Plastic that’s just clear isn’t enough for optical parts. You need transmission numbers. Haze specs. UV stability curves. Scratch resistance that survives assembly. And the material that wins on one axis often fails on another.
I’ve spent years matching transparent materials to optical applications — light pipes, sensor windows, display covers, lens elements. Here’s the thing: there’s no universal winner between PMMA, PC, GPPS, and PET. Each one dominates a specific set of requirements. Pick wrong and your part either yellows, crazes, scratches, or costs three times what it should.
Let me walk through the trade-offs with the numbers that matter.
PMMA (Acrylic) — The Clarity King
PMMA delivers 92% light transmission with haze under 1%. Nothing in the commodity or engineering space beats it for optical clarity. That’s why it’s the default for lenses, light guides, and display windows where seeing through the material is the entire point.
UV stability is outstanding — less than 1 ΔE after 1,000 hours of accelerated UV exposure. I’ve seen PMMA parts sit in direct sunlight for five years and still measure within spec. Most clear plastics can’t do that.
The catch is mechanical. Notched Izod impact is 20 J/m. Drop a PMMA lens on a concrete floor and you’re ordering a replacement. Pencil hardness hits 2H-3H, which means it resists scratching better than any other clear plastic on this list — but if the substrate flexes, the coating or the part itself cracks.
Processing is straightforward: melt at 210-250°C, mold at 40-80°C. Drying is mandatory — 80°C for 4 hours minimum. Skip it and you get silver streaks and bubbles in the optical zone. Cost index sits at 1.5x versus GPPS.
Use PMMA for: camera lenses, light guide plates, display windows, decorative trim that needs long-term UV stability.
Polycarbonate — The Tough One
PC gives you 88-90% light transmission. That’s 2-4% less than PMMA, and in a light pipe application that translates to measurable lumen loss. But nobody picks PC for its transmission. They pick it because notched Izod hits 700 J/m — thirty-five times tougher than PMMA.
I worked on a headlamp lens project where the OEM spec required impact survival at -30°C. PMMA shattered. PC passed on the first try. That’s the value proposition in one data point.
The downsides are real. UV yellowing measures 5-8 ΔE after 1,000 hours — the worst of the four. Unprotected PC outdoors turns yellow-brown within 18-24 months. You need UV-stabilized grades or hard coatings for any outdoor optical application. Pencil hardness is HB-F, so it scratches easily. Every scratch on a PC lens scatters light and reduces effective transmission.
Processing requires melt temps of 280-320°C with mold temps at 80-120°C. Drying is non-negotiable: 120°C for 4-6 hours, target moisture under 200 ppm. Wet PC hydrolyzes and loses 50% of its molecular weight. Cost index is 1.3x — cheaper than PMMA.
Use PC for: impact-resistant lenses, safety eyewear, headlamp covers, baby bottle components, riot shield windows.
GPPS — The Budget Option
General purpose polystyrene hits 90% transmission with haze under 1%. On paper it looks competitive with PMMA for half the properties. The headline number is cost index 1.0x — the cheapest clear plastic you can injection mold.
But here’s what the datasheet doesn’t emphasize: notched Izod is 20 J/m. Same as PMMA. GPPS is brittle, and when it fails it fails with sharp edges. Pencil hardness is 2B-B — soft enough that handling leaves micro-scratches. UV yellowing hits 3-5 ΔE after 1,000 hours.
The real limitation is chemical resistance. GPPS crazes on contact with alcohols, oils, and many solvents. A single cleaning wipe with isopropyl alcohol can ruin an optical surface. I’ve seen medical device prototypes fail because someone used an alcohol swab on a PS window.
Processing is easy: 180-260°C melt, 20-60°C mold. No drying required. Cycle times are fast. For disposable optics or short-life consumer goods where cost dominates every decision, GPPS is hard to beat.
Use GPPS for: disposable labware, toy optics, cost-sensitive display windows, packaging with optical windows.
PET — The Chemical Fighter
PET (amorphous grade) delivers 90% transmission with haze of 1-3%. The PET used in optical parts is PET-A (amorphous), not the semi-crystalline PET-G used for mechanical parts. It splits the difference between PMMA and PC on most properties and wins on chemical resistance.
UV yellowing is under 2 ΔE at 1,000 hours — significantly better than PC and GPPS, close to PMMA. Pencil hardness is H-2H, between PMMA and PC. Notched Izod hits 50 J/m — 2.5x tougher than PMMA but nowhere near PC.
The hidden tax is moisture sensitivity. PET absorbs moisture rapidly — 0.2-0.4% in 24 hours at 50% RH — and even small amounts cause molecular weight degradation during processing. Drying requirements are the strictest of the four: 150-170°C for 5-7 hours, target under 50 ppm moisture. I’ve seen molders run PET without adequate drying and scrap 30% of the first shift.
Process at 270-300°C with mold temps of 80-120°C. The narrow processing window and aggressive drying make PET harder to run than PMMA or GPPS. Cost index is 1.4x.
Use PET for: chemical-exposed lenses, food-contact optical parts, beverage dispenser components, medical device windows that see sterilants.
Side-by-Side Comparison
| Property | PMMA | PC | GPPS | PET (A) |
|---|---|---|---|---|
| Light Transmission | 92% | 88-90% | 90% | 90% |
| Haze | <1% | 1-2% | <1% | 1-3% |
| UV Yellowing @ 1000h | <1 ΔE | 5-8 ΔE | 3-5 ΔE | <2 ΔE |
| Scratch (Pencil Hardness) | 2H-3H | HB-F | 2B-B | H-2H |
| Notched Izod | 20 J/m | 700 J/m | 20 J/m | 50 J/m |
| Cost Index | 1.5x | 1.3x | 1.0x | 1.4x |
How to Choose
Optimize for optical clarity first. If your application needs the highest transmission and lowest haze with long-term UV stability, PMMA is the answer. That covers most lens, light guide, and display window applications where impact isn’t the primary concern.
Optimize for impact when the part might get dropped or hit. PC is the only choice here — nothing else comes close on notched Izod. But you need to design around the UV yellowing with either a stabilized grade, a hard coating, or a system that shields the plastic from direct sunlight. Budget for the coating.
Optimize for cost when the part is disposable or has a short service life. GPPS gives you 90% transmission at the lowest material cost with fast cycles and no drying. Accept the brittleness and chemical sensitivity as design constraints, not surprises.
Optimize for chemical resistance when your part sees solvents, cleaners, or sterilants. PET-A wins on chemical resistance while maintaining good clarity and moderate toughness. Accept the drying cost and process complexity.
I recommend getting drying capacity right before running PET or PC. Inadequate drying is the #1 cause of optical defects in both materials, and it’s entirely preventable with the right equipment.
A Note on Coatings
Every material on this list can be improved with hard coatings. A silicone hard coat on PC takes pencil hardness from HB-F to 2H — competitive with uncoated PMMA. Anti-reflective coatings boost transmission by 2-4%. UV-blocking coatings reduce yellowing rates by 60-80%.
The coating adds $0.10-0.50 per part depending on geometry and volume. For optical parts that need to survive real-world handling, it’s usually worth it.
Need help picking the right clear plastic for your optical part? Contact our engineering team with your transmission target, impact requirements, and operating environment. We’ll help you narrow it down.