How light guide plates are made: patterns, cutting and edge polishing

What a light guide plate does

LEDs are small, bright points of light. To make a whole screen or panel glow evenly, the light has to be spread out. In an edge-lit design, a row of LEDs shines into the edge of a clear plate. Inside the plate, the light bounces between the two faces and travels across it, trapped by total internal reflection. A pattern on one face breaks that reflection in a controlled way and sends light out toward the viewer: a little near the LEDs, more further away, so that the whole surface looks equally bright.

The plate is usually acrylic (PMMA), which is very clear, or polycarbonate where more toughness is needed. A reflector sheet sits behind it, and diffuser and prism films in front of it.

Five ways to make the light pattern

Printing and laser engraving need no tooling and suit flexible production. V-cutting gives a clean, ink-free pattern. Molding and roll pressing are the high-volume routes, and their quality depends entirely on the mold or roll.

The molds and rolls: where the diamond tool works

Molded and roll-pressed patterns are copies of a master surface. That master, a mold insert or a roll, is often machined with single crystal diamond tools on ultra-precision machines. Sharp diamond V-tools cut fine grooves and prisms, microns deep, into nickel-plated or copper surfaces. The shape and sharpness of the tool edge become the shape of every groove, and therefore the evenness of the light. The same technique is used for the rolls that make prism films, the brightness-enhancing films in many displays.

Because one mold makes millions of plates, the tool has to cut the whole pattern with a constant edge. A diamond that wears or chips during a long pattern leaves grooves that change from one side of the mold to the other, and that shows as uneven brightness.

Cutting the plate to size

Plates are cut from large sheets by sawing or CNC routing. That leaves rough, white edges. On three sides this may not matter, because they are covered by reflector tape or a frame. The light-entry edge is different.

The light-entry edge

All the light enters the plate through its edge, so the edge has to be optically clear and flat. A rough edge scatters light at the entrance and causes hot spots in front of each LED and dark areas between them. There are three common ways to finish it:

• Diamond edge milling or polishing: a machine with a single crystal diamond cutter cuts the edge clear and flat in one pass, without heat. This is the usual choice for optical edges.
• Flame polishing: fast and shiny, but the heat can cause stress and slight distortion, which is not ideal for an optical edge.
• Buffing: possible, but slow, and it tends to round the edge.

For more on finishing acrylic edges, see our guide to polishing acrylic.

Where Bulunmaz fits

Bulunmaz makes single crystal diamond tools for light guide plate work, in natural diamond and MCD: V-groove and micro-prism tools for molds and rolls, roll-mold turning tools, and edge-milling cutters for the light-entry edge, as standard tools or to your drawing, and resharpens them. Cutting V-grooves directly into the plate on a V-cutting machine is a different job; send us your machine and sheet details and we check case by case. See our diamond tools for light guide plates and molds.