Diffuser Lids and Even Glow for Light-Up Letters
The most common complaint about printed light-up letters is hot spots: bright dots where each LED shows through the lid. A good result is a smooth, even glow. You get it from four things: the lid, the depth of the letter, the strip, and how far the strip is from the lid. This guide explains how each one works, how to test them cheaply, and what to change when you see dots.
Why hot spots appear
An LED strip is a row of small, very bright points. If the lid is thin, clear or close to the strip, you see each point through it, like looking at a row of torches through tissue paper. To spread the light you need to scatter it in the lid and give it space to mix before it reaches the face.
The lid
A white lid printed at 100% infill, about 1 to 2 mm thick, is a good starting point for an even glow. White scatters the light. Solid infill avoids the pattern of an infill grid showing through. A lid that is too thick gets dim; one that is too thin shows the LEDs.
| Lid thickness | Brightness | Evenness |
|---|---|---|
| About 1 mm | Brighter | Good with enough depth; may show dots in shallow letters |
| About 1.5 mm | Balanced | A good all-round choice |
| About 2 mm | Dimmer | Very even; use a stronger strip |
Depth gives the light room to mix
The deeper the letter, the more distance the light has to spread before it reaches the lid, and the more even the glow. Deeper letters glow more evenly. A shallow letter with a close lid will usually show dots. If a hot-spotted letter must stay shallow, use a strip with more LEDs per metre, a COB strip or a thicker diffuser.
COB strips for shallow letters
A COB (chip on board) strip has so many tiny LEDs close together that it looks like a continuous line of light. It gives the smoothest result in shallow letters. A standard SMD strip with visible LED chips needs more depth. LED strips for channel letters compares them.
A reflective interior helps
Light that hits a dark wall is lost. If the inside of the letter is white or a light colour, the light bounces around and reaches the lid from many directions, which smooths the glow and makes it brighter. Print the body in white, or paint the inside with matte white paint before you fit the strip. Avoid glossy paint, which creates its own reflections of the LEDs.
The four lid styles
| Style | Effect on the glow |
|---|---|
| Flush cap | Even glow from the top face only; simplest to print. |
| Recessed face | The rim frames the glow and shades the face a little, so hot spots show less when you look at it from an angle. |
| Wrap-around | Light also comes out of the edges, which gives a halo on the wall behind; the lid has more area to print. |
| Open channel | No diffuser at all, so for neon flex or when you want to see the LEDs. |
Check the strip fits before you decide
The LED Letters tool flags areas too narrow for your strip. A narrow stroke of a letter may fit only one line of strip. Where the strip cannot run, the letter can look dark in that part. Make the letters larger or the strokes wider by choosing a bolder font.
Fixing hot spots step by step
- Check the lid first. Is it white, solid and at least 1 mm thick? A grey, translucent or thin lid is the most common cause.
- Add depth. If the sign allows it, increase the channel depth. Even 5 to 10 mm helps.
- Move the strip lower. Stick it to the back, not the wall, so the light travels further.
- Change the strip. A COB strip, or a strip with more LEDs per metre, removes dots at the source.
- Add a diffusion film. A sheet of thin diffusion film under the lid scatters light further, at the cost of some brightness.
Common mistakes
- Printing the lid in a translucent filament and expecting an even glow; it often looks patchy.
- Fitting the strip flat against the lid, so each LED shows.
- Using glossy paint inside the letter.
- Choosing a lid thickness without testing on a small sample first.
Questions people ask
What colour should the lid be?
White or a very light colour. Dark and coloured filament absorbs the light and looks dim.
Should the lid be translucent filament?
A natural or translucent filament can work, but it often looks patchy. A white lid at full infill is the most reliable.
Does the lid need support?
The lid prints flat. Print it face-down on the bed with no supports.
Why is my glow patchy near corners?
Corners are where the strip bends most and sits closest to the lid. Add depth there, use a COB strip or add a diffusion film.
Keep reading
How to Make 3D-Printed LED Channel Letters (Light-Up Letters)Design and print light-up channel letters: walls, depth, lid style, LED strip fit, wire holes, logo upload, and the STL, DXF and SVG files you get.Choosing LED Strips for 3D-Printed Channel LettersCOB or SMD, 12 V or 24 V, how bright, how much power, how wide: how to choose an LED strip that fits and lights 3D-printed channel letters, and what the LED Letters tool estimates.
How to Wire and Power an LED Neon Sign (Power Supply, Cutting, Connections)Choose the right power supply for an LED neon strip, work out the wattage from the strip length, cut and seal the strip, connect it safely and test it before you finish the sign.Written and checked by the Volucore team against the behaviour of our tools. Spotted a mistake or something out of date? Tell us and we will fix it.