Page 139 - Nov- Dec 2024
P. 139
FIGURE 1 - BINASAL AXIS NOTATION AND SIGN CONVENTION FOR DECENTRATIONS, X AND Y.
each eye so that decentration upwards and where z = x tan θ.
outwards is reckoned positive.
Hence, PR = y cos θ + x tan θ cos θ
Figure 2 represents the front surface of an = y cos θ + x sin θ.
astigmatic lens. O is the optical centre of the
lens. It is required to calculate the prismatic Now the vertical prismatic effect at R due to
effect at the point R. the cylinder is
Let: S = the power of the spherical component C x PQ or C x PR cos θ = C cos θ (x sin θ + y cos θ)
C = the power of the cylindrical component Adding the prism due to the sphere, we have:
θ = the axis direction in Binasal notation P = y.S + C cos θ (x sin θ + y cos θ).
V
x = the horizontal decentration of the OC P will be base up when its sign is positive and
V
base down when its sign is negative.
y = the vertical decentration of the OC
The horizontal prismatic effect at R due to the
P = the vertical prismatic effect at R cylinder is
V
P = the horizontal prismatic effect at R C x QR or C x PR sin θ = C sin θ (x sin θ + y cos θ).
H
The vertical prismatic effect at R due to the Adding the prism due to the sphere, we have:
sphere is simply, y.S and the horizontal prismatic
effect at R due to the sphere is simply, x.S. P = x.S + C sin θ (x sin θ + y cos θ).
H
P will be base out when its sign is positive and
The prismatic effect at R due to the H
cylindrical component of the lens is the product base in when its sign is negative.
of the perpendicular distance of R from the EXAMPLE:
cylinder axis, measured in cm, and the power of
the cylinder (PR x C in Figure 2), the prism base Find the vertical and horizontal prismatic
lying along PR. effects at a point 11 mm below and 2½ mm
inwards from the optical centre of the lens,
From Figure 2, PR = (y + z) cos θ R +6.00/-3.00 x 140.
LENS TALK THE INDIAN OPTICIAN | NOV-DEC 2024 | 133
