Why does my backlit graphic have hot spots?

The symptom is familiar: a regular grid of bright patches across the graphic, most obvious in large areas of flat mid-tone, invisible in busy image areas, and worse the closer you stand. It is one of the few print defects that looks like a printing problem and is not one.

What actually causes hot spots in a light box?

Light leaves an LED in a cone. Between the LED and the face, those cones spread and overlap, and once they overlap enough the illumination across the face is even. Depth is what buys that overlap. Halve the depth and you roughly halve the distance each cone has to spread, so the point at which neighbouring cones meet moves further apart than the LEDs themselves are spaced — and the gaps show.

The fabric is the second chance. A diffusing material scatters light sideways as it passes through, smearing each cone into its neighbours. A material that transmits light efficiently and scatters little passes the pattern straight through.

So the governing relationship is depth-to-pitch ratio against fabric diffusion. A frame with LEDs on 50 mm centres and 50 mm of depth is roughly a 1:1 ratio and will be forgiving. The same pitch in a 25 mm frame is 0.5:1 and needs real help from the fabric.

Which fabric should I use in a shallow light box?

The one that scatters most, which usually means the one that transmits least. Ranked by measured light through:

Fabric	Light through	Thickness
Rio	59 %	0.22 mm
Halo	48 %	—
Lumex	48 %	0.23 mm
Tesa	40 %	0.55 mm
Aureline	39 %	0.57 mm

Read as a brightness ranking that puts Rio on top. Read as a hot-spot-risk ranking it says the same thing in reverse, and the thickness column reinforces it: Rio and Lumex are around a fifth of a millimetre, Tesa and Aureline more than twice that. A thicker material has more depth in which to scatter.

That pairing — most transmissive and thinnest — is the reason Rio is the one to be careful with in a shallow frame, and the reason Tesa and Aureline are the safer picks there. To be clear about what that is: it is reasoning from the measured transmission and thickness figures, not a hot-spotting test result. Nobody publishes a hot-spot rating, because the number would only be true for one frame.

Can I fix hot spots without changing the fabric?

Usually, and often more cheaply.

Add a diffuser layer. A sheet of opal acrylic or a diffusion film behind the graphic does the scattering the fabric was not doing. This is the standard fix and it works, at the cost of some overall brightness and some weight.

Increase the depth. Most effective, rarely available — the frame is usually already bought.

Change the LED layout. Tighter pitch, or edge-lit with a light guide panel instead of direct backlighting. Edge-lit frames essentially do not hot-spot, which is why shallow retail boxes tend to be edge-lit in the first place.

Double the fabric. Two layers of the same material, or a plain white liner behind the printed face. Crude, free if you have offcuts, and surprisingly effective as a diagnostic even if you do not ship it that way.

Why does the same fabric hot-spot on one graphic and not another?

Because ink density changes local transmission.

Where the artwork is dark, less light gets through, and the LED pattern is hidden under the ink. Where it is light, more gets through and the pattern shows. A graphic that is mostly photographic detail hides hot spots well; one with a large area of flat pale colour — a sky, a white product background, a pastel brand field — is the worst case, and it is also the case that most often gets approved on screen without anyone thinking about the frame.

This is why the test that matters is not a swatch on a light table. It is your artwork, on your fabric, in your frame.

How do I test for hot spots before committing to a job?

Print a test file rather than a picture: a full-width band of flat 20 % grey, a band of flat 50 %, and a band of white. Flat tone is where the pattern shows, and three tones tell you where your threshold is.

Mount it in the actual frame, at the actual depth, with the actual LEDs. Then look at it from the distance the public will — hot spots that are obvious at arm's length frequently disappear at three metres, and if the graphic lives above a doorway that may be the end of the problem.

Photograph it. A phone camera at a fixed exposure is a more honest judge than the eye, which adapts and forgives.

Light-through figures and thickness for every fabric are on the product pages, and the light simulator on this site will show you the relative brightness of all six against a dimmer. For the part no number can answer, ask us for sample yardage and put it in your own frame.