Human Vision and Vision Correction (PHR 177)Course

1. Human Vision and Vision Correction (PHR 177)Course • Prof. Dr. Moustafa. M. Mohamed • Vice Dean • Faculty of Allied Medical Science • Pharos University • Alexandria • Dr. MervatMostafa • Department of Medical Biophysics • Pharos University

2. The Physics of light • Properties of Light • The Eye • Image Formation • Glasses

3. Light • The Electromagnetic Spectrum

4. The Electromagnetic Spectrum • Radio Waves - communication • Microwaves - used to cook • Infrared - “heat waves” • Visible Light - detected by your eyes • Ultraviolet - causes sunburns • X-rays - penetrates tissue • Gamma Rays - most energetic

5. Properties of Light

6. Properties of Light • Wave model – Classical sinusoidal wave – Can travel through a vacuum –Describes reflection, refraction, diffraction, interference, and Doppler Effect phenomena, etc. • Particle model – “photon” –Describes absorption and emission phenomena

7. The eyes mediate sight • Function – Sensory organ for sight –Detects light and converts it into neural responses that the brain interprets

8. Anatomy of the Human Eye

9. Eye Anatomy • Anatomy – Light enters the eye through the pupil – Photoreceptors (light-sensing cells) are located in the retina – Retina acts like the film in a camera • How are images formed?

10. Image Formation: Apertures • Apertures – “openings” • Basis of a pinhole camera – Dark box • small “pinhole” to let in light • Image screen on opposite side of hole – All light rays from a scene pass through single point (focusing)

11. The Pupil is an Aperture • Pupil –Opening in the center of the eyeball – Bounded by the Iris • The iris controls the size of the pupil –Opening through which light enters the eye

12. Image Formation: Apertures • To achieve a clear image on an image screen, the aperture must be very small • Problems: –Smaller aperture: • Fewer photons get through. • Ratio of pinhole diameter to image distance should be less than 1/100. –Image screen must be large. –Eye would have to be MASSIVE • Solution??

13. Lenses are the Solution to theAperture Problems • Lenses focus of the light waves past the aperture • Focuses the image on the screen • Allows for wider apertures • Produces smaller images

14. Lenses of the Eye • Cornea • Crystalline Lens • Primary function • – To focus the image on the back of the retina

15. Refraction • Bending of the path of a light wave as it passes across the boundary separating two media – Cause: • Change in the speed of the light wave • No change in speed = no refraction! Optical Density • Optical density of a material determines the speed of a wave passing through it • ↑ Op9cal density = ↓ Speed

16. Index of Refraction • Abbreviated as “n” • Indicator of optical density

17. Index of Refraction • refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. • It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium. • This can be written mathematically as: • n = speed of light in a vacuum / speed of light in medium.

18. Index of Refraction • Another common definition of the refractive index comes from the refraction of a light ray entering a medium. • The refractive index is the ratio of the sinesof the angles of incidence θ1 and refraction θ2 as light passes into the medium or mathematically:

19. Index of Refraction • For example, the refractive index of water is 1.33, meaning that light travels 1.33 times as fast in vacuum as it does in water. • As light moves from a medium, such as air, water, or glass, into another it may change its propagation direction in proportion to the change in refractive index. • This refraction is governed by Snell's law,

20. Snell’s Law –Quantitative answer to the question of “By how much does the light ray refract?” ni*sin(θi) = nr*sin(θr) • ni= index of refraction of incident media • nr= index of refraction of refractive medium • θi= angle of incidence • θr = angle of refraction • If ni = nr, then no refraction!!

21. Object-Image Relationship • Image location changes depending on object distance for a given lens’ focal length • The Lens Equation 1/f = 1/d object+ 1/d image • Distance-Size Relationship • = • Image size is limited by short image distance • Most vision restricted to small region of the retina

22. Problem • Retina is a fixed distance from the cornea-lens system (~22 mm or 2.2 cm) • Lens Equation – 1/f = 1/dobject + 1/dimage – In the eye, • dimageis fixed = distance between cornea lens system and the retina • dobjectis fixed = distance between the eye and the object being viewed • Solution??

23. The Solution is Accommodation • Accommodation –The ability of the eye to change its focal length (f) –Mediated by the lens and ciliarymuscles

24. Nearby Objects Have a longer dimage • Shorten the focal length Ciliarymuscles contract Squeeze the lens into a more convex (fat) shape Pushes cornea bulge out further = greater curvature • Distant Objects Nearby Objects • Have a shorter dimage • Lengthen the focal length • Ciliarymuscles relax • Lens assumes a flatter • (skinnier) shape • Cornea is not pushed out • = less curvature

25. Near Point and Far Point • Near Point • Closest point at which an object can be brought into focus by the eye – Ideally ~25 cm – Recedes with age (can lead to farsightedness) • Far Point • Farthest point at which an object can be brought into focus by the eye • Typically is infinity • Decreases with age

26. Hyperopia • INABILITY of the eye to focus on NEARBY objects • “Can see far” – no difficulty focusing on distant objects • Images of nearby objects are formed at a location BEHIND the retina • Near point is located farther away from the eye

27. Hyperopia: Causes • Shortened eyeball (retina is closer than normal to the cornea lens system) • Cornea is too flat • Lens can not assume a highly convex (fat) shape

28. Hyperopia: Correction • Need to refocus the image on the retina – Decrease the focal length of the cornea-lens system • Add a converging lens

29. Myopia • Inability of the eye to focus on DISTANT objects • “Can see near” – no difficulty focusing on nearby objects • Images of distant objects are formed in front of the retina

30. Causes of Myopia • Not usually caused by aging • Elongated eyeball (retina is farther away than normal from the cornea-lens system • Bulging cornea (greater curvature)

31. Correction of Myopia • Need to refocus the image on the retina – Increase the focal length of the cornea-lens system • Add a diverging lens

32. Presbyopia • “After – 40” vision • Progressively diminished ability to focus on near objects as one ages – Similar to hyperopia, but different cause • Cause = diminished power of accommodation due to natural process of aging – Reduced elasticity of the lens –Weakening of the ciliary muscles – Changes in lens curvature due to continued growth

33. Astigmatism • Most common refractive error • Blurred or sometimes distorted vision at any distance • Cause: – Irregularly shaped cornea or lens • More oblong than spherical • Refractive power differs between regions • Correction –Glasses • Lenses with different radii of curvature in different planes