ABO numbers, math, formulas
Prentice's Rule - ANS P = f x c
P = amount of prism correction
f = lens power (in diopters)
c = decentration (the distance between the pupil centre and the lens's optical centre, in
millimeters)
Cornea IOR - ANS 1.37
Tear Film, Vitreous IOR - ANS 1.33
Crystalline lens IOR - ANS 1.42
Optical Standard index of refraction - ANS 1.53
Crown Glass index of refraction - ANS 1.523
High index IOR - ANS 1.74
CR-39 IOR - ANS 1.49 - 1.50
Poly index of refraction - ANS 1.586
Air IOR - ANS 1.00
How fast does light travel through air - ANS 186,000 mi/sec
Minimum Blank Size (MBS) formula - ANS (GCD - PD) + ED
A + DBL + ED - PD
Frame PD - PD + ED
Dec. + ED
GCD = geometric center distance or A + DBL
PD = Pt. PD
ED = effective diameter
Dec. = Decentration
Focal length formula, diopters known - ANS 1/Dioptic Power
Dioptic Power formula, foci known - ANS 1/Focal Length (m)
Anisometropia - ANS Difference of 2 diopters between OD & OS
, What is the Radius of a 1 diopter curve? - ANS 530mm
Radius formula - ANS 530/Power = R
Power formula - ANS 530/Radius = Diop.
Vertical Decentration formula - ANS Seg.ht. - B/2
OC height - (B measurement / 2)
How much HORIZONTAL prism tolerance is allowed? - ANS 2/3 of a diopter
-or-
0.667
How much VERTICAL prism tolerance is allowed? - ANS 1/3 of a diopter
-or-
0.333
Base Curve tolerance - ANS +/-0.75
Sphere Tolerance up to 6.50 - ANS +/- 0.13
Sphere Tolerance above 6.50 - ANS +/- 2%
Cylinder Axis Tolerance >1.50 - ANS +/-2°
Cylinder Axis Tolerance >0.50 / less than or equal to 1.50 - ANS +/-3°
Cylinder Axis Tolerance >0.25 / less than or equal to 0.50 - ANS +/-7°
Cylinder Axis Tolerance >0.00 / less than or equal to 0.25 - ANS +/-14°
Drop ball test for Safety lenses - ANS 5/8ths in. diameter, weighing 0.56 oz. dropped 50ft
Frame PD - ANS A box + DBL
Total decentration - ANS Frame PD - Patient PD
A + DBL - PD (divide by 2 to get decentration per lens)
Base IN (MINUS) - ANS If lens PD is GREATER than pt. PD
Base OUT (MINUS) - ANS If lens PD is LESS than pt. PD
Thindex glass IOR - ANS 1.8
Prentice's Rule - ANS P = f x c
P = amount of prism correction
f = lens power (in diopters)
c = decentration (the distance between the pupil centre and the lens's optical centre, in
millimeters)
Cornea IOR - ANS 1.37
Tear Film, Vitreous IOR - ANS 1.33
Crystalline lens IOR - ANS 1.42
Optical Standard index of refraction - ANS 1.53
Crown Glass index of refraction - ANS 1.523
High index IOR - ANS 1.74
CR-39 IOR - ANS 1.49 - 1.50
Poly index of refraction - ANS 1.586
Air IOR - ANS 1.00
How fast does light travel through air - ANS 186,000 mi/sec
Minimum Blank Size (MBS) formula - ANS (GCD - PD) + ED
A + DBL + ED - PD
Frame PD - PD + ED
Dec. + ED
GCD = geometric center distance or A + DBL
PD = Pt. PD
ED = effective diameter
Dec. = Decentration
Focal length formula, diopters known - ANS 1/Dioptic Power
Dioptic Power formula, foci known - ANS 1/Focal Length (m)
Anisometropia - ANS Difference of 2 diopters between OD & OS
, What is the Radius of a 1 diopter curve? - ANS 530mm
Radius formula - ANS 530/Power = R
Power formula - ANS 530/Radius = Diop.
Vertical Decentration formula - ANS Seg.ht. - B/2
OC height - (B measurement / 2)
How much HORIZONTAL prism tolerance is allowed? - ANS 2/3 of a diopter
-or-
0.667
How much VERTICAL prism tolerance is allowed? - ANS 1/3 of a diopter
-or-
0.333
Base Curve tolerance - ANS +/-0.75
Sphere Tolerance up to 6.50 - ANS +/- 0.13
Sphere Tolerance above 6.50 - ANS +/- 2%
Cylinder Axis Tolerance >1.50 - ANS +/-2°
Cylinder Axis Tolerance >0.50 / less than or equal to 1.50 - ANS +/-3°
Cylinder Axis Tolerance >0.25 / less than or equal to 0.50 - ANS +/-7°
Cylinder Axis Tolerance >0.00 / less than or equal to 0.25 - ANS +/-14°
Drop ball test for Safety lenses - ANS 5/8ths in. diameter, weighing 0.56 oz. dropped 50ft
Frame PD - ANS A box + DBL
Total decentration - ANS Frame PD - Patient PD
A + DBL - PD (divide by 2 to get decentration per lens)
Base IN (MINUS) - ANS If lens PD is GREATER than pt. PD
Base OUT (MINUS) - ANS If lens PD is LESS than pt. PD
Thindex glass IOR - ANS 1.8
