Thickness Flashcards
What are the factors affecting lens thickness?
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)
What are the lens shape or frame?
influences the overall thickness partially because of the size of lens we use and how it sits on the patient
What are spectacle lenses cut from?
pre-made lens blank
What are blanks/uncuts?
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
What is the minimum size of cut?
-The smallest one of the blanks
what does the choice of diameter depend on?
prescription and frame choice.
What happens when you have. positive lens (i.e +6.00D) and have 2 uncut lenses one big and one small?
- 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 .
What do the blank sizes go up in?
5mm
What does the range of lens depend on?
- 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
What happens when you cut lens (size) from the same lens size diameter?
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
What is decentation?
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
what is a box centres?
frame measurement
what is a optical centre?
lens measurement
what happens if the patient is not looking through the optical centre?
- image is not sharp
- experience prismatic effect
What happens if optical centre is not at the same place as box centre?
- need lab to decentre
- by then finding out PD measurement which should coincide with the optical centre
What happens from decantation?
doesnt have to be moved
-alter the thickness of the lens
What happens when you cut a lens centrally?
thickness will be equal on Both sides
What happens if you have to shift the frame as having to move optical centre?
- thickness is different on both side- and don’t want this decentration to occur
How to calculate horizontal decentration ?
Decentration = (Distance between lenses - patients PD/distance )/2
what is the distance between lenses?
box centre
What is a box centre?
the centre of the frame of each lens
When do we decentre IN the lens?
if the PD is smaller than the box centre
When do we decentre OUT the lens?
if the PD is bigger than the box centre
What does decentration effect?
the minimum size uncut- smallest blank size you can USE FOR FRAME
How do you calculate the smallest lens you can use (MINIMUM size uncut)?
MSU = (mono decentration x 2) + ED + 2mm ED= measure to widest point of lens
How can we make lens thinner?
by choosing right frame
How do we calculate lens thickness?
we use sagitta (or sags)
What is the sag?
the depth of a curve at a fixed diameter/chord (diameter of lens)
What can we use the value of sag to do?
calculate edge thickness (e) or either centre of thickness (t)
So if we were to use a positive lens, what should we calculate?
-centre of thickness (t)
So if we were to use a negative lens, what should we calculate?
-edge thickness (e)
What does F1 have to do with?
S1
What does F2 have to do with?
S2- back surface
What is y?
radius
What is 2y?
diameter
What is r?
radius of curvature
What is the r equation?
r1= (1000(n-1)/F1 r2= (1000(1-n)/F2
What are the 2 ways to calculate sag?
- accurate
- approximate
What do we do to calculate sag?
We tend to be accurate-
What is the accurate sag formula?
s= r - √r2 - y2
What is the approximate sag formula?
s = y2F/ 2000 (n’ - 1)
- ONLY takes into account the overall power of lens
- doesnt takes into account what curve is on what surface
What is the equation for the centre of thickness for a minus and positive spherical round lens?
centre thickness of a Plano convex lens= t= e + s
centre of thickness of a Plano conch lens= t = e-s
What is the equation for edge thickness of a positive and minus spherical round lens ?
edge thickness for a Plano convex lens = t - s
edge thickness of a Plano concave lens = e= t + s
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
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 .
What is the equations for centre of thickness of a minus meniscus spherical round lens?
concave= t = e - (s2 -s1)
What is the equations for edge thickness of a curved minus meniscus spherical round lens?
e= t + s2- s1
What is the equations for centre of thickness of a convex meniscus spherical round lens?
t= e+S1- S2
Edge thickness of a convex meniscus ?
e= t- (S1- S2)
What is the difference from now using a miniscus lens?
have to first calculate S1 AND S2 before you use the equations
What do we need to calculate s1 and s2?
radius of curvature
y
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.
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
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
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
When you are calculataing accurately what does this mean?
you find out S1 AND S2
What happens if the shape of the lens is not round?
need to take into account the shape of the lens
y is going to change.
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.
- 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
- 55- √90.55^2-23^2 =
- 97mm
e=t+s
2.2+2.97= 5.17mm
When does the power of the lens changes:?
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
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.
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
What is the base curve?
back surface
What is the sphere curve?
front surface
What happens if your frames are not symmetrical?
If the frame is not symmetrical then y will change for different meridians- check which one you will need
What happens if your frames are not symmetrical?
If the frame is not symmetrical then y will change for different meridians- check which one you will need
