I wonder, does a glass flat lens on top of a TIR block-lens serves any purpose? It may prevent the acrylic from scratching in a scenario where the light is tossed with things like keys. But it can also shatter easier I imagine in similar situation or when dropped on a hard surface.
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IMO you’ve summed it up, scratch resistance with the caveat that it’s more likely to shatter than acrylic will. I have a couple of lights with a TIR and no lens, the TIR is recessed a few mm from the bezel and as I don’t put other things in the same pocket as my lights I’m not too worried about them getting scratched. In general I do prefer glass over the top though.
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It’ll be fine, I build probably hundreds of light with PC, PETG over tir optic, if there are any loses the are not noticeable by eye, as far as beam, no issues.
The one and only glass lens I’ve ever had break was on my Emisar D4 (original). I have no idea how it broke, I’d have thought support from the TIR behind would make the glass less likely to break. I found it in my backpack with the lens still attached but in 3-4 pieces.
I think for me, the scratch resistance aspect is only relevent for TIR optics which are hard to replace or a unique size. A 20mm optic in e.g. a S2+ is likely to be replaced a couple of times over the lifespan of the light, perhaps to change the beam when I update the emitter in a few years time etc.
For lights with no glass and hard to replace optics, I use clear self adhesive tape as a sacrificial layer.
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Agreed. Over time, an unprotected TIR will accumulate scratches and dirt, and I would guess that these damages will be more lossy than a piece of clear glass or other more scratch-resistant material. I see no downsides at all for adding such protection.
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new business idea
gorilla glass 
TIR protector
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Go nuts.
For me, the cost isn’t worth it. My lights don’t get rough use. I’ll continue to use transparent self adhesive tape/vinyl for lights I can’t assemble and that don’t have glass. For easy to replace optics, it’s no drama changing the optic in 3+ years or if/when I upgrade the LED…
Just needs a bit of research to find the correct size
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