To measure glasses

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).