Is LCD and LED LCD HDTV uniformity a problem?

There's an issue with all LCDs; can anything be done?

Geoffrey Morrison/CNET

Nearly every LCD TV on the market has a problem: uniformity. Certain areas of the screen are going to be brighter than other areas. On dark scenes, this can be visible and sometimes distracting.

So what causes it? What can be done?

The problem
All LCD TVs are essentially two parts: a backlight, and the Liquid Crystal "glass." The backlight, commonly CCFL or LEDs, creates all the light. As I'll explain below, "LED TVs" are just regular LCD TVs that use LEDs as their backlight. The LC glass is a complex sandwich of electrodes and liquid crystal whose sole job is to block the light created by the backlight. 

OK, so technically polarization filters block the light, the liquid crystal just twists the polarization, but for the ease of discussion, let's just say the LC "blocks" the light.

In the simplest of LCD TVs, the backlight creates a set amount of light. In more expensive TVs, the backlight may adjust depending on what is going on with the video. In other words, during a dark scene, the backlight may dim so the image appears darker than what would be possible with just the liquid crystal.

This LC/backlight system is never perfect, either due to manufacturing irregularities, design inefficiencies, or whathaveyou. Because it's not perfect, parts of the screen are going to "leak" light.

The backlight
CCFL backlights are a series of tiny fluorescent lights, like miniature versions of what's found in offices, stores, etc. Below is a look at the layers of a typical CCFL backlight TV. What you're seeing is the entire TV, spread out so that each layer is separate and visible.

The many layers of an LCD TV. The layer on the far left is the clear covering surface. The next layer (where you see the image) is the liquid crystal layer. The next layer (the bright white area in front of the gray plastic), is the diffusion layer. This is actually translucent "white" plastic. The back, far right, of the image is the backlight itself surrounded by the plastic frame of the TV. Geoffrey Morrison/CNET

The image on the left is the same layers as the previous image, but with the camera stopped down to show the bright areas. Here you can see the translucent diffusion layer in between the liquid crystal and backlight. The right image is the CCFL backlight. For a sense of scale, this was a 40-inch LCD, each CCFL is little more than an 1/8th of an inch in diameter. Geoffrey Morrison/CNET (via Home Theater magazine)

Ideally, the diffusion layer perfectly evens out the light from the individual CCFLs, so that the entire screen is a uniform amount of light. This is pretty tough to do, made harder by the number of CCFLs, the expense spent on the diffusion material, and other design factors.

It's possible then, on dark images, for the areas directly in front of a CCFL tube to be brighter than the areas between the tubes. Smaller LCDs may only have the CCFLs along the edges, making them perform similar to edge-lit LED models. Which brings us to...

All LED TVs are just LCD TVs

LED backlights, annoyingly and misleadingly referred to as "LED TVs," use tiny, efficient LEDs instead of CCFLs. How these are implemented has a dramatic effect on uniformity. Most LED LCDs today have their LEDs arranged along the edges of the screen (edge-lit). Some of these have their LEDs along just two sides (either top/bottom or on the sides), while others have LEDs on all four sides. Check out LED LCD backlights explained for more info on this.

If you've ever put a lit flashlight on a table, you have an idea of the problem created by putting LEDs along the edge of a TV. The area directly in front of the LEDs will be bright, while the area farthest from (in this case, the center of the TV) will be dimmest. In these cases, the LEDs fire across a surface with ridges (or something similar) that gets progressively taller towards the center. This way, the tall central surfaces can reflect the light that would otherwise just bounce back from the opposite side of the television.

I used my extensive artistic talents to draw this diagram:

This wondrous diagram shows a top-down cutaway view of the right half of an edge-lit LED LCD. The LED (yellow here, because white doesn't show up on a white background) fires along the width of the TV. The light guide (circular parts) reflect this light towards the screen. Done perfectly, the center of the screen (where the guide is tallest), is just as bright as the edges. Geoffrey Morrison/CNET

As cool as this is, it's not perfect.

Each of the above backlighting methods has the potential to have poor brightness uniformity in a unique way. For example, a side edge-lit LED could look like this, with a black screen:

An illustration of a common brightness uniformity issue with edge-lit LCD. Note how the sides of the screen closest to the LEDs "leak" a bit of light, causing the brightness of the screen to not be uniform. Geoffrey Morrison/CNET

A top/bottom edge-lit LED could look like this:

Geoffrey Morrison/CNET

Edge-lit with LED's all around:

Geoffrey Morrison/CNET

You get the idea. Potentially, the problem is less localized, leading to issues that could look like this (in the extreme):

An exaggerated illustration of a less localized brightness uniformity issue. Geoffrey Morrison/CNET

On the other side of the brightness scale, an edge-lit model can have poor uniformity with bright images too. In these cases, the center of the screen will be dimmer than the edges. These will look as you'd expect, given the location of the LEDs. For example, here's an illustration of what a edge-lit LED model with LEDs on all 4 sides could look like.

With a bright image, the center of the screen (the area farthest from the LEDs) can be noticeably dimmer. Geoffrey Morrison/CNET

With top/bottom, there could be a center band of dimness across the screen, or in the case of side edge-lit models, a vertical band.

Local dimming
Some high-end LED LCD models have their LEDs on the back of the TV, facing towards you. These "full array" LED backlights can have better uniformity, due to their more even spacing across the screen.

Early plasma screens had significant brightness uniformity issues, but in the opposite way. In those days, when trying to display a full white screen, they could look like this:

Old-school plasmas can have brightness uniformity issues as well. Newer models don't have this issue. Geoffrey Morrison/CNET

In my experience, the uniformity issues manifested itself with mild blotches of "discoloration," due to the demands of creating a full-screen white image.

These days, the design and electronics of plasmas has improved to the extent that you'd be hard pressed to see any brightness uniformity issues with plasma.

Related links
Is plasma HDTV burn-in a problem?
What makes a good HDTV?
How to read an HDTV review
Geoff's HDTV and Home Theater Resource Center and Infotacular

What can be done?
Well, for the end user, nothing. But of course manufacturers are spending lots of research dollars into making a better product, uniformity being one of those factors.

There are multiple issues that make a perfectly uniform LCD difficult. The thinner the TV (and therefore the less distance between the backlight and the LC), make a uniform brightness more difficult, given there's less space to diffuse the light evenly. That diffusion itself is problematic, as the better the diffusion, more light could be lost, and that's always a no-no.

Lastly, there's the cost. Brightness uniformity is just one element of performance, and one far down the list of importance (after cost, light output, contrast ratio, color, processing, etc.). A TV is designed to a price, and no matter what, that price is going to require some concessions in performance.

Brightness uniformity isn't something that's easily seen in a store showroom. Therefore, it's easy for engineers to concede some brightness uniformity for better light output, lower materials cost, or whatever else they need.

But even though it's not noticeable in a store (the only thing TV manufactures really care about), uniformity issues are easily seen at home. It's also often mistakenly assumed it's a "defect" with the television. If you watch dark movies, or ones with letterbox bars, it can also be extremely distracting.

This article stemmed from an e-mail exchange between a CNET reader from Maine and our man Katzmaier. The reader had returned several LCDs, as the brightness uniformity bothered him. To him, and to you if you're also bothered by this artifact, I'll offer this piece of sage advice: skip LCD. Get a plasma.

Got a question for Geoff? First, check out all the other articles he's written on topics like HDMI cables, LED LCD vs. plasma, Active vs Passive 3D, and more. Still have a question? Send him an e-mail! He won't tell you what TV to buy, but he might use your letter in a future article. You can also send him a message on Twitter: @TechWriterGeoff.

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