Week 1.05 Lens Form, Sags And Lens Thickness Flashcards
If they give us chord diameter in the question when working out sag what do we need to do?
We need to divide by 2 because chord diameter is 2y and we need just y
Calculate the sag of a surface with radius 80mm and chord diameter of 70mm
Chord diameter is 2y so y = 35mm
S = 8.06mm
What is the approximate sag formula and when do we use it
S = y^2/ 2r
Could be used when the lens aperture is small and the surface power is low
Concave lenses
Plano concave - one concave surface and one Plano
Bi-concave - two concave of differing radius
Equi-concave - two concave surface of equal radius
What is a meniscus form lense
Lens that is not flat form is curved or meniscus form
One surface is convex and one surface is concave
What is the formula to calculate edge thickness of a Plano concave flat lens
tc + S2 = te
S2 is the sag of the back surface (front is Plano)
What is the formula to work out edge thickness of negative/positive (meniscus) lens
Tc + s2 = s1 + te
S1 is sag of front surface
S2 is sag of back surface
Consider a negative spherical lens (in air) with a Plano front surface, BVP -8.00DS, diameter 50mm, refractive index 1.498, centre thickness 1.5mm. Plano front surface means F2 =BVP= -8.00DS. Calculate the edge thickness of this lens:
First calculate radius using surface power formula F2 =(n-n’)/r rearrange to get r = (n-n’)/F2
= 62.25mm
Use exact sag formula s2=r-√(r2 - y2) = 5.24mm (remember diameter is 2y and y is semi diameter)
Then use te = tc +s2
1.50 + 5.24 =6.74mm
Consider a negative spherical lens in air with front surface power +2.00DS. BVP -8.00DS, diameter 50mm, refractive index 1.498, centre thickness 1.5mm. Calculate edge thickness of this lens:
First work out radius of from surface using F1 = (n’-n)/r = 249mm
Then because you know the radius you can now work out sag 1 using s1=r-√r2 - y2) = 1.26mm
Then move onto back surface:
Work out F2 before you can figure out the radius because u don’t know what F2 is yet. BVP is not the same as F2 as its not a Plano surface
So use BVP formula and rearrange to get F2 = -10.004DS
Then use F2=(n - n’)/r and rearrange to work out radius of back surface = 49.8mm
Once you know the radius plug into sag formula to get s2 = 6.73mm
Then use the formula tc + s2 = s1 +te and rearrange to get te = 6.97mm
What happens to the sag and centre thickness of we increase the curvature of the lens
Larger sag of both surfaces and increased edge thickness
What is horizontal lens size
For each eye temporal y + nasal y = horizontal lens size
Outer lens edge to middle of pupil is temp y and inner lens edge to middle of pupil is nasal y
Is the distance between the centres the same as the patients PD
No, the distance between centres (distance between geometric centres of the two lenses) is usually larger than patients PD
How to calculate distance between centres
Distance between centres = box size + distance between lenses
So if DBL is 20mm
Box lens size is 50 x 30mm
Distance between centres = 50 + 20 = 70mm
How do you work out decentration of pupil with respect to lens geometric centre
Work out distance between centres first (box size + DBL) lets say that’s 70mm
Then do 70mm - px PD (64mm) = 6mm / 2 =3 mm
(DBC-PD/2) = decentration
Then work that out for each temporal and nasal values
For temporal box lens size is 50 so do 50/2 + 3=28mm
For nasal box lens size is still 50 so do 50/2 - 3=22mm
Remember temporal ADD the decentration and nasal MINUS the decentration as the temporal y is usually greater than nasal y
How to calculate temporal and nasal edge thicknesses:
Consider a negative spherical lens in air with front surface power +2.00DS, BVP -8.00DS, temporal y =28mm, nasal y = 22mm, refractive index 1.498, centre thickness 1.5mm. Temporal edge thickness will be?
First calculate radius of front surface. Rearrange F1=(n’-n)/r = 249mm
Then calculate S1 and use the temporal y as the y
= 1.58mm
Then calculate F2 using the BVP formula = -10.004DS
Then calculate radius 2 using F2=(n-n’)/r = 49.8mm
Then do S2 = 8.62mm (still use temporal y)
Calculate edge thickness of temporal side using tc +s2 = s1 +te = 8.54mm
Then repeat all these steps this time using the nasal diameter
Nasal thickness = 5.65mm
What is temporal and nasal decentration diameters if the patients PD is 64mm, DBL is 20mm and lens size is 58x34mm
Distance between centres = 58 + 20= 78mm
(78-64)/2=7mm
Temporal y = 58/2+7=36mm
Nasal y = 58/2-7=22mm
Keeping lenses thin for myopes
- consider high refractive index material
- Consider aspheric lens design
- choose frame with care - large box lens size bad for edge thickness
- try match DBC with px PD
- choose frame where BVD can be as small as possible
- choose frame where rims and side can hide some of edge thickness
- edge the lenses so that the bevel is at the front of the lens - prevents thick lens edge coming out the front
Keeping lenses thin for hyperopia
- consider high refractive index material
- consider aspheric lens design
- choose the finished lens blank with care
