Quantum dots: LCD technology takes on a new breath


The image on the TV screen is formed from small colored dots, three by one pixel in size. Today, most models use a technology based on liquid crystals that play the role of a shutter, becoming almost transparent at the right time and letting the light from the illumination system pass to the viewer.

Over the past ten years, the industry has made significant progress in manufacturing both liquid crystal films , and electronics, giving orders to change the state of molecules. This allowed achieving an impressive speed of image update, high contrast of the image, large viewing angles. However, liquid crystals are only part of the chain of conversion of electrical signals into visible light. Visible light! That's what developers are focused on today. In particular, the developers of Samsung with the new technology QLED, which is now used in the flagship TVs of the company.

In the struggle for better light

Existing modern technologies for backlighting the matrixes involve the use of LEDs. They give white light (the variant with three colored, red, green and blue LEDs is just a special case, their light is mixed in the right proportion to obtain white light). LEDs are economical, small-sized, easy to manufacture and install, relatively harmless in terms of ecology. However, LEDs have one significant drawback: the quality of the light emitted by them.

To obtain the RGB triad using a white light source, one more element is needed – a light filter. As a result of passing through it, the elements of the image get the desired color, but their brightness is naturally lost. Loss can be compensated by increasing the overall brightness of the backlight, though paying for such an improvement in energy consumption.

However, there is another problem that can not be solved by simply increasing the brightness. It is connected with the fact that the filter can only cut off unnecessary part of the spectrum, and add it not.

The intensity in different parts of the spectrum for LEDs varies greatly, and this prevents the correct color rendition in a wide range of brightness. By the way, the relative limitations of color coverage have the same roots. No filter, no algorithms pre-processing the signal will not allow to bring in the frame of the color that the screen is physically unable to show.

The output was found. Electroluminescence (the formation of light under the influence of electricity on various materials) in "classical" light-emitting diodes is good, but the best results can be achieved if we combine electroluminescence to obtain exciting radiation (by the way, not always in the visible part of the spectrum), and actually light for illumination "extract" thanks to the photoluminescence technology.

The blue LEDs produce waves with a large energy reserve. So large that it is enough to excite the phosphors, producing all the colors to produce white light by optical mixing. There is one nuance in photoluminescence: the phosphor emission spectrum depends very much on the size of its particles. Combining the necessary, engineers were able to achieve an excellent intensity ratio in the main parts of the spectrum.

Where did the term "quantum dots" come from? The thing is that very small particles of matter are shining, the units of their measurement are nanometers, billionths of a meter. To be precise, it is from two to ten nanometers. For comparison: the diameter of the erythrocyte, one of the main components of the blood, 7000-10000 nanometers.

 Quantum Dots

Fortunately, special accuracy for the distribution of quantum dots over the surface of the matrix is ​​not required, One cell needs a huge number of light-emitting elements. Today, production technologies allow the formation of areas of the right size quite simply and inexpensively, and the surface of the matrix can be not only even, but also curved.

Samsung Company in 2011 showed a display with a diagonal of 4 inches, and after just a few years, its engineers achieved an increase This indicator is more than an order of magnitude and industrial output with economically advantageous output of finished products.

The technology was named QLED. Studies show that it is not just cheaper in production, even compared to the "classic" LCD panels, but also provides a long service life of the products and a number of qualitative advantages. At the same time, Samsung is improving the material of quantum dots every year, for example this year QLED TVs with improved composition of quantum dots (formula and additional coverage) will be on sale.

Quantum dots and new quality standards

Quantum dot technology makes it possible to produce TVs that meet new requirements, the main of which – high brightness and enhanced color coverage. They are the basis of the recently emerged standards, primarily HDR 10. It requires that the brightness of the screen in nits or, as is customary in the C system, candelas per square meter, reaches 1000.

Quantum dot illumination systems allow such brightness , while maintaining another important image parameter – contrast. Thanks to photoluminescence, light output increases significantly, but not only efficiency.

As in the "normal" screen, white light is first divided into three basic ones in the QLED panel, and then the brightness of each sub-pixel is controlled by a matrix of liquid crystals to obtain the desired colors. However, due to the dips in the intensity of the white light spectrum from the LED backlight system, the matrix and the light filters must compensate for this defect. To preserve the correct color rendering, the overall brightness of the screen is artificially reduced. QLED, as already noted, has the best spectral composition of light and therefore fewer compromises are required from the matrix and light filters.

Quantum dots have made it possible to achieve bright and spectrally high-quality white light, breathing new life into the technology of producing liquid crystal screens.

Why did the movie industry demand such a high brightness? The task at all is not to turn your new TV into another lighting source for the living room. Simply, the general development of technology has allowed the introduction of HDR. HDR – enhanced dynamic range – improves the perception of a picture containing both very light and very dark areas, since they do not lose details and retain colors. In other words, the TV starts showing even minimal differences, for example, a thin texture of snow appears, details of clouds, leaves in the night forest, etc.

HDR-data contains not 8-bit information about each of the RGB channels but 10- bit, and in the future the bit capacity can grow. This allows to encode more gradations of both brightness and color (in fact, they are interrelated in the RGB model), which in practice means expanding the color gamut.

Since the technology of quantum dots makes it possible to improve the spectral composition of light, a panel with such a backlighting system does not only brighter, it is physically capable of displaying more shades. For example, the current generation of QLED in Samsung's flagship TVs displays 64 times more shades than with a color depth of 8 bits (16 million shades).

The improvement is noticeable in the transmission of very smooth brightness, but significantly different in color, smooth transitions, for example, clear sky before sunrise or sunset.

* * *

More recently, experts agreed that radiating imaging technologies have a clear advantage over blockers. However, quantum dots literally breathed new life into them. Today, the advantages of QLED, confirmed by the presence of real Samsung TVs on store shelves, give the buyer a real choice.

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