DISP - Progressive Lenses - Week 8 Flashcards
What happens to the range of clear vision with increasing near add power?
It is reduced
List 7 advantages of progressive lenses.
Improved cosmesis Complete range of working distances Convenient No image jump Thinner and lighter than other options More physiological
List 4 disadvantages of progressive lenses.
Aberrations in the peripheral parts
Some adaptation problems with some patients
More critical fitting than bifocals
More expensive
How are progressive lenses produced? Explain what the umbilic line is.
It is done by combining small segments of pheres decreasing in radius
The spheres are tangential to one another along the umbilic line
All other points are blended to make a smooth surface
Briefly describe how unwanted astigmatism can occur when making progressive lenses.
What is a solution to this?
When using conical curves, the radii decreases in the vertical meridian, but in some conical surfaces the tangential power was not matched in the saggital plane
If the intermediate and near powers are blended into the distance powers, and the distance power surface being lowered in height towards its periphery, distortions will be less severe
Is it possible to have no discontinuities in progressive lenses? Explain.
Inserting a solid ellipsoid will result in a surface with no discontinuities
Compare spherical and aspherical lens design for progressive lenses.
Spherical lenses may increase unwanted astigmatism
Aspherical lenses have less aberrations, better vision, and better cosmesis
Does skew distortion appear in progressive lenses as they do in bifcoals (as prism jump)?
Yes, this is due to increasing power resulting in increasing magnification
The transition is not as great as with bifocals or trifocals
What two diagrams illustrates how power varies across a lens?
Iso-cylinder power plot (for cyl power)
Iso-mean power plot (for distance and add)
Describe how edge thickness is reduced in progressive lenses.
The thickness difference is removed, which thins the prism from the lenses
This equalises thickness at the top and bottom of the lenses
What happens to astigmatism in the blended zone with increasing near add power in progressives?
It increases (linearly)
Consider an individual using multi-focal and reading glasses for office use. Describe their head and neck posture when trying to use these glasses for clear near vision.
Multi-focal - wearer lifts head to use intermediate lens area, resulting in a strained neck and shoulders
Reading lenses - wearer leans in towards to object to bring greater focus, resulting in unnecessary strain on the neck and back
What would a computer use progressive lens have the greatest viewing area of?
Intermediate, with a smaller reading near area, and distance viewing area being the smallest
List 5 individuals who would benefit from extended foxus lenses.
Experienced wearers of reading spectacles
First time presbyopes
Second pair for tranditional PPL wearers who need a larger internediate and near field
For presbyopes with specific vocational/occupational requirements
Contact lens wearers as an overcorrection for near vision
How is pantoscopic tilt measured?
View the frame from the side
Align the patients line of sight with one of the parallel horizontal reference lines
The tilt is measured from the vertical face of the spectacle
