Optics Flashcards
For patient wearing RGP CL if you steepen the base curve by 0.75D how should you adjust the contact lens power?
Prescription should increase by -0.75D
** remember SAM FAP Steepen add minus
Flattening the base curve of a lens induces?
Additional minus power of the tear film
**therefore to offset this power change need to ADD plus power to the Rx
**SAM FAP (Flat add plus)
What adjustments can be made to a GP lens if it’s loose and moves too much? (4)
Options:
1. Steepen base curve
2. Increase optic zone diameter
3. Increase overall diameter (OAD)
4. Narrow peripheral curve system or steepen it
Modifications to lens that fits too tight
- Flatten base curve
- Decrease OAD
- Decrease optic zone diameter
- Flatten peripheral curve
- Widen width of peripheral curves
What formula is used to calculate induced prism?
P=dF
d in cm
F in diopters
Tolerance for sphere powers +/- 6.50
+/- 0.13 D
(According to ANSI standards)
What does ANSI stand for?
American National Standards Institute
True or false
More strict tolerance for powers in lower power
True
Error tolerance for cyl powers of 2.00 DC or less
+/- 0.13 D
Over correction of near reading power will result in?
Reduced focal length
Causing patient to bring reading materials closer to
Antimetropia
One eye is myopic, while other is hyperopic
Refractive index of CR-39
1.498
* plastic
Refractive index of crown glass
1.523
Refractive index of Trivex
1.523
Abbe value of polycarbonate
30
* low abbe= high chromatic aberration
Abbe value of trivex
43-45
Abbe value of CR-39
58
Abbe value of crown glass
58
Frame adjustment
If right lens is in
Move right temple in (or left temple out)
Frame adjustment
If left lens is in
Move left temple in (or right temple out)
Frame adjustment
If right lens is out
Move right temple out (or left temple in)
Frame adjustment
If left lens is out
Move left temple out (or right temple in)
If frame not straight on face
Right lens is up how would you fix it?
Bend right temple up (or left temple down)
If frame not straight on face
If left lens is up
Bend the left temple up (or right temple down)
If frame not straight on face
If right lens is down
Bend right temple down or left temple up
If frame not straight on face
If left lens is down
Bend left temple down or right temple up
Drop ball test for dress lenses to be considered impact resistant
5/8 inch steel ball weighing 0.56 oz dropped from distance of 50 inches
Terrien’s marginal degeneration causes what type of astigmatism?
Against the rule (ATR)
True or false
Increasing index of refraction of lens material will allow for decrease in lens thickness
TRUE
* thinner lenses and may also lower specific gravity which minimizes overall weight of glasses
What is the far point when uncorrected?
Assume constant pupil size and relaxed accommodation.
OD: -6.75 DS
** take inverse of subjective refraction
1/ -6.75 =-0.148 or 14.8 cm
- if object is placed at this distance from the eye, pt will perceive a clear image if left uncorrected
Which ophthalmic instrument will provide the most accurate method of assessing spectacle lens warpage?
Lens clock
* used to calculate base curve of spectacle lenses, determine add power of Multifocal lens, determine slab-off prism, calculate thickness of GPCL
Which lens material has the greatest amount of reflection at its incident surface?
Polycarbonate n= 1.586 (refractive index)
Jackson cross cylinder
RX will have spherical equivalent of 0
Example: +1.00 -2.00 x 180
What factors contribute to development/progression of myopia in young children? (5)
- Family history of myopia (especially if both parents are myopic)
- Prolonged near activities on a consistent basis
- Presence of ATR astigmatism
- Children emmetropic or have less than 0.50 D hyperopia at school age
- Long axial length
What is the annual progression rate of myopia in children?
0.25 to 0.50 diopters annually
Marfan syndrome, ROP and Ehlers Danlos syndrome are most associated with what refractive error?
High myopia
Leber congenital amaurosis, cornea plans and nanophthalmos present with what refractive error?
High hyperopia
Normal A-scan length
22-25 mm
Patients with high myopia have axial lengths of ____mm or longer
26.5 mm
Which population have the highest predilection to pathological high myopia?
Japanese
Chinese
Jewish
Arab
* rare in AAs
What is a common characteristic of pathologic myopia related to the eye’s length?
Increased axial length
Increased axial length is a key indicator of pathologic myopia.
What is observed in patients with pathologic myopia concerning the lens?
Premature lenticular pacification
This refers to the early clouding of the lens in myopic patients.