23 Sep 2016 | Torben Rasmussen |

It has been 8 years since we published our first overview comparing two display technology. Back then, it was plasma and LCD but today plasma is no longer and a new competitor has arisen in OLED. In this overview we will look at the differences between the two technologies, pros and cons, and more.

We will start out with a short introduction of how each display technology works. We will be using the term “panel” because the display technology is one – although very important – component of a functional TV screen. Besides the panel, there are support structures such as a cabinet and a frame as well as lots of electronic circuits.

LCD panels

LCD panels have not changed fundamentally over the last 10 years. LCD is short for “Liquid Crystal Display” and describes the active element of the display that is made from liquid crystals. In an LCD display the light source is located behind the panel and emits light from a rack of LEDs (Light Emitting Diodes), whereas earlier, manufacturers would use more bulky fluorescent lamps (CCFL). The LEDs can either be placed either along the edges of the LCD panel (Edge or edge-lit LED) or behind the LCD panel (direct or backlit LED).

With edge-lit LED it is difficult to control the light intensity locally across the screen, as the LEDs emit light from the edges only. This light has to be reflected to cover the entire screen area with the help of a light guide. Therefore light intensity can only be controlled in bands. With backlit LED the diodes are placed behind the panel, which allows for better local control as the LEDs can be arranged in a grid and potentially be addressed individually or in zones. The highest number of diodes in an LCD display to date is 5200, which is only possible on a large 100” panel. Each diode will therefore illuminate approximately 6400 pixels (Ultra HD resolution).

Light is typically reflected through several plastic layers that serve the purpose of making light distribution homogeneous. These light diffuser layers will sit between the LEDs and the LCD panel, meaning that the complete backlight unit takes up a considerable amount of space, making the TV thicker.

LCD construction

To control light intensity for each of the basic colors, LCDs use liquid crystals inside each pixel. These crystals can be rotated in the cell by manipulating an electrical field, making it possible to adjust the light throughput. Once the light intensity of a basic color has been adjusted to its desired level, it passes through a color filter that filters out all wavelengths except red, green or blue. If you move close to the screen you can see these red, green and blue sub pixels that in pairs of three make up one pixel.

How fast an LCD panel can change color depends on how fast the liquid crystals can move into a new position. This determines the amount of blur/trailing you see in the picture. Ideally, you want the liquid crystals to assume the new position (color) instantaneously. However, that is not possible and it usually takes the liquid crystals a little time (measured in milliseconds) to do so, meaning that blurring/trailing can occur.

When light passes through the liquid crystals light intensity can no longer be sustained in all directions. If you think about an incandescent bulb it can emit equal amounts of light in all directions (it is called a Lambertian emitter) but in an LCD display the use of liquid crystals means that the panel will emit most of the light directly forward. Or in other words; light and color intensity will look strongest, and purest, only right in front of the screen. This phenomenon is important in order to understand the concept of “viewing angles”.

OLED panels

As you may know, OLED is a special kind of LED, namely an organic type (“O” for organic). We have previously explained how light is created in a LED (article only available in Danish, sorry) but since then we have witnessed the emergence of displays in TV-size formats, and the different types of OLED panel technologies that we discussed to lengths in 2009 have been reduced to only one. The OLED panel is in structure different to that of LCD since light intensity is no longer controlled with a LC (liquid crystal) layer. Instead, each single sub pixel is its own LED (or to be precise: OLED). In this structure several layers of the sandwich can be eliminated as each sub pixel emits white light.

This light – that is emitted separately for each individual sub pixel – then passes through a passive color filter, like with LCD, and out through the display. The method to regulate light intensity and the mix of red, green blue (RGB) involves controlling the intensity of the given OLED that is placed behind the red, green, and blue sub pixels, respectively. If you want a pure black color you simply turn off all sub pixels and if you want white you increase intensity to equal amounts in all of them.

LCD construction

The way that these OLED panels are produced allow manufacturers to make them considerably thinner than the LCD’s LED light source, which is why OLEDs are thinner than LCDs. If you peel off the LED backlight unit an LCD panel is also very thin but it requires a backlight to reproduce a picture.

It is important to emphasize that LG Display is currently the only commercial manufacturer capable of mass-producing OLED TV panels, which is why LG is controlling the development of OLED TV technology at the moment.

Technology comparison

In the tables below we summarize the characteristics of the two technologies.
Symbol explanation
Features specific to LCD technology
Features specific to OLED technology
Additional comments
about the category

Finishing remarks

The goal of this article is not to declare a winner or loser among two competing display technology but rather to outline advantages and disadvantages inherent to each technology. We leave it to you to decide which attributes matter the most. At the moment OLED TVs are still significantly more expensive than LCD TVs, but are expected to become more affordable over the next few years.

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