6: Prims .

why do we prescribe prism

to help a patient with heterophoria-

to correct fixation disparity 

to reduce double vision in some types of heterotropia 

to help with convergence insufficiency - headahces until double vision

to enalbe single binocular vision in a high power spectacle magnifier 

if seeing double vision, prism is used to move the images until can see single binocular vision

cross sectional view through a prism 

made from certain refractive material n’deviates light by certain angle- d 

effect on image position when looking through a prism

prisms deviate light towarda the base

the image appears to move towards the apex of prism

relationship between a, d , and n’

Deviation angle= (n’-1)a

equation useful for prsims where apical angle is 10 degrees or less

e.g a prism made from crown glass n’ = 1.523 with an a[ical angle of 10 degrees

d= (1.523-1) x 10 = 5.23 degrees

to work out eact deviation angle can use snells law

a prims made from a material of refractive index 1.523 has an apical angle of 2 desgrees. what is the deviation angle of the prism

deviation angle = (n’-1) a

(1.523-1) x 2 = 1.046

1.005 degrees

the prism dioptre 

can convert from deviation angle to prism power in dioptres 

a prism power of 1 will deviate light by 1 unit at a distance of 100 units, e.g 1cm deviation at 1 metre 

prism power = 100 x tan x d

prism dioptre calculations

e.g deviation angle is 2 degrees

prism power = 100 x tan(2)

prism power= 3.49

thickness of different prisms

gives the relationship between the thin and thick edges of a prism of given power, diameter and refarctive index - the workshop prism formula

another way of calculating prism power

prims thickness difference mm, g = thick egde - thin edge

P = prism power in prism dioptres

P = 100 (n’-1)g / D

g= PD / 100(n’-1)

worked example of thickness difference in prisms:

calculate the power P of a prism of 38 mm diameter made from opthalmic crown glass (n’ = 1.523) with a thinn edge of 3mm and a thick edge of 7mm

prism thickness difference, g = 7-3 = 4mm 

P = 100(n’-1)g / D 

P= 100 x (1.523-1) x 4  / 38 

P = 5.51 

calculate the prism thick ness difference , g of a 4 prism power of a 38mm diameter made from opthalmic crown glass of 1.523

g= PD / 100(n’-1)

g= 4 × 38 / 100(1.523 - 1)

g = 2.91 mm

so if the thin edge was 2mm the thick edge would be 4.91 mm

form of opthalmic prisms

can be produced in a curved meniscus form

form of opthalmic prisms - prism in a flat form 

these are generally trial case prisms and prism bar elements 

form of opthalmic prisms = curved form

this is the sort of prism that would be incorporated into the spectacle lens

measurement of prismatic power : hand neutralisation

whats done when we look through a trial case prism of known power

prism on top of cross, and seen that the horizontal line has been pushed away from base marking towards th apex

vertical line is unchanged

if horizontal line moves i mens the vertical displacement occured

usully only see horizontal line moving in base up or base down when prisms base is oreintated vertically

prims rotated it to be oblique 

base apex line is along the 45 degree marking 

horizontal and vertical line displaced 

hand neutralisation

we can hand neutralsie a prism by having a set of known powered prisms ad placing them over unknown prism until there is no displacement

base up down : vertical prism

vertical displacement occurs so horizontal line moves

base in out ; horizontal prism

horizontal displacement occurs so the vertical line moves

tangent scale- measurement of prismatic power

a scale placed some distance from the prism under test

displacement of the markings on a tangent scale indicates the prism power

see how much displacement

prism measurement by focimetry

an eye piece focimeter contains an astronomical telescope. this inverts the image, so base down prism will move the image in the eyepeice downwards, nnot upwards

first image shows no displacement

in second image, ring is upwards, so it would be base UP, has 2 prism dioptres 

final image shows horizontal prism, its 2 prism dioptres 

HORIZONTAL prism

on image on right, if dealing with a right eye it would be 2 prism dioptres base OUT

if left eye, it would be 2 prism dioptres base IN

all right eyes

draw line from middle of target and see where it meets the ring

for 1. just more than 2 prism dioptres base DOWN , and 2 prism dioptres base OUT

2. 4 prism dioptres base up, 3 dioptres base IN

3. 2.5 prism dioptres base UP, 2 base OUT

prism measurement by focimetry- use of loose trial case prisms

prim thinning used on vary focal lenses even if prism is not neede d

loose trial case prims can be placed into the optical path of focimeter to move the image of the target to a more convenient location 

e.g to centre the image in a progressive addition lens with prims thinnning 

prism thinning affs ase down to a PAL 

prism thinning

if a lot of prism thinning is needed a trial case prism is used to neutralise the thinning so an accurate measurement can be made

example in picture shows 2 base down; a normal value for prism thinnng in a progressive addition lens

a prism compensator sits behind focimeter telescope and can dial up howveer much prism you want. used to also centre the image

why do we use prism thinning for those who dont have prism

when making progressive lenses, the lens is thicker at the bottom becase the near vision part adds more curvature , hich can make it look thicker and hevaier at bottom 

by adding small equal amounts of base down prim the optical cenre is slightly shifted to reuce thickness difference 

standard notation for base oreintation

triangle should be patients nose

base ou tin the RE would have an axis notation of 180 degrees, and base out for the ledt eye would be 360 degrees

how do we provide prism in a spectacle lens?

prism power can be induced into a single vision lens by:

• working the prismatic element into the lens (worked prism)- forming a wedge onto lens. only done if cant decentre 

•  by decentering the lens - moving optical centre to another position

the lens has to have a sufficient refractive power for this to be possible 

what are the reasons why we cant decentre the lenses 

if its a vary focal or bi focal 

if its aspheric 

or if theres not enough refractive power in the lens to achieve the amount of prism u want 

effect of decentration of an opthalmic lens

when a patient looks through a point other than the optical centre of the lens, a degree of prismatic power will be induced as light rays bend , prentices rule can be used to calculate this

opictal centre- point where light passes undeviated

minus lens (concave) decentration effect

iif optical centre i sin the mddle

if the pateint looks upward, theyrenow looking through the upper part of the lens which acts like a base up prism

this means image is shifted downward toward the apex

prentices rule 

P = cF 

primsatic power is equal to the decentration x refractive power of lens in that meridian 

c is measured in cm 

decentration- where the lens is moved 

lens is moved downwards by 0.8cm 

base direction of positive lens

this lens is 2 prisms with the bases together

base down and out are positive

base direction of negative lenses

considered as 2 prisms but their bases are opposite, instead their apexes are together

• base up and in are negative 

applies to decentration and base direction from calculation 

prism is toward the apex opposite the decentration

decentering a positive power lens

if we decentre the positive lens

should induce a base down prism

upper part of positive len behaves as a base down

lower part of the lens behaves like base up

apex of prism goes down which is base down prism

decentering a negative  lens 

will induce a base up prism 

since the lens is moved downward, eye is looking through the upper part

rays entering the eye are deviated downwards so image is upward on the retina, making it base up

positive lenses and relationship to direction in prism

decetering a lens in a given direction moves the base of the prism in that direction 

eg upwards decentration gives base up inwards decenteration gives base in 

out decentration gives base out 

negative powered lenses and relationship to direction in prism

decentering a lens in a given direction moves base of prism in opposite direction 

eg

upwards decentrartion gives base down 

horizontal prism 

positive = base out 

minus = base in 

vertical prism

positive = base down 

minus = base up 

decentering: R +2.00 / -4.00 × 180

decentering this lens inwards gives a base in prism

decentering the prism upwards induces base down prism as in the meridian the power is -2

prismatic effect of cylindrical and sphero cylindrical lenses

prismatic effects due to decentration of cylindrical and sphero cylindrical lenses requires careful consideration

plano / -3.00 × 180

decentering this lens vertically will induce prism according to prentices rule 

decentering the lens horizontally i.e along the axis of the cylinder , will induce no prism as F is plano along horizontal line 

decentering this lens along some other orientation will induce prism power,

+4.00 / -1.00 × 180

determine cross cyl of the lens

F in vertical meridian = +3.00

F in horizontal meridian = +4.00

working out prisms for cylinder axes that are not 90 or 180 ; oblique cylinders

e.g decentering plano / -3.00 × 160 horiznotally or vertically

-3.00 acts along 70

if this lens was decentred vertically, a smaller vertical prismatic effect than -3.00 decentration will be induced

a horizontal prismatic effect will also be induced

prismatic effect of a decentred sphero cylindrical lens 

calculates how much prism youre getting horizontally and vertically when cyls are not 90 or 180 

out is plus, in is minus

xS is prentices rule 

first part of formula is for cylinder as its not on its principal meridian

worked example- consider R: +4.00 / -1.00 × 5 decentered upwards by 2cm ad out by 1.5cm. calculate induced prism

horizontal prism:

feta for right eye will be 180- cyl axis = 180 -5 = 175

x = +1.5 and y is -2H = (1.5 x sin 175 + (-2) x cos 175)  sin 175 x (-1) + 1.5 × 4.00 

H = +5.81 

as positive horizontal its base out so its 5.81 base out 

roughly similar to the power along horizontal axis (4)

so to see if correct use 4 × 1.5 = 6 which is close enough to 5.81

R +4.00 / -1.00 × 5 decentred upwards by 2cm and outwards by 1.5cm. calculate induced prims of vertical prism

S= +4.00

C= -1.00

feta = 180 - 5 = 175 as its a right lens

x= +1.5 y = -2

(1.5 x sin 175 + (-2) x cos 175 x (-1.00) + (-2) x 4.00

V = -5.88

minus vertical so base up

prismatic effects of oblique cylinders

displaced vertically

so biggest prismatic effect is verticallu, only some horizontal is induced

working out how much decentration is needed for a given prism power, for when cylinder axis is not at 90 or 180

worked example numer 1 for horizontal 

worked example number 1 vertical

upwards decentration givees base down prism

prismatic effects of complete spectacles

for higher prescriptions, tilt of a spectacle frame can induce significant amounts of unwanted prism, leading to vertical diplopia

consider a patient with R -10.00 DS, L -10.00 DS. Their frame is bent accidnetly resulting in upwards decentration of the right lens by 3mm and downwards deventration of left lens by 3mm 

calculate the prismatic effect 

P = cF 

0.3 x -10.00 = 3 base down for the right eye, and 3 prism power base up for the left eye 

vertical differential prism = 6 

yhis would most likely produce vertical diplopia 

consider a patient with R +2.00 DS and L +1.50 DS

the frame is bent, resulting in decentration of the right lens upwards by 3mm and decentration of L lens downwards by 3mm. calculate the vertical prismatic effect

P = cF

RE = 0.3 × 2 = 0.6 base up

LE = 0.3 × 1.5 = 1.05 base down

total vertical prismatic effect = 0.6 + 0.45 = 1.05

dividing prism ; how prism is prescribed when dispensing

if all prism is in the right eye it would look very thick at the bottom as 8 up

right lens will be heavier than left

prism power can be divided between the eyes to even out the wight and cosmetic appearance of the 2 lenses

solution could be:

R -1.00 DS 4 base up

L -0.75/ -0.25 × 175 4 base down

rules for dividing prism

for vertical prism the opposite of base up in one eye is base down in the fellow eye

for horizontal prism the opposite of base out in one eye is base out in the fellow eye

examples of dividing prism

R = -2.00 DS 4 base IN 

L= -1.75 / -0.25 × 15 

could divide the prism as:

R -2.00 2 base IN 

L -1.75/-0.25 × 15 2 base IN 

know its horizontal as its base in ; can be base in or out for horizontal 

split: R -2.00

L -1.75 / -0.25 × 15 4 base up

divided by

R -2.00 2 base down

L -1.75 /-0.25 × 15 2 base up

vertical as up; can be up down for vertical

R -2.00 DS 4 base IN 6 base UP

L -1.75 /-0.25 × 15

split prism

R -2.00 DS 2 base IN 3 base UP 

L -1.75 / -0.25 × 15 2 base IN 3 base DOWN

R -2.00 DS 10 base DOWN

L -1.75 / -0.25 × 15 6 base OUT

split prism

R -2.00 DS 3 base OUT 5 base DOWN

L -1.75/-0.25 × 15 3 base OUT 5 base UP

compounding prism power 

horizontal and vertical prism can be combined into a single value, with the base direction specified using a protractor scale and standard notation

compounding prism power:

R +0.50 DS 4 base IN 3 base UP

L +0.75 /-0.25 ×90

the prismatic power in the right eye can be compounded from a horizontal and vertical element into a single prismatic component , as follows:

single power prism will be the hypotenuese of the triangle

identifying base direction using prottractor scale; from prev flashcard

axis pointing to 45

makes sense as 45 for right eye is up and in

calculating exact resultant power and base direction

a² + b² =c² 

so c = square root of 4² +3² = 5 compound prism dioptre 

to work out angle 

place a right angle triangle on the diagram 

point O is the geometric centre of lens 

angle is the angle of base oreintatino 

from tan feta = opposite / adjacent ( can use sOH cAH tOA)

= ¾ 

so feta is 36.9 degrees 

another compound prism:

R +0.50 DS

L +0.27 /-0.25 × 90 2 base IN 4 base DOWN

a² + b² = c²

c²= 4² + 2² ; square root

= 4.47

tan = opp /adjacent

= 4/2 = 2

63.4 degrees

axis is 180 + 63= 240 degrees 

resolving prism power

rotary prism

found on phoropters 

reffered to risley prims 

device contains 2 prismatics elements of equal power 

allows individual prisms to contra rotate to eachother as a wheel is turned

the whole arrangement van be rotated about the optical axis of the phoropter to alter base setting 

 

fresnel prism

a special type of prism, formed from lots of little prisms made in a sheet

usually made from flexible plastic sheet. they can be cut t osize with scissors and stuck to the front surface of spectacle lens

temporary prism correction if someone for example has a stroke

good for those with large prismatic corrections,