As many AVS members know, LCD TVs need a light source to illuminate the image, and modern LCDs use LEDs as that light source. The LEDs can be mounted along the edges of the screen—an orientation called edgelighting—or they can be located behind the LCD panel in a 2-dimensional array. Edgelighting allows the set to be super-thin, but it often results in uneven illumination, which is especially evident in dark scenes. On the other hand, full-array backlighting requires a bit more cabinet depth, but it generally produces more even illumination across the screen.

Another benefit of full-array backlighting is the possibility of local dimming, in which the LEDs behind dark portions of the image are dimmed while the LEDs behind bright parts of the image are brightened. This feature is generically called full-array local dimming (FALD), and it can dramatically improve the perceived contrast and black level of the display. (Many edgelit sets claim to have "local dimming," but it's often not as effective as actual FALD.) In all FALD LCD TVs, the LEDs are grouped into zones, and the LEDs in each zone are dimmed and brightened as a group.

An ideal FALD-based LCD would have just as many zones as an OLED or plasma—one per pixel. Unfortunately, that is impossible, since LEDs are physically much larger than LCD pixels. However, based on my experience, I'd like to see thousands of local-dimming zones on premium TVs, not merely hundreds. Additionally, I've become skeptical of TVs that feature only one or two dozen FALD zones. I've found those TVs perform no better than edgelit designs, which feature ever-improving algorithms to help hide the machinations of the pseudo-local dimming.

A the recent Value Electronics shootout , LG's 55EC9300 OLED and Samsung's long-in-the-tooth PN65F8500 plasma tied for best TV. At this year's event, emissive displays ranked much higher in the vote tally when compared to the LCD-based screens. The calibrators who ran the show lauded the image quality of plasma and OLED displays, where every single pixel acts as a local-dimming zone.

In this picture from the shootout, you can see plasma, OLED, FALD LCD, and edgelit LCD TVs.
The most impressive display I saw at CES last January was Vizio's Reference series . When I viewed a 65-inch prototype, I saw an image that reminded me of plasma or OLED. The Reference series features a 10-bit panel with 384 dimming zones, which beats the zone count of the well-regarded Sharp Elite PRO-60X5FD. A number of FALD-based UHDTVs—from various manufacturers—make do with fewer zones. For example, HDTVtest's Vincent Teoh says the anticipated Panasonic  AX900 will feature 128 zones in the 65-inch model, and (oddly) only 32 zones in the 85-inch version. The AX900 prototype I saw at CES came remarkably close to matching the image quality of a plasma.

At CES 2014, Panasonic's AX900 prototype (on the right hand side) looked a lot like the ZT60 plasma (on the left).

Vizio's impressive 120-inch Reference Series UHDTV sports 384 local dimming zones. Photo by Scott Wilkinson.

Unfortunately, most manufacturers will not disclose the number of local-dimming zones in their TVs. Toshiba announced a high-end FALD set that looked great on the show floor at CES, but the company is tight-lipped about the zone count. The same goes for Samsung and Sony . Fortunately, there is an easy way to count zones on your own. In a darkened room, turn the TV's contrast and backlight controls all the way up. Then, watch the following video full-screen and count the rows and columns as the zones light up. Multiply the two numbers, and you've got a TV's zone count. Alternately, you can use a computer to drag the mouse pointer along the screen's edges while displaying a full-screen black field. Each zone will light up as the cursor passes through it.
This year, I paid close attention to local dimming because I was frustrated with how obvious its effects were on my Vizio  M3D550KD, which is an edgelit model with only 12 dimming zones, six per side. When I use that TV to watch letterboxed content, the imprecise dimming zones spill over into the black bars. I experimented with physically masking the letterbox bars ; after enjoying some success with that I wound up buying a plasma .

With its imprecise edgelit dimming, the Vizio M3D550KD can't keep the letterbox bars dark.

I also checked out a Vizio E602i-B2, which, like all 2014 Vizio LED-LCD TVs, is true FALD. However, it has only 16 local-dimming zones, and the implementation is not what I'd call elegant; I found it to be more distracting than the pseudo-local dimming in my older M-series. With the new E-series, I could see the large rectangles of each zone dim and brighten as the camera panned through a static scene—it was intolerably distracting. The larger, fuzzier zones found on the M3D550KD tended to hide those problems. I returned the Vizio within 24 hours of picking it up, largely as a result of its inferior FALD implementation. I'm glad I returned that Vizio , because that was the trip where I saw a Samsung PN64F8500 plasma on clearance—I bought it right away.

A high zone count offers no guarantee that local dimming won't draw attention to itself. At the Value Electronics shootout, I observed Sony's $25,000 XBR-85X950B flub a Harry Potter torture test—a dark scene filled with mist. As the camera panned, the mist pulsated as the dimming zones tried to maintain peak contrast. With Sony's flagship UHDTV, it looked as if a swarm of fireflies was hiding in the mist; it reminded me of the local-dimming effects I saw on the Vizio E. By the same token, a low zone count and/or edgelighting does not mean that the effect of local dimming will always be obvious. Panasonic 's AX800U and Samsung's HU9000 both feature edgelit local dimming that doesn't call attention to itself. According to Panasonic, superior algorithms get the credit for the unobtrusive nature of its local dimming.

I'd like to see full arrays with thousands of local-dimming zones combined with better algorithms; it's the only way LCD might compete with OLED and plasma image quality. What are your thoughts about this? Will advancements in FALD allow LCD to compete with emissive displays? Also, if you have a TV that features local dimming, how many zones do you count?