Middle and peripheral thickness of the spectacle lens calculation during manufacturing
Introduction
Central thickness of the spectacle lens is not only influenced by proper base chose but also with rear curvature calculation. Currently we use software programs for these calculations. Every lens producer uses their own software. We write down in software sphere and cylindrical power, axis, addition, prismatic and lens diameter. Further we can choose appropriate refractive index of the material, lens diameter and basis of the semi-finish. The software enables to calculate appropriate lens decentration and peripheral lens thickness (Vesely et al. 2012).
Earlier was the calculation of the rear optical power (𝛗´2) made manually. According to lower placed formula where 𝛗´1 is front surface optical power (basis), 𝛗´c is total lens optical power, n is refractive index of the material and d smallest thickness of the lens (for plus lenses it is usually 0,5 mm, for minus lenses it is usually in center 1,7 mm).
Further we need to calculate front and back lens curvature. If we know optical surfaces of both sides (𝛗´1 a 𝛗´2) and refractive index we can use lower formulas:
Further we need to calculate height of the lens sagitta according lower formulas:
Now we can calculate final central (for plus) and peripheral (for minus) thickness according lower formula:
(e – peripheral thickness for minus lens, t is substituted with 1.7 mm)
(t – central lens thickness for plus lens, e is substituted with 0.5 mm)
Goals
- Calculate central thickness of the plus lens
- Confirm the calculation with schematic drawing
- Calculate edge thickness of the minus lens
- Confirm the calculation with schematic drawing
Equipment
Spectacle lens, sphere-meter, calculator.
Methods
Calculate central thickness of the plus lens
With help of sphere-meter measure given lens basis. After that calculate rear lens optical power. Peripheral edge thickness substituted with 0.5 mm. Further calculate radius and sagitta of the front and rear surface. Finally, we can calculate central lens thickness.
Confirm the calculation with schematic drawing
Draw the plus lens. With sphere-meter and thickness-meter measure back surface and lens thickness of given lens a check if the measurements refer to previous calculations.
Calculate edge thickness of the minus lens
With help of sphere-meter measure given lens basis. After that calculate rear lens optical power. Central thickness substituted with 1.7 mm. Further calculate radius and sagitta of the front and rear surface. Finally, we can calculate peripheral lens thickness.
Confirm the calculation with schematic drawing
Draw the plus lens. With sphere-meter and thickness-meter measure back surface and thickness of given lens a check if the measurements refer to previous calculations.
Results
Calculate central thickness of the plus lens
𝛗´2 =
r1 =
r2 =
s1 =
s2 =
t =
Confirm the calculation with schematic drawing
Calculate edge thickness of the minus lens
𝛗´2 =
r1 =
r2 =
s1 =
s2 =
t =
Confirm the calculation with schematic drawing
Discussion
During final lens parameters calculation is central and peripheral thickness very important. Peripheral lens thickness influences lens cutting for frames with nylon or for drilled frames. Central thickness influences consistence and stability of the minus lens. We can say that if we have higher central lens thickness we have more stable lens, but this lens will have higher peripheral lens thickness which is unaesthetic.
Conclusion, notes, comments
Does the lens diameter play the role for central lens thickness? In which case?