Dispensing Flashcards
What’s the formula to calculate the curvature of a lens?
R (curvature) = 1/r ( radius in meters) (units of m^-1)
What’s the formula for calculating surface power of a lens?
F = (n’ - n) / r(meters) (units of D)
n is what light is going from and n’ is what the light is going into
How to calculate focal length of a lens in (m)
f = n/F (meters)
In air, n = 1
What’s the formula for spherical equivalent?
Sphere power + (cylindrical power / 2 )
How do you find the position of the circle of least confusion?
you do the reciprocal of the spherical equivalent and the unit is m so if you want mm then you divide 1000 by the spherical equivalent
for CR-39 what is
the refractive index
density
abbe number
reflectance as a percentage
1.498
1.3
58
4.0
for crown glass what is
the refractive index
density
abbe number
reflectance as a percentage
1.523
2.5
59
4.3
for polycarbonate what is
the refractive index
density
abbe number
reflectance as a percentage
1.586
1.2
30
5.2
What is the formula for the curve variation factor? CVF? (this helps work out the likely change in thickness)
(n old - 1)/ (n new -1)
how do you calculate transverse chromatic aberration (TCA)?
prismatic effect/ abbe number of the lens material
= P/V
what is the formula for prismatic effect?
distance from the OC (cm) x power of the lens (D) this is prentices rule
whats the formula for reflectance?
p= ((n’-n)/(n’+n))squared
for trivex what is
the refractive index
density
abbe number
reflectance as a percentage
1.532
1.1
46
4.4
whats the formula for deviation?
d= a(n-1) where a is the angle opposite to the right angle of the prism and d
how do you find the prism dioptre using the apical angle?
P = 100 tan d
how do you find the resultant magnitude of a prism?
by doing Pythagoras theorem
what’s the formula to resolve a prims and find the horizontal and vertical components?
H = prism power * cos theta
V = prism power * sin theta
how do you calculate prism at a point on a sphere?
P= distance from optical centre (cm) * power of the lens
how do you calculate ocular refraction?
K (ocular refraction) = 1/k (far point)
how do you calculate dioptric length of the eye?
K’ (dioptric length) = 1 (this number is usually always n which is 4/3) / k’ (axial length)
whats the formula relating ocular refraction to the power of the reduced eye?
K (ocular refraction) + Fe (power of the reduced eye) = K’ (dioptric length of the eye)
what’s the formula to calculate compensated power?
Fnew = Fold / 1+-(d x Fold)
What is the accurate sag formula?
s= r - square root of r squared - h squared
what is the approximate sag formula and when can you use it?
is s is small in relation to y:
r≈ y^2/2s
what is the sag formula that relates to the power of a lens:
s ≈ y^2F / 2(n’-n)
how do you work out centre thickness for
plano convex lens
plano concave lens
plano convex:
t=e+s
plano concave
t=e-s
how do you work out the centre thickness of any meniscus lens?
t = e + s1 - s2
what is the formula for minimum size uncut?
MSU = greatest distance across eye shape + 2* decentration + 2mm for wastage
whats the formula for sag factor?
y^2 / 1000
how do you calculate sag of the entire lens?
sag factor * power of the lens
how do you calculate typical edge of thickness for glass or plastic?
e = 2.0 - (0.2 * power of the lens)
what is the sag formula when solving for r?
(s squared + y squared)/ 2s
whats the formula for CL power?
CL = F/1-dF where d is the BVD in meters and F is the power
whats the formula for lens to centre of rotation?
vertex distance + cornea to centre of rotation
Give a formula for thickness of the anti-reflective coating of a lens
t𝑓 = λ/4𝑛𝑓
-t𝑓 = the thickness of the anti-reflective coating
-lambda is the wavelength of light you want to eliminate
-nf is the refractive index of the film
What is the formula for amplitude condition?
n𝑓 = √𝑛g
-nf = refractive index of film
-ng = refractive index of the lens
What spectacle prescription would you expect for an emmetrope that has now become aphakic?
-spectacles: +10 to +12D
-CL: +15D
-Intraocular lens: +18 to +21D (this one is more dependent on the patient)
what does prentice’s rule state?
The prism power of a lens at any point on its surface is equal to the distance away from the optical centre in centimetres times the power of the lens in dioptres
how do you find the position of the image formed by a lens system made of two thin lenses in air separated by a distance assuming the object is distant
by finding the back vertex power using the equation:
F1+F2-(d.F1.F2) / 1-d.F1
whats the equation for front vertex power?
F1+F2-(d.F1.F2) / 1-d.F2
What is the formula for light transmitted to the eye through a lens?
p= ((n’-n)/(n’+n))^2
where n = 1
what is the formula for sag factor?
y^2/1000 (this tells you the thickness per dioptre)
What is the formula for power of the reduced eye (Fe)?
Fe = (4/3 - 1)/ r
what is the standard value of power for Fe?
60D
what is the standard value for axial length (k’)?
22.22mm
