1. Physiological optics 15 th lecture Dr. Mohammad Shehadeh

2. Contact Lenses Advantages They are safer than spectacles for many sports and occupations because they do not fall off, break, fog up, become spattered in rain or impair the use of protective eye wear Contact lenses are optically superior to spectacles in some cases. They reduce or eliminate the aberrations associated with spectacles used to correct high refractive errors. They also reduce the aniseikonia associated with anisometropia and high degrees of astigmatism

3. Uses used diagnostically and during surgery to view the fundus and trabecular meshwork. A number of non-refracting contact lenses are used in ophthalmology: bandage contact lenses (precorneal membranes) are used therapeutically for ocular surface disorders to protect and promote healing and to relieve pain. A painted contact lens may be worn to improve the appearance of a small or an unsightly blind eye to provide an artificial iris in aniridia. A type of contact lens with an attached electrode is used to perform electroretinography (ERG).

4. Geometry The refracting power of the optical (central) zone of a contact lens is determined by its anterior and posterior curvatures, thickness and refractive index. A contact lens is described as spherical when it has the same radius of surface curvature in each meridian. Cylindrical refractive errors may be corrected by contact lenses in which the front surface, back surface or both are toric

5. Torsion of a toric contact lens may be prevented either by incorporating an up to 2.00 D base-down prism to weight the lower pole of the lens, or by removing the lower 0.5–1 mm of the lens (truncation) to allow it to sit on the edge of the lower eyelid

6. Truncated contact lens.

7. The posterior surface of the optical zone is defined by its posterior central curvature (PCC), also known as the base curve. The base curve is measured by its radius in millimetres or by its dioptric power in air. The shape of the posterior surface curvature of a contact lens should conform closely to the aspheric surface of the cornea to ensure a correct fit. An aspheric shape may be created by encircling the optical zone with one or two concentric zones of increasing radius of curvature to produce a bicurve or tricurve contact lens;

8. the junctions between the zones are made smooth by a process called blending. This has been superseded by computer- controlled production of a precise aspheric curve. The base curve and the diameter of the contact lens determine how tightly the lens fits the cornea and how easily it moves with each blink.

9. Corneal contact lenses have a smaller diameter than the cornea on which they are supported. Scleral (haptic) contact lenses have a peripheral rim which is supported by the sclera; they are now rarely used, although they are easier to handle because of their large (25 mm) diameter. The corneal surface is oxygenated by the tear film. Contact lenses made of gas-impermeable materials may therefore incorporate fenestrations, slots or grooves to facilitate the circulation of tears behind the lens.

10. Contact lenses used to correct high refractive errors present problems because of their greater thickness0 and weight. The tendency of the upper eyelid to grip the thick upper edge of a high power minus (concave) lens and cause it to ride high is countered by a peripheral bevel. The weight of a high power (convex) plus lens that causes it to drop to a lower position can be countered by a minus peripheral carrier portion which tends to be lifted by the upper lid.

11. Tear Lens and Astigmatism The refractive index of the precorneal tear film (1.333) almost equals that of the cornea (1.3375). Optically, the tear film neutralises corneal surface irregularity, and the refractive power of the corneal surface is effectively that of the tear film–air interface

12. The tear film between the posterior surface of a contact lens and the anterior surface of the cornea is known as the tear lens. If it has uniform thickness it has plano power. A steeper base curve (that is, a more vaulted contact lens) increases the axial height of the tear lens to make it more strongly positive; the converse makes it more negative. The tear lens allows a spherical contact lens to neutralise corneal astigmatism. The base curve of the contact lens should be the same as the corneal surface curvature in the flattest meridian, so that where the cornea is steeper the tear lens is thicker and neutralises the astigmatism. It is therefore convenient to express the prescription using negative cylindrical powers because only the spherical component need be prescribed

13. Whereas rigid lenses can normally correct large degrees of corneal astigmatism, soft contact lenses neutralise no more than 1.00 D in this way because they tend to adopt the shape of the cornea. Corneal astigmatism of more than 1.50–2.00 D is therefore an important limitation to the use of soft contact lenses. Astigmatism arising from the crystalline lens or an implanted intraocular lens will only be neutralised by a front surface toric contact lens

14. Differences between Contact Lenses and Spectacles Field of View A contact lens moves with the eye and therefore allows good vision in all positions of gaze. The distortions which occur when looking through the periphery of a spectacle lens do not occur. When the pupil is dilated, a rigid contact lens may cause a halo effect because of refraction through the peripheral zone of the lens or adjacent tear film.

15. Hypermetropic patients reduce their field of view by wearing spectacles because the lens periphery has a prismatic effect with the base towards the visual axis. When they change to contact lenses they do not need to move their eyes so far to see the same overall field of view. The opposite applies to myopic patients whose spectacles increase the field of view because of a prismatic effect with the base away from the visual axis

16. Most anisometropia is axial, and changing from spectacles to contact lenses in such cases produces image magnification (and improved visual acuity) for myopic patients and image minification for those who are hypermetropic. Aniseikonia is reduced with contact lenses compared with spectacles

17. Optical Aberration Correct contact lens fitting ensures that the lens remains almost centred in all positions of gaze and that on blinking any lens movement is not excessive. This minimises the oblique aberration which occurs looking through non-axial portions of the lens and allows good visual acuity in peripheral gaze.

18. Accommodation and Convergence Spectacle lenses which are centred for distance induce a prismatic effect when the eyes converge for near vision. No such effect occurs with contact lenses, which remain centred. Myopic spectacles have abase–in prismatic effect which reduces the amount of accommodation and convergence required for near. A change to contact lenses therefore demands greater convergence and accommodation which may cause eye strain in presbyopic myopes. The unequal prismatic effect of anisometropic spectacles is eliminated by contact lenses.

20. Prisms It is possible to incorporate up to 3 dioptres of prism power into a corneal contact lens without making it too thick to be practical. The weight of the prism rotates the contact lens so that the prism is always base down. This makes horizontal prismatic correction impossible and limits the prism to one lens only. Carefully fitted scleral lenses allow incorporation of vertical or horizontal prism up to 6 prism dioptres divided between the two lenses.

21. Tint Contact lenses may incorporate a slight blue tint to make them more visible for easier handling and retrieval. They may also have a deeper green, blue or brown tint (sparing the centre) to make the iris appear a different colour

22. Bifocal Contact Lenses Presbyopic, pseudophakic and aphakic patients need to achieve optical correction for more than one object distance because their accommodation is reduced or absent. The contact lens alternatives available to these patients without accommodation are the wearing of spectacles over contact lenses, bifocal contact lenses or monovision. Monovision entails fitting one eye (usually the one with better vision) with a distance contact lens and the fellow eye with a lens which corrects the near vision. Patients must learn to adapt to having to concentrate on the clearer image from one eye. Binocularity and stereopsis are diminished.

23. Various bifocal and multifocal contact lens designs are available: annular, aspheric, segmental and diffractive. Annular bifocal contact lenses have a central zone which usually corrects for distance, surrounded by an annular zone for near. In down gaze, the contact lens rises relative to the cornea, placing the near portion in front of the visual axis.

24. In an aspheric multifocal contact lens the central part of the lens corrects for distance and there is a gradual transition in power to the peripheral portion which corrects for near.

25. Segmental bifocal contact lenses incorporate the near addition over the lower portion of the lens. The eye looks through the distance portion in the primary position. In down gaze, the contact lens rises relative to the cornea, placing the near portion in front of the visual axis. Segmented bifocals must be prevented from rotating by truncation or by ballasting with a base-down prism.

26. Diffractive bifocal lenses have concentric diffraction rings on their posterior surface which are designed to focus equal amounts of light from distant and near objects. The image is less bright than with a single- focus contact lens and this may be a problem in dim illumination

27. Keratoconus The increased corneal surface curvature in keratoconus causes increasing myopia and irregular astigmatism. Mild cases are amenable to spectacle correction but myopia and astigmatism frequently progress to the extent that only rigid contact lenses allow satisfactory vision. Historically, haptic contact lenses were used but newer corneal lenses are designed to sit on the cone. In severe keratoconus when the cone is too steep or scarred, contact lenses may not be appropriate and corneal grafting may be necessary.

28. Other Optical Problems Associated with Contact Lens Wear Short-Term Problems If the posterior surface of the contact lens is too flat, it will move excessively on the cornea and the edge of the lens may cross the visual axis and cause the vision to fluctuate with each blink. Movement of the upper eyelid during blinking over a soft contact lens presses it against the cornea and causes a temporary fluctuation in the visual acuity.

29. The quoted power of a soft contact lens denotes its power when it is suspended in saline at room temperature. Small changes occur when the lens is in use: the lens moulds to the surface curvature of the cornea, the evaporation of water increases the refractive index and an increase in temperature increases the curvature (the latter two factors increase the negative power of the lens). The dry atmosphere of an aircraft cabin may cause blurred vision during and after a flight because of the increased evaporation of water from soft contact lenses

30. Long-Term Problems Corneal warpage is the change in corneal curvature induced by wearing a contact lens which is not associated with corneal oedema. It regresses after the lens is removed over hours or days. During this time, any spectacles which were once accurate no longer compensate for the altered shape of the cornea and spectacle blur results. Warpage is more pronounced and of longer duration when more rigid lenses are worn. It is important not to perform refractive surgery or biometry (to estimate intraocular lens power) or to prescribe spectacles before these changes have stabilised