LED Strip for Lightboxes: Pitch, Depth and Uniformity
How module pitch, box depth and acrylic grade set lightbox uniformity. A pitch table by depth, the 85% target, and why adding modules makes hotspots worse.
HFY Applications Engineering · Published October 27, 2026 · 3 min read
Lightbox uniformity is set by the ratio of module pitch to box depth, not by module brightness. A 100 mm box with 3 mm opal acrylic needs modules at about 110 mm pitch to reach 85% edge-to-centre uniformity; the same box with 5 mm acrylic tolerates 150 mm. Widening pitch beyond the table produces a hot centre and dark edges that no extra module fixes.
Key points
- Uniformity is a geometry problem: pitch divided by depth predicts the result.
- The target for fascia signage is ≥ 85% edge-to-centre luminance.
- Adding modules in the centre of a patchy box makes it worse, not better.
- Driver headroom of 20% is not optional — a sealed cabinet reaches 60 °C in summer.
The geometry that decides everything
A lightbox is an optical cavity. Modules emit into it, the back and sides reflect, and the acrylic face diffuses what arrives. The variable that predicts uniformity is:
pitch ÷ depthBelow 1.2, the box reads even. Between 1.2 and 1.8, it reads acceptable at 85–90% uniformity. Above 2.0, the face shows a visible grid.
Pitch table by depth and diffuser
Measured on a 1,200 × 800 mm test box with white back panel, 0.9 W side-emitting modules:
| Box depth | 3 mm opal acrylic | 5 mm opal acrylic | 3 mm clear + diffuser |
|---|---|---|---|
| 80 mm | 90 mm pitch | 110 mm | 100 mm |
| 100 mm | 110 mm | 130 mm | 120 mm |
| 150 mm | 140 mm | 170 mm | 150 mm |
| 220 mm | 180 mm | 200 mm | 190 mm |
| 300 mm | 200 mm | 220 mm | 210 mm |
Modules per square metre falls from about 27 at 80 mm depth to about 13 at 300 mm. That is the cost difference between a box designed from optics and one specified from habit.
Why the centre gets hot
If a box is patchy at the edges, the instinct is to add modules along the perimeter. This often makes the result worse, because the modules are then too close to the acrylic at the edge and produce a bright band there while the centre remains under-lit.
The correct moves, in order:
- Reduce pitch evenly. Redistribute the same number of modules over a tighter grid.
- Move modules away from the face. Depth is free; distance is what averages the light.
- Check the back panel. A grey or unpainted back absorbs 40% of the light that should be reflected to the edges.
- Check the acrylic. Clear acrylic without a diffuser will never be even, whatever the pitch.
Zigzag routing and connectors
A row-by-row layout needs four feeds and twelve connectors on a typical 1,200 × 800 mm panel. A zigzag FPC with 3-LED cut units runs the whole panel from one feed. Since connectors are the most common failure point, removing them removes most of the warranty exposure.
The trade-off is that zigzag routing requires the layout drawing to be followed exactly at installation. Mixed packing, or a fitter improvising the pattern, produces a box that measures unevenly even though the modules are correct — see the lightbox case study for how much this cost one fabricator.
Power and heat in a sealed box
A 1.5 m² box at 20 modules per square metre and 0.9 W per module draws 27 W. Two things follow:
- The driver should be rated at least 34 W (27 ÷ 0.8), and mounted outside the cabinet if possible.
- Internal temperature in summer reaches 55–60 °C, which reduces module life to about 30,000 hours against the 50,000 hour rating at 25 °C.
If the box is sealed and outdoors, specify IP65 modules and a 3 mm drain in the base. The failure we see most is not water on the face but 5 mm of standing water at the bottom of a cabinet with no drain.
For side-emitting and zigzag products see lightbox and side-emitting strip; for the driver side, power supply sizing covers the derating rule.
This article describes general manufacturing practice for machined and fabricated components. It is not a substitute for a drawing-specific engineering review — tolerances, material selection and process capability depend on the individual part.