Test your screen for dead pixels

The screens, and what each one finds

• White — dead pixels, which stay black, plus dust, smears and any mark on the glass.
• Black — stuck pixels, which stay lit, and backlight bleed around the edges. Best judged in a dark room.
• Red — only the red sub-pixels are lit. A dark dot here means that pixel’s red has failed.
• Green — the easiest screen to spot a fault on — the eye is far more sensitive to green than to red or blue.
• Blue — the hardest. Blue faults are genuinely difficult to see, so give this one longer than the others.
• Cyan — green and blue lit, red off. A pixel that looked wrong on red and looks fine here has confirmed which sub-pixel failed.
• Magenta — red and blue lit, green off. The pair to cyan for isolating a green fault.
• Yellow — red and green lit, blue off — the last of the three, and where a blue fault becomes obvious.
• 50% grey — uneven backlighting, patches of tint, and the cloudiness usually called mura.
• Smooth ramp — banding. A good panel fades evenly; a weaker one shows visible steps across the gradient.
• Step wedge — crushed ends of the range. You should be able to tell all sixteen bands apart; if the darkest few or lightest few merge, contrast or brightness is set too far.