2. Best form lenses
• The ideal lens should correct all aberrations
which may be due to:
• A. Rotation of the eye.
• B. Optical aberrations of lenses themselves.
• It is impossible to find a lens correct all
aberrations, but there are lenses which
minimize these operations and so termed;
best form lenses.
3. Meniscus Lenses
• The meniscus form is the best of all form which
minimize the two types of aberrations.
• In the meniscus forms the standard surface is
called the base surface and the other surface is
called the combing surface (which is ground to suit
the ordered prescription).
• Standard forms of meniscus types were used in
order to reduce the number of grinding tools.
Thus,manufacturers produce a definite number of
forms.
4. Base &combining surfaces
• Therefore, standard plus base surface usually
+6.0D power as front curve was ready made
as semi-finished block.
• Other alternative base surfaces could be
+9.0D and +12.0D for high powers.
• E.g. if +2.0D lens needed it should have +6.0D
front base curve and – 4.0D combining back
curve.
5. Transposition
• Transposition is expression used to show the
minus or plus form used in lens prescription.
• The simple transposition was to change the
sign of cylinder and accordingly the sphere
power and axis degree will change.
• E.g. if you want to change:
• +2.0Dsph/ +1.0 CX 180 to minus cylinder
form.
• We have three steps
6. • 1- Add the sphere to cylinder to get the new
sphere ( Algebraic summation).
• +2.0 + (+1.0) = +3.0 this is the new sphere.
• 2- Only change the sign of cylinder
• - 1.0 cylinder instead of +1.0 cylinder.
• 3- Change the axis 90 degree (e.g. when axis
degree less than 90 add, but when axis more
than 90 degree subtract)
• 180 – 90 = 90
• The new form will be
• +3.0D sph/ - 1.0 CX90
7. Notation of cylinders
• It is known that the effect of cylinder lens is not
only depend upon the refracting power, but also
on the position of its axis which always lie
perpendicular to the power.
• In the standard notation, the observer is facing
the patient, zero at observer left and the scale is
read below the horizontal, or 180 is at left and
the scale is read above the horizontal.
• However, this notation is same for left and right
eyes.
8.
9. Lens prescription(1)
• Spectacle lenses can be written directly as
following:
• Name:------------------------ Date--------
• Glasses for distance (PD = 62mm)
RE +2.0/+1.0 CX 90
LE +2.5/+0.5 CX 75
Photo gray lenses
For reading add +1.5D
signature------------
10. Lens prescription (2)
• Spectacle lenses can be written directly as
following:
• Name:------------------------ Date--------
• Glasses for constant use (PD = 60mm)
RE -3.0 Dsph.
LE -2.5/-0.5 CX 85
Plastic lenses with yellow colour tint 25%
signature------------
11. Lens prescription (3)
• A pre-prepared prescription paper can be used.
Name:----------------------------------------------------- Date:----------------------
PD =
64mm
OD (RE) OS (LE)
SPh Cyl Axis SPh Cyl Axis
Constant
Distance +1.00 -0.25 180 -0.75 ---------- ---------
Near +3.00 -0.25 180 +1.25 ----------- ---------
Remarks: photo-brown bifocals Sign:
