Why does projection onto glass work in the first place?

A question many ask – and one with a beautiful answer

When people first hear about projection onto glass facades, they often have the same objection: That can’t possibly work. Glass is transparent – a projected image would simply pass straight through it.

The objection is valid. And that’s precisely why it’s worth taking seriously: Anyone who understands why projection onto normal glass does not work will also understand why a wafer-thin film changes everything.

Why we see anything at all

An image is only visible if light from a surface reaches our eye. A cinema screen works because its white, slightly rough surface reflects the projector light in all directions. Physicists call this diffuse reflection: Every point on the screen becomes a small light source that radiates into the entire hall – which is why all viewers see the same image, no matter where they are sitting.

Therefore, it’s not the projector’s light that matters, but what the illuminated surface does with it.

Why normal glass fails

Glass does the opposite of a screen with light: It lets it pass through. Over 90 percent of incident light passes through a clear pane almost undisturbed – after all, that’s precisely what glass was invented for.

If you aim a projector at a shop window, the following happens: The image passes through the pane and lands somewhere behind it – on the floor, on the back wall, in nothingness. Only a faint shimmer remains on the glass itself, caused by the few percent of light reflected from the surface.

And this reflection is no help: It is directional, like a mirror. Only those standing at exactly the right angle would see anything at all – everyone else would see nothing.

[Illustration: Diagram – Projector beam passes through clear glass, only minimal directional reflection]

What the film changes

A projection film adds exactly what the pane lacks: a scattering layer.

Microscopically small structures in the film – in the case of PDLC films, unordered liquid crystal droplets – ensure that incident projector light does not pass through but is refracted at millions of tiny interfaces and distributed in all directions. The smooth, permeable pane optically transforms into a screen: Each pixel now glows independently and is visible from almost every angle.

Everyone who has seen light fall on frosted glass knows the result: The surface absorbs the light instead of letting it pass through.

[Illustration: Diagram – the same projector beam hits the film layer and is diffusely scattered in all directions]

The apparent contradiction: scattering yet transparent

The crucial question remains: How can the same surface be crystal clear during the day?

The answer lies in switchability. Scattering only occurs as long as the crystals in the film are disordered. When an electrical voltage is applied, they align themselves, their light refraction adapts to the surrounding material – and the interfaces optically disappear. The light no longer encounters resistance: The pane is transparent.

So, the film is not a compromise between a screen and a window. It is – electrically switchable – both consecutively: a window during the day, a screen at night.

[Comparison image: the same shop window pane in day mode (transparent) and projection mode (brilliant image)]

Why the result looks so brilliant

Projection onto film even has an advantage over classic screens: Operation usually takes place in the evening, when little ambient light falls on the surface. The projected image hardly competes with external light – contrasts appear deep, colors glow, and the surface seems to radiate from within.

Large dark image areas remain almost invisible, which is appealing from a design perspective: Content can appear to float freely on the facade.

Conclusion

Projection onto glass works not despite, but because of physics: Normal glass lets light pass through – a scattering film layer forces it to become visible. Diffuse reflection turns the pane into a screen, and electrical switchability turns it back into a window.

The solutions from Lumerax are based precisely on this principle – with films that master both states in high quality. The basic article in this series shows how the overall system is constructed.

Further articles

How does projection glass work?
What is PDLC film?
Can you still see through it during the day?