Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

DISPENSING > Thickness > Flashcards

Thickness Flashcards

1
Q

What are the factors affecting lens thickness?

A 
Lens thickness can be related to CENTRE thickness or EDGE thickness.
Ø  Back vertex power (Fv’)
Ø  Surface powers
Ø  Refractive index
Ø  Lens form
Ø  Lens shape
Ø  Decentration
Ø  Minimum size uncut
Ø  Minimum centre/edge thickness (mechanical stability)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What are the lens shape or frame?

A

influences the overall thickness partially because of the size of lens we use and how it sits on the patient

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What are spectacle lenses cut from?

A

pre-made lens blank

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What are blanks/uncuts?

A

these lens disc which haven been cut yet so can be pitched to be in any frame
-are made in a range of different diameters

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is the minimum size of cut?

A

-The smallest one of the blanks

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

what does the choice of diameter depend on?

A

prescription and frame choice.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What happens when you have. positive lens (i.e +6.00D) and have 2 uncut lenses one big and one small?

A
  • The width of the lens frames are the same however the lens thickness is different.
  • with positive lenses - if you have the smallest lens size (uncut lens) it would affect the thickness without changing any change in materials .
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What do the blank sizes go up in?

A

5mm

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What does the range of lens depend on?

A
  • the lab which you are sourcing the lens from
  • the material that the lens been made from
  • majority they are range from 15mm-70/75mm
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What happens when you cut lens (size) from the same lens size diameter?

A 
—  For a negative lens –
A larger lens size has
thicker edges
A smaller lens size has thinner edges
  For a positive lens –
A larger lens size has thinner
edges
A smaller lens size has thicker edges
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is decentation?

A

relationship of the box centre of the frame and the optical centre of the lens

  • you want the patient eye to sit at the box centre
  • optical centre nd box centre should coincide
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

what is a box centres?

A

frame measurement

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

what is a optical centre?

A

lens measurement

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

what happens if the patient is not looking through the optical centre?

A
  • image is not sharp

- experience prismatic effect

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What happens if optical centre is not at the same place as box centre?

A
  • need lab to decentre

- by then finding out PD measurement which should coincide with the optical centre

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What happens from decantation?

A

doesnt have to be moved

-alter the thickness of the lens

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What happens when you cut a lens centrally?

A

thickness will be equal on Both sides

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What happens if you have to shift the frame as having to move optical centre?

A
  • thickness is different on both side- and don’t want this decentration to occur
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

How to calculate horizontal decentration ?

A

Decentration = (Distance between lenses - patients PD/distance )/2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

what is the distance between lenses?

A

box centre

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What is a box centre?

A

the centre of the frame of each lens

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

When do we decentre IN the lens?

A

if the PD is smaller than the box centre

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

When do we decentre OUT the lens?

A

if the PD is bigger than the box centre

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

What does decentration effect?

A

the minimum size uncut- smallest blank size you can USE FOR FRAME

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

How do you calculate the smallest lens you can use (MINIMUM size uncut)?

A 
MSU = (mono decentration x 2) + ED + 2mm
ED= measure to widest point of lens
26
Q

How can we make lens thinner?

A

by choosing right frame

27
Q

How do we calculate lens thickness?

A

we use sagitta (or sags)

28
Q

What is the sag?

A

the depth of a curve at a fixed diameter/chord (diameter of lens)

29
Q

What can we use the value of sag to do?

A

calculate edge thickness (e) or either centre of thickness (t)

30
Q

So if we were to use a positive lens, what should we calculate?

A

-centre of thickness (t)

31
Q

So if we were to use a negative lens, what should we calculate?

A

-edge thickness (e)

32
Q

What does F1 have to do with?

A

S1

33
Q

What does F2 have to do with?

A

S2- back surface

34
Q

What is y?

A

radius

35
Q

What is 2y?

A

diameter

36
Q

What is r?

A

radius of curvature

37
Q

What is the r equation?

A 
r1= (1000(n-1)/F1
r2= (1000(1-n)/F2
38
Q

What are the 2 ways to calculate sag?

A
  • accurate

- approximate

39
Q

What do we do to calculate sag?

A

We tend to be accurate-

40
Q

What is the accurate sag formula?

A

s= r - √r2 - y2

41
Q

What is the approximate sag formula?

A

s = y2F/ 2000 (n’ - 1)

  • ONLY takes into account the overall power of lens
  • doesnt takes into account what curve is on what surface
42
Q

What is the equation for the centre of thickness for a minus and positive spherical round lens?

A

centre thickness of a Plano convex lens= t= e + s

centre of thickness of a Plano conch lens= t = e-s

43
Q

What is the equation for edge thickness of a positive and minus spherical round lens ?

A

edge thickness for a Plano convex lens = t - s

edge thickness of a Plano concave lens = e= t + s

44
Q

A +7.00DS lens circular plano convex blank is 70mm in diameter, n=1.5.
— Determine the centre thickness if it is knife-edged using the approximate sag formula

A

s = y2 F 2000 (n’ – 1)
35^2 x 7.00/ 2000(1.5-1)
= 8.58mm
knife edged means the lab has made it so there is no edge thickness - use approximate as e = 0 .

45
Q

What is the equations for centre of thickness of a minus meniscus spherical round lens?

A

concave= t = e - (s2 -s1)

46
Q

What is the equations for edge thickness of a curved minus meniscus spherical round lens?

A

e= t + s2- s1

47
Q

What is the equations for centre of thickness of a convex meniscus spherical round lens?

A

t= e+S1- S2

48
Q

Edge thickness of a convex meniscus ?

A

e= t- (S1- S2)

49
Q

What is the difference from now using a miniscus lens?

A

have to first calculate S1 AND S2 before you use the equations

50
Q

What do we need to calculate s1 and s2?

A

radius of curvature

y

51
Q

Worked example
— A +9.00DS meniscus lens is made in crown glass (n-=1.523) with a 60mm diameter and 1mm minimum edge thickness. The base curve is -3.00DS. Accurately calculate the centre thickness.

A

— Find the nominal front curve +12.00DS

Find front curve (S1)
Find S1 = r - √ r2 - y2
—  r1 = 1000(n-1)/ F1
 = 1000(1.523-1)/ 12 = 43.58mm
—  S1= 43.58 - √43.58^2 – 30^2 = 11.97

Now find S2

r2= 1000(1-n) /F2
= 1000(1-1.523)/ -3
=174.333mm
S2 = 174.33mm - √174.33^2 - 30^2
=2.60

use: t= e+S1- S2

t= 1 + 11.97 -2.60= 10.37mm thick

52
Q

Worked example
— A -6.00DS lens is made with a +4.00 front curve with a 48mm diameter. The refractive index is 1.523 and the centre thickness is 1.2mm. Using the accurate equation find the edge thickness.
— Find the nominal back curve -10.00DS

A

S1

r1= 1000(1.523-1)/4
=130.75mm
s1= 130.75mm - √130.75^2- 24^2
=2.22mm

r2= 1000(1-1.523)/-10
=52.3mm
s2= 52.3mm -√52.3^2-24^2
=5.83mm

use : e=t +s2-s1

e= 1.2 + 5.83 - 2.22
= 4.81mm

53
Q

When you are calculataing accurately what does this mean?

A

you find out S1 AND S2

54
Q

What happens if the shape of the lens is not round?

A

need to take into account the shape of the lens

y is going to change.

55
Q

Worked example
— Mrs Jolie has chosen an oval frame where the lenses are 52x46. The lens –5.50D is made in plano concave form in CR39 (n=1.498). If the centre thickness is 2.2mm, find, accurately, the maximum and minimum edge thickness of this lens.

A
  • This lens is a minus lens so what will overall have the thickest part to it- the horizontal because it shows on the diagram (check slide) and the widest cut out blank is always the thicker at the edges in a minus lens.
  • radius of curvature is the same in this example as power of lens does not change

use: e= t + s
t= 2.2mm
NEED TO FIND OUT s1 and s2

Sag horizontal

r= 1000 (1-1.498)/ -5.50
=90.55mm
s=r-√r2 -y2 
90.55-√90.55^2 –26^2=3.81mm
 e=t+s=2.2+3.81=6.01mm

Sag vertical

— s=r-√r2 -y2 —

  1. 55- √90.55^2-23^2 =
  2. 97mm

e=t+s
2.2+2.97= 5.17mm

56
Q

When does the power of the lens changes:?

A

if a toric lens

  • radius of curvature is different from one to another- we know that toric lenses have 2 powers which are 90 degrees apart.
  • still use same equations but more time consuming
    • This means that there are two sags of the back surface, s2 base and s2 cross
57
Q

Worked example
— Accurately calculate the thick and thin edges of the following lens made in crown glass (n=1.523) with a diameter of 60mm and a minimum centre thickness of 1mm.
Ø -6.00/-2.00 x 90 Sphere curve +4.00DC.

A

Ø Toric transpose the prescription
Ø Calculate sags for F1 and F2
Ø Calculate e thick and e thin using e= t + s2- s1

-When you toric transpose sphere curve is F1 and base curve is F2 and then also cross curve is F2

r1= 1000(1.523-1)/4
=130.75mm
s1= 130.75 - √130.75^2 - 30^2
= 3.49mm

r2= 1000(1-1.523)/-10.00
=52.3mm
s2= 52.3 - √52.3^2-30^2
=9.46mm

r2= 1000(1-1.523)/ -12.00
=43.58mm
s2= 43.58 -√43.58^2-30^2
=11.97mm

e thin = e= t + s2- s1
1+9.46-3.49= 6.97mm

e thick = e= t + s2- s1
1+11.97 - 3.49= 9.48mm- thickest one is bigger number

58
Q

What is the base curve?

A

back surface

59
Q

What is the sphere curve?

A

front surface

60
Q

What happens if your frames are not symmetrical?

A

If the frame is not symmetrical then y will change for different meridians- check which one you will need

61
Q

What happens if your frames are not symmetrical?

A

If the frame is not symmetrical then y will change for different meridians- check which one you will need

DISPENSING (19 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions