LCD, OLED and QLED screens: a short guide on the differences

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Liquid chrystals, organic Led or quantum dots: the evolution of screens technology defines the quality, and also the life of the current digital signage solutions. In this article we will look at their meaning, characteristics and benefits.

LCD, LED, OLED, and QLED are the key technologies of digital signage solutions. They not only allow the creation of ultra flat screens of any shape and size, but guarantee a very high quality image resolution both indoor and outdoor.
Here is a brief guide on characteristics and differences.

What is LCD technology.
LCD, acronym of Liquid Cristal Display, is a technology that uses the optical properties of special organic substances (i.e. a carbon based compound). This compound allows the reduction of the display surface. The fluid is inserted between two glass surfaces coated with thin film transistors (TFT) that activate an electric field. Each transistor controls a small area of the panel, identifiable as a pixel (or a subpixel in colour screens). LCD screens can be used in two different modes. In the first mode, called transmissive, the display is lit on one side and viewed on the opposite side.
In this case light is positioned behind the screen and goes through the liquid crystals that act as a filter, allowing only the desired colour to get through. Although it uses more energy, this type of screen is brighter, and offers better viewing in dim light, while it becomes less clear in bright light (this is ideal for indoor use). The second mode, called reflective, uses less energy since it takes advantage of the light present in the environment, reflected by a mirror behind the screen. Contrast is thus lower in comparison with transmissive LCD, since the light has to go through the filter twice. The size of a LCD screen can vary from a few dozen millimetres to over 100 inches. The screens have an average life span of 50.000 hours.

What is LED technology
LED (Light Emitting Diode) technology consists of a series of semiconductor diodes that emit luminous radiation when electrical current passes through. This technology is supplanting the fluorescent tubes in the LCD segment, and does not use mercury, unlike some methods of backlighting. There are two main applications of LED technology: Edge Lighting (or Edge LED) and Full Array Backlighting (or Full LED).

Edge lighting
LEDs are positioned along the edges of the screen, so the light is distributed throughout the whole screen, and point inwards. Although the thickness is minimised, the depth of the black is not strong enough, and, when looked at closely, the perimetral area of the screen appears more lit compared to the central one.

Full-array backlighting (or Full matrix LED)
In this case LEDs are distributed evenly behind the LCD panel and are pointed towards the viewer, making for brighter colours and hightened contrast. One of the advantages of full-array backlighting can be observed in local dimming mode, meaning that each LED, or a specific LED area, can be turned on or off independently from the rest of the screen, in order to give the best  results in terms of brightness and darkness in that specific area.
The benefit of LED technology is the improved quality of image definition, with deeper blacks and better contrast, thus guaranteeing a longer life and less energy use.

What is OLED technology
OLED is the acronym of Organic Light Emitting Diode, which is the working principle used for this kind of display. The monopolar nature of the organic material layers that make up the OLED screen causes the electricity to go in only one direction. The first OLED models were very simple: a film of organic material was placed between two electrodes (anode and cathode); by applying voltage to the two electrodes the passage of electricity in the organic layer would cause the emission of light. These type of electrodes, however, were impractical since they required extreme precision in the production phase. For example, an imperfect alignment could cause big loss of energy, and consequently inefficient displays. Subsequent improvements led to the production of high luminosity OLED displays fed by low voltage, equal to 10 volt. Nowadays an OLED display is made of various overlapping layers: on a first transparent, protective layer is positioned a transparent conduction layer that acts as anode; then three organic layers are added: one for the holes injection, one for the transport of electrons, and between them the three fluorescent materials (red, green, blue) positioned to form one layer made of many elements, each of which is formed by the three coloured microdisplays. A final reflecting layer, that acts as cathode, is added. Despite the many layers, an OLED display is very thin.

Unlike LCD, with OLED technology it is possible to make colour displays that can emit their own light and do not require other components for illumination. This offers many advantages: not only the reduction of energy costs, but also the creation of ultra thin displays, that can be even bent and rolled up. The only drawback of this technology is the short life span: 5000 hours against the 60.000 hours of traditional monitors.

What is QLED technology
QLED is the acronym of Quantum Dot Led. Created in the late 90’s, it is a type of screen based on nanocrystals that act as semiconductors. Today quantum dot technology is the last frontier in research and development, supporting large, flexible screens that should not wear out as the OLED version. One of the initial applications of this technology was the filtering of LED lights for the backlighting of LCD screens. The properties and the performance of QLED screens are determined by the dimension and composition of quantum dots. The quantum dots, with dimensions ranging from 2 to 10 nanometres, can be either photoactive (photoluminescent), or electroactive (electroluminescent). In this way they can be integrated in the displays architecture. Screens made with quantum dot technology stand out for the depth of colour produced by the changing brightness. The advantages are the natural colours, the limitless life of the panel and reduced energy use.

Laura Zanotti, Journalist and Technical Writer

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