This document discusses various aspects of ophthalmic prisms, including: - Types of prism bases including plano, base-up, base-down, base-in, base-out, and oblique settings. - Prismatic effects occur when light passes through any part of a lens outside its optical center, with the base directed towards the thickest part. - Power of a prism is measured in prism diopters and represents the angular deviation of an object's image. Decentration occurs when the visual axes do not align with the optical centers, introducing unwanted prismatic effects.

1. Dispensing (7)
Dr. Atif Babiker Mohamed Ali
Faculty of Optometry
2023
MTC

2. Ophthalmic prisms
• A plano prism: is a
transparent material
with both surfaces plane
and so the prism is
without focal power.
• Usually plano prisms
included in every
ophthalmic trial set.

3. Base-apex line
• Any straight line passing
from apex to the base is
called base-apex line.
• This line is important to
fix the direction in
which deviation
produced.

4. Prism base setting
• It is conventionally used the base of the prism
to specify it is setting in relation to the eye (
this known as base setting).
• Two vertical settings Base-up & Base-down .
• Two horizontal settings Base-in & Base-out .

5. Oblique setting
• The oblique settings are
specified by the use of
standard axis notation of
cylinders followed by up
or down.
• For example; “base 150
up” means that the base-
apex-line lies along the 150
degree axis meridian and
the direction of the base
being upwards above the
horizontal.

6. A convex lens is considered as prisms placed base to base
A concave lens is considered as prisms placed apex to apex

7. Prismatic effects
• It is important that when the glasses are fitted the optical
centers should correspond to visual axes of the patient’s eyes.
• When light passes through any part of lens out side its optical
center the effect is that of a prism.
• Always the base of the prism is directed towards the thickest
part of the lens.

8. Power of prism
• Power of prism is the deviation expressed in
prism dioptre (∆).
• 1 ∆ is calculated as an object situated one
meter away gives an apparent displacement of
1cm to object image vertically.

9. Prismatic effect of spherical lenses
• Usually incident parallel
rays on a spherical lens
surface meet at one point
(focus).
• The action of lens in
deviating incident rays is
termed its prismatic effect
(v = cxf ).Thus
• P = C X F
• P (in prism dioptre) = C (in
centimeters) X F (in
diopteres) .

10. Lens Centration
• When the two eyes are directed to an object at infinity the
visual axes will be parallel.
• Thus the distance between the centers of pupils should be the
same as distance between optical centers of lenses when
used for distance vision.
• In this condition the accurate measurement of pupillay
distance is very important.

11. Decentration
• Decentration is a process in which the visual
axes of the eyes are allowed to pass away
from the optical centers of spectacle lenses.
• This practically means the pupillary distance
(PD) is not equal to the distance between
centers of glasses (centration distance or CD).
• Mathematically the PD ≠ CD so the prismatic
effect will appear and increases or decreases
according to the difference (C in centimeters).