Lens Thickness and Base Curves

Written by Optician Club on . Posted in Conventional Progressive Lenses, Freeform Rx Lenses, Optician Tools

Mechanical Factors Associated with Lens Thickness and Base Curves:
1. Lens thickness
2. Plate height (or bulge)
3. Frame shape and glazing ease
Flatter base curves result in thinner, cosmetically superior lenses (thinner centers in plus powers, thinner edges in minus powers), which are also lighter in weight. However, traditional eyeglass frames are commonly shaped for a nominal base curve of 6.00 diopters, which means that if the base curve of the lens is too flat or too steep, it may not fit securely in the frame (modern edgers have minimized this problem to some extent). Further, if the lens is too flat, eyelash clearance may become an issue or even contact between the lens and the cheeks or brow.

Extremely steep and extremely flat curves may also be difficult to fabricate using certain surfacing techniques. For instance, traditional, biaxial (diamond wheel) generators do not cut back curves below 3.00 diopters. And steep curves become difficult to fine and polish. Also, when fabricating a back-surface progressive lens using free-form surfacing, if the front base curve is too flat, the near zone on the back of the lens will become convex, resulting in a potato-chip-like shape that some lens designers prefer to avoid.

The maximum thickness of a lens, for a given prescription, varies with the form of a lens. Flatter lens forms are slightly thinner than steeper lens forms, and vice versa. Since the lenses are thinner, they also have less mass—making them lighter in weight as well. In addition to lens thickness, varying the lens form will also produce significant differences in the plate height, or overall bulge, between lenses of the same power. Essentially, plate height is the height of a lens as measured from a flat plane.
Lens Thickness and Base Curves
Plus lenses with flatter plate heights do not fall out of frames as easily, which is especially important with large or exotic frame shapes. In addition, flatter plate heights are also more cosmetically pleasing than steeper, bulbous ones—particularly in plus powers.
A reduction in plate height will also provide a significant reduction in the magnification associated with plus lenses. Since a flatter plate height brings the back surface closer to the eye, smaller Vertex Distance, the minification associated with minus lenses is also reduced slightly. This gives the wearer's eyes a more natural appearance through the lenses.
We can evaluate the maximum thickness, plate height, and weight for a range of lens forms to demonstrate the effects of lens form upon cosmesis for a given prescription. The table, below, represents a range of +4.00 D lenses in hard resin plastic, edged to a 70-mm diameter and a 1-mm minimum edge thickness.
 
+4.00 D Lenses
Base Curve
Center
Plate
Weight
10.00 D Base
6.9 mm
15.3 mm
21.7 g
8.00 D Base
6.3 mm
11.7 mm
19.5 g
6.00 D Base
6.0 mm
8.7 mm
18.3 g
4.00 D Base
5.9 mm
6.0 mm
17.7 g
Note how the lenses become gradually thinner, flatter, and lighter in weight as the base curve is reduced—or flattened. The table, below, represents a range of -4.00 D lenses in hard resin, edged to a 70-mm diameter and a 2-mm minimum center thickness.
 
-4.00 D Lenses
Base Curve
Edge
Plate
Weight
6.00 D Base
8.7 mm
16.4 mm
25.4 g
4.00 D Base
7.8 mm
12.8 mm
24.0 g
2.00 D Base
7.3 mm
9.7 mm
23.2 g
0.00 D Base
7.0 mm
7.0 mm
22.8 g
 
Again, the lenses become gradually thinner, flatter, and lighter in weight as the base curve is reduced. In summary, flatter lens forms provide the following mechanical and cosmetic benefits, check Lens Power range limited by base curve:

Lens Thickness and Base Curves

  • Flatter (less "bulge")
  • Thinner center thickness (plus) or edge thickness (minus)
  • Lighter in weight
  • Less magnification (or minification)
  • Better frame retention (in plus powers)

Consequently, even for free-form lenses that are optically optimized (and not necessarily all free-form lenses are), the choice of base curve is still important. You'll find that many free-form lens suppliers will generally use "pucks" (semi-finished lens blanks with spherical base curves) with base curves that are not entirely unlike the base curves of traditional lenses in order to produce finished lenses with a lens form or bend similar to traditional lenses. These lenses will often be somewhat flatter in order to improve cosmetics, like traditional aspheric lenses, but generally steep enough to avoid the optical and mechanical issues described above.

 

 

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