Measuring prisms in spectacles Flashcards
how are + and - lenses considered consideration of + and - lenses
+ lenses are two prisms that are joined at the base
- lens - 2 prisms joined at the apices
- because the optical centre is either where the apices are joining or where the base is joining
where is there o prismatic effect
there is 0 prismatic effect at the optical centre
optical centre is where the image is deviated compared to the object
so the object is undeviated through the optical centre
if you lift the lense up you are no longer looking though the optical centre
the light will therefore be deviated towards the base
image is deviated towards the apex
what is the difference between hand neutralisation and ret
if you were doing hand neutralisation and you see a with movement add + and if you were doing ret you would record the + lens that you picked up from the trial lens case
with hand neutralisation we want to know the strength of a unknown spectacle lens
so for hand neutralisation say you got a + 4 with movement you wouldn’t write + 4 as the result you would write -4 because it is the neutralising lens
how to spectacle wearers experience prismatic effects
because of the effects of decentration
i.e. when we read - we read in a depressed position so
for hypermetropes looking below the optical centre you will get prism base up
for myopes we get a prism base down
define decentration
it is the difference between where the lens needs to be for a prism (mono ipd) - and where the lens is (optical centre)
difference between the two = how much a Lens has been decentred
what us prentices rule
prism diopters is equal to c x f lens
c = distance from where we are looking vs the optical centre and we multiply that by the focal power of the lens in diopters this relates prism diopters to diopters
a patient that is 8mm below the optical centre - f lens = +4.00 diopters calculate the prismatic effect
we need the amount of prism and the base direction
8mm = 0.8 cm
prisms diopters = 0.8 x (+4.00) = 3.2 diopters
its a + lens and we are looking through the optical centre so it is base up
below the optical centre = base up
a patient wears
right eye +4.50 / -2.00 x 180
left -3.50ds
mono ipd:
right 32 mm , left 33mm
distance to the optical centres - mono optical centre
right 37mm
left 40mm
calculate the horizontal prismatic effect in primary position
measure the vertical prismatic effect when looking 11mm below the optical centre
right eye 37-32
mono ic - mono ipd = 5mm which is 0.5cm
p= cf lens
0.5 x (-4.50) = 2.25 base in
mono ic is bigger than the mono ipd so the prism has moved laterally and therefore it is base in
the optical centre is at 32mm but the optical centre is too far out so they are incorrectly made for 0 prismatic effect in the horizontal direction so we need to know the difference between the two which is 5mm and 0.5cm
and it is at 180 so we are calculating the horizontal prism
the focal power along the horizontal meridian = 180
LE - the difference between oc and mono ipd = 7mm
so it has to be 0.7cm
left eye = prismatic effect = p = cf lens
so 0.7 x 3.5ds = 2.45f base in - because the optical centre has been displaced laterally
the total prismatic effect is the prismatic effect of the left and right eye added together
2.25 + 2.25 = 4.7 base in
if you wanted to measure the vertical prismatic effect when looking 11mm below the optical centre
question is asking for the prismatic effect in the vertical meridian - power in the vertical meridian is the sphere and cylinder added together which is equal to 6.5
p = cflens - 1.1 Which is 11mm in cm and that is equal to c x (sphere and cylinder added together which is flens for this part of the question) which = 1.1 x 6.5 = 7.15 it is in the vertical meridian so it can only be base up or base down
for the left eye
left eye dosnt have an axis so you just leave it as it is so it will be 1.1 x 3.5 which is equal to 3.85
and then you subtract the two values for the prismatic effect
what is the difference between mono ipd and mono ic
mono ipd = where the eyes are which is where the optical centres needs to be for zero prism
mono ic = where the optical centres really are in the specs
right eye +4.75/ -1.00 x90
left eye +8.50 / -3.00 x 90
ipd = right -34mm and left = 33mm
oc = right - 28mm ,left - 26mm
calculate horizontal prismatic effect in primary positon and verrtical prismatic effect 12mm below
step 1
p= c x f lens
so c is equal to the amount of decentration so for the right eye
it would be 6mm which is equal to 0.6 cm
then f lens would be +4.75 +- 1.00 in the right eye and that would give you +3.75 (sphere + cylinder to get the horizontal meridian power)
so 0.6 x 3.75 would give you 2.25 base in
left eye
decentration = 0.7 (33-26) which is equal to 7mm and 0.7 cn
f lens would be +8.50 + - 3.00 = 5.50
so total prismatic effect for right eye would be 0.7 x 5.50 = 3.85
so total prismatic effect would be 3.85 + 2.25 = 6.1
to work out vertical prismatic effect 12mm below:
you would add the sphere and the cylinder which would give you 3.75 for the right eye and then multiply that by 1.2
and for the left eye you would add the sphere and cylinder and multiply that by the 1.2 and then add the prismatic effect of the left and the right eye to Give you the total prismatic effect
calculate the mono ic that gives you 2 diopters base in right and left
right eye = +4.75 / -2.00 x 90
left eye = +3.75 x / -1.00 x 90
ipd right eye = 30mm , left eye = 29mm
prism diopters = p= cf
rearrange to find c = p /f
prism base = 2
it is saying base in right and left which tells you that it is along the horizontal meridian
- start by adding the sphere and the cylinder in order to get the horizontal meridian because it is at 90
so for the right eye
2/ 2.75 which is equal to 0.73cm which is equal to 7.3mm
mono ic = ipd - oc which for the right eye is 30-7.3= 22.7mm
