Prisms Flashcards
Use of prisms
Can be used to both measure defects (of the extrinsic ocular muscles/their innervations) and relieve some symptoms caused.
Prism definition
A transparent optical medium bounded by 2 plane polished surfaces, which are inclined at an angle to one another.
Principal section
Section of the prism perpendicular to the refracting surfaces.
Apical angle
The angle between the two refracting surfaces (in the principal section).
Units of measurement
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.
Prism equations
P = 100 tan d
or
P = 100 (X x Y)
Deviation equation (in degrees)
d = (n - 1) a
Finding x (the separation)
Prism dioptres- x = P x y / 100
Deviation in degrees- x = y x tan d
Apical angle in degrees- x = y x tan d
Prism thickness
g = (e thick - e thin)
or
g = P x diameter / 100 (n - 1)
Compounding prisms
Combining two or more prisms to form a single resultant prismatic effect.
Using Pythagoras to find single resultant.
Using Trigonometry to find base direction.
Pr, Ph and Pv
Pr = resultant prism
Ph = horizontal prism
Pv = vertical prism
Resultant prism equation
Pr = {Pv2 + Ph2 (square root)
Axis equation
0 = tan-1 Pv/Ph
Direction equations
+
Top left = 180 - axis
Top right = leave as is
Bottom left = 180 + axis
Bottom right = 360 - axis
Resolving prisms
Pv = Pr sin 0
Ph = Pr cos 0
Compounding prisms
1) resolve each prism into its vertical/horizontal components
2) combine the vertical and horizontal components
3) combine total effects
Principals rule (Prismatic effect)
P=cF
Derivation of prentices rule
P = 100 tan d
P = 100 (x / y)
P = 100 (c / fâ)
P = 100 (c x F)
P = c x F
Positive lens base direction (prismatic lenses)
Opposite to the direction the patient looks.
Negative lens base direction (prismatic lenses)
The same direction as the direction the patient looks.
Horizontal and vertical prismatic effect
Pv = cv x Fv
Ph = ch x Fh
Resultant prismatic effect
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
Vertical differential prism effects
More than 1P may not be tolerated leading to;
-diplopia (blurred vision)
-headaches
-eye strain
-epiphora (watery eyes)
-closing one eye
To find decentration
P = cF becomes c = P/F
Positive base direction (decentration)
We decentre in the same direction as the required prism.
Negative base direction (decentration)
We decentre in the opposite direction to the required prism.
Resultant decentrations
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
Minimum size uncut
MSU = finished lens size + (2 x decentration) + wastage (usually 2mm)
