How module spacing and acrylic face quality directly affect the appearance and performance of channel letters and light boxes
1. Introduction
The quality of an illuminated sign or light box is ultimately judged by what the viewer sees from the outside — a smooth, evenly lit face with consistent colour and brightness from edge to edge. Achieving this result depends on two factors that are frequently underestimated by installers: the spacing between LED modules inside the sign, and the grade of acrylic used for the face.
Both decisions are made during the build, before the sign is ever powered on. Both are difficult or impossible to correct after the fact without dismantling the sign. Getting them right at the planning stage is the difference between a sign that looks professional and one that reflects poorly on both the fabricator and the end client.
PART ONE: LED MODULE SPACING
2. How Module Spacing Affects Light Uniformity
Each LED module produces a cone of light that spreads outward from the point source. As that light travels through the depth of the sign — from the module on the back wall to the acrylic face — it expands. The deeper the sign, the wider the cone spreads before it reaches the face, and the more the light from adjacent modules overlaps and blends.
When modules are placed too far apart relative to the depth of the sign, the cones of light from neighbouring modules do not overlap sufficiently by the time they reach the face. The result is a series of bright spots directly in front of each module, with darker areas — known as hot spots and shadow banding — visible between them.
This effect is immediately obvious to anyone viewing the sign, particularly at night or in lower ambient light conditions. It gives the sign an unfinished, low-quality appearance that cannot be corrected after installation without removing and repositioning the modules.
⚠ IMPORTANT: Hot spots and shadow banding caused by incorrect module spacing are not a product defect — they are an installation error. Repositioning modules after the sign is built is time-consuming and costly. Always calculate spacing correctly before installation begins.
3. The Relationship Between Depth and Spacing
The general rule in the signage industry is straightforward: the shallower the sign, the closer together the modules must be placed. The deeper the sign, the more spacing is permissible because the light has more distance to spread and blend before reaching the face.
As a practical guideline for 1.2W LED modules:
| Sign / Box Depth | Max Module Spacing | Recommended Spacing | Notes |
| 30 – 50mm | 75mm | 50 – 60mm | Very shallow — modules must be close |
| 50 – 75mm | 100mm | 75 – 90mm | Shallow channel letter depth |
| 75 – 100mm | 125mm | 100mm | Standard channel letter depth |
| 100 – 150mm | 150mm | 120 – 130mm | Medium light box depth |
| 150mm+ | 200mm | 150 – 175mm | Deep light box — spacing can increase |
Note: These are general guidelines for 1.2W modules. Higher-power modules with wider beam angles may permit slightly greater spacing. Always test with a mock-up before committing to spacing on large installations.
✔ TIP: When in doubt about spacing for a shallow sign, err on the side of placing modules closer together. The cost of a few additional modules is always less than the cost of rebuilding a sign that has visible hot spots.
4. The Consequences of Incorrect Module Spacing
Placing modules too far apart for the given sign depth produces a range of visible and practical problems:
• Hot spots — bright circular pools of light directly in front of each module, clearly visible through the acrylic face, particularly on white or light-coloured faces.
• Shadow banding — darker bands or patches between modules where the light cones do not overlap, creating an uneven, striped appearance.
• Colour inconsistency — in RGB or colour-changing applications, gaps between modules cause the colours from adjacent modules to be visible as distinct zones rather than blending into a single uniform colour.
• Perceived poor quality — to the end client and the viewing public, an unevenly lit sign looks cheap and unprofessional regardless of the quality of the materials used elsewhere.
• Difficult and costly rectification — adding modules to a sign that has already been assembled, wired, and installed requires the sign to be taken down, the face removed, additional modules mounted and wired, and the sign reinstalled. This is rarely economical.
5. Varying Depth Within a Single Sign
A particularly common challenge arises in channel letters and shaped light boxes where the depth of the sign is not consistent throughout. The upright strokes of a letter may be deep, while the curved sections or thin strokes are shallower. A light box with a shaped face may have greater depth at the centre than at the edges.
In these situations, a single module spacing that works well in the deepest sections of the sign will be too wide for the shallower sections. The shallow areas will exhibit hot spots and banding, while the deep areas may appear uniform. The sign will have inconsistent brightness from one area to another.
The correct approach is to adapt the module spacing to the local depth of each area of the sign:
• Map the depth variations of the sign before placing any modules. Identify the shallowest sections and calculate the required spacing for those areas.
• Use closer spacing in shallow sections and wider spacing in deeper sections. This will result in more modules in some areas, which must be accounted for in the power supply calculation.
• Consider reducing depth variations during fabrication where possible — a more consistent depth throughout the sign makes module placement significantly simpler and produces a more uniform result.
⚠ IMPORTANT: Never use a single fixed module spacing across a sign with varying depth. The shallowest point in the sign always dictates the minimum spacing requirement for the entire layout.
PART TWO: ACRYLIC FACE QUALITY
6. Why Acrylic Grade Matters
The acrylic face of a channel letter or light box is not merely a protective cover — it is an active optical component. Its job is to receive the light from the LED modules inside the sign, diffuse it evenly across the face, and transmit it to the viewer with consistent colour and brightness. The grade and quality of the acrylic has a direct and significant effect on how well it performs this function.
Not all acrylic is equal. There is a substantial quality difference between generic or low-grade acrylic sheeting and purpose-made signage acrylic from reputable manufacturers — and that difference is clearly visible in the finished sign, especially when viewed at night when the sign is illuminated.
7. Cast Acrylic vs Extruded Acrylic
Acrylic sheeting is manufactured by one of two processes: casting or extrusion. Each produces a materially different product with different optical and physical properties.
| Property | Cast Acrylic | Extruded Acrylic |
| Manufacturing | Poured into moulds and cured slowly | Pushed through a die at high speed |
| Optical clarity | Superior — very consistent clarity | Good, but may have minor striations |
| Thickness tolerance | Very tight (±0.3mm) | Wider tolerance (±0.5mm or more |
| Light transmission | More consistent across the sheet | Can vary slightly across the sheet |
| Colour consistency | Excellent — colour is stable and even | Good, but can vary between batches |
| Surface quality | Smooth, uniform surface | Can exhibit surface ripple or waviness |
| UV resistance | Excellent — resists yellowing | Good but generally lower than cast |
| Machinability | Excellent — cuts and polishes cleanly | Acceptable — can chip on edges |
| Cost | Higher | Lower |
| Best for | High-quality signage, backlit displays | General use, interior, lower budgets |
8. The Effect of Low-Grade Acrylic on Illuminated Signs
When low-grade or generic acrylic is used as the face of an illuminated sign, the visual problems are compounded by — and sometimes more visible than — any issues with module spacing. Even with perfectly placed modules behind it, a poor-quality acrylic face will compromise the finished result in several ways:
• Uneven light transmission — variations in thickness or internal stress within low-grade acrylic cause some areas of the face to transmit more or less light than others, resulting in a mottled or patchy appearance when illuminated.
• Hot spot amplification — low-grade acrylic with poor diffusion properties makes hot spots from LED modules more visible, not less. A high-quality diffuse acrylic helps blend and spread the light, masking minor spacing imperfections.
• Colour shift and inconsistency — cheaper acrylic may not have consistent colourant distribution, causing the colour to appear lighter or darker in different areas of the same face panel.
• Yellowing and UV degradation — low-grade acrylic exposed to sunlight will yellow and discolour within months to a few years, causing the sign to look aged and poorly maintained.
• Surface distortion — extruded acrylic of poor quality can exhibit surface waves or ripples that distort the reflected and transmitted light, making the sign face look warped or uneven.
⚠ IMPORTANT: Specifying low-grade acrylic to reduce costs is a false economy. A sign that yellows, shows hot spots, or has uneven colour will be a source of client complaints and potentially require early replacement of the face panel — at greater total cost than using quality acrylic from the outset.
9. Choosing the Right Acrylic for Illuminated Signs
For all illuminated channel letters and light boxes, the following acrylic specifications are recommended:
• Use purpose-made signage acrylic from reputable manufacturers. These products are specifically formulated for backlit applications with optimised light transmission and diffusion properties.
• Specify cast acrylic for any application where light uniformity and colour consistency are critical — including high-profile commercial signage, retail displays, and externally illuminated fascias.
• Select the correct diffusion level for the sign depth. Shallower signs benefit from acrylic with a higher diffusion rating to help mask module spacing. Deeper signs can use lower diffusion acrylic with a higher light transmission percentage.
• Match the acrylic thickness to the sign size — larger face panels require thicker acrylic to resist bowing under temperature and wind load. A standard minimum is 3mm for letters and small light boxes; 4–5mm for larger panels.
• Verify UV stabilisation for all externally installed signs. Confirm with the supplier that the acrylic carries UV stabilisers suitable for prolonged outdoor exposure in the local climate.
✔ TIP: For shallow channel letters (50–75mm deep), always use a high-diffusion acrylic face. The diffusion layer compensates for closer module spacing requirements and helps achieve a smooth, even glow across the entire face with no visible hot spots.
10. The Combined Effect: Spacing and Acrylic Working Together
Module spacing and acrylic face quality are not independent variables — they interact with each other. The correct combination of both produces a sign with excellent, professional-quality illumination. An error in either introduces visible problems that the other cannot fully compensate for.
| Module Spacing | Acrylic Quality | Result |
| Correct | High quality | Excellent uniform illumination — professional result |
| Correct | Low quality | Patchy transmission, possible colour inconsistency, UV degradation risk |
| Too wide | High quality | Hot spots partially masked by diffusion, but still visible in shallow signs |
| Too wide | Low quality | Hot spots clearly visible, uneven colour, rapid deterioration — poor result |
| Too close | High quality | Excellent uniformity — slight over-illumination cost, best possible result |
The message from the table above is clear: the only combination that consistently produces a professional result is correct module spacing combined with high-quality acrylic. Cutting corners on either — or both — produces a visible, lasting compromise in the quality of the finished sign.
11. Summary
• Always calculate module spacing based on the shallowest depth in the sign — not the average or maximum depth.
• Use closer spacing in shallow sections of signs with varying depth. Account for additional modules in your power supply calculation.
• Specify cast acrylic for high-quality illuminated signs, particularly for external applications and where colour uniformity is critical.
• Match diffusion level to sign depth — higher diffusion for shallow signs, lower diffusion with higher transmission for deep boxes.
• Never reduce acrylic quality to save cost — the long-term consequences of yellowing, uneven transmission, and client dissatisfaction always outweigh the short-term saving.
12. Need Assistance?
For advice on module spacing, acrylic specification, or any aspect of your illuminated sign build, please contact The Lighting Zone:
Email: admin@thelightingzone.co.za
Tel: +27 10 590 6101
Website: www.thelightingzone.co.za