Electronic components, Electronics, Light

Liquid Crystal Display

I receive requests to explain how the liquid crystal display (LCD) works. Which is this post’s subject.

Light polarization

Before understand LCD it is necessary to understand light polarization. The light is an electromagnetic wave, therefore has electric and magnetic fields perpendicular to the propagation direction. The non-polarized light has your electrical field scattered in random planes.

Exist polarizing filters which allow only that only one of the light’s electric fields pass through. Vertical polarizing filter allow only electric field and horizontal polarizing filter only let pass horizontal field.

Polarizing filters has long chain molecules and the alignment of molecules which determinate the light polarization’s direction. If the alignment of molecules is horizontal, the axis polarization is vertical and vice-versa.

Liquid Crystal

The liquid crystal is a substance in a physical state between the solid and the liquid, and your molecules are positioned almost in the same direction.

The liquid crystal can be divided in others physical states.

Nematic phase is a state similar to liquid, but the molecules appoint to the same direction, in the smectic phase the molecules get arranged in layers one above another, in the cholesteric phase or twisted nematic the molecules orientation in each layer remind a spiral, like showed in the figure below.

Other phase is the columnar which forms discs, but it is only for discotic liquid crystal or with disc shape molecule. Liquid crystal for display are calamitous or has elongated molecules.

The display

The crystal liquid most important property is the direction change with the passage of an electrical current. A crystal in cholesteric phase submitted to an electrical current change to the nematic phase in many degree depending on the applied tension.

When an electric field is not applied there is light passage, with electric field there is not light passage. In a LCD, each pixel is controlled separately electronically. In the next post about electronics, I will show how a LCD is manufactured.

About Pedro Ney Stroski

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