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Dispensing

- 1. Dispensing (3) Dr. Atif Babiker Mohamed Ali Faculty of Optometry 2023
- 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.
- 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: