dispensing 1 Flashcards
what are the issues of having a high power Rx?
-the lenses are thicker so more conspicuous
-heavier since more volume of lens
-spectacle magnification/ minification making faces appear larger/ smaller
what are the 4 types of plus lenses?
-equi-biconvex
-biconvex
-planoconvex
-meniscus forms
what are the 4 types of minus lenses?
-equi-biconcave
-biconcave
-planoconcave
-meniscus forms
How do you calculate CVF?
(n’-1)/(n-1)
what measurements do you need for HI Single vision (SV) lenses?
Mono. PD
Heights of pupil centre (Dispenser’s Rule applied – DV only)
BVD
what measurements do you need for High index (HI) bifocals (BIF)
Binocular PD (Mono. PD if Rx>8D or face asymmetrical)
Inset
Heights at lower limbus
BVD
what measurements would you take for high index varifocal lenses (PPL)?
Mono. PD
Heights of pupil centre
BVD
how do you find densitiy
density = mass per unit volume
how do you work out specific gravity?
density of a material relative to the density of
water (which is 1g/cm3)
why should BVD be kept to a minimum?
-maximise FOV
-reduce distortion
-reduce chromatic aberration
why is it important to correct centration?
- correct Rx most effective in front of pupil
- reduces unwanted differential prism
- reduces possible formation of ghost
images
what are the effects of inwards decentration of high power lenses
-increased temporal edge thickness in negative lenses
-increased nasal edge thickness in positive lenses
why can dispensing in high myopia be a problem?
because of edge thickness
what can help dispensing lenses with high myopia?
-lens material and form
-high index lenses
-high v value (abbe number)
-AR (anti reflection) coating
-aspheric or lenticular lenses
how can you improve frames in high myopic glasses?
-little to no decentration frame
-small eye size
-plastic rimmed and thicker rimmed metal frames support and help hide edge thickness
what are polynomial lenticulars vs standard lenticulars
polynomials have a gradual change between margin and centre whereas standard have a sudden change between margin and centre
what is the aperture of a lens?
the area in the centre of the lens that have the Rx
what are lenticular lenses?
reduced aperture lense
what are the benefits of lenticulars
-good vas
-good FOV
-controlled aberrations
-improved edge thickness and mass
-allows wider frame selection
-help remove minifcation of the face
what are the main concerns of dispensing for a high hpermetrope?
-nasal edge thickness
-centre thickness
-weight
what kind of prescription are lenticulars dispensed for?
+-12D
what helps high hypermetropia lenses?
-lens material and form
-HI lenses
-high V value
-AR coating
-aspheric or lenticulars
-small blank size
what helps dispensing frames for high hypermetropes?
little/no decentration
-small eye size,
good fit to avoid
slipping as ‘front heavy’, consider regular, fixed pad bridge and strap
bridges
-choose lightweight frame material
what affects can high hypermetropia lenses have and how can these be improved?
jack in the box effect, can be reduced with lenticulars
what kind of patients would be good candidates for bifocal lenticulars
high powered presbyopes
what coatings are used in sports eyewear?
-MAR (anti reflective coating)
-hydrophobic coatings
-smudge resistant coatings
what filters are used in sports eyewear?
-image enhancement
-UV and IR protection
what categories does sports eyewear need to cover?
-visual correctiom
-maximum field of view
-safety so has impact resistant
-protection against foreign bodies like dust/dirt
-practical
what are the 7 benefits if wearing contact lenses instead of spectacles in sport?
Wider field of view
Reduced magnification/minification
Enhanced depth perception
Fewer aberrations and reflections
Lenses not exposed to environmental conditions e.g. rain,
dirt etc.
Protect against UV
Lesser risk of damage or loss
what features should glasses being worn by people who play racquet sports have?
-lenses should be made of polycarbonate / trivex
-close fitting nylon goggles
-silicone bumpers/ side shields as they can take the impact on bridge and temples if hit providing impact resistance and prevent fractures of the orbit
-should ideally be worn with a headband to keep in place
for skiing and snowboarding, what material should spectacle lenses be made of?
polycarbonate as it provides robustness and has anti-fogging properties