LED TV Repair

    LED TV Explained

    Having gained popularity among the HDTV-buying public in recent times, LED TV is in itself a misnomer.

    These LED televisions are actually not true LED displays; instead, as a result of some creative (and perhaps misleading) advertising by manufacturers and retailers, an "LED TV" in today's market usually refers to an LCD TV that uses LED backlight rather than conventional cold-cathode fluorescent lamps (CCFL) to illuminate its screen. For the purpose of maintaining technical accuracy, we try to use the terms "LED-backlit LCD TV", "LED-based LCD TV", "LED LCD display" or "LED LCD TV" whenever possible in our reviews when alluding to LCD televisions with LED backlighting.

    There are 3 main variants of LED backlighting technology, namely local-dimming LED, edge-lit LED, and direct-lit LED without local dimming.

    Local Dimming LED TV

    When LED backlighting was first introduced on LCD panels, its goal is to produce deeper blacks than what could be achieved through CCFLs. The way traditional LCD TVs produces black is by "blocking" the always-on fluorescent backlight. As you can imagine, this somewhat inefficient method generally results in a lighter shade of black due to light leakage.

    So some smart TV engineers started thinking, what if we could turn off the backlight on the LCD screen as necessary for black-level reproduction? Cue the birth of LED-backlit LCD televisions — LEDs which can be switched on and off are installed behind the LCD matrix in place of the more space-and-energy-consuming fluorescent bulbs. What this means is that a specific area on screen can be darkened (by switching off the LEDs) separately from other brighter areas (where the LEDs are switched on), contributing to reduced light leakage and substantially deeper blacks.

    How local-dimming LED TV works
    How local-dimming LED works — backlight zones can be dimmed or brightened separately

    But because it is too complicated and expensive to assign an individually adjustable LED to every pixel (for example a full HD 1920 x 1080 LCD panel would require in excess of 2 million LEDs), to keep costs down the LEDs are deployed in independently controllable blocks/ arrays of "dimming zones". This inevitably entails some compromise in the form of circumferential flaring/ blooming around the brighter elements overlaying a dark background, as a direct consequence of light from brighter areas on screen spilling/ bleeding into adjacent darker zones.

    Circumferential flare or blooming
    Circumferential blooming seen on a first-generation local-dimming LED LCD TV

    In spite of circumferential blooming, generally speaking black levels on local-dimming LED-backlit LCD TVs (may also be known as "rear-lit LED TV", "direct LED TV", "full-array LED TV" or "dynamic LED TV") are noticeably deeper than those seen on CCFL-backlit LCD televisions. And because TV makers have been working hard to increase the number of dimming zones as time and technology advances, blooming is significantly less of a problem these days.

    That said, less and less full-array local-dimming LED television models are launched on the market with each passing year, due to their high manufacturing costs (and hence lower profit margins) compared to edge-lit LED televisions.

    Edge LED TV

    Local-dimming LED-based LCD TVs are expensive to make, and their superior picture quality isn't always appreciated by cost-conscious consumers. So manufacturers are focusing most of their R&D and marketing resources on edge-lit LED TVs, where white LEDs are mounted along the edges of the screen to illuminate a cleverly arranged array of reflective plates which then steers the light forward and out of the screen.

    How edge LED TV works
    An illustration of how Edge LED works — the whole backlight dims or brightens together

    Because less LEDs are required for this approach, edge-backlit LED-based LCD televisions (a.k.a. "edge LED TV", "side-lit LED TV" or "global dimming LED TV") offer the following advantages over other HDTVs:

    • A thinner design is possible, making them aesthetically more appealing;
    • Improved energy efficiency leading to lower power consumption, hence more eco-friendly;
    Ultra slim design of edge LED TV
    An ultra slim edge-lit Samsung LED TV measuring 3cm in depth

    However, their sheer slenderness means that edge-lit LED-based LCD TVs usually manifest worse screen uniformity and weaker audio performance. And due to the global nature of any backlight adjustment (i.e. the entire backlight has to be dimmed down or brightened up together), edge LED panels suffer from poorer black levels (compared to local-dimming versions) when displaying sequences with mixed brightness, and lower perceived contrast when displaying predominantly dark content.

    Since 2011, some TV manufacturers like Samsung, LG and Sony started introducing LED TVs that employ edge LED lighting, but allow for some degree of independent light control (pseudo-local-dimming) to achieve deeper blacks and brighter whites, while keeping in check the thickness of the chassis, the power consumption, and of course the production costs. However, among video enthusiasts who value image quality, true full-array local dimming is still widely regarded as the pinnacle of LED backlight technology.

    Local-Dimming LED vs Edge-Lit LED

    Here's a table summarising the pros and cons of local-dimming versus edge-lit LED backlighting systems:

    Black level in mixed brightness content Generally deeper Generally lighter
    Contrast in predominantly dark content Generally higher Generally lower
    Physical depth of TV chassis Thicker Much thinner
    Screen uniformity Usually better Usually worse
    Circumferential blooming Present however minimal Non-existent
    Audio performance of internal speakers Generally better Generally flimsier
    Energy consumption Higher Lower
    Manufacturing cost and retail price More expensive Less expensive

    Although edge-lit LED LCD televisons are cheaper, slimmer and more energy-efficient, local-dimming LED-based LCD TVs still hold the upper hand as far as image quality is concerned. In fact, most local-dimming LED TVs are capable of matching plasma HDTVs in almost every aspect of picture quality bar viewing angle and to a lesser extent motion resolution.

    Direct-lit LED without local dimming

    Usually found on budget or entry-level models, this is a variant where non-local-dimming LEDs are arranged behind the LCD screen instead of along the edges. As a result, the side profile is typically thicker, but on the upside, screen uniformity is generally better. TVs with these rear-mounted static LEDs perform just like a traditional CCFL-backlit LCD TV in terms of picture quality.