Just how see-through can EVA foam be? I ran some tests and the results may surprise you (but almost certainly won’t).

For a project I’m considering, it would be neat if there was visible light coming from inside of the prop. But thick EVA foam definitely blocks all light, and if you’re going to smear layers upon layers of primers and paints on top of it, well… you’re definitely not going to see any light coming through it.

But if the foam is think enough, low-density enough, and the amount of layers on top of it are few enough, perhaps some light might be visible?

Well, yes. Or I wouldn’t be writing this post.

Using 2mm low-density EVA foam (less than or equal to 50 kg per cubic meter — or 1mm of high-density EVA foam of 100kg/m³, the results would be very similar) gives the best result. You probably shouldn’t use a low-density EVA foam that’s thinner than 2mm, because that just doesn’t have much structural integrity (although you could get away with 1mm high-density foam). But “best” isn’t really that great.

Here’s the sample piece I prepared for this test, again using 2mm low-density (45kg/m³, specifically) EVA foam:

To facilitate easier testing, I split my sample piece up into five different areas, from left to right:

2+0	Two layers of gray Poly-Props SEAL Prime primer, and no paint
2+1	Two layers of primer, 1 layer of brushed-on titanium white paint
2+2	Two layers of primer, two layers of white paint
2+3	Two layers of primer, three layers of white paint
2++	Two layers of primer, three layers of white, plus more paint

I set up a digitally programmable 5V LED strip (WS2812B) and placed the foam sample piece at a 3cm distance from the fully bright white LEDs to prevent obvious highlight spots. This is the result:

Interesting!

2+0	Lots of light still passes through this, but it’s quite speckled.
2+1	Some light still passes through, but it’s already noticeably less.
2+2	With two layers of white, very little light makes it through.
2+3	With three layers of white, the light is all but stopped.
2++	A whole lotta nuttin.

These results probably aren’t very surprising to anyone who’s given this much thought, but it’s good to get real-world confirmation. But even with this much material, is it at all possible to get any light to come through? Yes, actually. Sort of. Here’s a similar test with the same piece of sample foam, but with the LEDs almost directly behind the foam:

These results mirror much of what the previous test show, although the last and second to last areas now do have some light coming through. But the resulting visible light comes through only in such tiny spots that it’s just not very practical.

So where does that leave you if you want to have light pass through, and you want to use only EVA foam, and tiny highlights aren’t enough for you?

My only suggestion here is to use white EVA foam and keep the area you want light to shine through as free as possible of both paints and primers. Without all the extra materials sitting in the way and literally taking up the light hitting it, the performance of thin EVA foam for this particular purpose is actually surprisingly good:

If none of this is acceptable, then EVA foam definitely isn’t the material to use for your project, and you should look into alternative materials instead.

I hope this little test has helped you out!

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