Introduction to LCD, LED, OLED, and QLED display technologies

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Display technology has been going on through an aggressive evolution since last few decades. This evolution has been driven by a consumer market that made televisions, computers and now smartphones, the basic need of the day. Once the technology, passed the era of Cathode Ray Tubes, LCD and Plasma emerged as the contending successor among displays. Plasma displays discontinued in 2014 after OLED displays took their place. Over this entire long journey, two technologies LED and LCD are still racing to contest against each other with their futuristic versions of OLED and QLED respectively. To avoid any confusion, it is worth mentioning that most LED TVs or monitors nowadays marketed as LED TVs or monitors are LCDs  with LED backlight, while the term OLED is a reference to an entirely different display technology. Quantum Dot LED (QLED) is improved LCD technology, fairing well compared to once elite-class OLEDs.

LCD Vs LED Vs OLED Vs QLED

Fig. 1: Typical Image of LED TV

In context to display screens, resolution, and type of display are the two most common questions before any buyer. With the advent of 2019, display screens with maximum 4K resolution (available as Ultra HD) are readily accessible in the market. Full HD and HD resolution screens are affordable alternatives for constraint budget. Well, the type of display – LCD/LED, OLED or QLED is another dilemma commonly faced by any buyer. Even if going with maximum resolution, i.e. 4K Ultra HD, it is available in all the three display technologies (LCD/LED, OLED and QLED). So, once settled with the desired screen resolution, it is then all about choosing the perfect type of display. This is the point where the majority of buyers have little knowledge or just are not aware at all. So, let us understand what LCD, OLED, and QLED display technologies are?   

LCD and LED-backlit LCD

LCDs  are made up of a backlight, one or two polarizing filters, liquid crystal array, and color filter. The liquid crystal array is sandwiched between two glass substrates. Some LCDs have a single polarizing filter before liquid crystal array while some have two where liquid crystal array is sandwiched between two polarizing filters. When LCD is powered, its white backlight switches on and liquid crystals rotate polarized light to block or pass light through them. The color filters divide light into red, green and blue light components which are blocked or passed by the liquid crystal array to blend the colors into an image. Each pixel has red, green and blue sub-pixels with liquid crystal serving as a light valve. By light coming from different sub-pixels, different colors are produced. In LCD, all the pixels are illuminated simultaneously, and the backlight remains on constantly.

Earlier LCDs used to have Cold Cathode Fluorescent Lamps (CCFLs) for backlight, while LED displays used to have LED backlight with an LCD panel. Nowadays, all TVs and monitors sold with LED label are LED-backlit LCDs. The LCDs using CCFLs are obsolete. So technically, LCD or LED displays now are LCD panels with LED backlight that are sold as LED TVs or monitors.

The LCD panels come in three types with descending contrast ratio and image quality as follow – In-Panel Switching (IPS), Vertical Alignment (VA) and Twisted Nematic (TN). The IPS LCDs have the best contrast ratio and image quality compared to VA while TN LCD panels have the worst among three. Therefore, if choosing an LED TV or monitor (which is LED-backlit LCD), IPS must be the preferred choice.

The LED/LCD TVs can have two types of backlighting – full-array and edge. LED TVs with full array backlighting have LEDs behind the LCD panel spanning across the entire length of the panel. LED TVs with edge lighting have LEDs only at the edges (top, top and bottom, left, and right or all four corners) for the backlight. Edge-lit LED TVs are slimmer while full-array lit LEDs are brighter, although some Edge-lit LEDs now have advanced technology that alongside giving the advantage of being slim has brightness comparable to full-array lit LEDs.

For better contrast ratio, LED TVs use local dimming. The dimming of the backlight to match the contrast of the picture is called local dimming. All full-array LED TVs carry this feature which makes their image quality as good as OLEDs and Plasma displays. Local dimming comes available in some edge-lit LED displays but is not as effective as in full-array LED displays. The edge-lit LED displays not having local dimming feature often have light coming out from the edges of the TV or monitor with low brightness at the centre of the screen.

OLED

OLED (Organic Light Emitting Diodes) displays replaced the once popular plasma displays. OLED is an emissive display technology. OLED is made up of a panel of pixel-sized organic compound which emits light in response to electricity. OLED display can individually control light coming from each pixel. On OLED display, color from the pixels is illuminated by electrochemical reactions of the organic compound. Until 2017, only LG was manufacturing OLED TVs at premium price tags. Now, Sony also has a line of OLED TVs in production. OLED displays are expensive due to design complexity and are mostly used only in high- end consumer products.

QLED -   

Developed and trademarked by Samsung, first QLED TV was announced at CES 2017. QLED is advanced LCD technology having quantum dots to enhance color quality. The white backlight in LED/LCD TVs is not white. The LED panel is made up of blue LEDs coated with yellow phosphorous. This panel produces a white like light but not exactly the white light as produced by traditional CCFLs. The light emitted by the LED matrix has an uneven intensity of red, green and blue components which affect the picture quality of conventional LED TVs with generally low-temperature white light at the background. QLED fix this problem with quantum dots. QLED has a quantum dot enhancement film (QDEF) between liquid crystal matrix and pure blue LED backlight. QDEF is a sheet of film saturated with quantum dots. Quantum dots are nanocrystal semiconductors that glow colors depending upon their size (which is at the atomic level). QDEF is saturated with quantum dots glowing red and green light which combines with the blue backlight to produce a pure white light. This white light has the equal intensity of red, green and blue colors, so that color filters of the LCD panel can efficiently filter it. This way, QLED has far better color quality and image blending compared to contemporary LED TVs.

QLEDs have the brightness like LED TVs with the advantage of a pure white backlight and image quality as good as or even better than OLED. The year 2018 has already witnessed the emergence of a new race between LCD/LED and OLED technologies with QLED giving a tough fight to OLED displays. Being based on LCD technology, QLED is expected to edge out OLED in the price war.