Backlight unit

An LCD panel can't make light, so a TV puts a light box behind it: the backlight unit. Its job is to turn dozens or hundreds of bright LEDs into a smooth, even white glow across the whole screen, with no hot spots and little waste.

It's a stack inside a shallow tray. The back chassis, a steel pan, holds everything and helps spread heat. LED strips are fastened across its floor. Over them lies a reflector sheet, a bright white film with a hole for each LED, which bounces backward-heading light forward again. A short distance above the LEDs sits the diffuser plate, a milky plastic plate that blurs the points of light into a broad glow. On top go thin films: a diffuser sheet to soften any remaining bright spots, then one or two prism sheets that bend wide-angle light toward the viewer, making the screen brighter from the front.

This layout, with LEDs right behind the screen, is called direct-lit. Some thin TVs are edge-lit instead, with LEDs along the edges shining into a clear acrylic light guide. Mini-LED TVs use thousands of tiny LEDs in zones that dim separately for deeper blacks. OLED TVs need no backlight at all.