Most of the TV sets on the market today are of the LCD type with LED lighting (popularly called LED TV). However, there are several types of LED TV sets, which is one of the main reasons why prices vary so much between different models.
Admittedly, it does not have to be a problem if the picture is completely black since the TV can then dim the backlight. However, if the image contains both light and dark parts, it will be difficult for the TV. Then it cannot lower the backlight but instead must try to block as much of the light as possible in the dark areas. This usually results in the darker parts becoming more greyish than black. Premium TV LEDs have a solution to the problem in the form of “local dimming” (discussed later in this chapter).
The LEDs that illuminate the LCD panel of an LED TV can be placed in several different ways. The most common thing today is that they are located along the edges of the TV (called “edge-lit”). The light they generate is controlled behind the LCD panel and evenly distributed behind it. One of the biggest advantages of this technology is that the TV sets can be made incredibly thin.
The other solution is to place the LEDs directly behind the LCD panel (called “back-lit”). This solution is used in some of the cheapest and some of the most expensive TV models. The solution makes the TV somewhat thicker, but can also provide a great advantage for the contrast (explained later in this chapter).
It is not possible to say that one lighting technique is better than the other. The problems with uneven lighting that edge-lit TVs suffered from initially have less impact over time. Now the image quality is rather determined by how well worked the lighting is. An edge-lit TV from the premium segment may well have more even lighting than a back-lit TV from the budget segment.
The problem of uneven lighting is usually called “clouding”. This is because the light distribution gives the image cloud-like, bright areas. Unfortunately, the problem is not something that can be fixed, but you can minimize the experience of it by changing the TV’s picture settings.
The clouding problem is most evident in dark scenes, which can make it difficult to notice at a demonstration at a major electronics store (without any dedicated and authentically illuminated home theater). It is therefore recommended that all TV buyers read several reviews about an intended TV before the purchase!
How tangible the clouding problem is can also vary between different copies of the same TV model. It is also not usually considered to be a fault covered by the warranty. People who are sensitive to this type of problem should therefore carefully check the TV retailer’s terms of open purchase.
Another problem that edge-lit TVs can suffer from is bleeding. This means that excessive light is transmitted along the edges or in the corners. Bleeding is most evident in dark scenes (just like clouding).
Many premium TV LED models work with something called “local dimming” to give us the wonderful contrast we want. As the name reveals, the technology is based on being able to dim the backlight in selected areas in order to obtain a deeper blackness (without at the same time having to darken the rest of the image). The TV’s built-in computer constantly analyzes the image and adjusts the backlight as needed.
Local dimming is not a standard technology, which means that different TV models’ local dimming solutions work differently. Backlit TV models generally have a better chance of adjusting the lighting than the edge-lit ones. If a backlit TV is to show, for example, a full black night sky, it can dim all LEDs except those behind the moon. How good the result will be varies depending on how accurately and with what intelligence the TV can adjust its lighting as well as how many LEDs the backlight consists of. If it consists of only a few LEDs, local dimming produces a glorious effect.
Of course, the best thing would have been to have a LED behind each pixel. Since such a solution had required over two million LEDs (for a Full HD TV), it is currently too expensive to implement.
The TV manufacturers do not usually indicate how many LEDs the lighting consists of on their different models. Such a figure had also not said much since the intelligence of the system also affects the end result. Therefore, just how well a local dimming solution works is only reviews that reveal.
For obvious reasons, an edge-lit TV is more difficult to dim the lighting in selected regions. In recent years, the development has progressed at a rapid pace and today there are many TV sets that are really good at local dimming. However, the variation between different models is great, so just like with backlit TVs, it is recommended to read reviews to find out how well a specific model is performing. For a TV demonstration that is not performed in an authentic home environment, it is difficult to judge the result.
Quantum dots can be seen as the next evolution of the LCD panel. To meet HDR image requirements, screens with a wide color spectrum and high brightness are required. This can be achieved through the use of quantum dots.
Quantum dots are nanocrystals with the unique ability to pass through different wavelengths of light (different colors) depending on size. This allows the colors to be regulated by the size of the nanocrystals.
Today’s quantum dots
Quantum dots can refer to two different things. Today’s technology uses quantum dots to improve the backlight of the LCD panel. Ordinary LCDs use white LEDs with a yellowish light, making it difficult to get completely accurate colors. Quantum dot screens instead use blue LEDs. The quantum dots are then used to change the light to red, green or to retain the blue color.
In this way, the colors become more accurate (cleaner) than if a color filter with yellow backlight had been used. The quantum dots also block less light than a color filter. This makes the screens more energy efficient and bright.
Several manufacturers use quantum dots, including Philips and Samsung. Samsung’s technologies are called SUHD and QLED, where QLED is the latest technology. Instead of illuminating each pixel from one direction, the pixels are illuminated in a multiple-angle QLED screen. It makes the screen brighter and, above all, the viewing angles get better (the colors do not change as much when the screen is viewed from the side). The screens also use a new metal alloy around the quantum dots which gives deeper blackness.
Quantum dots of the future
The second definition of quantum dots is often called the Quantum Dot Light Emitting Diode (QD) LED and is a real contender for OLED. The great advantage of OLED is that individual pixels can be turned off and thus high contrast can be achieved. By using quantum dots that glow when set to voltage, the pixels in a QD LED display can emit light without the need for backlighting. It also allows individual pixels to be turned off, thus providing the same contrast as with an OLED screen.
The technology is similar in many ways to OLED. They use different methods to achieve roughly the same thing. QD-LED displays are not available on the market at this time, and it is, therefore, difficult to evaluate the technology.