How bifocal lenses are produced - learn more about the differences in producing glass or plastic bifocal lenses.
To produce glass bifocal lenses, a lens pressing and a so-called button are required.
The circular button consists of two segments with different refractive indices. The top part of the button – known as its crown – has the same refractive index as the base lens, i. e. as the distance portion, while the lower part – the subsequent near portion – has a higher refractive index.
At the surfaces of the dividing edge which is later visible, the two segments are smoothed and then permanently fused in a furnace. The back surface of the button is ground and polished. The function of the button crown is to protect the dividing edge of the near portion when the button is being milled and to complete the distance portion above the near portion.
In this process a round hollow is ground into the front surface of the distance portion and then polished. This displays the radius required for the addition.
The button is placed into the hollow of the base lens and both are then passed through a melting furnace for a period lasting approx. 4 1/2 hours. Depending on the type of lens involved, the temperature totals 670 °C to 760 °C. The button assumes a "pasty" consistency and fuses with the base lens.
A base lens with plane surfaces is used for this technique. In this case, the back surface of the button determines the addition. The base lens is heated to beyond its softening point and the cold button is pressed into the red-hot base lens using a punching device. The pressure required for this process corresponds to a weight of approx. 6 tons.
After cooling, the button and the base lens remain permanently fused. The button crown is now totally invisible, as it fuses with the base lens to form a homogeneous glass block. The projecting part of the button is removed by milling. The resultant bifocal blank is then ground and polished on its front and back surfaces, and is then coated.
To produce plastic bifocal lenses, a hollow with the shape of the segment is made in the concave back surface of the casting die. This means that the casting displays the curved segment and palpable segment edge typical of plastic bifocal lenses. The dioptric power of the segment is achieved solely by increasing the surface curvature, and not by the use of a material with a higher refractive index as in glass bifocal lenses.