Monitor Test (Dead Pixels, Gamma, Banding)

About the Monitor Test (Dead Pixels, Gamma, Banding)

Check a monitor, laptop, TV, tablet or phone screen with full-screen test patterns, before you buy, return or adjust it. The test steps through screens of solid colours for dead and stuck pixels, black and white level steps in the style of the broadcast PLUGE signal, a gamma check at 25, 50 and 75% brightness (and for red, green and blue alone), smooth gradients that show banding, grey fields for uniformity and backlight bleed, one-pixel lines, checks and text for sharpness and scaling, 100% and 75% colour bars, a viewing-angle board and moving blocks for response time and motion blur, with the refresh rate your browser paints at.

Every pattern is drawn in your screen’s own pixels, and each screen tells you what to look for and what it means. Move with the arrow keys, a click, a tap or a swipe; the screen changes only when you ask, at most about three times a second, so nothing flashes. The test works offline and records nothing.

Common uses

• Checking a new or second-hand monitor, laptop, TV or phone within its return period.
• Setting brightness, contrast and gamma before photo or video work.
• Comparing screens in a shop or between models, with the same patterns.
• Showing a fault (bleed, clouding, a stuck pixel, smearing) to a seller or repair shop.

Black and white level

The broadcast PLUGE signal (ITU-R BT.814) puts stripes about 2% below and 2% above black on a black background, and the brightness control (black level lift) is set so that the blackest stripe just disappears while the brighter one stays visible. A web page sends full-range RGB, in which nothing is darker than black, so the black-level screen shows black, the stripe 2% above it (level 5) and one at 4% (level 10), and steps from 1 to 25 below them: set the brightness so the background is as black as it gets and the 2% stripe is only just visible.

The white-level screen does the same at the top: steps from 230 to 254 on white should all stand out. If several low steps merge into black or the top steps into white, the screen or the computer may also be using a different RGB range (full 0–255 or limited 16–235) than the other expects; most TVs and graphics drivers have a setting for it.

How the gamma check works

A display turns each level into light with a power curve: a level v gives about (v ÷ 255)^γ of white’s luminance. Lines one pixel apart, one white and one black, give exactly half of white’s light, whatever the gamma, so a solid patch blends into them when its own light is half too: at level 255 × 0.5^(1/γ). The rows at 25% and 75% use one lit line in four and three in four. Computer screens and the web use sRGB, whose curve is close to gamma 2.2 (the 50% match is level 188; CSS Color 4 gives the formula); video monitors in dark studios follow ITU-R BT.1886, an exponent of 2.4.

The lines must fall on the screen’s own pixels: use 100% zoom and the native resolution. The page sizes its canvas in device pixels; where a browser cannot (some scaled screens), it says so on the screen.

Banding, uniformity and bleed

The top grey gradient uses all 256 levels a web page can show, one band each; the dark and light strips stretch the shadows and highlights so steps there are easier to see. Visible steps come from colour settings, night light, colour profiles that clip, or panels with fewer shades. Web pages draw these patterns with 8 bits per colour, so the test cannot show whether a 10-bit path works.

The grey fields show uniformity: compare the nine areas around the small marks for darker corners, clouds or a tinted side. On black in a dark room, light leaking in at the edges of an LCD is backlight bleed and stays put when you move; IPS glow changes with your viewing angle. OLED screens should be completely black.

Sharpness, colour and viewing angle

Lines and checks one pixel wide should look crisp up close and flat grey from a distance; blur or moiré means the picture is being scaled, and bright or dark halos around the black and white bars mean the screen adds sharpening. The text rows show how small text renders. The colour bars at 100% and 75% should be clean and evenly lit, without colour bleeding. On the viewing-angle board, move your head: IPS and OLED panels change little, TN panels darken or invert from below, and VA panels shift mid-greys away from straight on.

Motion and refresh rate

Three blocks cross the screen — white on black, black on white, and grey on grey, a change many panels are slower at — at a speed you choose, with ticks every 32 pixels to judge how far a smear reaches. Follow a block with your eyes: a trail behind it is slow pixel response or the motion blur every hold-type screen has; a bright or dark fringe is overshoot from aggressive overdrive. The screen also shows the refresh rate the browser paints at, measured from its own frame timing; power saving, a background tab or a variable refresh rate can make it lower than the screen’s maximum. Measuring response times in milliseconds needs a high-speed camera or a sensor.

The screens change only when you ask and at most about three times a second, and no screen flickers, because flashing more than three times a second can trigger seizures (W3C WCAG: Three Flashes or Below Threshold).

Limitations

• You judge the results by eye: the page cannot measure light, so colour accuracy and exact brightness need a colorimeter.
• Patterns are drawn with 8 bits per colour in the standard colour range; HDR and 10-bit modes are not tested.
• iPhones do not let web pages go full screen, so Safari’s bars stay visible; some TV browsers cannot either.
• The refresh rate shown is how often the browser paints, which can be lower than the screen’s maximum.

Privacy

Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.