The simple answer is just about everything!
If there is one question that we get asked more than any other at the moment it's probably "what's the difference between an LED and an OLED TV?"Despite the similarities in their names the two technologies are very different and, in reality, an OLED TV has more in common with a plasma TV than it does with an LED TV. The confusion really stems from manufacturers using the term 'LED' TV in their marketing over the last five years when, in actual fact, there's no such thing as an LED TV. What they actually mean is a TV that uses an LED backlight to illuminate an LCD panel, so the term they should be using is LED LCD TV. Of course, that's not very catchy and doesn't sound like something exciting and new, meaning that many consumers have been buying 'LED' TVs thinking it's a different type of technology when, in actual fact, they use LCD panels that have been around for decades.
This marketing misdirection to emphasise the use of an LED backlight was fine until OLED came along, when suddenly the manufacturers had consumers wondering how this genuinely new technology differed from the 'LED' TV that they'd just bought. It certainly made things difficult for the manufacturers themselves, who were hoisted on their own petard as they tried to explain why OLED was different from the 'LED' TVs that they had been aggressively marketing for the last few years. In this article we'll go through the technologies behind both LED LCD TVs and OLED TVs, describing how they work and why they differ from one another. Hopefully this will help you make a more informed decision the next time you decide to buy a new TV.Although manufacturers have been referring to 'LED' TVs for years, there's actually no such thing as an LED TV.
The majority of the TVs that are currently available to buy use a Liquid Crystal Display (LCD) panel. As the name suggests, this panel is comprised of a layer of liquid crystal that is sandwiched between two polarised filters and then illuminated by a light source behind the panel. The image above shows the basic construction of any LCD TV with the backlight at the rear, then the TFT (Thin Film Transistor) Array, the liquid crystal layer and the colour filter. When an electrical current is passed through the liquid crystal layer, the alignment of the crystals in each pixel changes from closed to open, thus allowing the light from the back light through. However, any LCD TV struggles to deliver deep blacks because the light source is being passed through the panel and, even when the pixels are closed, some light will still leak through.
When LCD TVs were first released they used a Cold Cathode Fluorescent Lamp (CCFL) as a backlight, which is essentially the same light source you would find in a kitchen strip light. Whilst a CCFL backlight was very effective, it did make the TV itself quite deep and so, in an effort to make them slimmer, the manufacturers started using LEDs as a light source instead. LED stands for Light-Emitting Diode and the definition of a diode is a two terminal electronic component. In the case of an LED, when an electrical current is passed between the two terminals it glows, much like a light bulb. LEDs are smaller, brighter and more energy efficient than a CCFL backlight and they can produce purer colours, so their use offered a number of benefits. However perhaps the most obvious benefit from the perspective of both manufacturers and consumers was that LCD TVs using an LED backlight were much thinner. The reason for this is that the manufacturers could position the LEDs along the sides or top and bottom and then bounce the light off mirrors behind the panel, thus illuminating it.
These ultra-slim LED LCD TVs proved to be hugely popular with consumers, kicking off an arms race amongst manufacturers as they tried to produce ever thinner TVs. There are even some recent LED LCD TVs that have positioned the LEDs at the bottom, along with the speakers, connections, power supply and electronics, creating panels that are only 1cm deep. Unfortunately these ultra-thin LED LCD TVs have a weakness because the use of mirrors can result in backlights that are sometimes uneven or cloudy with bright corners and edges where the LEDs are located. Recently there has been a move towards using direct LED backlighting, where the LEDs are positioned directly behind the panel. This does mean slightly deeper TVs but the backlight uniformity and brightness can be significantly improved. This approach does sometimes result in banding, where the LEDs are actually visible behind the panel but, on the whole, the direct approach is the best way of illuminating an LED LCD TV.
Despite the similarities in their names, an OLED TV actually has more in common with a plasma TV.
OLED by comparison is a self-illuminating technology, just like plasma, which means that the panel itself produces its own light, rather than being illuminated by a light source behind the panel. OLED has been evolving for a number of years and LG, in particular, have invested a great deal of money in its development. There are many, including AVForums, who feel that OLED is a revolution in terms of display technology and represents the future of television. So, how exactly does OLED work and what benefits does it actually offer? Well, an Organic Light-Emitting Diode (OLED) is a device which is composed of a layer of organic compounds which emit light in response to an electric current. It's essentially the same principle as the LEDs used in an LED LCD TV, except that in the case of an OLED TV the entire panel is effectively the LED rather than just a light source behind it.
So with an OLED TV the conducting material is composed of a layer of organic compounds within the panel itself which glow when a current is passed between the two terminals, and since one of these terminals is transparent it creates a screen, as seen in the image above. This basic OLED technology has been around for a number of years but it's the recent development of active-matrix OLEDs that has resulted in the creation of high definition and Ultra HD 4K panels and larger screen sizes. This active-matrix is a thin-film transistor grid which sits at the back of the panel and can switch each individual pixel on or off, thus allowing an unprecedented level of control over the image. So for example an Ultra HD image has a resolution of 3840 x 2160 pixels, which means it's composed of over eight million pixels and thanks to the active-matrix, each pixel can be individually illuminated.
The huge advantage that an OLED TV has over an LED LCD TV is that, due to its self illuminating nature, it can deliver incredibly deep blacks because when the current to a pixel is turned off the pixel goes completely black. In fact an OLED TV can go even blacker than a plasma TV because although plasma is also self-illuminating, when the pixel is turned off it doesn't go completely black, there's still a slight after-glow. As we've already mentioned, an LED LCD TV struggles to deliver deep blacks because the backlight will still leak through the panel, even when the pixels are closed. The ability to deliver really deep blacks means that an OLED TV has a very wide dynamic range - which is the difference between absolute black and peak white - thus giving images greater impact. These incredibly deep blacks can mean that OLEDs struggle with gradations in areas just above black, resulting in the crushing of some shadow detail. It's possible that this is a limitation of the technology but careful setup can help in this area
Aside from the blacks, the other obvious advantage when it comes to an OLED TV is that the panel is also incredibly thin. We have reviewed OLED TVs that use panels less than 5mm deep and, more importantly, this ultra-slim design is achieved without sacrificing any screen uniformity. The major disadvantage that OLED TVs have at the moment is that they remain difficult, and thus expensive, to manufacture. The production yields are gradually improving and prices are falling but a mature technology like LCD is much easier to manufacture and thus cheaper. As a very new technology OLED also has a few question marks surrounding its long term performance and reliability. There's also the possibility of screen burn or image retention, although the manufacturers appear to have taken effective measures to mitigate this potential issue.Whilst both technologies have their strengths and weaknesses, OLED is generally considered to be superior.
An LED LCD TV isn't just cheaper with a more established performance record, it also has a number of other advantages over OLED. First of all an LED LCD TV is much brighter than an OLED TV, which can be very useful in rooms with ambient light; although conversely an OLED TV performs much better in a dark room or one with very little ambient light. The increased brightness also helps LED LCD TVs support High Dynamic Range (HDR) content by delivering the increased peak brightness. However dynamic range isn't just about brightness, the black levels are also important in order to deliver that range between dark and bright. Since black levels are an inherent weakness of LED LCD TVs, the manufacturers have developed a number features designed to help improve the performance. The first feature designed to enhance black level performance was the development of VA (vertical alignment) LCD panels which can block out more light and thus improve the native black levels.
The other feature developed to address the inherently poor blacks on LED LCD TVs is called ‘local dimming’, which breaks the image down into a number of different zones - as shown in the image above - the more zones the more effective the local dimming. The TV's processing analyses the image, turning off certain zones when that part of the image is dark. Whilst this does improve the perceived black levels, due to the limited number of zones that can be dimmed it also introduces additional problems like ‘haloing’, where a light object against a dark background appears to glow. The only way to completely eliminate such issues on an Ultra HD 4K TV would be to have 8 million individual dimming zones, one for each pixel, which is obviously impractical. However, due to its self-illuminating nature that's exactly what an OLED TV does do, with each pixel individually lighting up or going completely black.
We mentioned VA panels earlier and although these do help improve an LED LCD TV's native blacks, they also have a downside - a limited viewing angle. What this means is that as you move off-axis from the centre of the image it loses luminance (brightness) and contrast performance, becoming washed out. This isn't a problem if you're sat directly in front of the TV but if there's more than one viewer, the off-axis performance becomes very important. As the image above shows, OLED TVs deliver a much better off-axis performance, retaining more brightness and contrast as you move away from the centre. This means that for multiple viewers an OLED TV delivers a more consistent brightness and contrast performance, which can be important with HDR content.
The development of quantum dot technology has resulted in LED LCD TVs delivering a wider colour gamut than OLED is currently capable of but in many respects OLED can still deliver a superior colour performance. First of all, just as with brightness and contrast, an OLED TV is capable of a superior colour performance off-axis. Secondly because an OLED TV is self-illuminating it is also capable of more accurate colours in darker scenes. In the same way that an LED LCD TV's blacks are affected by light leaking through from the backlight, the same issue affects the colour accuracy. This is important because colour performance isn't just about how big the colour gamut is but also how accurate the colours are at different brightness levels.
So there you have it, an OLED TV is a completely different type of display using an exciting new technology and should not be confused with an ‘LED’ TV which is nothing more than a misleading marketing name for an LCD TV that uses an LED backlight. An OLED TV uses self-illuminating organic compounds to deliver superior images when compared to LCD TVs, with better blacks and higher contrast ratios. It also offers wider viewing angles and whilst there are still some question marks surrounding the lifespan of the panels themselves and the possibility of screen burn or image retention, the biggest issue at the moment is the difficulty of production. Due to this OLED TVs are still relatively expensive but their prices are dropping all the time and it's only a matter of time before they reach mass market acceptance.
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