20,23 - SVL Flashcards
Overall process for making lenses is done in what steps?
- Raw material
- Cut/grind/cast surfaces
- Polish
- Apply coatings
- Cut to shape and edge/glazing
- Fit into frame
Define Lens Bending
Changing the front and back surface curvatures (and thereby power) whilst maintaining overall power. This helps minimize aberrations and maintains good cosmetic + practical properties such as thin lenses.
What is Oblique Astigmatism when it comes to lens aberration
- Describe how it can be reduced
Oblique Astigmatism is a higher order aberration that happens when eye doesn’t look down centre of spectacles. Light rays hit at a different angle to the surface causing different equivalent refractive power.
When plotting power at X axis, and base curve at Y axis, a Tscherning’s ellipse is formed where there’s zero OA.
Wollaston – Steeper base curve
Ostwald – Flatter base curve
Percival – Never fully eliminate OA but always COLC on retina.
Zeiss Punktal – No OA but needs 15,000 blanks in stock. Individual base curve for every power using curve solutions
Torics typically are +ve or -ve? Front or back?
- Why?
-ve (back surface)
- F1 is constant (spectacle magnification less different between meridians)
- Bevel can be placed at consistent distance from front surface. Cosmetically better.
- Most multifocals/bifocals have additions at front.
- Describe the differences between stock and specially worked lenses (6 total)
- Stocks prebuilt blanks that only need to glaze + edge
- Stocks usually only SV
- Stocks precoated or tinted
- Stocks low power, up to -2.00DC
- Stocks only prism via decentre
- Stocks fast + cheap
What are aspheric lenses?
- How to describe asphericity?
Rotationally symmetric but not spherical. Uses conic sections. Eccentricity describes uncirculariness and has a changing surface curvature.
nP values.
P > 1 = Olate ellipse
P = 1 = Circle
0 < P < 1 = Prolate ellipse
P = 0 = Parabola
P < 0 = Hyperbola
How to find aspheric sag? Equation?
x = (y^2)/(r0 + sqrt[r^2 - p * y^2])
x = sag
y = effective radius (half diameter)
r0 = radius of curvature at lens centre [r = (n-1)/F]
p = shape factor
Note: Sag is distance from front to back of whole lens
Advantages of aspherics?
Disadvantages?
Main use?
- Reduce lens thickness when p < 1 (and weight)
- Reduce OA (Good for larger power positive lenses which can’t have OA corrected by spherical surfaces)
- Power is varied away from optical centre meaning lens centration extra important
Anytime a high plus lens is needed e.g. Aphakia (missing internal crystalline lens)
- Corrects their missing +20DS without having much OA.
Not having OA happens is good and it works because aspheric surface lenses have inherent astigmatism that counteracts the OA at the periphery. This however, does induce spherical reductions at the periphery.
Describe the use of aspherics in hyperopia vs myopia
Mainly for hyperopia. Due to use of plus lenses which have thick centres. You use a flat base curve with an aspheric hyperboloidal curve at the front. This makes it lighter and thinner and have less OA.
Minus lenses have thin centres so making them thinner doesn’t help as much. Spherical base curves are flat anyway so aspheric doesn’t make much of an advantage.
Aspheric lens fitting considerations?
Any off-axis power errors due to tilt/decentration is significant and noticed as peripheral blur. We should be extra percise with fitting.
- Use monocular PDs
- Fit to pupil centres
- Allow 1mm drop in optical centre height for each 2 degrees of pantoscopic tilt.
Patient may need to adapt.
- Tell them to look through centre.
Flatter surfaces may increase surface reflection awareness
- Use AR coatings.
Describe the use of prisms in aspherics
Aspheric only uses grinding.
Compared to spheric lenses, aspheric lenses generally…
1) Lens thickness and shape?
2) Weight?
3) SPectacle magnification?
4) OA?
5) Off-axis optical quality?
6) Reflections?
7) Prism prescription methods?
1) Thinner + flatter
2) Lighter
3) Less
4) Neutralised at every power (spherics only can for –23.00 and +7.00DS)
5) Poor due to spherical power drop when off-centre
6) More noticeable
7) Grind only (no decentration possible)
Purpose of AR coating on higher minus lenses?
reduce power rings
Is FOV changes more of an issue w/ +ve or -ve lenses? How to fix?
+ve due to magnification causing ring scotoma. Tell Px to move head and keep eyes centred.
What distortion appears in high +ve lenses?
Pincushion
