GP Lens Optics Flashcards
What are 4 benefits of using rigid lenses?
1) Best vision quality
2) Good for astigmatism
3) Good for presbyopia
4) Good for Irregular corneas like keratoconus
How do you find the power of a contact lens in the exploded model? (formula)
1) Find power of front curvature:
F= (n2-n1)/ r (in meters)
2) Find power of back curvature:
F= (n2-n1) / r (in meters)
3) Add the front and back surface powers together.
Front Surface power + Back surface power = total power
If we make the BC of a contact lens flatter than corneal K, the tear film would become more minus. In order to maintain the same power in the contact lens, what must we change on the contact lens?
We steepen the front surface radius of curvature.
If we make the BC of a contact lens steeper than corneal K, the tear film would become more plus. In order to maintain the same power in the contact lens, what must we change on the contact lens?
We must flatten the front surface radius of curvature.
How do we find the total power of the contact lens, tear film optical system
Total power = Contact Lens power + Tear film power
If we change the radius of curvature by 0.1 mm, how much does that change the power by?
0.50 D
If the radius went 0.1 mm FLATTER, how much power would that change?
-0.50D
If the radius went 0.1 mm STEEPER, how much power would that change?
+0.50D
What is the index of refraction we use for the tear film?
n= 1.3375
How do you switch from base curve radius of curvature (mm) to diopters?
337.5/ r (in millimeters)
What Base Curve power do we use to make calculations? (True or clinical)
We use the “Clinical” BC power that uses the keratometer index.
What does the “clinical” base curve power tell us?
Tells us the power of the front of the tear film.
What will the back surface of the tear film always match?
Match the curvature of the cornea
How do you switch from power to radius of curvature(mm) ?
337.5/ Power
Is a low K value flat or steep, more plus or minus?
Low K value = flatter = more minus
Is a high K value flat or steep, more plus or minus?
High K value = steeper = more plus
What K value is more power and steeper?
K: 44.00
K: 42.00
K: 44.00
True or False:
GP lens can correct all of the refractive cylinder?
False, it will only correct the corneal cylinder but not the lenticular cylinder.
How do you find the residual astigmatism?
RA= Refractive Cyl - K cyl
How much residual astigmatism is allowable?
0.75D or less
How much residual astigmatism is not okay?
1.00D or more
What 3 things does an over-refraction help us do?
1) Refine the power
2) Determine the best acuity
3) Find the actual Residual astigmatism
When do we need to vertex an over-refraction? What is the equation?
If over-refraction is 4.00D or more.
(Power)/ 1-d(Power)
d in mm
