Orthoptics I - optics, refractive error, prisms and lenses

What is light vergence?

the angle formed by rays of light that are not perfectly parallel
- can be divergent (from one point into different directions)
- can be convergent (start from different points and meet)
calculated as the reciprocal of the distance from the point to the source/focus

How are divergent rays vergence calculated?

1/distance between source to focal point
distance is measured backwards and given a negative value

How are convergent ray vergences calculated?

measured in the same direction of light travel = positive value

What is the unit of vergence?

described in diopters
so, the reciprocal of any distance can be described as dioptres as 1D = vergence at 1m

What are the two classes of reflection?

regular or diffuse

What is regular reflection?

when a parallel pencil of rays are incident on a highly polished surface it is reflected in a single definite direction

What is diffuse relfection?

light falling on a less polished surface it is scattered in different directions

What is the Law of Reflection?

the angle of incidence is equal to the angle of reflection on a highly polished surface resulting in a reflected image that is the same size as the true object

What is Snells law of refraction?

a formula used when quantifying the amount of refraction when light passes between two mediums
- n1sintheta1 = n2sintheta2

What is a refractive index?

value assigned to a medium to describe how quick a light would pass through it in comparison to a vacuum

If light were to enter water from the air, and then exit back into the air, how would the angle of emergence compare to the initial angle of incidence?

they would be equal

What is total internal reflection?

a phenomenon in which all the light incident on a boundary is reflected back into the original material

results with any angle of incidence greater than the critical angle

What is the critical angle?

the minimum angle of incidence beyond which internal reflection occurs

How can the critical angle be calculated?

sinthetaC = n2/n1

How do polarising filters/lenses work?

allows one plane of light to pass through
e.g. light reflecting on a body of water has a change vibration preference, becoming horizontal, so polarising lenses are orientated vertically to block the horizontal waves

What is refractive error of the eyes?

the eyes inability to focus light at the fovea, typically due to axial length, corneal curvature and crystalline lens

What are the implications of a high astigmatism?

different powers across different meridians, 90degs apart
- may be interpreted as double vision as one meridian may be more blurred than another

What is accommodation?

change in dioptric power of the lens to maintain focus at different distances
- closer the object of interest = increased accommodation

How are presbyopic patients treated?

convex lenses to allow clear vision at near due to reduced ability to accommodate

How can refractive correction be calculated in dioptres using axial length?

n'/AL x 1000 - 60
e.g.
1.33/22.22 x 1000 - 60 (the power of an emmetropic eye) = -0.2D

What does a prism do?

deviates an incident ray of light (and disperses light) - measured as an angle from the incident ray to the emergent ray

Where is light displaced towards in a prism?

the base of the prism

What is the prism tangent scale?

prism dioptres = 100 tan d
1 cm of displacement at 1m
generally 1PD = 0.57degs of displacement

What is the prentice position?

the patients line of sight is perpendicular to the surface of the prism

What is frontal plane position?

the rear surface of the prism is aligned with the frontal plane of the position
- does not take into consideration the angle of deviation

What is the minimum deviation position?

where the patients line of sight makes an equal angle with each surface position

Why is it important to know what position a prism bar is calibrated for?

if you use the wrong position you could induce the wrong strength

How can prisms be used in ophthalmic lenses?

can be added into the lenses alongside refractive correction - assumes the optical centre is being used to view through otherwise a prismatic effect will be seen (could be done intentionally)

How does the prismatic effect of a decentred prism increase?

increases with the power of the lens and the amount of decentration

What is Prentice's rule?

calculates the amount of induced prism
PD = decentration (cm) x lens power (D)

What are Fresnels prisms?

a flexible plastic sheet consisting of many small prisms that can be stuck onto a lens

What is the maximum strength fresnel prism that can be prescribed?

20D

How do you fit a Fresnel prism?

onto rear surface of glasses with the smooth surface in contact with glass
trace around the lens onto the plastic sheet and cut out
clean the lens and place a small amount of water on the surface of the lens
this will stick the fresnel on, then squeeze out excess water

At what strength does the back vertex distance need to be documented on a spectacle prescription?

+/-5D

How do you calculate the strength of a read add?

1/dioptric power of add
e.g. a 2.50add will be for 40cm

What is lens effectivity?

the change in vergence of light that occurs at different points along its path, calculated as D2/1000 x mm of change

What is back vertex distance?

distance from the back of the lens to the front of the eye
- important as it changes the power of the lens

What is done with the change in effective power of a lens?

added or subtracted to the original lens power

small BVD for a concave lens requires...

less power

large BVD for a concave lens requires...

more power

small BVD for a convex lens requires...

more power

large BVD for a convex lens requires...

less power

What is the BVD of a contact lens?

0 (its literally touching the eye)

What does a retinoscopy cross tell us?

the power needed to correct refractive error in each meridian, objectively

How do you determine a prescription from a retinoscopy cross?

highest plus/least minus = sphere
difference in power between meridians = cylinder
axis from the highest plus/least minus = axis
subtract the working distance from the sphere only (1.5D for 2/3m, 2D for 1/2m)

How do you transpose a prescription from -cyl to +cyl?

add the sph and cyl together = new sphere
flip the sign on the cyl
change axis by 90degs

What material factors impact dispensing of lenses?

optical quality (higher Abbe number = better)
impact resistance
weight (specific gravity)
thickness (impacts refractive index, glass is best but is heaviest)
UV protection

What is a "blank"?

piece of lens that the desired lens shape is cut from
- bigger blank size = thicker

What does the optical centre of the lens need to correspond to?

pupillary distance

How do you ensure the optical lens is aligned with pupillary distance?

through decentration, which requires a larger blank size
- if boxed centre distance (BCD) larger than PD, nasal decentration is needed