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We were unimpressed with the black level and total contrast ratio of the Vizio VF550XVT. The greyscale gamma curve was smooth, just like we like it, but was a bit steeper than ideal. All of the tests in this section were administered using DisplayMate software.
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Black Level (5.22) 
We measured the Vizio VF550XVT's black level at 0.36 candelas per meter squared (cd/m2). This is an unimpressive black level, higher than the average of 0.25 cd/m2 we've seen for all televisions we've reviewed, although it is slightly better than the 0.41 cd/m2 we measured from the Vizio SV470XVT.
Peak Brightness (7.70)
Peak brightness is the opposite of black level, it represents the brightest white that a television can produce. The Vizio VF550XVT's peak brightness was measured at 251.3 cd/m2. This is probably a bit lower than we'd like to see. Our average peak brightness for all televisions we've reviewed comes in significantly higher than this at 295.76 cd/m2, and this includes Plasma television that usually have a much lower peak brightness. The Vizio SV470XVT for example achieved a peak brightness much higher than the VF550XVT at 374.93 cd/m2. That said 251.3 cd/m2 is plenty bright for most situations.
You should note that the peak brightness we measure is constrained by our calibration of the television. If you really want a much brighter screen you can achieve this by cranking up the contrast control on the television. Just note that in the process you will lose all kinds of color accuracy and detail in your scenes. We calibrate for maximum accuracy of colors and greyscale, not to produce the brightest white.
Contrast (5.49)
Contrast tells you how wide the range between a television's deepest black and brightest white is. As such we just take the two numbers from the scores above and divide them to produce the Vizio VF550XVT's contrast ratio of 698:1. This is not a very impressive contrast ratio, as you may have guessed as we were not particularly impressed by either the black level or peak brightness of the VF550XVT. In fact the VF550XVT's contrast ratio is the lowest we have measured to date.
What this means in practice is that the range in which the VF550XVT can make fine adjustments along the greyscale is relatively limited. The larger the contrast ratio a television has the wider this range the more likely your eye is able to pick up subtle changes in the greyscale. This is important for identifying fine detail in many scenes.
As we mentioned above the VF550XVT's poor contrast ratio is partly the result of our calibration decisions, which are focused on getting the maximum accuracy out of both color and greyscale. As we discuss in our calibration section we found ourselves setting the contrast of the VF550XVT much lower than we would have liked because turning it up further erased fine detail at the higher end of the color spectrum. If you really just want to boost the contrast ratio you can do so by turning the contrast number on the television up much higher than our calibrated setting, but of course you will run into the same problem with lost detail and color accuracy that we were correcting for.
Tunnel Contrast (8.54)
Some televisions have trouble holding the black level steady as black takes up less and less of the display. This is what we're looking for in this test as we measure the black level with the percentage of black on the screen ranging from 5% black to 90% black. Below you can see our results in a chart.
What we found on the VF550XVT is that although the black level did rise a bit as the percentage of black on the screen decreased, the increase was minimal. We saw black level go from 0.36 cd/m2 to 0.56 cd/m2. Although significant in percentage terms it's not going to have a huge impact on your viewing experience.
White Falloff (9.80)
In this test we are doing the opposite of our tunnel contrast test; that is we are looking at what happens to the peak brightness as we reduce the amount of white on the screen. Below you can see the results of our testing in a chart.
As should be clear from the chart there was almost no variation in the peak brightness on the Vizio VF550XVT. This is what we expect to see from an LCD television, white falloff is usually a much larger problem with Plasma television's like the Samsung PN50A760.
Uniformity (7.13)
Here we are looking for how uniform black and white on the display are. To do this test we put up a totally black and totally white screen and check for areas of inconsistency. With the Vizio VF550XVT we found that the white screen was smoothly uniform with no obvious blotches and only minimal dimming at the corners and the edges. The black screen, however, did not fare as well. We noticed significant leakage at the corners and edges of the display, with these areas much brighter than the center of the screen. These areas of brightness extended over a large portion of the display, creating blotches that were easily seen. This is a problem because it means that you're unlikely to get consistent black levels across the display, something that you will likely notice in very dark scenes.
Greyscale Gamma (7.19)
Greyscale gamma refers to the curve along which changes in greys are made by a display from the deepest black, through middle greys all the way up to the brightest white. The Vizio VF550XVT did a reasonable job in this test, with a gamma of 2.67, just a bit higher than our ideal of 2.2. Below you can see a graph of the VF550XVT's gamma curve.
You'll note that the curve is very smooth, with no obvious bumpiness or plateaus that would indicate that there are areas where changes in the greyscale occur abruptly or in directions we would not expect. Below we graph the same curve logarithmically, which gives you a better idea of how the curve actually appears to our eyes as human eyes can only discern multiplicative changes in intensity rather than arithmetic. What that means is you actually need to double the intensity, rather than just increasing it by a fixed amount, for the human eye to see the change.
Plotted logarithmically you'll note once again that the VF550XVT's performance is relatively smooth. On some television's you'll see significant jumps or zig zags on this curve, not so with the VF550XVT.
That said the gamma curve is a bit steeper than we would like, what this means is that changes along the greyscale occur more aggressively than ideal. This is likely related to the limited contrast ratio we saw above as well as a more aggressive gamma curve means that any change you make along the greyscale takes up more "space" than a shallower gamma curve would.
Resolution Scaling (7.78)
Although the Vizio VF550XVT is a 1080p television, which means that it can handle the highest quality of HD content, you will find that this level of content is actually rare, limited to Blu-Ray discs. In practice you will find yourself watching a wide variety of standard definition and high definition content, which is available in three major formats apart from 1080p: 480p, 720p and 1080i. In this section we look at how the VF550XVT handles content at these different resolutions.
480p (7.60)
Commonly known as standard definition this is the resolution of old-style televisions and is still what you get from a lot of sources including DVDs and standard definition broadcasts. On the VF550XVT we saw the standard issue with overscan using 480p content. This means that about 3% of the screen on the top, bottom left and right are all slightly cut off. As you would expect from a lower resolution we had some trouble reading very small text on the display, but we didn't see any problems with moire patterns.
720p (8.00)
This is a common HD standard that is used by many sports broadcasts and online HD content like what you would get from the iTunes store. Sometimes overscan is a problem with 720p content, but we were pleased to see that this wasn't the case with the Vizio VF550XVT. We did still have some legibility issues with text on the screen, but otherwise resolution was not a problem and neither were moire patterns.
1080i (7.75)
This is the most common type of HD content that you actually receive from your cable or satellite provider. 1080i content is used instead of 1080p because the "i" in 1080i stands for interlaced. This means that what appears on your screen is interlaced between two separate signals of 540 vertical lines. This means that broadcasters only have to send half the data that a 1080p signal would require.
1080i content on the Vizio VF550XVT looked good, there was no overscan or moire problems and we were able to discern a much smaller font size on the display. We did notice a problem that we had noticed with the display when viewing 1080p content as well, namely a greenish tinge to some patterns of alternating black and white lines. Although it won't cause problems with standard content most of the time, it will crop up occasionally so we deducted a point for this.
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