LCD and OLED Displays

TECHNOLOGY EXPLAINED

The most commonly available displays on the market today are LCD and OLED, with older technologies such as CRT and plasma being less and less common or even phased out altogether. OLED has been showing up more and more in recent years, with it gaining popularity with Samsung’s AMOLED displays dating back to the original Samsung Galaxy smartphone. These days we are beginning to see OLED used on TVs and computer monitors, albeit at quite a cost.

The LCD, or liquid crystal display, consists of a pair of fluorescent tube backlights (and possibly more on larger displays), which along with a diffuser for more even lighting shines through various layers including polarizing filters, a TFT (thin film transistor) array, liquid crystal laye and colour filter. Each pixel gives colour by controlling how much of the backlight was allowed through to each pixel on the colour filter, but as the backlight had a static brightness across the entire display you would end up with pixels which should be pure black glowing a shade of grey in the dark and only moderately high colour accuracy.

A technology which was widely touted in the late 2000s and early 2010s is the so-called LED display, but it is the backlighting rather than pixels themselves which use LEDs. These LEDs are normally white and can be either along the edge or as a matrix behind the entire panel. One advantage it offered on certain displays only is the ability to have multiple brightness zones, otherwise known as local dimming, where individual groups of backlight LEDs could be set to various brightness levels simultaneously as opposed to a single brightness level across the entire display at once. More advanced displays often used for photo editing and the like use RGB LEDs, resulting in a much wider colour gamut.

The next more widespread advantage was in power consumption, with LEDs using far less power than fluorescent tubes. The last major advantage is that the smaller LEDs and the lack of a transformer, which fluorescent tubes require to get the approximate 1,000v they require, resulted in a thinner panel than one lit by a fluorescent tube.

LCD displays work in various manners, with names for these technologies including TN, VA, IPS, AFFS and more. All of these displays work on the same basic principle of the backlight being shone through various layers, but the way these technologies work to change the pixel colours works in unique manners and will therefore be covered by future articles. These articles will ultimately be linked here.

OLED, or organic LED, works differently in that each pixels glows rather than relying on a backlight. This allows for better colour reproduction, as a pixel’s brightness is not affected by its neighbouring pixels. The displays can also be flexible, as we’re expecting to see on several upcoming cellphones. Additionally, each pixel being its own independent light source means that backlight bleed is almost zero, and a panel showing pure black in the dark can be effectively invisible.

The pure level of black means that the contrast ratio is effectively infinite, as the contrast is measured as a ratio of how many times whiter pure white is than pure black. On a traditional LCD display you might see a contrast level of around 1,000:1, meaning that white is 1,000 times whiter than black. The limiting factor here is that the black isn’t pure black. With pure black such as on an OLED display, it doesn’t have any white in it, and therefore even if you were to multiply the whiteness by 100,000 it would still be pure black. As you can multiply the whiteness by infinity and still have pure black, the contrast ratio is infinite.

LCD displays can sometimes be found offering incredibly high contrast ratios, such as 1,000,000:1 or even higher, but the caveat here is that it is a dynamic contrast ratio. This means that the black is measured with the backlighting at its lowest level and the white with the backlight at its highest level. These two brightness levels cannot appear on the screen simultaneously. With most images on a computer monitor having at least some pixels displaying pure white, the LEDs have to be running at full intensity which makes the dynamic contrast ratio little more than a marketing gimmick for computer use. On TVs, however, it can drastically increase the perceived dynamic range and contrast ratio.

OLED seems like the obvious choice, but it has several major downsides at the moment. The first is price, with OLED displays costing several times that of their LCD counterparts. The second is that they can suffer from motion blur, with LCD panels offering much faster response times. The last downside is that OLED displays can suffer from image retention or burn-in, where displaying the same static image for extended periods of time causes the pixels to permanently display a ghost of the image.

CRT, plasma and OLED suffer from this phenomenon. The CRT on an ATM will often have image retention of the logo or text displayed while the ATM is waiting to be used, and a plasma TV might show it where a channel logo or marquee text bar as used by most news channels uses, or at an airport where the layout of the information is largely the same. An OLED display used in a cellphone can suffer from image retention in areas such as the signal and battery levels, home screen icons or onscreen keyboard.

LCD avoids this for the most part, but can still be susceptible to image persistence. Image persistence is similar in as much as high contrast parts of the image being displayed for extended periods of time is retained, but this normally disappears after a while. Even in extreme cases, displaying a moving image or rapidly flashing various colours should remove all traces of image persistence.

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