Are the designations ‘CR' and ‘Hard Resin' essentially the same thing ?
Generally speaking yes. Both CR-39™ (A registered trademark mark of PPG, which stands for Columbia Research's 39th version of the formula) and Hard Resin (the generic term for standard plastic lenses) are made of a clear, water like liquid (diethleneglycol - allycarbonate) that are part of the thermosetting family of resins. In order to turn into lenses, the resin must be poured into a glass mould and then cured or polymerised in an oven and the resin hardens. Many companies have their own formulations and processes to make Hard Resin lenses.
How is polycarbonate different from CR-39™ ?
Polycarbonate is a tough, dimensionally stable, thermoplastic, while CR-39 ™ / Hard Resin is a thermoset polymer . Polycarbonate is processed by injection moulding. Polycarbonate pellets are heated, injected into stainless steel moulds under pressure and compressed to form a lens. All polycarbonate lenses must be scratch coated, as the uncoated material is susceptible to abrasion. Polycarbonate is lighter in weight and more impact resistant than Hard Resin. Hard Resin tends to be easier to edge and can be tinted (dyed) to a wider variety of lens colours.
What is the difference between "Bent Sheet" and "Injected" Polarized Polycarbonate?
Bent Sheet (thermoformed) polarized polycarbonate lenses are made by placing polarized film between two thin, flat sheets of polycarbonate material. A lens is formed by cutting out a particular size from the sheet and then heating the lens on a bending tool to achieve the required base curve (thermoforming). Injected polarized polycarbonate lenses are made essentially the same as standard polycarbonate lenses (injection moulding) with the introduction of the polarizing film in the lens mould, instead of lamination. The result is the injected lens provides superior Class 1 optical performance with the film as part of the finished lens.
What is TR90 ?
TR-90 is a superior compound which has no risk of toxicity or allergy and is FDA and BGA approved. It is extremely light weight and designed to flex making it resistant to stress cracking and material fatigue. It is free of plasticizers and extremely stable to UVA exposure in all types of temperature and humidity conditions. Strong resistance to chemical products such as solvents, alcohol, cosmetics and organic acids gives TR-90 maximum durability in outdoor environments.
What is an optically-decentered lens ?
Optically-correct decentred lenses provide superior optical clarity. They achieve this by undergoing a process where the thickness of a deeper base curve lens is horizontally tapered from the bridge area and becomes thinner at the outer edge of the lens near the hinge. This process eliminates prismatic distortion by shifting the optical sweet spot of each lens from the mid-point of the lens to directly in front of the eye.
