Display Testing Explained: How We Test Monitors and TVs

Viewing Angles, Uniformity, Pixel Response and Input Lag

Viewing Angles

The more monitors we test, the more we can see that off-axis viewing performance is dependent not only on pixel structure (IPS, PLS, TN, and so on), but backlight technology as well. The anti-glare layer makes a difference too.

In this test, a picture is worth one thousand words. We set a Panasonic Lumix camera to manual exposure and zero its white balance to each individual monitor. No settings are changed between shots. The top and side photos are taken at a 45-degree angle off-axis. Then, the three images are assembled into a composite. It’s a good approximation of what the eye actually sees when viewing a monitor off-center.

  • Patterns used: Gray Steps (horizontal and vertical)
  • Panasonic Lumix DMC-LX7, manual exposure
  • Off-axis angle: 45 degrees horizontal and vertical

Screen Uniformity: Luminance

To measure screen uniformity, zero and 100-percent full-field patterns are used, and nine points are sampled. First, we establish a baseline measurement at the center of each screen. The surrounding eight points are then measured. Their values get expressed as a percentage of the baseline, either above or below. This number is averaged. It's important to remember that we only test the review sample each vendor sends us. Other examples of the same monitor can measure differently.

Black field uniformity is also known as light bleed. If it’s visible, it shows up as light areas on an otherwise black screen. If the value is under 10 percent, we consider the monitor essentially perfect with no visible problems.

In the white field test, our benchmark is the same 10 percent. Few displays score higher than that.

If a display has a uniformity compensation feature, we run the test with it off and on and compare the results.

  • Patterns used: 100-percent White Field, 0-percent Black Field
  • Where appropriate, we compare measurements with Uniformity Compensation On and Off
  • Results under 10-percent mean no aberrations are visible to the naked eye

Screen Uniformity: Color

To measure color uniformity, we display an 80-percent white field and measure the Delta E error of the same nine points on the screen. Then we subtract the lowest value from the highest to arrive at the result. A smaller number means a display is more uniform. Any value below three means a variation that is invisible to the naked eye.

As in the white uniformity test, it’s rare that a display shows any color shift from zone to zone. The larger screens of HDTVs are more susceptible to errors here.

  • Pattern used: 80-percent Gray Field
  • Where appropriate, we compare measurements with Uniformity Compensation On and Off
  • Results under 3 Delta E mean no color shift is visible to the naked eye

The Tests: Pixel Response and Input Lag

To perform these tests, we use a high-speed camera that shoots at 1000 frames per second. Analyzing the video frame-by-frame allows us to observe the exact time it takes to go from a zero-percent signal to a 100-percent white field.

The pattern generator is placed at the base of the monitor so our camera can capture the precise moment its front-panel LED lights up, indicating that a video signal is being received. With this camera placement, we can easily see how long it takes to fully display a pattern after pressing the button on the generator’s remote. This testing methodology allows for accurate and repeatable results when comparing panels.

Here’s a shot of our test setup. Click on the photo to enlarge.

The brighter section of the camera’s screen is what appears in the video. You can see the lights of the pattern generator in the bottom of the viewfinder. We flash the pattern on and off five times and average the results.

When we test monitors with refresh rates greater than 60 Hz, we have to use a PC as the signal source. We use the same white field pattern and trigger its appearance with a mouse movement. That motion is recorded with the high-speed camera to see precisely how long it takes the screen to fully render after the mouse is moved. This test method is also run five times and the result averaged.

The response chart shows only how long it takes for the panel to draw a full white field from a black screen. To calculate the total input lag, we first time the period between initiating the signal to the beginning of the refresh cycle. Then we add the screen draw time to arrive at the final result.

  • Pattern used: 100-percent White Field
  • Camera: Casio Exilim EX-ZR100 set to 1000 fps (1 frame = 1 millisecond)
  • Video analyzed frame-by-frame to calculate result
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