How do LED's and LCD's differ?

This question brings back memories of the first LED digital watches. They were able to display the time but you had to press a button to activate it because to allow it to display the time continuously would run the battery down. It didn't matter. It was cool to have one!

It wasn't long until LCD watches came on the scene and LED watches were a thing of the past. This transition to LCD's also happened to LED calculator displays. Eventually LCD based screens replaced TV and monitor picture tubes.

Plasma displays preceded LCD's and coexisted for a time with digital tube TV's. Basically, a plasma display is a bunch of tiny neon-like cells that are fired up by electrical currents. A single pixel has three plasma cells that emit the required red, green and blue light by means of phosphors. To me this idea was an intermediate concept between the original color picture tube and LCD screens. Plasma displays suffer from higher heat output and are therefore less efficient. The original plasma displays eventually suffered burn-in, but they could be made in very large sizes. When LCD screens were made larger, this advantage was lost.

LED stands for Light Emitting Diode. A diode is a two-lead, pn-junction electronic device that allows current to pass in one direction only (current flow from the p-side anode to the n-side cathode). It turns out that when a suitable voltage is applied to the leads of an LED diode, electrons combine with holes (see my post on the transistor) and produce light. The first LED's emitted red light, but since then, technology has produced LED's that emit light across the spectrum, including infrared and ultraviolet. LED based lamps have begun to replace incandescent bulbs because of the fact that they are much more efficient. LED bulbs use red, blue and green LED's in tandem, or use phosphors to obtain the color mix from a UV-LED.

LCD stands for liquid crystal display. The reason it's called this is because it uses the fact that liquid crystals can change their opacity because of an electrical charge. The way a liquid crystal works is more complicated. It depends upon a property of light that allows a wave of light to be twisted. They call this idea polarization and it's because light waves can oscillate in more than one direction. Light can act as a wave of electromagnetic radiation and as particle, a photon. The wave property of light is something that can be bent and twisted. Think of Polaroid sunglasses. They use a polarizing filter to prevent UV radiation from penetrating the glass that is coated with the filter material. Since light waves oscillate in both the horizontal and vertical directions, a polarizing filter can stop one of the directions. Light reflected off of a shiny surface is strongly polarized and therefore partially blocked by the filter, so the idea works.

Some brilliant people saw the utility of this polarizing concept. All one needs is a chemical whose molecules become aligned in the direction of an applied electrical field. These chemicals are called liquid crystals. Liquid crystal molecules twist in an electrical filed. So, what you need for this to work is a thin layer of this liquid crystal substance between two thin transparent electrodes and two polarizing filters, one parallel and one perpendicular to the axis of light transmission. Keep in mind that an LCD needs a backlight to work. The backlight is blocked according to the amount of voltage applied to the transparent electrodes. Of course these liquid crystal cells must be tiny, basically the size of a pixel and there must be some way to electrically address these pixels arranged in an array in order to make an LCD screen. Most LCD screens use active-matrix LCD's in which each pixel cell has its own transistor, and a LED light panel backs up the array. Color displays are made by incorporating color filters on pixels much like color televisions did in that age of tubes.

The science behind the LCD display would require an entire book. My aim here was to show the difference between the two technologies. These two amazing inventions have completely changed the way we see television and movies, and how we see the world on our cell phones.

Thanks for reading.