Phys And Optical Characteritisics Of Lenses 3 Flashcards
Find the vertical prism induced in the right eye when a patient looks 10mm below the optical center of the following lens
OD +2.00 -1.50x 135
Do the power cross and find where it is at 90
- 25BU
- 25x10/10=1.25
A binconvex lens is clocked with the following properties: Sag of front surface=4.5mm Sag of back surface=3.2mm Diameter=44mm N=1.50 What is the power of each lens surface?
F=(2(n2-n1)S/(h^2)
- 298D
- 6D
Remember to change the -6.6 to a plus since it is BICONVEX
Using the lens clock to differentiate between progressive and single lens
After initial measurement, lens clock can be oriented along the vertical meridian to check for variance in power across the front surface
Total power of the lens when read from a lens clock
Sum of the front and back surface powers, however, this ignores thickness. Can cause the back vertex power of plus Rx to be underestimated when adding surface powers
Lens thickness
Describes the edge or center thickness of a spectacle lens
Minus lens thickness
Thicker in the periphery than the center
A fixed center thickness
Plus lens thickness
Thicker in the center than edges
A fixed peripheral thickness
Approximate sag formula
S=h^2/2r
What equation is used to estimate the sag from the surface power without calculating radius
S=(Ph^2)/(2(n-1))
Front surface sag
S-(PH^2)/(2000(n-1))
Back surface sag power
(PH^2)/(2000(1-n))
What sign are Tc and Te always
Positive
If there is a flat surface, than S1 or S2 will be
0
Change in thickness for plus lenses
It is positive. Center thickness is greater than edge thickness
- h is half the lens diameter
- BC or lens form have no impact on this formula
-5.00D N=1.6 Diameter=56mm Center thickness=1.5mm Calculate edge thickness
-5x28^2/2000(1.6-1)=-3.3mm
Must make it positive so it is 3.3mm
3.3+1.5=4.8mm
Calculate maximal edge thickness Rx=-2.00 -2.00 X 146 Center thickness=2mm Diameter=52mm N=1.42 Assume optical center is at geometric center
Want the thickest to be at the highest power meridian so doing a lens cross, what would make -4.00 our highest meridian
(-4.00(26^2))/(2000(1.42-1))=-3.219
Make it positive 3.219
3.219+2=5.219
Calculate center thickness Rx=+5.00 -2.00 x 49 Minimum edge thickness=2mm Diameter=69mm N=1.7 Assume optical center is at geometric center
+5.00 is our highest power meridian based on the lens cross
+5(34.5^2)/2000 (1.7-1)=4.25
4.25+2= 6.25mm
What are the 3 factors affecting lens thickness
Lens power
Lens diameter
Refractive index
Lens power and lens thickness
As lens power increases, thickness increases
Lens power and RI
As RI increases, thickness decreases
Eyesize and lens thickness
If eyesize increases, thickness increases
Base curve and thickness
increased BC increases thickness
Changing the base curve for a -8D lens will not change
The amount of light from the front surface
-it will change the edge lens thickness, the retinal image size, the oblique astigmatism
Rx=-5.00 -4.00 x 45
N=1.65
Diameter=54mm
Central thickness=1mm
What is the power in the horizontal meridian?
What is the thickness at the temporal edge?
P=S+C(sintheta)^2
-5+(-4x0.5)=-7.00D x 180
- 7(27^2)/2000 (1.65-1)
- 3.9mm
3. 9mm+1MM=4.9mm
Rx=1.00 -3.00 x 180 Diameter=60mm N=1.5 What is the power at 30 degrees? What is the edge thickness at 30 degrees
-1+(-3x 0.25)=-1.75D
- 1.75(30^2)/2000(1.5-1)
- 1.575
1. 575+1=2.575mm
Rx=-3.75 -2.50 x 120 Center thickness=2mm Diameter=50mm N=2.6 Calculate the thickness at the temporal edge of the lens, assuming the optical center is at geometric center
Calculate the max edge thickness
S+C(sintheta)^2
-5.625 x 180
(-5.625(25^2))/2000(1.6-1)=2.93
2.93+2=4.93mm
-6.25 is the highest powered meridian
Plug into equation and get 3.26+2=5.26mm
Isothickness curves
Curves drawn on a power cross to show the curves on which thickness is the same
- two regions on the same curve will have the same thickness
- lines close together=thickness is changing quickly
What does a spherical lens look like on an iOS thickness curve?
Circular and centered on the optical center
What does a cylindrical lens look like on isothickness curves
Straight lines parallel to the axis
What does a spherocylindrical lens look like on isothickness curves
Circles, ellipses, hyperbolas, or straight lines
Consists of small rows of equally powered prisms and yielding the same optical effect as that of a conventional prism, but less than a mm in thickness
Fresnel prism
What are fresnel prisms useful for
Short term treatment of binocular vision problems and for evaluating whether prism can eliminate symptoms related to vertical imbalance
Formula for combining prisms
- if prisms are perpendicular, use Pythagorean theorem,
- if both base up, take the difference between the two
What is the total prism that results from combining 2 BU and 1 BO
Square root of (2^2 +1^2)
2.23PD
What is the total prism that results from combining 2 BU and 1BU
1 BU
Calculating thickness of a Plano prism
Thickness of a prism varies with distance from the apex
The change in thickness:
(Zh)/(100(n-1))
Using thickness difference to determine prism power
- rearranging the previous formula to get the prism thickness
- the geometric center of the concave lenson the right contributes no prism. Thus, prismpower can be determined by the difference in edge thickness using the abovefiormual
Calculate horizontal prism at geometric center Rx=-5.75Ds Diameter=50mm N=1.6 Temporal edge thickness=2mm Nasal edge thickness=5mm
Z=(100x3x(1.6-1))/(50)
3.6 PD
Vertical prism
Must take into account the amount of vertical prism induced into each eye
If the amount of vertical prism in each eye is different
There is a vertical imbalance or binocular prism included
-result of having differencevertical primsatic effects in each eye at a give gaze position
To find vertical imbalance
Find the difference in the amount of vertical prism in each eye
A patient has a RX calling for OD +5.ooD and +3.00D spherical lens, how much vertical imbalance is induced when the patient looks down 10mm to read?
5x10/10=5BU
3x10/10=BU
Net 2 BU
