high power dispensing

• issues to the px of having high power Rx

  Minus lenses thicker on edge and plus lenses are thicker in centre 

  • thicker lenses so more conspicuous 
  • Heavier since more volume of lens
  • Spectacle magnification/ magnification making face appear larger/ smaller 

  
• High index lenses (HI)

  RI, TCA

  RI def. = the ratio of the velocity of light in a vacuum to its velocity in a specified medium 

  RI varies with λ

  Generally, TCA increases with RI (measured using a V value)

  BS 7394 part 2 classifies index of lenses - eg plastic HI = n between 1.64- 1.74
• 


  CVF

  Curve variation factor (CVF) used to calculate how much thinner a HI lens will be cf. standard thickness of same power 

  HI used to reduce the thickness (not necessarily weight) of lens cf. standard index lens

  Plastic lenses - % reduction in edge thickness compared to CR39 

  CVF  (1.498-1) / (n-1)

  For 1.586; 15% dec. CVF= 0.85

  Glass lenses - the percentage reduction in edge thickness compared to crown glass 

  CVF = (1.523-1)/(n-1)

  For 1.700 , 25% dec. CVF= 0.75
• Weight depends on density and specific gravity 

  Density = mass/ volume 

  Specific gravity = density of a material relative to the density of water (1g/cm3)

  Reflectance (% reflection at each surface)
• key values of materials


  ...

  
• Single vision 


  • mono PD, to reduce chance of inducing prism
  • Heights of pupil centre (dispenser’s rule applied)- DV only
  • Dispensers rule - pantiscopic tilt- every 2 degrees, drop height by 1mm; heights not for near vision as px looks through bottom of lens anyway 
  • BVD
• bifocal vision


  • Binocular PD (mono. PD if Rx>8D or face asymmetrical)
  • Inset - difference between near pd and distance pd/ 2 (usually 2mm per eye)
  • Heights at lower limbus 
  • BVD
• PPL, varifocal vision 


  • Mono PD 
  • Heights of pupil centre
  • BVD - anything above 5 take BVD 
• BVD


  distance from corneal apex to visual point on lens

  BS 2738 part 3 - BVD should accompany a Rx if power of lens in any meridian is ± 5.00 D and above 

  • effective power of lens changes as it is moved away from or closer to the eye ie - lenses become more positive if they are moved away from the eye 
• to compensate for a change in BVD

  F new = F old / 1 ± (d x Fold)  

  Use + for increase in BVD or - for decrease in BVD

  d (change in BVD) expressed in metres and consider both meridia
• Why should we keep BVD to a minimum

  • maximise FOV
  • Reduce distortion 
  • Reduce chromatic aberration 
  • Plus lenses reduce the retinal image with a drop in VA; they reduce the spec. Mag of px eyes
  • Minus lenses increase the retinal image with an increase in VA;  reduce the spec min of px eyes
• what happens if the BVD increases in a minus lens

  the minus lens becomes weaker and so a stronger lens needs to be ordered to compensate for the change due to effectivity (opposite for +ve lens)
• importance of centration 


  correct centration is important as

  • correct Rx most effective in front of pupil 
  • Reduces unwanted differential prism
  • Reduces posible formation of ghost images
• equation for decentration and its effects


  BCD / 2 - each mono PD

  required when the BCD is not equal to the PD

  • apart from when decentration is used to produce prescribed prism
  • effect of inward decentration on high power lenses are due to increase in temporal edge thickness in -ve lenses and increase in nasal edge thickness in +ve lenses
• dispensing with high myopia

  • main concern is edge thickness 
  • Lenses helped by lens material and form , HI lenses, high v- value , AR coating, aspheric or lenticular
  • Frames helped by little to no decantation, small eye size, plastic rimmed and thicker rimmed metal frames support and help edge thickness
  • Be wary of excess nasal edge thickness obstructing nose pads 

  
• Dispensing lenticular lenses for high myopia 

  • lenticulars are reduced aperture lenses (aperture in centre has Rx and margin at edges has little or no Rx)

  Benefits to patients are

  • good VA
  • Good FOV
  • Controlled aberrations 
  • Improved edge thickness and mass
  • Allows wider frame selection
  • Cosmesis is okay and helps remove minification of face 

  Generally are dispensed above -12D

  • dispensed as standard lenticular or polynomials (graduated)

  
• dispensing high hypermetropia


  • main concerns are nasal edge thickness, centre thickness and weight 
  • Lenses are helped by lens material and form, HI lenses, high v- value, AR coating, aspheric or lenticular, small blank size 
  • Frames are helped by little to no decantation, small eye size, good fit to avoid slipping as front heavy, consider regular fixed pad bridge and strap bridges, choose lightweight frame material 
• dispensing lenticulars for high hypermetropia

  Benefits to Px

  • good VA and FOV
  • Controlled peripheral abberations 
  • Improved edge thickness and mass
  • Wider frame selection
  • Good cosmesis and eps to remove minification of face 
  • Reduces jack in box effect 

  Generally are dispensed above +12D (frequently dispensed prior to IOL introduction)

  Dispensed as standard lenticulars or polynomial (graduated)

  Dispensing bifocal lenticulars 

  • good option for high powered presbyopes 
  • Exact measurements needed - mono PD, heights of seg. Top, inset and BVD

  
• why is frame selection vital

  • must consider matching size of aperture with eye size
  • plastic frames (or strap bridge) to balance weight of spex across bridge of nose
  • limiting eye size and wide sides