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What are the benefits of each? It 'seems' plexi has better light transmission from what I've read. Any other materials I should be looking at to cover my lens in the front of the housing?
TIA
Andy
 

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They're the same thing. Really. Both are polycarbonate, both have essentially identical optical properties, but "Lexan" (it's a brand name, not a material type) is technically "stronger" than average plexi. Personally, I haven't ever seen anyone with cracked plexi or lexan on any lights as they're usually not taking hits as they're recessed and mostly protected, so it's probably not the main concern. Glass will almost always have higher optical losses unless it's particularly pure optical glass, because it has relatively high iron impurities in it (why most glass sheets look green on the edges).
 

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Best bet while keeping cost in mind is a decent brand acrylic like Acrylite. This will come in at about 92%. It's rather cheap and comes in enough thickness options to get what you need. I've seem a couple brands claiming 93% in their spec sheets but couldn't find any suppliers with actual prices instead of "quote me" crap. The really nice stuff is AR acrylic, abrasion resistant, which has a coating to help resistant hazing and scratches. The minimum thickness and ordering size is too much unless you're doing a product run.
 

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Lexan and Plexiglass are not the same plastic. Lexan is a trademark name for polycarbonate. Plexiglass is a trademark name for poly methyl methacrylate, commonly referred to as acrylic or PMMA.

For LED bikelight use, my preference is acrylic though I usually use polycarbonate as I have access to remnant material. Acrylic, even uncoated, has better fine scratch resistance than polycarbonate. The light transmission of acrylic is just slightly better than polycarbonate. Not enough difference to see.

Do you really need a cover? If using reflectors, you must. If using TIR optics then it is maybe. I use a cover if the optic has recessed "pockets" in the face. Those "pockets" collect dirt that severely reduces the function of the optic. They are kind of tough to clean on the trail. If the optic has a smooth or lightly textured face, I omit a cover and gain ~10% output. If a bare optic becomes badly scratched, they are cheap to replace. It helps to design a housing that is easy to change optics on.
 

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kan3 said:
Aren't vlt values generated using both sides of a material? So a 92% vlt material would only cause a 4% lose in output when used as cover.
I am not an optical engineer by any stretch of the imagination, but I am pretty sure the 8% loss would be the total. In this case there are two surfaces of the material that the light must transit. One is the outer face of the side adjacent the LED and the other is the inner face of the side opposite the LED.

TIR optics make use of the fact that the inner surface of plastic can be quite reflective.

One thing I also seem to recall is that optical efficiency is the product of the individual efficiencies. ie. 90% efficiency of the optic and 92% efficiency of a cover give an 83% optical efficiency.
 

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I just installed to high CRI XPGs in a housing with a 2 mm lexan lens.

Before installing it, I spent some time playing with the just the emitters with the bare carclo optics on a heat sink.

I can tell a difference in light output.

I am going to knock the lexan out, move the heat spreader forward, and 'nano glue' the two optics together and to the housing.

Something else I have noticed is that there was a good amount of useful spill from the edges of the optic. I cut a lens window 1 mm larger than the optics. I think a 5 to 10mm oversized lens might be better. It a compromise between heat sinking and spill, I suppose.
 

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P220C said:
Something else I have noticed is that there was a good amount of useful spill from the edges of the optic. I cut a lens window 1 mm larger than the optics. I think a 5 to 10mm oversized lens might be better. It a compromise between heat sinking and spill, I suppose.
Troutie makes use of that "wasted" light in his Liberator design.
 

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Ofroad'bent said:
Lexan is also impossible to cut into shape with scissors, so it's a saw-and-sand job.
What thickness of acrylic is the "Official MTBR" material?
1 mm will cut with good scissors

I started off using 2 mm in my first lights then 1.5 for the Liberators easier to heat and bend than 2 mm .

and just this morning some 1 mm thick dropped through the mail box
for a protection layer in front of reflectors and textured optics 1 mm is fine .

I cut all mine with knife with a concave blade in it like these
 

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Ofroad'bent said:
Lexan is also impossible to cut into shape with scissors, so it's a saw-and-sand job.
What thickness of acrylic is the "Official MTBR" material?
I use .75mm lexan as I got a remnant piece at the job. It cuts pretty easily with good scissors. I don't worry much that it scratches more easily than acrylic as the scratches could be polished out or the cover replaced with a new one.

Acrylic is too brittle to cut with scissors and must be saw cut or milled.
 

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I have some 1mm and 1.75mm lexan that I use scissors on to cut.
The 1mm is easy to cut into shapes, but the 1.75mm, not so easy.
I can cut it, but only in straight lines. I use a burly pair of Fiskars scissors.
With cheap scissors, it would be a no go.
I have a pair of flush cutters, made for electronics with a short 1/2 inch cutting edge that work really good for cutting lexan.
It's very slow going and you have to make sure each cut is lined up properly, but they work very well.
 

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I got this tip from this forum: replacment lenses for velder masks are made from polycarbonate (aka lexan). Cheap and easily available. They are thin and can be cut with the tip of snapp-off knife.

Arne
 
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