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I've got a led headlight for illuminating a small spot region at 30cms distance from my head.

I've let it fall and the glass lens chipped. So I unscrewed the lens and got a piece of acrylic rod which is sold as having 92% light transmittance.

I turned on my lathe a copy in acrylic as close as possible of the shape of the original glass lens, and polished it to a completely transparent look.

When I screwed the new lens to the headlamp case, it worked perfectly, the spot size of the light is the same as the original lens, at least visually, but for me is like the light is not so bright as it is with the glass lens...

Why is that so? Transparent acrylic is worse than glass for transmitting light in a collimated beam?

Thanks!

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No, you shouldn't notice the difference because of light transmission losses. As you stated the acrylic rod was sold as 92% transmittance (makes sense), therefore at worst (assuming glass with 100% transmittance, which it is not) you would see a loss of 8%. This is not something you would notice.

What may be the cause is the fact that acrylic usually has a slightly lower refractive index, therefore a lens of the same shape but acrylic instead of glass, will not produce the same optical magnification! So The spot should be somewhat larger. Perhaps this is not noticeable, but remember that area is power of 2.

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