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An LED light box panel is a sheet of acrylic doing two jobs at once. It has to pass light through, and it has to spread that light out enough that nobody can see the individual LED modules sitting behind it. Most of the industry lands somewhere between 3 mm and 5 mm for indoor light boxes, and moves to 8 mm or more once the unit goes outdoors or gets big.

Thickness is the number buyers ask about first, and it is the least decisive of the four variables. Panel thickness, the diffusion grade of the sheet, LED pitch and box depth decide the result together. Get three of them right and the fourth can still ruin the panel.

Quick pick: thickness by light box type

Light box type	Panel	Thickness	Why
Slim indoor unit, up to about 600 × 900 mm	Opal white	3 mm	Enough diffusion at this size, keeps the unit thin
Standard indoor light box	LED diffusing opal	3–5 mm	Hides hotspots at normal viewing distance
Large indoor panel, over 1.2 m	LED diffusing opal	5 mm	Flatness, plus more room for light to scatter
Edge-lit panel with engraved light guide	Clear, laser engraved	3–6 mm	Clarity drives the light guide, diffusion does not
Outdoor or large format	UV stabilised opal	8–10 mm	Wind load and long term bowing
Replacement fluorescent diffuser	Opal	2–3 mm	Shallow fitting, weight matters more than stiffness

Three millimetres is the most common answer for small indoor work and five millimetres is where we would start for anything a person stands in front of at close range. Anything past about 8 mm is rarely about light at all. It is about the panel not moving.

Diffusion and transmission pull against each other

This is the part that gets skipped. A panel that scatters light well is usually a panel that passes less of it. The two properties run in opposite directions, so "make it thicker" is not a free fix.

Clear acrylic transmits around 92% and scatters almost nothing. Put LEDs behind it and you see LEDs. Opal white sits in the 40–55% range and diffuses well, and that combination made it the default light box face. The grades formulated specifically for LED diffusion reach about 70% transmission while still scattering hard enough to kill hotspots, and they cost more for that reason.

Grade	Light transmission	Where it belongs
Clear acrylic	About 92%	Light guide plates, box bodies, edge-lit panels
LED diffusing opal (Opal 050 type)	Up to about 70%	Shallow light boxes, anything under 20 mm deep
Standard opal white	40–55%	Deeper boxes, 40 mm and up
Translucent coloured	20–60%, varies by colour	Decorative panels and brand colour work
Opaque white	Close to 0%	Nowhere in a light box. It blocks light rather than spreading it

That last row catches people out. Standard opaque white acrylic looks like the right material and is not. Behind a lit LED strip it produces a bright patch in the middle and near darkness at the edges. If the product listing does not say "light diffusing" or "LED opal", you have the wrong grade for the job.

The other half of the trade: a thicker opal panel diffuses better and transmits less. Past roughly 5 mm you are buying evenness with brightness, so a box that was marginal at 3 mm can end up dim at 8 mm with the same LEDs inside it.

LED spacing and box depth decide more than thickness does

We have seen plenty of panels specified at a sensible thickness that still showed dots, because the LEDs behind them were too far apart for the depth available. Light needs distance to blend. A shallow box with widely spaced modules has nowhere for that to happen, and no sheet on the market fully repairs it.

Panel	Box depth	LED spacing	Grade needed	Hotspot risk
3 mm	Under 20 mm	8–12 cm	LED diffusing opal	High
3–5 mm	20–40 mm	8–12 cm	LED diffusing opal	Medium
5 mm	20–40 mm	10–15 cm	LED diffusing opal	Low to medium
5 mm	Over 40 mm	10–15 cm	Standard opal white	Low
8 mm	Over 40 mm	Up to 15 cm	Standard opal white	Low

The spacing figures come from common practice in side lit builds. The risk column is our read of how these variables interact, not a measured figure, and it assumes the LEDs sit at least 12 mm back from the panel. Closer than that and every row moves up a level.

There is a design tension hiding in this table. Slim boxes look better on a wall and cost less to ship. Deep boxes light better. When a customer asks for a very thin body and also wants to save money on LED density, the panel is the part that gets blamed, and it is rarely the part at fault.

Outdoor and large format work

Outdoor changes the calculation. Now the sheet is carrying wind load and sitting in direct sun, so 8 mm is the usual floor and 10 mm is not unusual on anything over a metre. The grade has to be UV stabilised, or you will be replacing panels for yellowing long before the LEDs go.

Acrylic expands at roughly 7 × 10⁻⁵ m/m·°C. On a two metre panel that is real movement across a summer, so leave 1 to 3 mm between sheet and frame and seal with silicone rather than a rigid adhesive. Panels bonded hard into a frame tend to crack at the corners in hot climates, and this shows up sooner in humid markets than in temperate ones.

Weight is the reason acrylic replaced glass here. At about 1.19 g/cm³ against roughly 2.5 for glass, it is a little under half the weight for the same volume.

What thickness costs you in freight

Thickness is not only a lighting decision. Sheet packs by volume, so panel area per cubic metre of cargo scales almost exactly with the inverse of thickness.

Thickness	Weight per m²	Panel area per m³ of cargo space
3 mm	About 3.6 kg	About 333 m²
5 mm	About 6.0 kg	About 200 m²
8 mm	About 9.5 kg	About 125 m²

Calculated from 1.19 g/cm³. Real packing never reaches these figures because of pallets, gaps and protective film, but the ratio holds: you get roughly two and a half times more panel area out of the same container at 3 mm than at 8 mm. For an export order, that is usually the largest single cost line affected by the thickness choice, and it rarely gets raised before the order is placed. Send us the panel size and the thickness you are weighing up and we will run the landed figure for your route.

What to put in the enquiry

• Give the thickness and the grade. "5 mm acrylic sheet" is not a specification, it is half of one.
• Ask for the transmission figure rather than a colour description. "Milky white" tells a supplier nothing measurable.
• State the box depth and the LED pitch. A good supplier can work backwards from those; without them they are guessing.
• Ask for the tolerance, and ask whether it runs both ways. Cast acrylic carries the looser thickness tolerance, commonly quoted at about ±10% of nominal, while extruded holds roughly ±5% or better. That runs opposite to what most buyers expect, since cast is the more expensive sheet.
• Say indoor or outdoor, and give the largest panel size.
• If you are importing, ask how many square metres of that thickness fit the container you are paying for.

Tolerance shows up twice in a light box. The panel has to sit in the frame or channel it was cut for, and the thickness you actually receive is what sets the diffusion, not the number on the label.

How to check a sample before you commit

Judge the panel powered up. A sheet on a sample table looks flat and clean and tells you nothing about the thing that matters. We have watched a slim retail sign sit on a desk looking perfect, then light up showing every diode in a row.

Stand back a couple of metres and look at it from an angle, not straight on. Check the corners and the centre separately, since weak reflectors and short mixing distances show up at the edges first. Take your own photographs rather than trusting the supplier's, because a camera exposure will flatten uneven light into something that looks fine.

Frequently Asked Questions

What thickness acrylic is best for an LED light box?

For most indoor light boxes, 3 mm to 5 mm. Use 3 mm on small units where the panel is short enough not to flex, and move to 5 mm once the panel is large enough that flatness becomes a concern. Outdoor units and anything over about a metre across generally start at 8 mm.

Is thicker acrylic better for hiding LED hotspots?

Only up to a point. A thicker sheet gives light more distance to scatter, which helps, but opal acrylic also transmits less as it gets thicker. Past roughly 5 mm you are trading brightness for evenness, so if the box is already marginal, going thicker can make it dimmer without fully solving the dots.

Can I use 2 mm acrylic in a slim light box?

You can, if the LEDs are closely spaced and the sheet is a genuine LED diffusing grade. In a shallow box with widely spaced modules, 2 mm will usually show the diodes. We would not specify it on anything a customer looks at from arm's length.

What is the difference between opal white and LED diffusing acrylic?

Transmission, mainly. Standard opal white runs 40–55% and diffuses well enough for deeper boxes. LED diffusing grades reach about 70% while still scattering hard enough to remove hotspots, which is what makes them the only reliable choice in a box under about 20 mm deep.

How far should the LEDs sit from the acrylic panel?

At least 12 mm where the design allows it, and more is safer. The closer the LED sits to the panel the harder it is to hide, regardless of how good the diffusing grade is.

How far apart should LED modules be?

Around 8–12 cm for 3–5 mm panels, and 10–15 cm at 5 mm. Wider spacing needs either more box depth or a stronger diffusing grade. Narrower spacing is more forgiving of a thin panel, at the cost of more LEDs and more heat.

Do I need cast or extruded acrylic for a light box?

Extruded is cheaper and is fine for indoor work where the sheet stays flat. Cast is the better choice outdoors and for anything that gets thermoformed, and it gives cleaner edges when laser cut or routed. On thickness, the two run opposite to what most people assume: cast is the looser sheet, commonly quoted around ±10% of nominal, while extruded holds closer to ±5%. You will occasionally see a supplier publish the reverse, so read the tolerance off the datasheet rather than trusting the general rule. The two grades are compared in more detail in how to choose acrylic sheet: cast vs extruded.

Can acrylic light boxes be used outdoors?

Yes, with the right sheet. Use a UV stabilised grade, go to 8 mm or thicker, seal the enclosure properly and leave expansion room in the frame. Untreated acrylic will chalk and yellow in direct sun well before the LEDs wear out.

How much does an acrylic light box panel weigh?

About 3.6 kg per square metre at 3 mm, 6.0 kg at 5 mm and 9.5 kg at 8 mm, based on a density of 1.19 g/cm³. That is a little under half what the same area of glass would weigh.

Will a thicker panel make my light box dimmer?

Yes, slightly, for the same grade and the same LEDs. Opal sheet transmits less as thickness goes up. If the box is bright enough at 3 mm, moving to 5 mm costs you a little output and buys evenness, which is usually the right trade. Moving to 8 mm for optical reasons is rarely worth it.

Whichever thickness you settle on, check the transmission figure on the datasheet and judge the panel with the power on. That is the only test that tells you anything.

See our acrylic sheet range, light box materials, or contact Jin Yu for samples and pricing.

Published by Jin Yu (Foshan Jin Yu Advertising Material Co., Ltd.), a supplier of acrylic sheet and light box materials based in Foshan, China.