2. Definitions
• Dispensing is a science
of making and fitting of
spectacles.
• The spectacle is an
optical appliance
comprising lenses and
a frame.
3. The lenses
• The lens may be
defined as a portion of
a transparent substance
bounded by two
polished surfaces.
4. Introduction
• Eye care professionals (optometrists and
ophthalmologists) are trained to determine
the specific corrective lenses that will provide;
• The clearest, most comfortable and most
efficient vision, avoiding double .
• They can test to see if the corrective lenses
are appropriate.
5. Lens Correction
• A corrective lens is a lens worn in front of the
eye, mainly used to treat Myopia, Hyperopia,
Astigmatism, and Presbyopia.
• Glasses or "spectacles" are worn on the face a
short distance in front of the eye.
• Contact lenses are worn directly on the surface of
the cornea.
• Intra ocular lenses (IOL) are surgically implanted
most commonly after Cataract removal.
6. Task of Opticians
• The dispensing optician
task is transformation of
written prescription
into wearable and
acceptable glasses.
• The spectacle should be;
• Comfortable
• Attractive
• Provide the visual
improvement
prescribed.
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7. Lens surfaces
• When parallel rays of light incident on a lens
surface each individual ray may be bend to a
different degree and all the rays perhaps meet
at one focus. The distance of this focus from
the lens surface depends on:
• 1. Curvature of surfaces.
• 2. Optical density (refractive index).
• 3.Wave-length of light.
8. Classification of lenses
• A. Spherical lens: is a portion of
sphere and therefore refraction is
symmetrical about the principal
axis.
• B. Cylindrical lenses: both surfaces
of lens may be represented as
portions of cylinder.
• C. Sphero-cylinder: a lens in which
the curvatures of the two
meridians at right angle are
different.
9. The power of lens
• The power of lens is measured by reference to its
focal length; and this depends on surfaces
curvature, lens thickness, and refractive index of
lens material.
• A lens with focal length one meter is referred as
having a refractive power of one dioptre (1D).
Therefore, dioptres are reciprocal of focal length
in meter.
• E.g. if focal length is 20cm then F=100/f cm or
100/20 = 5D, if focal length is 50mm then
F=1000/f mm or 1000/50 = 20D.
