What is the semi finished lens?
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.
Round top bifocal lens:
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This lens is also called a CFR (Croma Free Round - In reference to the glass fused lenses), and a kryptok lens. This lens has been offered in widths ranges including 22, 25 and 28mm, however some sizes and mediums in this lens have been phased out and are no longer available. This lens gives the user a narrower initial reading range up top, because of its round top.
Bifocal lens can be called a multi purpose lens. It has 2 different fields of vision in one visible lens. The larger of the lens usually has the prescription necessary for you to see for distance. However, this can also be your prescription for computer use or intermediate range, as you would normally be looking straight when you view through this particular part of the lens. The lower part, also called the window, typically has your reading prescription. Since you generally look down to read, this is the logical place to put this range of vision assistance.
A simple way of wearing bifocals, is to put them on and focus for distance through the upper segment, then, without moving the head (this is most important—no head movement) to lower the eyes to read a book held 18" away and down at an angle of 30 degrees. One thus uses the lower segment for reading. This exercise must be repeated over and over again so that the brain and the eye adapt to a new system of reading.