Light it Up: How Your TV Produces an Image

When it comes to shopping for a new television, manufacturers will flood you with a tsunami of glowing prose hyping all of the advantages their sets offer. But these claims can be a puzzle to potential buyers, who understandably haven’t made a study of TV technology. The latter is perhaps most confusing with regard to how a TV produces a visible image; that is, how it lights up the screen. Here are some of the key facts.

LED Televisions Many if not most sets on the market today are advertised as LEDTVs. But there’s no such thing (at least not yet) as an “LED television.” The devices in such sets that create the image are called pixels (picture elements). A 4K set has 2160 rows of pixels from top to bottom, each row with 3840 pixels from left to right (typically referred to as a resolution of 3840 x 2160). That’s a total of about 8 million pixels, and each of these pixels is an LCD.

But LCDs create no light on their own; all they can do is turn on or off in response to the source. This is sufficient to produce an image, but without some sort of light behind the LCD panel you wouldn’t see much, certainly not enough to be useful. In olden days (15 years or so ago) the light source for LCD flat screen sets was invariably fluorescent tubes. But soon it was found that LEDs (light emitting diodes) offered significant advantages over fluorescents, and before long LEDs dominated the backlighting chores. LEDs are now ubiquitous in flat screen sets, but all they do is provide the backlighting. The sets still use LCDs to produce the picture. But soon after LED backlighting was introduced some industry wag decided that the term “LCD” had grown tired and “LEDTV” had more marketing bling.

LED backlighting can be used as true backlighting (with the LEDs behind the screen) or as edge-lighting (with the pixels located along the sides or the top and bottom (and sometimes just the top or the bottom!). In order to spread the LED light evenly across the LCD panel, diffusors are also used. Uneven diffusion can be the source of a number of visible issues, one of which has been dubbed the dirty screen effect (DSE in tweet-speak), where black areas of the image are cluttered with uneven, cloudy-looking grays. These are most visible when the picture fades to total black and the viewing room is dimly lit or totally dark.

LEDs are still too large to be used as the pixels themselves in consumer-sized displays. Someday it may be possible to make LEDs pixel-sized, but for now such designs are either still in the laboratories of TV makers or used in huge commercial displays such as the Jumbotrons in sports stadiums. For consumer sets, LEDs simply serve to provide the backlighting for LCD designs.

Quantum Dots Quantum dots are microscopic semiconductor nanocrystals. When hit by light, the emit light of their own at a wavelength determined by the size of the dot. As with LEDs, they are used today as backlighting only, mostly in higher end LCD models. In such sets, a blue LED is used as the light source and the quantum dots are sized to produce red and green light of the proper wavelength when hit by the blue light of the LED. Viola, red, green, and blue light! Quantum dots offer a number of advantages over LED-only backlighting, such as higher efficiency and a wider color range.

As with LED-only designs, quantum dots may someday be used as the pixel structure itself in a Quantum dot LED set, but those days are still in the future. At least one TV maker, Samsung, is using the designation QLED in some of its high-end designs, but don’t be confused; these sets are still LCD models with quantum dot lighting.

OLED When the source of the light is in the pixels themselves, the display is called self-emissive; that is, no backlighting is needed. No LCD sets of the sort described above are self-emissive, though if we get to the point where the LEDs (or quantum dots) themselves make up the pixels they would be (though then they’d no longer be LCDs!). But for today, the only self-emissive displays available to the consumer are OLEDs (Organic Light Emitting Diodes). Plasma sets were self-emissive as well, but are no longer made.

The advantage of a self-emissive display is that it can shut off the light to each pixel whenever that part of the image needs to be fully black. OLEDs can do this, though plasmas never quite got to full black because they needed a small residual charge on their pixels so they were ready to light up instantly when called on. OLEDs can produce deep, rich blacks down to the individual pixel level.

Local Dimming While the pixels in an LCD panel can turn on and off, they can’t do so perfectly. Some light always gets through. LCD designs therefore can’t usually go totally black when the source calls for it. A grayish background is always there on dark scenes. To help with this, a technology called local dimming was developed. But it’s the source of much confusion.

Done best, local dimming is configured with the LEDs or quantum dots positioned behind the screen, not at its edges, and divided into a number of individually controllable zones. When the source calls for part of the screen to be darker, or totally black, the set can dim or darken that particular zone. The number of zones can vary (often with the price of the set!), but done well this technique can offer remarkably results. Some of the best LCD sets on the market use this type of dimming.

Enthusiasts dub this type of dimming as Full Array Local Dimming (FALD). But some designs use it with edge lighting instead. FALD requires more LEDs/quantum dots than edge lighting, and that costs money. Some of these edge-lit local dimming compromises work better than others, but don’t ever assume that they will equal the results produced by FALD.

Watch out for specs that refer simply to “local dimming,” or even, local dimming by zones—this might simply mean that the zones are at the edges of the screen. Also note that the term “backlighting” is generic with LCD designs, and can mean either true backlighting or edge-lighting if not further qualified. Don’t be reluctant to ask if the backlighting and local dimming on a set you’re considering is full array by zones behind the screen, but don’t be surprised if your “sales associate” has no idea of what you’re talking about! And don’t be surprised if even some very expensive sets use edge-lit local dimming. It might be better than other examples of the breed (Samsung, for example, uses edge lit dimming in its flagship “QLED” sets), but we haven’t yet seen an LCD model with edge-lit local dimming that can equal well-implemented FALD.

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