High Prescription Dispensing L7 Flashcards
What is a high prescription?
- BS states that BVD has to be part of the rx when rx>=/-5.00D - includes cyl also
- high index lenses often suggested for rx >+/-4.00D
What do we look for in good lenses?
- should provide good vision (FOV, aberrations, rx)
- should be cosmeically acceptable to px eg thickness on temporal edge
- should be wearable - if theres too much thickness on nasal edge, lab has to push nose pads in more, and so any initial adjustments made during dispense will not stand on collection
How does the form of the lens affect vision?
- curves should aim to remove a particular aberration - aspherics
- should be kept as close as possible to existing lens form
- minus base toric for minus lenses, plus base toric for plus lenses
- takes into account thick lens equations and BVD
How does the form of the lens affect cosmesis?
- steeper curves = thicker lenses
- minimum center thickness
- aspherics
- appearance through the lenses
How does form of the lens affect wearability?
larger base curves are more difficult to glaze
How can the material of the lens affect the following?
1) Vision
- v value
- can influence lens curves and therefore reflection
- ghost images
2) Cosmesis
- reflections
- lens thickness
3) Wearability
- specific gravity
- thickness
4) SAFETY
How do surface treatments affect:
1) Vision
- MAR
2) Cosmesis
- MAR
- tints/ mirror coating
- hydrophobic coat
- UV coat
- scratch resistant coats
- transition lenses if theyre photopobic
High Minus lenses
-can be diff to glaze
- reduced aperture lenses available :
oval aperture = for rectangular frame cosmesis
round aperture
profile aperture = produced by polishing, fits whatever shape you choose, best cosmetically
-available as lenticulars or blended lenticulars -manufactured by solid designs, bonded or hand edged
What are flattened lenticulars?
- flattened full aperture lenses
- can be flattened or plano flatened
- consider flat f1
- part in centre = aperture - where vision is
- part around edge = margin, where you have reduced/limited vision, not too much of a problem for minus rx patients as tehy have increased FOV
- when margin is completely flat, you have more reflections
What are blended lenticulars?
=aperture curve is blended with marginal curve with a convex curve
- cosmesis is good
- not good visually - blended area leads to lots of astigmatism as they cross the margin - minus pxs have wide FOV, so theyre fine
What are high positive lenses?
- poor FOV leads to a ring scotoma and a jack in the box effect
- reduced aperture lenses available
- available as button, blended or polynomial
- available as bifocals and progressives
- should be uv400 coated - most people using these are aphakic
zonal aspheric blended margins - better cosmetically but poor vision through area of blending
polynomial blending - better cosmetically and better vision - no aberrational astigmatism and eliminates roving ring scotoma and jack in the box effect
How do we alter rx for BVD?
- can use step along or F2/1000
- tells you how much each mm of power alters the effective power of the lens
Near Vision Effectivity error
- need to calculate at near as light not from infinity
- need to take into account the diff performance from the thick trial lens form to a thick spectacle lens
- results in lenses being under plussed
Field of view
- can be calculated by
tanx=y(37-F)/1000
-where x = half FOV
-affected by rx - myopes have increased FOV and hypermetropes smaller - affected by radius of curvature - steeper curves reduce FOV
-vertex distance - decreased increases FOV
Benefits of lenticular lenses
Fitting - close to eye to increase FOV - consider BVD/panto, trial frame set to frame characteristics
Glazing - easier to glaze, more frame choices to choose from
Cosmesis - better
Wearability - lighter
