Lens material is often decided late in a project and regretted early in the field. Four families dominate outdoor luminaires: acrylic, polycarbonate, optical silicone, and glass. Each of them solves a different problem, and none of them wins on every axis. The right choice depends on where the fixture will live for the next ten years, on how it will be cleaned, and on what happens if it is struck, baked by the sun, or sprayed with salt water, not simply on the peak transmission figure printed in a datasheet.

Optical-grade PMMA, or acrylic, is the default material for injection-moulded TIR optics. Its transmission is high, typically around 92 percent, and it can be moulded to tight tolerances so the optical surfaces need no secondary machining. UV-stabilised grades hold their clarity for years outdoors, which is why most landscape and architectural optics are acrylic. The limits are mechanical and thermal. PMMA is relatively brittle, its impact resistance is modest, and it softens at temperatures that high-power modules can reach, so the thermal design has to keep the optic cool. Some solvents and aggressive cleaning agents will also craze the surface over time.

Polycarbonate trades a little optical performance for toughness. It survives impacts that would shatter acrylic, holds up better at elevated temperatures, and is the usual choice for vandal-prone car parks, industrial sites, and public realm fixtures. The cost is lower transmission, roughly 88 to 90 percent, plus a tendency to yellow under UV unless the grade is stabilised or hard-coated. Optical silicone sits at the high-performance end: it tolerates heat close to the LED, resists UV and thermal cycling, and stays flexible under vibration, but it is more expensive and harder to mould into complex TIR geometry at high volumes.

Glass, usually borosilicate, is the most chemically and optically stable option. It does not yellow, resists scratching and most cleaning chemicals, and keeps its transmission over decades, which makes it attractive for heritage projects, coastal installations, and harsh industrial environments. It is also heavy, fragile, and dependent on a robust frame. A practical selection rule is this: acrylic for well-behaved residential and landscape optics, polycarbonate where impact or vandalism is the dominant risk, silicone for compact high-power designs, and glass where chemical exposure or a very long service life outweighs weight and cost. In every case, ask for UV test hours, transmission data, and the impact rating, not just the material name.