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
what are the qualities of polycarbonate lenses?
-anti fog
-scratch resistant
-impact resistant
-very flexible
-can be thermoformed to any shape
for skiing and snowboarding what coating should you put on the spectacles?
100% UV protection - mirror coatings good for reflecting back UV and reducing glare (do not use polarised lenses as not advised for safety because they remove the contrast making it harder to see details in snow surface)
what should skiing and snowboarding glasses be made of?
nylon, propionate or rubber so they dont go brittle in the cold
how do skiing and snowboarding add protection?
they have large wrap around goggles / masks to shield the eyes
for skiing and snowboarding glasses, what filter should be used?
BS EN 12312 states the use of Filter Category 4 i.e. at least 15% LTF
for skiing and snowboarding glasses, would you use tints?
yes as they improve contrast
when are tints available in diving masks?
When using 4mm CR39 to increase contrast
what are the features of a diving mask?
-Full mask with polycarbonate frame and
silicone skirt with well-fitting seal and nosepiece
-With tempered infinity glass flats when no Rx
-Available with Rx glazed insert giving wide
FOV
-Available in custom Rx glass-bonded
lenses
-Usually have option to have gauge reader as BIF
-Sometimes limited to standard OCs (would need to compensate for BVD
change)
when dispensing for children, what should a well fitting frame have?
the frame PD should be the same as the patient’s PDs
when dispensing glasses for a highly myopic child, what should you look for in the glasses you choose?
-plastic frame to hold the lens and hide some edge thickness frames
- wide sides help hide edge thickness and give durability
-rectangular in shape
when dispensing glasses for highly hyperopic children, what should you look for in the glasses you choose?
-a lens as small as possible to try and minimise the blank size (MSU)
-round oval shape
what can sport bands be used for in paediatric dispensing
to keep the frames in place
what do sprung hinges do in glasses?
make the frame more durable
what 3 lens properties are important in paediatric dispensing?
-impact resistance and durability
-that they are relatively thin and lightweight so better comfort and both child and parent can be happier with cosmetic outcome
-they protect against UV as UV radiation dangerous to developing eyes and skin
how much UV protection should children’s glasses have in nm
at least 380nm up to max of 400nm
how can you make communicating with children more effective?
-Use familiar words - use short sentences
-Think about the order of words in sentences
-Ensure you are at eye level when you speak to your px
-It is best to speak directly to the child whilst including the parent
-Try and keep the conversation light, laugh with the px and have fun
-It is important they feel welcome in
practice so that they are happy to return
-Try giving the child a sticker as a reward when they leave
what are prismatic readers?
ready readers with base IN prism
what do prismatic readers do?
Relieves eye strain when focussing on
close objects
Aids convergence
Good for occupations requiring
prolonged NV at short working distance
e.g. tailor, jeweller etc.
who is a ptosis prop for?
pxs with a droopy upper eyelid which can be due to trauma to the levator muscle or damage to the oculomotor nerve. this may be caused by diabetes or brain tumour
what is a ptosis prop? what is it for?
Metal silicone-covered support fitted to upper back plane of spex, holds up droopy upper lid as a short term measure for those awaiting treatment e.g. botox
what is the negative of a ptosis prop?
Pxs unable to blink freely so lubricant drops needed
what measurements are needed for a ptosis prop?
length of support (A) and distance from back plane of spex to upper lid (B)
who can be dispensed with myopia control spectacles?
teenagers/ children who have increasing myopia
name the 2 myopia control spex on the market
HOYA introduced MiYOSMART 1st followed by Essilor who introduced Stellest
what is a feature of myopia control glasses?
they have either highly/ slightly aspherical lenslets
what are the 4 types of protective eyewear?
-afocal one-piece
-box type goggles
-cup type goggles (used very infrequently as they are uncomfortable)
-face shields or visors
what is housing and oculars in protective eyewear?
housing = frames
oculars = lenses
what are box type goggles made of?
housing is usually polyvinyl chloride (pVC) and oculars made of polycarbonate or cellulose acetate and have no Rx
what are face shields or visors useful for?
allow for good protection against splashes and flying particles
what are afocal focal one piece protective eyewear? What are they made of?
wrap around eye protection - they are the most basic and they generally manufactured in only one size - made of polycarbonate
what do most protective eyewear protect against?
mechanical impact
why are frames in protective eyewear usually made of plastic not metal?
as its safer
on protective eyewear, what should side shields be made of?
a transparent and vented material which is normally polycarbonate
what standard does protective eyewear need to conform to and where was this standard derived from?
he European standard EN 166 (2002), from the practical test of to drop the ball from a height of 1.3m
what practical test does the oculars and housing of protective eyewear need to meet for increased robustness S
Oculars - 22mm diameter steel ball of 43g @ 5.1 m/s
Housing - 6mm diameter steel ball of 0.86g @ 12m/s
what practical test does protective eyewear need to meet for low energy impact (F)
6mm diameter steel ball of 0.86g @ 45 m/s
what practical test does the oculars and housing of protective eyewear need to meet for medium energy impact (B)
6mm diameter steel ball of 0.86g @ 120 m/s
what practical test does the oculars and housing of protective eyewear need to meet for high energy impact (A)
6mm diameter steel ball of 0.86g @ 190 m/s
how should protective eyewear oculars be marked?
-manufacturers mark
-Optical class (1 = continuous, 2 = occasional, 3 = intermittent) – 1 most common
-If present, impact grade (usually S, F, B or A)
There may be a Kitemark, which is not a regulatory requirement, but a
manufacturer’s stamp of quality.
how does housing of safety spectacles need to be marked?
-Manufacturer’s mark,
offering traceability
(plus Kitemark, not a
requirement).
-EN 166 (relevant standard).
If present, impact grade (F,B or A as no marking on housing are
required for Grade S, increased robustness).
-CE / UKCA mark - products must meet legal requirements before sold
within the EU
what materials are used for plastic protective oculars?
-polycarbonate
-trivex
-columbia resin 39 (CR39)
what is the most commonly dispensed ocular material?
polycarbonate
what is the positives and negatives of using polycarbonate as ocular material for safety specs?
Advantages
-highest impact resistance of all safety ocular materials
disadvantages
-low abbe number so colour fringing
-very soft
-resistance to common chemicals is negligible
what is the positives of using trivex as ocular material for safety specs?
what is the positives and negatives of using polycarbonate as ocular material for safety specs?
what are the two methods of toughening glass and what is the preferred method of the UK?
-heat toughened glass and chemically toughened glass
-heat toughened is preferred
what us vital when you dispense safety specs?
making patient’s case records
once supplied and fitted, what happens if safety specs get damaged?
no repairs should be carried out on the eyewear. Should be returned to the original manufacturer and replaced to maintain the manufacturers guarantee.
what does a spectacle frame consist of?
-sides x2 and 1 front
-dowel screw: connects sides to front
-bridge - sits across px nose
-lugs x2- connects front to hinge near px temples
what are the types of bridge in metal frames?
-nose pads
-comfort bridge
-strap bridge
-W-bridge (has no pads - unusual)
in metal frame, what are the nose pads made of? how do they work?
generally silicone
-push in or screw in so fully adjustable
in a metal frame, how does a comfort bridge work and what is it made of?
built into frame and spreads weight of spex across the nose
-silicone
in metal frame, how does strap bridge work, who is it made for?
- either push-in or
screw-in - good for px’s with
shallow bridge e.g. px’s who have DS so can replace nose pads
what is a tip?
the plastic end of a metal side?
what are the most common type of bridge in plastic frames?
fixed pad bridge however this cannot be adjusted due to the pads being part of the frame
what are the types of bridge in a plastic frame?
-fixed pad bridge
-keyhole bridge
-regular bridge
-asian fitting
in a plastic frame, what patients may benefit from a keyhole bridge?
those who find bridge fit difficult, otherwise become more common as fashion statement
be able to identify all the different bridges for metal and plastic frames
ok - powerpoint can help
in a plastic frame, what does the Asian fitting consist of?
pads that are part of the plastic front so good for those with lower crest (flatter nose)
In a plastic frame, what is a regular bridge?
no pads, bridge follows contours of nose but this is bridge type is unusual
geometric center definition
the center of the opthalmic lens which is the point on the horizontal midline halfway between the two lens-bordering vertical lines
what is the A measurement?
the horizontal length of the lens also known as the eye size
what is the B measurement?
vertical length of the lens
what is the C measurement?
the width of the lens itself along the horizontal midline
what law must you obey when fitting glasses and why?
fitting triangle. It makes sure that the bend is on the right and left earpoint and the bridge sits nicely on the top of the nose, the frame sits correctly
what two types of plastics are used in glasses?
-thermoplastic
-thermosetting
what is the significance of cellulose acetate in glasses?
used to be used commonly in the UK but has been made illegal since it releases harmful gases when heated
what patients are stainless steel and titanium frames good for?
those who are sensitive to nickel as they are hypoallergenic
what is the ED?
effective diameter = distance from the gemoetric center to the apex of the lens bevel that sits farthest from the center times 2 (diagonal measurment)
-good for measuring blank size
what is the DBL?
distance between the two boxes when both lenses are boxed in the frame
what do the three numbers on the side of the frame tell you? e.g. 51box19 145
eyesize (51); DBL (19); total length of side (145)
Boxed centre
The intersection of the vertical and horizontal centre lines of the rectangular box containing the lens shape.
Horizontal lens size
Distance between the vertical sides of the rectangle containing the lens shape.
Vertical lens size
Distance between the horizontal sides of the rectangle containing the lens shape
Horizontal centre line (HCL)
An imaginary line drawn through the geometric centres of the rectangular boxes that just encompass the two lenses of a front.
Distance between centres
Distance between the geometric centres of each lens.
Distance between lenses
Distance between the nearest points of the apices of the two lenses.
Bridge width
Minimum distance between the pad surfaces of the frame measured along the bridge width line.
Bridge width line
Line parallel to and 5 mm below the horizontal centre line.
Bridge height
The vertical distance from the bridge width line to the intersection point of the vertical symmetry axis with the lower edge of the bridge (crest height plus 5 mm).
Crest height
The vertical distance from the HCL of the front to the midpoint of the lower edge of the bridge.
Bridge projection
Minimum horizontal distance from the back plane of the frame front to the centre of the back of the bridge.
Distance between rims (DBR)
The horizontal distance between the nasal surfaces of the rims measured at a stated distance below the midpoint of the lower edge of the bridge, for example, ‘DBR 20 at 10 below crest’.
Apical radius
The radius of the arc forming the lower edge of the bridge viewed perpendicularly to the back plane of the front
Frame temple width
Distance between the sides 25 mm behind the back plane of the frame front.
Frame head width
Distance between the sides at the ear points.
Dowel point
The centre of the bottom of the dowel hole.
what are the three types of earpoint?
-drop end of side
-of a curl side
-of a straight side
what is of a drop end of side ear point?
he midpoint of the arc of contact between the bend of the side and the circle which fits it.
what is of a curl side ear point?
The point on the lower edge of the side at the beginning of the curl.
what is of a straight side ear point?
The point on the lower edge of the side which is assumed to make contact with the ear.
Line of the side
A straight line through the dowel and ear points.
angle of let-back
The horizontal angle between the inner surface of the fully opened side, adjacent to the joint, and a normal to the back plane of the frame front.
angle of side
The vertical angle between a normal to the back plane of the front and the line of the side when fully opened.
Overall length of side
Length from the dowel point to the very end of the end tip.
length to bend
Length from the dowel point to the ear point.
Length to drop
Length from ear point to the very end of the end tip
length to tangent
Distance from the dowel point to the tangent to the inner surface of the curl at rest which is perpendicular to the line of the side.
how do you check frame fit from the front?
- Ensure frame is level
- Frame sits comfortably on bridge of nose
- Complete nose pad is in contact with nose
- Lower rims don’t rest on cheeks
how do you check a frame fit from the sides?
- LTB is correct – sides long enough to rest on patient’s ears with side just
past the top of ears - Length of drop not too long, should be between 35-40mm (for adults)
- Pantoscopic angle (angle of side with front) – lower rim should be
angle inwards 5 – 10o - BVD is correct, back of spectacle lens correct distance away from eye
How do you check frame fit from above?
- Should be slight bow to front of frame following the curve of head
- Frame should not be twisted so one lens is closer to the face than the other
(BVD would be incorrect) - Temple width is correct – sides should grip head snugly, not too tight/loose
What’s the differences between thermoplastic and thermosetting?
-thermoplastic can be reformed with heat whereas thermosetting cannot
-thermoplastics have a long linear chain whereas thermosetting have a 3D structure
-thermoplastics form by addition polymerisation whereas thermosetting form by condensation polymerisation
-thermoplastics are soft, weak while thermosetting is hard and strong
thermoplastics are soluble in organic solvents whereas thermosetting is insolule in organic
-thermoplastics can be reclaimed for wastes whereas thermosetting polymers cannot be reclaimed
For cellulose acetate,
-what actually is it?
-What temp does it soften at?
-how can joints be applies?
-what kind of colour patterns does it come in?
-rigid blocks of acetate sliced into sheets, and is inexpensive and most common of materials
-57 degrees
-Joints heat inserted, pinned or screwed
-Colour extruded (same colour throughout), laminated (2-tone) or dyed (see through) - but discolours with age
name a glasses frame material similar to cellulose acetate
cellulose nitrate but is now illegal in the uk
for cellulose nitrate
-what is the flashpoint?
-what colour is it?
-how do its properties change over time?
-only 70 degrees so highly flammable
-colour laminated
-with age becomes brittle as surface cracks and crazes and the material yellows and smells of camphor if scratched
for optyl
-what kind of material is it?
-how do the sides join?
-what temp does it soften at
-how is it made
-what are its positives
-what symbol does it have on its frame
-Epoxy resin material registered by Safilo
-First 15mm of plastic sides reinforced, fully reinforced or metal sides
-Plastics memory and softens at 88°C – hold adjustment in place
until cool and set
-Injection moulded
-lightweight and claimed to be
hypoallergenic
-Symbol which appears on frame is Ω
for SPX
-what is it
-how is it made
-what temp does it bend at
-SPX is a co-polyamide made by Silhouette (SPX marked on frames)
-Injection moulded and coloured
-Adjust at 95°C but shrinks if over-heated (and so lenses usually inserted cold)
what are the positives of SPX as a frame material
-Flexible and lightweight
-Hypoallergenic
-Sides always reinforced
what material are most rubbers used in spectacle frames?
silicone rubbers
what are the positives of silicone based rubber spectacle frames?
-Unbreakable i.e. highly durable
-Soft and pliable
-Strap can hold frame in place
-No hinges or metal parts thereby reducing chances of facial injury
-Lightweight
-Available in many solid colours
what does the British Hallmarking Act of 1973 do?
prevents gold, silver, platinum
and palladium to be used in products unless they are
hallmarked so need
Authorisation stamp
Descriptions permitted if it relates to colour of the item
what are base metals?
non-precious metals situated under precious metal coating and can be alloyed with precious metals to give strength
why is pure gold not often used in glasses frames?
as its expensive and heavy
what are the advantages of using gold as a frame?
-keeps colour well
-resistant to corrosion
for gold frames, how are they made?
made by coating a base metal of nickel, silver or bronze with gold, by welding or electrolysis
what is the most common base metal in frames?
nickel
what are the positives and negatives of using nickel in glasses frames?
-positives are easily adjusted and pretty flexible
-negatives are turns green e.g. if in contact with sweat
how can patients with nickel allergy wear nickel frames?
-use plastic tips and bridges
-have inert coatings on the frame
how are nickel silver frames assembled and coloured
-assembled by soldering and colour applied using electroplating or lacquering
what do nickel silver frames consist of?
approx 20% NI and mostly Cu (have no Ag)
why are stainless steel frames uncommon?
as they are difficult to manufacture
what are the positives of stainless steel frames?
-Lightweight and corrosion resistant
-Rigid yet flexible
-Hypoallergenic if pure stainless steel
how are stainless steel frames made?
-assembled by welding
-colour applied by electroplating
why are titanium frames expensive to produce?
as the titanium is mined as an ore
what are the two types of Ti frames?
Pure – 98% of frame parts Ti
β Ti – 80% Ti (and other metals e.g. aluminium, iron, tin, etc. and titanium alloys e.g. Ni
and Co
how are titanium frames assembled?
by cutting and pressing followed by laser soldering and colour
galvanised or ion plated
what are the positives of titanium frames?
-40% lighter in weight than other metals and flexible
-Corrosion and abrasion resistant
-Can be laser soldered if broken
-Hypoallergenic
what are memory metals?
unrelated alloys which return to original shape even after extreme distortion but will snap eventually if flexed through sharp angles
give some memory metals free of nickel and or titanium
Cu-Al, Ti-Co
whats the most common memory metal
nickel titanium
give 4 more recent glasses frame materials
-TR90 Nylon: high durability and lightweight
-wood/ faux wood like bamboo: able to withstand shock
-recycled materials like acetate: eco friendly
-3d printed bioplastics like polyamide: not fully biodegradable
