Prisms

Question 1

Use of prisms

Answer 1

Can be used to both measure defects (of the extrinsic ocular muscles/their innervations) and relieve some symptoms caused.

Question 2

Prism definition

Answer 2

A transparent optical medium bounded by 2 plane polished surfaces, which are inclined at an angle to one another.

Question 3

Principal section

Answer 3

Section of the prism perpendicular to the refracting surfaces.

Question 4

Apical angle

Answer 4

The angle between the two refracting surfaces (in the principal section).

Question 5

Units of measurement

Answer 5

One prism diopter is an angle whose tangent is 1/100.
One prism diopter is the power of a prism that produces 1cm of displacement at a distance of 1m.

Question 6

Prism equations

Answer 6

P = 100 tan d
or
P = 100 (X x Y)

Question 7

Deviation equation (in degrees)

Answer 7

d = (n - 1) a

Question 8

Finding x (the separation)

Answer 8

Prism dioptres- x = P x y / 100
Deviation in degrees- x = y x tan d
Apical angle in degrees- x = y x tan d

Question 9

Prism thickness

Answer 9

g = (e thick - e thin)
or
g = P x diameter / 100 (n - 1)

Question 10

Compounding prisms

Answer 10

Combining two or more prisms to form a single resultant prismatic effect.
Using Pythagoras to find single resultant.
Using Trigonometry to find base direction.

Question 11

Pr, Ph and Pv

Answer 11

Pr = resultant prism
Ph = horizontal prism
Pv = vertical prism

Question 12

Resultant prism equation

Answer 12

Pr = {Pv2 + Ph2 (square root)

Question 13

Axis equation

Answer 13

0 = tan-1 Pv/Ph

Question 14

Direction equations

Answer 14

+
Top left = 180 - axis
Top right = leave as is
Bottom left = 180 + axis
Bottom right = 360 - axis

Question 15

Resolving prisms

Answer 15

Pv = Pr sin 0
Ph = Pr cos 0

Question 16

Compounding prisms

Answer 16

1) resolve each prism into its vertical/horizontal components

2) combine the vertical and horizontal components

3) combine total effects

Question 17

Principals rule (Prismatic effect)

Answer 17

P=cF

Question 18

Derivation of prentices rule

Answer 18

P = 100 tan d
P = 100 (x / y)
P = 100 (c / f’)
P = 100 (c x F)
P = c x F

Question 19

Positive lens base direction (prismatic lenses)

Answer 19

Opposite to the direction the patient looks.

Question 20

Negative lens base direction (prismatic lenses)

Answer 20

The same direction as the direction the patient looks.

Question 21

Horizontal and vertical prismatic effect

Answer 21

Pv = cv x Fv

Ph = ch x Fh

Question 22

Resultant prismatic effect

Answer 22

Pv = cv x Fv
Ph = ch x Fh

Pr = (square root) Pv2 x Ph2

0 = tan-1 (Pv / Ph)

Direction depends on quadrant;
0-90 degrees stays the same
90-180 = 180 - 0
180-270 = 180 + 0
270-360 = 270 + 0

Question 23

Vertical differential prism effects

Answer 23

More than 1P may not be tolerated leading to;

-diplopia (blurred vision)
-headaches
-eye strain
-epiphora (watery eyes)
-closing one eye

Question 24

To find decentration

Answer 24

P = cF becomes c = P/F

Question 25

Positive base direction (decentration)

Answer 25

We decentre in the same direction as the required prism.

Question 26

Negative base direction (decentration)

Answer 26

We decentre in the opposite direction to the required prism.

Question 27

Resultant decentrations

Answer 27

cv = Pv / Fv
ch = Ph / Fh

Cr = (square root) cv2 + ch2

0 = tan-1 (cv / ch)

Direction depends on quadrant;
0-90 = same
90-180 = 180 - 0
180-270 = 180 + 0
270-360 = 360 - 0

Question 28

Minimum size uncut

Answer 28

MSU = finished lens size + (2 x decentration) + wastage (usually 2mm)