Semi finished lens:
What is the semi finished lens?
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The starting point for freeform production is a semi-finished lens, also known as a puck due to its resemblance to an ice hockey puck. These are produced in a casting process that is also used to manufacture stock lenses. The semi-finished lenses are produced in a casting process. Here, liquid monomers are first poured into moulds. Various substances are added to the monomers, e.g. initiators and UV absorbers. The initiator triggers a chemical reaction that leads to hardening or "curing" of the lens, while the UV absorber increases the UV absorption of the lenses and prevents yellowing.
FT-28 optical lenses:
FT-28 offers a wide field of forward vision and allows the user to see objects at close range without changing spectacles. This lens gives the wearer a wide range of long-distance vision and, at the same time, allows viewing of objects at close range such as meters, navigation screens and expressway tickets. This type can also be used for sports such as golf.
1) The most popular fused bifocal today has a D-shaped near segment rotated 90 degrees so that the flat part of the "D" is facing up. For this reason, D-seg bifocals also are called "flat-top" (FT) or "straight-top" (ST) bifocals.
2) This is a D segment bifocal lens. It has the benefit that the optical centre of the near vision part of the lens i.e. the part of the reading area which gives the very best vision, is situated at the top of the reading portion. Also the widest part of the reading area is just below the dividing line, and is the part of the lens the wearer would use.