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ov1004 dispensing > lenses > Flashcards

lenses Flashcards

1
Q

<p>PLUS LENS</p>

A

<p>- convex

- thicker in middle
- thinner on edges
- magnifies images
- converges light
- AGAINST movement</p>

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2
Q

<p>MINUS LENS</p>

A

<p>- concave

- thicker on edges
- thin in middle
- diverges light
- minifies
- WITH movement</p>

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3
Q

<p>OPTICAL CENTRE</p>

A

<p>- where light goes through, no prismatic effect</p>

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4
Q

<p>1.54 - 1.64</p>

A

<p>mid - index</p>

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5
Q

<p>1,48-1.54</p>

A

<p>low index</p>

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6
Q

<p>1.64 - 1.74</p>

A

<p>high index</p>

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7
Q

<p>dispersion</p>

A

<p>chromatic abberation</p>

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8
Q

<p>high dispersion =</p>

A

<p>high chromatic abberation</p>

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9
Q

<p>high V value</p>

A

<p>low TCA</p>

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10
Q

<p>low v value</p>

A

<p>more TCA</p>

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11
Q

<p>V value 45 & above</p>

A

<p>low dispesion/ low tca</p>

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12
Q

<p>V value lower than 39</p>

A

<p>high dispersion / more TCA</p>

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13
Q

<p>CROWN GLASS (RI)</p>

A

<p>1.52</p>

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14
Q

<p>POLYCARBONATE (RI)</p>

A

<p>1.586</p>

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15
Q

<p>CR39 (RI)</p>

A

<p>1.498</p>

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16
Q

<p>CROWN GLASS ADVANTAGES</p>

A

<p>- high v value so low CA

| - scratch resistant</p>

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17
Q

<p>CROWN GLASS DISADVANTAGES</p>

A

<p>- brittle

- high specific gravity (heavy)
- low refractive index (more abberations)
- tendency to fog/shatter</p>

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18
Q

<p>Greatest impact resistance?</p>

A

<p>Polycarbonate</p>

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19
Q

<p>POLYCARBONATE DISADVANTAGES</p>

A

<p>LOW v value so more dispersions</p>

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20
Q

<p>advantages of plastic materials ( high index)</p>

A

<p>thinner & lighter than other plastics</p>

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21
Q

<p>TRIVEX qualities</p>

A

<p>- high impact resistance

- high v value
- low specific gravity</p>

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22
Q

<p>Which lens blocks 100% of UV-A and UV-B below 394nm</p>

A

<p>Trivex & polycarbonate</p>

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23
Q

<p>True or False: Trivex is more resistant to chemicals than any other plastic lens material.</p>

A

<p>True</p>

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24
Q

<p>In order for a patient to have problems with chromatic aberration, the amount must reach \_\_\_\_\_Δ.</p>

A

<p>0.25</p>

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25
Q

<p>Chromatic Aberrations more of a problem for what type of lenses</p>

A

<p>PLUS ( thicker)</p>

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26
Q

<p>best choice for lens protection but scractes the easiest?</p>

A

<p>polycarbonate</p>

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27
Q

<p>how to reduce chromatic aberrations in polycarbonate</p>

A

<p>anti- reflective coating</p>

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28
Q

<p>how to improve the impact resistance of glass</p>

A

<p>thermal & chemical tempering</p>

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29
Q

<p>which is faster- chemical or thermal tempering</p>

A

<p>thermal</p>

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30
Q

<p>which is more resistant</p>

A

<p>chemical: 2-3 x more resistant</p>

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31
Q

<p>what is specific gravity</p>

A

<p>ratio of a material's weight compared with the weight of an equal volume of water</p>

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32
Q

<p>high index lens is heavier bc</p>

A

<p>contains more lead</p>

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33
Q

<p>thermoplastic example</p>

A

<p>polycarbonate</p>

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34
Q

<p>thermosetting example</p>

A

<p>CR39</p>

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35
Q

<p>material with highest dispersion</p>

A

<p>polycarbonate</p>

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36
Q

<p>why are plastic materials less subject to fogging with changes in temperature and humidity?</p>

A

<p>less thermal conductivity</p>

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37
Q

<p>what is a Trivex lens?</p>

A

<p>a tri-performance lens material:

1. improved impact resistance
2. high Abbe value/lower dispersion
3. low specific gravity</p>

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38
Q

<p>Anti reflection coating</p>

A

<p>- thin layer/material applied to surface of lens to reduce unwanted reflections: increases the amount of lens passing through the eye</p>

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39
Q

<p>why do plastic lenses need scratch resistant coatings?</p>

A

<p>- typically soft materials that scratch easily</p>

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40
Q

<p>how to estimate the power of a lens</p>

A

<p>1. thickness: thicker the lens, higher the power

2. speed of movement: slower the speed, higher the lens power</p>

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41
Q

<p>how to neutralise a positive lens?</p>

A

<p>- add a minus lens of equal power</p>

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42
Q

<p>optical centre</p>

A

<p>- vision is best at this point

| -limited aberrations</p>

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43
Q

<p>toric lens are used for</p>

A

<p>astigmatic people</p>

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44
Q

<p>what happens if curve in lens is flat</p>

A

<p>PLANO</p>

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45
Q

<p>PLANO LENS</p>

A

<p>no powe</p>

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46
Q

<p>function of prism</p>

A

<p>deviate light from original path</p>

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47
Q

<p>prentice rule</p>

A

<p>the relationship between the power of the lens, the distance of the eye away from the optical centre and prismatic effect it gives.</p>

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48
Q

<p>prentice rule equation</p>

A

<p>P=cF
P= prismatic effect
c= distance from optical centre of the eye (cm)
F= back vertex power of lens in Dioptres

We ignore + and -
This equations shows the importance of correct centration</p>

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49
Q

<p>how to measure durability</p>

A

<p>materials are tested with the drop ball test and graded in terms of impact resistance
. effected by the elasticity of the material
- if elasticity of material is good then when hit by an object , the lens will be able to bend and flex and absorb some of the energy from impact</p>

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50
Q

<p>refractive index of material refers to</p>

A

<p>- density of material

| - higher the refractive index, denser the material</p>

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51
Q

<p>lowest index for plastic</p>

A

<p>cr39 - 1.498</p>

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52
Q

<p>lowest index for glass</p>

A

<p>crown glass: 1.523</p>

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53
Q

<p>refractive index refers to</p>

A

<p>- density of material

| -high refractive index : higher density</p>

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54
Q

<p>more dense</p>

A

<p>more light refracts because flatter curve: behave like mirrors and reflect more light</p>

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55
Q

<p>flatter curvature</p>

A

<p>thinner lens</p>

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56
Q

<p>polycarbonate is a...</p>

A

<p>THERMOPLASTIC

| can be moulded&remoulded by heating and cooling</p>

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57
Q

<p>what does v value influence</p>

A

<p>- how much tca patient will experience</p>

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58
Q

<p>dispersal</p>

A

<p>is looking at when incident light hits the lens , how much its split into its colour components</p>

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59
Q

<p>when is TCA experienced</p>

A

<p>when patient looks off-axis: not optical centre</p>

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60
Q

<p>lower v value =</p>

A

<p>more dispersion</p>

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61
Q

<p>How much TCA can we tolerate</p>

A

<p>0.1 prism dioptres but varies with each patient</p>

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62
Q

<p>what do surface reflections do</p>

A

<p>- dramatically reduce transmission of white light with high refractive index materials</p>

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63
Q

<p>what is UV cut off</p>

A

<p>- light that is not reflected from the lens surface passes through
-some is absorbed by lens material instead of being transmitted
100-380 nm</p>

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64
Q

<p>curve variation factor</p>

A

<p>how much thinner is a material compared to crown glass</p>

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65
Q

<p>coating a lens is dependant on</p>

A

<p>material of lens</p>

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66
Q

<p>does glass require a hard coat?</p>

A

<p>no : hard enough, doesn't need it
- resistant to abrasion and scratches

abrasion ( rubbing with fingerprints)
scratching is from larger particles</p>

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67
Q

<p>what is a hard coat</p>

A

<p>- clear coating protects from abrasion and scratches

- should never refer to as scratch proof</p>

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68
Q

<p>2 types of plastic</p>

A

<p>thermoplastic: - include polycarbonate and long chain molecules within the material don't cross-link therefore they can be heated are remoulded

thermosetting: set in a mould with a chemical reaction which cross-links molecules within the material
e. g. high index plastic( always hard coated) and CR39</p>

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69
Q

<p>polycarbonate properties</p>

A

<p>high impact resistance

| scratches easily so may choose to hard coat</p>

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70
Q

<p>hard coat properties

btw dont have much flexibility: will crack if too thick</p>

A

<p>- must be 0.5-10 microns

- spin coated: expensive, not used much, NO TINT
- vaccum: thinner, harder coat, expensive, NO TINT
- IN MOULD
- dip dyed: lots of lens can be coated at once
- labs clean, important to control humidity & temperature</p>

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71
Q

<p>why develop AR coating?</p>

A

<p>-light reflected from lens is light that isn't reaching the eye , this means image is not clear and this can give rise to reflections and ghost images</p>

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72
Q

<p>what does reflection of lens cause?</p>

A

<p>light scatter - which reduces contrast sensitivity - which is how well Px sees definition between target and it's background

- ghost images which causes visual disturbances
- reduction of light transmittance leading to asthenopia ( eyestrain )</p>

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73
Q

<p>what does anti-reflection coating do ?</p>

A

<p>clear coating added to lens to protect against reflections from artificial light which cause glare and dazzle
e.g. computer , car headlights</p>

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74
Q

<p>what do ghost images cause ?</p>

A

<p>- cause visual disturbances to the Px

- causes px to see images of their eye or eyelashes
- can be reflections from the front or back surface
- they can be distracting to wear and give glassy appearance to the lens</p>

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75
Q

<p>relationship between refractive index and ghost image</p>

A

<p>higher the refractive index higher the intensity of ghost image</p>

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76
Q

<p>ghost image will not be seen if</p>

A

<p>if outside px field of view</p>

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77
Q

<p>how do we reduce ghost images</p>

A

<p>1- reducing BVD- reducing amount of light getting to lens

2- angling the lens - to sit closer to lens

3- changing the base curve - to make lens and more curved as we know flatter curves give more reflection

4- applying anti-reflection
coating</p>

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78
Q

<p>types of AR coating</p>

A

<p>single layer: can result in 5% of reflections so 95% of light getting through to patients eye

multi layer: reduce reflections to 1% or even less</p>

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79
Q

<p>why do we coat both sides of lens</p>

A

<p>ghost images and reflections can come on both front and back surface</p>

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80
Q

<p>anti reflective coatings work on principle of</p>

A

<p>destructive interference</p>

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81
Q

<p>what is constructive interference</p>

A

<p>two waves combined and in phase to make 1 large wave</p>

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82
Q

<p>what is destructive interferancw</p>

A

<p>two waves of light of equal amplitude and out of phase will cancel each other out</p>

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83
Q

<p>conditions for single layer coats to work</p>

A

<p>-path condition: -wave reflections to be exactly out of phase from the front and the back of the coating it must be 1/4 wavelength thick - this can only occur for one wavelength at a time and we know that visible light is composed of many wavelength

- for this coating to be most efficient the thickness of the coating must be different for all different wavelength ( not possible ) so we use light in middle of spectrum
- 550 nm</p>

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84
Q

<p>how is anti reflection coating visible to people</p>

A

<p>- have a bloom which is visible to observers - so if you tilt lens back and forth in front of artificial light source you can see slight sheen of colour on surface of lens- what you're seeing is light being reflected
- light being transmitted through to eye is the one you can't see - this means that you can spot single layer coating which has a purplish bloom to it because light its letting through is the central wavelength ( yellow light )</p>

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85
Q

<p>what is the conditions that must be met once path condition is satisfied for single layer coats to work ?</p>

A

<p>- once path condition is satisfied then waves will be out of phase but they need to be of equal amplitude

2- amplitude condition - ensures waves are of equal strength</p>

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86
Q

<p>what is the function of multi-layer coating ?</p>

A

<p>allow us to reduce reflectance of more than one wavelength resulting in less reflection and more light reaching px eye</p>

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87
Q

<p>when do we use multilayer coatings</p>

A

<p>on plastics</p>

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88
Q

<p>why is harder to coat plastic with single layer coating ?</p>

A

<p>- single layer coatings are quite brittle and they tend to crack when applied to a flexible plastic material</p>

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89
Q

<p>what does a multilayer anti-reflection coating contain ?</p>

A

<p>will include hard coat so px doesn't need to pay extra</p>

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90
Q

<p>what is adhesion coat ?</p>

A

<p>on top of hard coat and will help the thick and thin layer coating stick</p>

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91
Q

<p>what are thick and thin pair of coating made of ?</p>

A

<p>formed of titanium oxide and silicone dioxide</p>

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92
Q

<p>thick pair function</p>

A

<p>cancel central spectrum of light</p>

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93
Q

<p>thin pair function</p>

A

<p>cancel blue and red reflections</p>

94
Q

<p>what are the multilayer coating in order ?</p>

A

<p>hydrophobic ( lens will be easier to clean )
thick pair ( cancel the central spectrum of light )
thin pair ( cancel out reflections from blue and red end of spectrum )
adhesion coat
hard coat</p>

95
Q

<p>why are multi layer coating harder to produce ?</p>

A

<p>- require a sterile environment for manufacture

| - can be vacuum coated , evaporation coated or sputter coated ( simpler process and smaller equipment )</p>

96
Q

<p>what are the disadvantages of anti-reflection coating ?</p>

A

<p>- any dirt , water or skin oil will reduce the effectiveness of the coating
- AR coating exaggerate the contrast between clear and dirty area
smudges on lens reduce transmission by 4%
- deteriorate in heat
- coating needs to be fully intact to give good vision</p>

97
Q

<p>why do we dispense tints</p>

A

<p>- protection from uv light
- ^ glare/discomfort

glare: light that causes discomfort- watery eyes

- visual comfort
therpeutic + medical
- cosmetic
-contrast filters: increasing contrast of objects so can see clearer for e.g in a sport like a football use a green tint etc</p>

98
Q

<p>Who can benefit from a tint?</p>

A

<p>-a patient that could have early signs of cataract- which elevate their discomfort.</p>

99
Q

<p>What do tints protect against?</p>

A

<p>certain wavelengths of light such as infrared- which can be damaging to the eye.</p>

100
Q

<p>Why do we need protection from the different wavelengths of light - UVA ?</p>

A

<p>linked to cataracts and retinal damage: deeply penetrating</p>

101
Q

<p>UVB :</p>

A

<p>cataracts and photokeratitis</p>

102
Q

<p>Why do we need protection from the visible spectrum ?</p>

A

<p>disability and comfort glare</p>

103
Q

<p>Why do we need protection from Infrared ?</p>

A

<p>800nm + causes heat cataracts and retinal/choroidal burns</p>

104
Q

<p>Do all sunglasses have UV protection ?</p>

A

<p>No

| -UV is a separate clear coating and you can then go onto adding a tint to it as it is not always provided.</p>

105
Q

<p>Why are we more concerned about UV coating a tinted lens?</p>

A

<p>-pupil size increase due to dark tint and more light is let into the eye- change the ocular dynamics of the eye</p>

106
Q

<p>What can the amount of light entering the eye be altered by ?</p>

A

<p>- absorption

| -reflection</p>

107
Q

<p>What are mirror coatings supplied with?</p>

A

<p>Supplied with a dark tint to prevent the wearer seeing themselves in the mirror coating.</p>

108
Q

<p>What do mirror coatings do?</p>

A

<p>- they have Tint depth which also influences the amount of reflection
- they Don’t wear well- can easily be scratched affecting them</p>

109
Q

<p>How can you describe the tint?</p>

A

<p>- State the tint by colour

- Depth in percentage
- Light transmission factor (LTF) - FOR how light or dark you want the tint to be so by telling how much light you want transmitted through from this factor
- Absorbance (ABS) - to tell how much light is absorbed by the lens

85% LTF = light lens as 85% of light being transmitted</p>

110
Q

<p>ABS (ABSORBANCE) for 85% ltf</p>

A

<p>15% of light absorbed by lens</p>

111
Q

<p>What colour tint does a patient choose?`</p>

A 
<p>-To enhance contrast?
Drivewear
Sports tints
-Personal preference?
-Cosmesis?
Frame choice
-Protection
Certain colours provide protection</p>
112
Q

<p>N tint</p>

A

<p>neutral, grey tint</p>

113
Q

<p>why is N tint good</p>

A

<p>filter out all wavelengths roughly the same they don't distort colours much whereas a contrasting tint will.
-So when a patient sees through them there is no distortion</p>

114
Q

<p>Who is a N tint good for ?</p>

A

<p>drivers- don't want traffic light colours being distorted

| -Architectures</p>

115
Q

<p>what tints are available</p>

A

<p>- Fixed

- Graduated
- Photochromic
- Polarised</p>

116
Q

<p>fixed tints</p>

A

<p>same colour throughout lens

- tint absorbs 1mm into material
- just front surface</p>

117
Q

<p>What are the methods of tinting fixed tints?</p>

A

<p>-Plastic- dip dying
- Glass- solid or laminated

glass material = cannot dip dye</p>

118
Q

<p>-Plastic- dip dying

| - Glass- solid or laminated</p>

A

<p>Solid - the tint is introduced to the lens when it's molten- a tint is solid throughout the material/ lens
- if patient wants green tint add iron to molten mix

-Laminate the lens- a laminated tint is a thin sheet which is laminated or vacuum coated onto one surface of the lens- a tint is only one surface of the lens.</p>

119
Q

<p>What do we do when deciding between a solid and laminated tint?</p>

A

<p>Lamberts law</p>

120
Q

<p>what is lamberts law</p>

A

<p>thicker the material, darker the tint

- 8 : laminiated is better
plano: solid tint</p>

121
Q

<p>what is a solid tint</p>

A

<p>dispersed in the whole lens/material</p>

122
Q

<p>what is a laminated tint</p>

A

<p>only at one surface of the lens.

| -not affected by tint at all - equal across the lens</p>

123
Q

<p>What is a graduated tint ?</p>

A

<p>- single OR dual coloured
- darker at top, GRADUATING to a lighter colour at bottom

advantage: good for reading for elderly patients</p>

124
Q

<p>What is a photochromic tint?</p>

A

<p>- Darken due to the energy provided by UV.- use energy to alter the structure- when hit UV light turns dark

- Darken quicker than they return back to light - on a sunny day will turn dark MORE and less UV means less dark.
- No separate sunglasses required
- Tint variable
- Temperature dependent- work better in cold environments than hot
- Full UV400 protection - if remove this will limit how it works - not as well
- Less reactive behind a UV filter- e.g. car windscreen.</p>

125
Q

<p>What benefits and advice do you think you should give your patients regarding
 Darken quicker than they return back to light</p>

A

<p>- may think they are faulty so make sure u tell patient first
- possibly health and safety hazard if they come inside - wouldnt be able to suddenly see so take care when walking into buildings</p>

126
Q

<p> No separate sunglasses required (tint)</p>

A

<p>-save money

| -conviencince - no swapping glasses</p>

127
Q

<p> Less reactive behind a UV filter- e.g. car windscreen</p>

A

<p>- get glare - better outside the car

| - may not get as dark when driving</p>

128
Q

<p> Residual colour with age</p>

A

<p>- older styles they get old after time (get yellow tint after a while ) - may affect vision in darker conditions.</p>

129
Q

<p>What can photochromatic lenses be made in ?</p>

A

<p>-Glass- which contain
Silver halides- separate when activated by UV - when separated they go darker and recombine and get lighter
Equi-tint can be used on high prescriptions
-Plastic- use
Pyrans and oxazines in the 100-150 microns of the front
the surface of the lens
Molecules rotate on activation by UV- don't split and as they rotate they become darker</p>

130
Q

<p>Why Polarised lenses?</p>

A

<p>-Unpolarised light- light which vibrates in all directions

- Polarised light- it maintains in the one direction- which can cause glare and mirror effect
- Brewster angle- if angle between the refracted and reflected beam is 90° then the reflected beam will be completely plane polarised.</p>

131
Q

<p>Why is polarised light not good?</p>

A

<p>can cause glare and mirror effect e.g when driving</p>

132
Q

<p>What is polarized light?</p>

A

<p>polarized in the plane parallel to the
reflecting surface
e.g surface of water</p>

133
Q

<p>How can we polarise lenses?</p>

A

<p>- Axis of the filter set vertical

- BS tolerance 5 degrees
- Stretched iodine crystals
- darker tint more polarisation occurring
- </p>

134
Q

<p>How can we tell our patient is wearing a polarised lens?</p>

A

<p>- do that by thinking about the effect of holding up another polarised lens in front of them

- if you have 2 polarised lens - it will let some light through - the picture will be less clear- contract reduced by still able to see the image
- if by rotating the filters at right angles to one another - block both vertical and horizontal meridian- won't see a image</p>

135
Q

<p>What are IR lenses?</p>

A

<p>- need to use with glass lens

- Exposure from sun, furnaces and with explosions!
- Linked to heat cataract, retinal and choroidal lesions
- Cant protect with CR39
- Rayban G-15- ferrous oxide- green tint - glass lens</p>

136
Q

<p>Can UV damage be reversed</p>

A

<p>no - protection required all year round: summer & winter</p>

137
Q

<p>are children more vulnerable to UV</p>

A

<p>yes</p>

138
Q

<p>do darker tinted lenses provide more protection</p>

A

<p>no</p>

139
Q

<p>field of view</p>

A

<p>total area patient can see when eye is in primary position- i.e. patient is looking ahead</p>

140
Q

<p>what is real field of view ?</p>

A

<p>it is obtained when a spectacle lens is glazed into the frame</p>

141
Q

<p>what is field of view influenced by ?</p>

A

<p>- frame and therefore lens shape

- vertex distance
- power of the lens - only in real field of view</p>

142
Q

<p>what are frame markings found in spectacle lenses ?</p>

A

<p>- horizontal lens size x distance between right and left lens ( mm )
e.g. 50x18 50*18 50 □ 18

- total length of side ( mm ) from screw to the tip taking out the bend
e. g. 135 mm

- manufacturer / design / suppliers name or trademark
- frame model
- frame colour</p>

143
Q

<p>what box centre distance ?</p>

A

<p>- measuring from the centre of one box to the centre of another box
- add horizontal lens size and distance between lenses

- this measurement that can be used to work out any decentration that we are going to use to place the optical centres of the lenses in front of the patient's pupil.
- The patients pupillary distance would match the box centre distance.</p>

144
Q

<p>what are the three groups for lens mounting ?</p>

A

<p>1. full rim - rim surrounds whole of lens

2. rimless - no rim surrounding lens

3. supra - frame covers part of lens
when covers bottom and not top - this can be good for reading</p>

145
Q

<p>what does mounting type determine ?</p>

A

<p>determines the edge of lens which we need to glaze to allow lens to remain securely in frame and not fall out</p>

146
Q

<p>when do we use BEVEL and MINI BEVEL lens edge ?</p>

A

<p>used for full frames</p>

147
Q

<p>when do we use flat lens edge</p>

A

<p>rimlesss flat style</p>

148
Q

<p>when do we use supra lens edge ?</p>

A

<p>edge not covered must be held in place this can be done by a carving a groove into lens edge and thin clear wire is placed into groove and secured to frame</p>

149
Q

<p>what are ways rimless lens mounting can be produced ?</p>

A 
<p>rimless edge can be
flat
scalloped
facetted
inlaid</p>
150
Q

<p>what are frame components ?</p>

A

<p>-pads

- bridge
- rim - incases lens
- side - extension from front of frame that fits around the ear
- joint - where side of frame and front of frame meet</p>

151
Q

<p>what is bridge ?</p>

A

<p>the bridge forms the main connection between the right and left rims . it may or may not have pads
different designs for plastics and metal frames</p>

152
Q

<p>what is pads</p>

A

<p>extension or attachments to the bridge which form the bearing surface of the frame for the nose
larger pad = greater weight distribution</p>

153
Q

<p>what are the bridge types ?</p>

A

<p>. fixed pad bridge - plastic frames only , pads moulded into rim and same material as frame

. regular bridge - plastic frames only
no moulded pad shapes

. keyhole bridge - plastic frames only
only pads resting against patient's nose

. pads on arms - metal frame only</p>

154
Q

<p>what are pad on arms ?</p>

A

<p>. metal frame only
. pads can be screw on or push in
. can be saddle / comfort / strap bridge with same material attachment
. greater surface area touching nose = more comfortable</p>

155
Q

<p>what are pads on arms made from ?</p>

A

<p>. silicone - flexible , light weight , soft

| . vinyl/acetate - light weight , rigid , cheaper</p>

156
Q

<p>what are some examples of sides ?</p>

A

<p>what are some examples of sides ?</p>

157
Q

<p>protective eyewear protect against</p>

A

<p>penetrating trauma & chemical burns</p>

158
Q

<p>what is impact resistance?</p>

A

<p>the ability of a material to withstand a high force or shock applied to it over a short period of time</p>

159
Q

<p>how are materials tested?</p>

A

<p>- materials are tested with the drop ball test and graded in terms of impact resistance</p>

160
Q

<p>what is the affect of elasticity?</p>

A

<p>- the elasticity of material is what give the material its impact resistance
- elastic material is able to absorb some of the shock/force that hits it and lens doesn't break</p>

161
Q

<p>PROTECTIVE EYEWEAR

| how should the frame and lens be ?</p>

A

<p>- the frame needs to be loose enough that if the lens is flexing during this period of elasticity that its got somewhere to go
- lens isn't glazed too tight</p>

162
Q

<p>what are types of protective eyewear</p>

A

<p>- spectacles that are glazed with prescription

- some can be worn on top of spectacles ( temporary measure )
- googles ( protect around ocular area ) - can also be ventilated
- visors - protect the whole head</p>

163
Q

<p>A screw comes out of the side of a PX safety glasses that he uses for work and asks to put a new one in - what do you say?</p>

A

<p>no, as this will void the manufacturer guarantee

| - you need to send back to manufacturer who will re-test the unit and ensure that its meeting standards</p>

164
Q

<p>what are lens markings</p>

A

<p>lens markings show that if a lens has met standards but only found if applicable</p>

165
Q

<p>what are examples of lens markings?</p>

A 
<p>. scale number ( filter lenses )
. manufacturers mark - always relevant
. optical class
. mechanical strength symbols
. field of use
. scratch resistance
. resistance to fogging
. radiant heat</p>
166
Q

<p>what is protective eyewear considered as ?</p>

A

<p>considered as a single unit- must be tested as such</p>

167
Q

<p>protective eyewear

what are scale numbers</p>

A

<p>- higher numbers= darker filters

| - consists of code number and a shade number</p>

168
Q

<p>what is optical class</p>

A

<p>- 1,2 or 3 - indicates the optical quality of the ocular
- how close the prescription is what is should be
1= in tolerance with what the prescription is expected to be . +/- 0.66 D
- as numbers get bigger tolerance is lower</p>

169
Q

<p>what is mechanical strength ?</p>

A

<p>- looks at impact resistance

e. g bearing test
- can be marked with various codes
- most safety spectacles are marked with an S or F

- S= increased robustness
e. g. CR39- not very tough
- F= low energy impact
e. g. polycarbonate

- B= medium energy impact
e. g. goggles

- A = high energy impact
e. g visors or face shield or some goggles</p>

170
Q

<p>what materials make a good safety lens material?</p>

A

<p>polycarbonate-
trivex-
glass - must be treated to meet S standard</p>

171
Q

<p>what are the pros and cons of using PLASTIC lenses as protective eyewear?</p>

A

<p>- more elastic than crown glass

- impact resistance depends on material
- impact resistance is affected by coating
- CR39 can break into hooked shards</p>

172
Q

<p>what are pros and cons of using GLASS as protective eyewear?</p>

A

<p>- untreated is not suitable for protective lenses

- can be heat or chemically toughened
- laminated</p>

173
Q

<p>what are pros and cons of using TRIVIEX as protective eyewear?</p>

A

<p>higher v value compared to polycarbonate

- UV cut off to 400nm
- tinting not possible
- good chemical resistance
- n=1.54
- lowest specific gravity</p>

174
Q

<p>what is a compression envelope ?</p>

A

<p>- outside of the lens is put into compression - molecules at the lens surface closer together compared to the centre

- allows lens to become stronger
- heat and chemical toughening both produce compression envelopes</p>

175
Q

<p>what does force cause in an untreated glass?</p>

A

<p>untreated glass- force causes compression at blow and release tension on the other side</p>

176
Q

<p>what are the two main methods of toughening a glass lens ?</p>

A

<p>- heat toughened

| - chemically toughened</p>

177
Q

<p>How is heat toughening carried out?</p>

A

<p>- lenses must be glazed before toughening

- requires a minimum centre/edge thickness of 2.4 mm
- lens is heated just below melting point 650 c
- thicker lens= heated longer
- variable heating time
- outer glass cools more rapidly than centre
- if it does smash then blunt pieces
- rejection rate compared to chemically toughened
- can be recognised by Maltese cross on a polarising filter</p>

178
Q

<p>what is compression envelope caused by ?</p>

A

<p>- caused in difference in cooling between the surface and centre</p>

179
Q

<p>what should you remember when supplying protective eyewear?</p>

A

<p>- if protective eyewear is supplied as industrial prescription protective eyewear then the employer is required to assess the workplace risk

- you need ti supply to the company order form
- if you do perform a workplace assessment you must keep detailed case records of the assessment</p>

180
Q

desirable properties of frames

A
  • ease of production
  • lightweight
  • strong, non flammable
  • easily glazed, shape retention flexible
  • cosmetically acceptable
181
Q

What are some types of plastic frames?

A
  • Cellulose acetate
  • Optyl
  • SPX
  • Cellulose propionate
  • Acrylic/PMMA
  • Carbon Fibre
182
Q

What are some properties of cellulose acetate?

A
  • Inexpensive
  • — Fairly light- density 1.27
  • — Always has reinforced sides
  • — Adjust at 57degrees
  • — Needs reinforcement or can warp and become soft
  • — Joints can be heat inserted, pinned or screwed
  • — Can react with skin- can be metal joints so can lead to rash.
183
Q

How can cellulose acetate frame be cut?

A

Can be cut from a block or poured semi-molten into a mould- this is stronger as the molecules are not cut.

184
Q

What are some more properties of cellulose acetate ?

A
  • Solid colouring- thicker frames bolder colours, can laminate with clear sheet for transparent colours
    -colouring achieved by water based dyes
    -— Lacquered and polished for shine.
    — To recognise- colour all through and where sides meet frame front you can see it has been cut- rough edge.
185
Q

What is a disadvantage of the cellulose acetate?

A

-— Discolours with age- Can react with skin- white

deposits and material becomes rough to touch

186
Q

How is the cellulose acetate plastic adjusted / fitted?

A
  • forced in a semi- molten state into a mould. This latter method gives a stronger frame, as the fibres, or molecules, are not cut through, but bend around curves.
187
Q

what are some properties of optyl?

A

-— Side only reinforced for the 1st 12mm- not really
required- decoration.
—- Can be laminated or dyed then polished.
— -Joints are injection moulded
—- Hypoallergenic
- 30% lighter than cellulose acetate
— -Memory plastic- softens at 88degrees- need to hold adjustment in place until it has cooled or it will return to its original shape

188
Q

What are some properties of SPX?

A

-— Sides always reinforced
-— Joints are injection moulded
— -Can be dyed or lacquered
— -Adjust at 95oc- beware!
-Shrinks if overheated- ‘cold glaze only’.
-— Impact resistant
—- Flexible
— -lightweight

189
Q

What are some properties of cellulose propionate?

A

-— Sides always reinforced
-— Joints are injection moulded around hinges
-— Can be dyed or lacquered after cutting so where side meets front is smooth
-— Adjust at 67-70oc Beware! Shrinks if over heated so ‘cold glaze’
-— Good stability- but hard to adjust
-— Lightweight
— -Hypoallergenic

190
Q

What are some properties of Acrylic/PMMA?

A

-Sides unreinforced, often metal
-— Joints are pinned
— -Frame front usually laminated, sides dyed- bright colours used
-— Very brittle so only used for supra styles (often upswept)
—- Needs to be heated to a high point before any adjustment therefore hard to adjust

191
Q

What are some properties of carbon fibre

A

— Nylon composite (20% carbon, 80% nylon) often with metal sides
— Joints always pinned
— lacquered
— Cant be heated- so can not adjust and difficult to glaze
— Very inflexible
— Strong
— light

192
Q

What are some properties of cellulose nitrate?

A

Blocks of nitrate sliced into sheets
— Sides always reinforced
— Joints always pinned
— Laminated or crystal (yellows quickly though)
— Beware- highly flammable!! DO not adjust
— Smells of camphor if scratched (if in doubt test in an inconspicuous place)
— Joints can rust and turn green
— Brittle- can crack with age

193
Q

what to consider when looking at metal frames

A
  • Consider the fit- the style of bridge

- — Consider possible adjustments

194
Q

What are some properties of gold?

A

— Gold filled- Fraction of gold content marked in parts per 1000 marked on the bridge.
— Rolled gold- Has a base metal (nickel silver/ bronze) core. Gold content encircled and marked on the bridge or the sides.
— Keeps colour well —Soft
— Resistant to corrosion
— Hypoallergenic unless underlying base metal exposed.

195
Q

properties of nickel silver

A

— Most common metal used for frames- inexpensive
— Contains copper, nickel, zinc and manganese (no silver!) — Parts assembled by soldering
— Colour can be electroplated or lacquered (can wear off) — Easily adjusted
— Flexible
— Some corrosion resistance
— Nickel can react to the skin

196
Q

properties of nickel titanium

A

Memory metal
— 50% titamium
— Only used for bridge and sides (not joints) — 23% lighter than silver nickel

197
Q

What are some properties of stainless steels?

A 
— Assembled by welding
— Strong and flexible
— Coloured by electroplating — Hypoallergenic
— Corrosion resistance
— Often no end tips
— Rigid and flexible
— Can be welded
198
Q

What are some properties of aluminium ?

A

— Less widely used
— Expensive
— Lightweight
— Rigid so hard to adjust and repair (can only be laser soldered)
— Hypoallergenic
— Shiny and can be coloured by anodising
— Parts for frame all cut from one piece of material

199
Q

What are some properties of titanium?

A 
50% lighteer than silver nickel
-hard to repair
-hypoallergenic
-flexible
-corrosion resistant
usually marked
200
Q

what are things to consider for lens materials for children ?

A
  • hard coating- always recommended - comes as standard with cR39- increases durability of lens
  • anti-reflection coating- less often dispensed to children as they may not realise their potential benefits- don’t sit in front computer for long time
  • more suitable for teenagers
  • UV coating
  • tint- can be useful for children when they go on holiday
  • photochromic- not very suitable for children under 16
  • polarised- not very suitable for children under 16
201
Q

what are things to consider with frame selection for paediatric dispensing

A
  • frame fit is very important- if frame does not fit can the correct adjustments be made
  • narrow down the selection by size
  • discuss with the patient and the carer
  • stock up to date brands
  • try and let the child have the final choice and allow them to try on
202
Q

what things to consider for frames for babies ?

A

soft plastic ( nylon )

  • lenses should be flatter
  • flexible
  • moulded
  • no hinges
203
Q

what are some feature of specialist frames for children ?

A
  • spring joints- for older children - people more rough with glasses
  • soft on skin/non irritant
  • grip on back/strap
  • adjustable length to bend
  • soft nose pads
  • variant bridge sizes
204
Q

what are bridge options for infants and toddlers ?

A
  • bridge should conform to child’s nose
    1. moulded bridge
    2. saddle bridge
    3. strap bridge
  • absorbs impact
  • needs less frequent adjustment
205
Q

what is spectacle magnification

A
  • is the ration of the retinal image size in the corrected eye, to that in the uncorrected eye
206
Q

how does a positive lens affect mag

A

magnifies

207
Q

how does a negative lens affect mag

A

decreases mag

208
Q

where is thickness in positive lens ?

A

middle

209
Q

what does it mean if spectacle magnification is greater then 1 ?

A
  • for a hyperope it is greater than 1 (magnification)
210
Q

what does it mean if spectacle magnification is less than 1?

A
  • for a myope it is less than 1 (minification)
211
Q

what are ways to increase spectacle magnification ?

A
  • increase power of F1
  • increase thickness
  • decrease n
212
Q

why is it important to calculate spectacle magnification?

A

why is it important to calculate spectacle magnification?

213
Q

what does anisometropia cause ?

A
  • double vision
  • eye strain
  • headache
214
Q

what are the glasses options for presbyopia?

A
  • single vision lenses
  • single vision lenses allow a patient to see the prescription through the whole lens but only allowed to see one distance at a time

. full lens dedicated to prescription
. two separate pair
. not very useful for people who multitask
. when you have reading prescription on you won’t be able to see anything in the distance

215
Q

what are bifocals?

A

bifocals - focuses at distance and near

  • use distance and near prescription
  • much more convenient for patient’s allowing them to watch tv and read at same time
216
Q

what are trifocals?

A

trifocals - focuses at distance, intermediate and near

- modified form of bifocals which allows to focus at three distances

217
Q

when are bifocals dispensed?

A
  • bifocals are dispensed when there are problems with accommodation
  • usually due to presbyopia
  • can also be due to other problems with accommodation such as in children or if there is an issue with the lens
218
Q

what are the advantages of bifocals ?

A
  • bifocals allow clear vision at any 2 distances
  • wide reading area- can range in size - so can order a wider or smaller segment
  • more convenient than 2 separate pairs
219
Q

what are the disadvantages of bifocals?

A
  • visible line between distance vision and reading area, so indicative of patient’s age
  • smaller seg diameters have limited field of view compared with single vision
  • patients over 55yrs will start to require intermediate correction on top of distance and near
    e.g. computer screen
    may need to recommend trifocal or progressive lens
  • prismatic effect- induced by segment
  • condition called jump -
220
Q

how are bifocals described ?

A

bifocals are described by their segment shape and size

-e.g. D28
D= shape
28 = size - means at its widest point the segment will measure 28mm across

221
Q

what are the measurements we need to take to order bifocal lenses?

A
  • need to measure the patient so that the bifocal segment sits in a comfortable place for reading

. need to record

  • distance binocular PD
  • near binocular PD
  • heights
222
Q

what is the geometrical inset?

A
  • the distance between the distance OC and the midpoint of the segment diameter ( based on the near OC)
223
Q

how to calculate how much inset is required for a patient?

A
  • you need to take away your near PD from your distance PD and divide by 2
  • ( distance - near ) /2
224
Q

how to measure near PD?

A
  • make sure text is held where the patient will read
  • replace with accommodative target
  • measure
225
Q

how to record segment height?

A
  • this measurement is referring to where the top of the bifocal will sit in reference to the lowest tangent of lens
    e. g. 24 mm from lowest tangent
  • lowest tangent can mean different things to different people
  • can mean the point at which the lens meets the frame
  • can mean the bottom of the frame it self so including the depth of frame going around lens
  • can measured directly below where the distance optical centre would be
  • can be measured from lowest point of frame
  • due to this confusion you will be required to use segment top position
226
Q

what does an appropriate segment height mean?

A
  • means that the patient will easily be able to access the segment itself - but its position will not affect the when they’re looking straight ahead
227
Q

what happens if height of segment is too high ?

A
  • if too high - segment interrupts distance vision
228
Q

what happens if height of segment is too low?

A

struggle to get near vision

229
Q

where do we want to place segment ?

A
  • it is best to place the segment at the lower limbus of the pupil
230
Q

when will we want to place segment higher or lower than the lower limbus?

A
  • might want to take segment higher if the patient does a lot of near vision work and is generally slight down
  • might want to take segment slightly lower if the patient is rarely going to use near vision portion and is more concerned with the distance

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