Display Testing Explained: How We Test Monitors and TVs

Brightness, Contrast and Calibration

Brightness and contrast are, in our opinion, the two most important factors in perceived image quality.

High-Contrast Versus Low-ContrastHigh-Contrast Versus Low-Contrast

Higher contrast ratios are preferable since the lower a display's contrast ratio, the more washed out the picture appears. Given the data we’ve collected over the past two years, we’ve settled on a ratio of 1000 to 1 as a benchmark for computer monitors. Most can come close to or slightly exceed this value. HDTVs usually post far higher contrast numbers (some as high as 20,000 to 1). To arrive at the final result, we simply divide the maximum white level by the minimum black level. Obviously, the best monitor in the comparison group is the one with the highest contrast ratio.

To read about that concept in depth, please check out Display Calibration 201: The Science Behind Tuning Your Monitor for a brief treatise on imaging science.

Our tests begin with the panel in its factory default configuration. Before making any color adjustments whatsoever, we measure zero and 100-percent signals at both ends of the brightness control range.

Uncalibrated: Maximum Backlight Level

With the display's brightness control turned all the way up, this test uses full-field white and black patterns to measure white level, black level, and contrast. The contrast ratio is calculated by dividing the black level into the white (W / B = CR). We do not raise the contrast control past the clipping point. While doing this would increase a monitor’s light output, the brightest signal levels would not be visible, resulting in crushed highlight detail. Our numbers show the maximum light level possible with no clipping of the signal.

What we’re looking for in this test is whether the panel meets the manufacturer’s spec, and if it’s bright enough for its intended application. For instance, we like to see lots of light from gaming monitors (especially those with a blur-reducing backlight strobe, which cuts output by at least half). Meanwhile, professional studio screens don’t need to be as bright. However, photographers needing to use them on-location would consider a brighter screen to be a better fit for the situation.

  • Patterns used: Full White Field, Full Black Field
  • Monitor should meet or exceed manufacturer's stated maximum brightness value
  • Better displays will exceed 1000:1 contrast

Uncalibrated: Minimum Backlight Level

For the minimum brightness tests, we turn the backlight to its lowest setting and measure the white and black field patterns again. There really isn’t a better or worse result here. We believe 50 cd/m2 is a practical lower limit. Anything under that results in a dim picture that can cause eye fatigue even in a completely dark room. The purpose of this test is to see if the monitor’s contrast ratio remains constant throughout the entire luminance range. Some monitors become too dark for practical use. In those cases, we suggest a minimum setting for the brightness control that results in 50 cd/m2 output.

  • Patterns used: Full White Field, Full Black Field
  • The minimum white level should be at or close to 50 cd/m2
  • Contrast ratio should remain the same regardless of the backlight setting

After Calibration to 200 cd/m2

Since we consider 200 cd/m2 to be an ideal point for peak output, we calibrate all of our test monitors to that value. In a room with some ambient light (like an office), this brightness level provides a sharp, punchy image with maximum detail and minimal eye fatigue. On some monitors, it’s also the sweet spot for gamma and grayscale tracking.

In a dark room, many professionals prefer a 120 cd/m2 calibration. If a monitor’s contrast is consistent, it makes little to no difference on the calibrated black level and contrast measurements.

Calibration often reduces contrast slightly. If we measure a significant difference, we weigh the reduction against the improvement in color accuracy. A few monitors are color-accurate without adjustment, and therefore best left uncalibrated to maximize contrast.

  • Patterns used: Full White Field, Full Black Field
  • Calibrated contrast should be as close as possible to uncalibrated contrast

ANSI Contrast Ratio

Another important gauge of contrast is ANSI. To perform this test, a checkerboard pattern of sixteen zero and 100-percent squares is measured. This is somewhat more real-world than on/off readings because it tests a display’s ability to simultaneously maintain low black and full white levels, factoring in screen uniformity. The average of the eight full-white measurements is divided by the average of the eight full-black measurements to arrive at the ANSI result.

The ANSI pattern is designed to test intra-image contrast. Its overall average level is 50 percent, representing a typical picture. It’s a good indicator of the quality of a display’s grid polarizer. That is the part most directly responsible for controlling light bleed between pixels. Even in the best monitors, the ANSI value is usually a little lower than the calibrated one.

  • Pattern used: Checkerboard (8 Full-White, 8 Full-Black)
  • Test performed after calibration to 200 cd/m2
  • The ANSI Contrast Ratio should be nearly equal to the on/off value
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  • Chetou
    Thank you for this writeup. Though I do find 200 cd/m2 retina scorching on any monitor and viewing conditions, especially with large screens.

    Do you take your measurements in a dimmed/dark room? Even with the meter flush with screen, some light can pass through the glass on the sides.
  • Chetou
    double post
  • cpm1984
    Gotta ask about your 'total lag' measurements - they seem to be generally much higher than what TFT Central measures. For example, the Dell UP3214Q has 97ms total lag with Toms':
    http://www.tomshardware.com/reviews/benq-pg2401pt-24-inch-monitor,3848-10.html

    but it has only 29ms lag at TFT Central (and only 25ms in 'game mode'):
    http://www.tftcentral.co.uk/reviews/dell_up3214q.htm

    Most of the monitors you review seems to have total lag around 80-100ms, which seems really slow. Slow enough that you'd notice the mouse lagging when you move it around. Yet I can't feel any appreciable lag on my Dell 2713HM (but I have no way of measuring....)
  • cpm1984
    Gotta ask about your 'total lag' measurements - they seem to be generally much higher than what TFT Central measures. For example, the Dell UP3214Q has 97ms total lag with Toms':
    http://www.tomshardware.com/reviews/benq-pg2401pt-24-inch-monitor,3848-10.html

    but it has only 29ms lag at TFT Central (and only 25ms in 'game mode'):
    http://www.tftcentral.co.uk/reviews/dell_up3214q.htm

    Most of the monitors you review seems to have total lag around 80-100ms, which seems really slow. Slow enough that you'd notice the mouse lagging when you move it around. Yet I can't feel any appreciable lag on my Dell 2713HM (but I have no way of measuring....)
  • ceberle
    Anonymous said:
    Gotta ask about your 'total lag' measurements - they seem to be generally much higher than what TFT Central measures. For example, the Dell UP3214Q has 97ms total lag with Toms':
    http://www.tomshardware.com/reviews/benq-pg2401pt-24-inch-monitor,3848-10.html

    but it has only 29ms lag at TFT Central (and only 25ms in 'game mode'):
    http://www.tftcentral.co.uk/reviews/dell_up3214q.htm

    Most of the monitors you review seems to have total lag around 80-100ms, which seems really slow. Slow enough that you'd notice the mouse lagging when you move it around. Yet I can't feel any appreciable lag on my Dell 2713HM (but I have no way of measuring....)


    TFT Central uses the SMTT software which measures only the actual display lag. Our test indicates the total time it takes for a user input to translate to the screen. We account for the lag inherent in the input source which in our case is a pattern generator. While this device may be slower than the average mouse or keyboard it is completely consistent and repeatable. If we used a mouse-to-PC test, timing could shift if a driver were updated or we changed video boards. Every monitor we've measured over the past two years has been tested with the same Accupel signal generator.

    Our principal goal is consistency from monitor to monitor. By having the exact same equipment and test parameters, we can ensure that results from last year are comparable to more recent tests.

    -Christian-
  • KevinAr18
    The response time testing method is done wrong and is very misleading. The tesst do not always reflect the real response of the monitors.

    Why?
    Monitors have thousands of different response times, not a single number. Testing just one or two transitions can easily give very innaccurate results; the one transition you tested may be much slower or faster than the other thousand or more transitions that exist. In order to find out if the monitor is any good, you must test a wide range of transitions.

    Want proof?
    See:
    http://www.xbitlabs.com/articles/monitors/display/viewsonic-fuhzion-vx2268wm_4.html#sect1
    In "standard" mode, the black to white transition is low, while almost all the others are high. (Note: the 255 to 0 transiyon is hidden behind the higher ones.) If you had tested this monitor using your current method, you would have concluded it is VERY fast, when, in fact, "standard" mode is actually slow! This is how they used to lie on the box specs: by using only a single number that is fast, while the rest are slow... and your tests only support the lie!


    Examples of how to test response times:
    Xbitlabs tests a range of 72 transitions spread out evenly; the also show the overshoot error rate for all those transitions:
    http://www.xbitlabs.com/articles/monitors/display/samsung-sa850_8.html#sect0
    TftCentral tests 30 transitions & the corresponding overshoot errors:
    http://www.tftcentral.co.uk/reviews/content/benq_bl3200pt.htm#gaming


    If you want, compare your tests of the BenQ BL3200 to tftcentral:
    http://www.tomshardware.com/reviews/benq-bl3200pt-qhd-monitor,3898-9.html
    The tftcentral tests show that the monitor has response time problems of 41ms for the 0-50 transition, but better numbers (6-10ms) otherwise. Your single number tests do not reveal any of these issues.
  • KevinAr18
    Sorry for the aggressive tone in my previous comment. I should have taken more time to write it up nicely.

    This issue with response time testing has always been a problem on tomshardware (even before you ever started writing reviews here), however this is the first time I got a good chance to contact the person that writes the articles. I hope you will be able to look into the response time tests at some point. Sadly, there is literally only two sites on the internet (that I know of) that test response times correctly: xbitlabs & tftcentral. In fact, even tftcentral used to test response times wrong (for many years). xbitlabs was the original site I know of that began testing response times correctly.

    I apologize for not being able to write in more detail right now, but here's two helpful links:
    Response times:
    http://www.xbitlabs.com/articles/monitors/display/lcd-testmethods_5.html
    Overshoot errors:
    http://www.xbitlabs.com/articles/monitors/display/lcd-testmethods_6.html
  • dovah-chan
    You can't really fully benchmark response times accurately as it varies amongst monitors such as the overall overclocking headroom varies by CPUs that are even of the same model.

    Also thanks for this article. It's n-not like I was one of the people who requested it. I'm just glad to see that we are listened to. >__<
  • Drejeck
    I sense a disturbance in the force.
    Is some monitor being reviewed right now? Hope it's a G-sync near 300 euros
  • Blazer1985
    Hi christian!
    I have a request :-)
    Would it be possible for you to compare some cheap chinese tv and monitors to the expensive western branded ones?
    I know that you usually get what you pay for but there might be a sweet spot somewhere in between and would be nice to know :-)
    Especially today that we are beginning the switch to 4k and the prices vary so much (ex. Haier 50" uhd @600€).
    Thanks!
  • KevinAr18
    Anonymous said:
    You can't really fully benchmark response times accurately as it varies amongst monitors such as the overall overclocking headroom varies by CPUs that are even of the same model.

    The part I like the most about "benchmarking response times" is that even with minor variations between each monitor, the numbers are very useful in finding out if a particular model is any good. (Personally, I think that the variations between monitors of the same model may be so minor, that testing is still very useful, but I would need lots more data to say with more certainty :)).

    However, instead of me just making comments, I think you may appreciate a good explanation, since I failed to provide one before; sorry about that. :) So... here goes.


    To be clear, the problem that I was referring to with tomshardware's benchmark was basically this: tomshardware tests only one out of thousands of transitions, which may tell you nothing about the monitor's real response times!

    The best way to understand what I am referring to (and why it is so important) would be for me to try and explain the details.

    Consider these details about transitions & response times:

    1. An 8bit monitor uses 256 shades of grey to produce each color. 0 = black; 255 = brightest color (white); 22= very dark grey, etc...
    2. Response time is the measure of how long it takes to change from one shade to another. Say from 0 (black) to 156 (greyish).
    3. A little bit of math shows that if you have 256 shades, there are 65,280 possible transitions. [ (256*256) - 256 = 65,280 ]
    4. Each transition has a different response time; that means an 8bit monitor has over 65,000 different response times!
    5. Consider this example:

    source: http://www.xbitlabs.com/articles/monitors/display/zalman-zm-m240w_4.html#sect3
    Notice how the 255 to 0 transition (white to black) has a response time of ~2ms.
    However, look how many other transitions are maybe 14ms or more!

    Sidenote: overdrive:
    1. Long ago, they dicovered that they could increase response times by applying more voltage to certain transitions (or maybe less in some cases?? not 100% sure).
    2. This "overdrive" method can improve response times a lot (you want it on a monitor). However, if not tuned correctly, too much voltage may be applied and it may "overshoot" the correct color and create bad image problems that are sometimes worse than having slow response times.
    3. This means any response time testing must also test for overdrive errors (commonly called "RTC error").


    The problem with tomshardware (and most sites):
    Currently tomshardware tests only the 255 to 0 (white to black) or the 0 to 255 (black to white) response time and ignores all the other 65,000+. As you can see from the earlier picture, if you test only one transition and not the others, you may end up with a response time that is 100% useless and misleading.
    Also, tomshardware does not test for overdrive errors, which, in some cases, can sometimes be more important than response times

    The correct way to test response times:
    Obviously, you don't want to test all 65,000+ transitions. However, you can test a range of them. Xbitlabs tests 9x9 transitions or 72 transitions scattered evenly around ((9*9)-9 =72).
    This testing reveals some very interesting things. Consider these examples:

    http://www.xbitlabs.com/articles/monitors/display/20inch-6_15.html
    This monitor has a white to black transition of 2ms and a black to white transition of 6ms, but 55+% of the time it is really 16+ms! Tomshardware would have reported false (low) response times if they had tested this monitor using their current method.


    http://www.xbitlabs.com/articles/monitors/display/acer-gd245hq-lg-flatron-w2363d_9.html#sect0
    The response time on this one varys a lot, but you'd never know that without the tests done like on xbitlabs.


    http://www.xbitlabs.com/articles/monitors/display/20inch-4_23.html
    This picture is actually not response times but overdrive errors that I mentioned earlier. What this shows is how MVA panels often have MAJOR image problems when dealing with 0 to 96 and/or similar nearby transitions, but is good most other times. AKA, it means you'd see bad glowing on some dark scenes but a good picture the rest of the time. Note how tomshardware tests reveal none of this important data.