shown with permission
Window colours have mirror symmetry

Birefringent glass or plastic splits light into two polarised components

Thin film interference

Rays reflect inside a thin film of plastic. The two (or more) emerging rays have travelled different path lengths and their wave crests might be in-phase or out of phase or in some in between condition.

The in-phase condition depends (like birefringence) on viewing angle and colour.

We see patches of colour but in general they are weaker than those from birefringence.

Soap film colours are from thin film interference.


Laminated toughened glass is anisotropic and this causes it to be birefringent.  Light entering it splits into two distinct rays which are polarised and refracted differently.

The two rays have slightly different optical path lengths as they traverse the glass.  On emerging, their wave crests can be in phase and combine to give a bright colour. They could also be out of phase giving less or no light. The phase condition depends on the wavelength (colour) and the viewing angle.    We see coloured patches!

The colours are irregular because the anisotropy is generated as the glass is heated and cooled to laminate and toughen it.  The mirror symmetry of the colours on the two windshields speaks volumes that they are the result of anisotropy and strains deliberately introduced during manufacture.

Birefringence is best seen by illuminating the material with plane polarized light and then viewing or photographing it through a second polariser.  The colours can often be seen without deliberate polarization.   The multi-layered windows act in themselves as partial polarisers.

Passenger windows are also birefringent and can generate bands of false colour especially on photographs.

Car windscreens have toughened laminated glass – they show similar (but weaker) colour patterns.

Flight deck windshields have much to withstand.

There are extremes of temperature, +30 to -50C within 20 minutes or so after a hot weather take-off. Pressure differentials of a significant part of an atmosphere. 500mph winds. Bird and hail strikes.

They must not fail. Or, at minimum, they must fail without loss of pressure or structural integrity.

Their designs vary, but essentially they have (1) non structural glass or acrylic, (2) thick load bearing glass or acrylic layers, (3) inter-layers of plastics and (4) a heating layer of a thin conducting film or grid. A typical window will have 5-6 or more layers. The glass is laminated like that of a car windscreen, designed to crack in a controlled and safe manner.

The colours are generated by the laminated glass (birefringence) and possibly also by thin plastic layers (thin film interference).
A thin film of water from the rain might contribute but that is unlikely given the symmetry of the colour patterns.