How to Light

Two-minute explainer: Liquid crystal lenses

There is a hierarchy in what lighting designers want a lighting fixture to do:

1. Direct its light towards a given subject.
2. Manage the shape of the light beam (circular, oblong, rectangular, patterned).
3. Be able to adjust the shape of a light beam within a projector.
4. Be able to adjust the light beam in situ.

In the history of lighting, we’ve seen those three states manifest in these ways:

1. Put a lamp in a tin can – the light goes forwards.
2. Put a reflector around the lamp to create a light beam of a given diameter.
3. Put a lens in front of the lamp and reflector to enable that beam to be shaped.
4. Put a motor inside the projector to move the lamp assembly in the housing.

This describes a classic theatre projector – or a later-generation architectural projector. And they all have one thing in common: altering the characteristic of a light beam has always required a mechanical solution, because something has to move, whether that be the lamp, the reflector or the lens.

The refractive index is different for the long and short axes of liquid crystal molecules. Due to these differences, when the molecules align to the shaped field, a gradient of refractive index – hence a lens – is created.

The liquid crystal lens removes that problem.

We’re used to seeing liquid crystal displays (LCDs) on TV screens and the information displays of electronic devices. They are based on the principle that an electrical field acting on a liquid crystal layer will alter the optical character of that layer.

When there is no electrical charge applied to a layer of liquid crystals, it may appear transparent – but a charge will cause the molecules to align differently, thus changing the refractive index of the liquid crystal lens. The beam distribution of a light fixture using liquid crystal lenses is determined by the electrical field applied to the lens. The ‘at rest’ condition permits light to travel directly through the liquid crystal layer (the layer is typically only around 50μm thick). The strength of the electrical charge determines the change in the molecular alignment, thereby creating a fully adjustable light beam.

An important feature of this is that the light beams can be adjusted remotely, without needing direct access to the projectors. This can be achieved in a number of ways, but the most popular is via a Bluetooth-enabled app that enables control from a mobile or tablet.

 

Thanks to Lensvector for the use of the graphic.