Visual Optics I

1. Visual Optics I Chapter 2 Schematic Eyes

2. How do we handle the optics of something this complex? http://www.eyedesignbook.com/ch6/fig6-1bBG.jpg

3. Objectives Dealing with the eye as an optical system Reducing the complexity of real eyes to manageable schematic eyes Representing ametropia with schematic eyes

4. Schematic Eyes Page 2.1 • Simplified paraxial representations of the optics of real eyes

5. Schematic Eyes Page 2.1 • Simplified paraxial representation of the optics of real eyes • Assume that all ocular surfaces are perfectly centered (single optic axis)

6. Non-centered Optical System Centered Optical System Optic Axis

7. Schematic Eyes Page 2.1 • Simplified paraxial representation of the optics of real eyes • Assume that all ocular surfaces are perfectly centered (single optic axis) • To use paraxial optics, rays must be limited to the paraxial region • Standard emmetropic schematic eyes are derived from the average constants of large numbers of real emmetropic eyes • Simulate ametropia (myopia, hyperopia, astigmatism) by varying the constants of standard emmetropic schematic eyes • Define three different schematic eyes, from complex to very simple • Rule of thumb: use the simplest schematic eye that will adequately represent the task/problem being considered

8. Overview – Optics of the Eye http://www.capioeye.co.uk/eyeinfo/anatomy/eye.gif

9. Define “emmetropia” • A healthy eye needing no correction for distance or near vision • A healthy eye needing no correction for distance vision • An eye needing no correction for distance or near vision • An eye needing no correction for distance vision

10. Q1. Removal of which surface would make this eye myopic (near-sighted)? • Anterior cornea • Posterior cornea • Anterior crystalline lens • Posterior crystalline lens

11. Q1. Removal of which surface would make this eye myopic (near-sighted)? • Anterior cornea • Posterior cornea • Anterior crystalline lens • Posterior crystalline lens The posterior cornea is the only negative refracting surface in the eye

12. Q2. Flattening (to plane) of which surface would make the eye extremely hyperopic (far-sighted)? • Anterior cornea • Posterior cornea • Anterior crystalline lens • Posterior crystalline lens

13. Q2. Flattening (to plane) of which surface would make the eye extremely hyperopic (far-sighted)? • Anterior cornea • Posterior cornea • Anterior crystalline lens • Posterior crystalline lens The anterior cornea carries about 2/3 of total ocular power

14. Q3. If anterior chamber depth was decreased (all other parameters unchanged) the emmetropic eye would become hyperopic: • True • False

15. Q3. If anterior chamber depth was decreased (all other parameters unchanged) the emmetropic eye would become hyperopic: • True • False Treating the eye as a thick lens with the cornea as F1 and crystalline lens as F2 : Decreasing anterior chamber depth, decreases the value subtracted from (Fcornea + Fcr lens)

16. Gullstrand #1 Exact Eye Page 2.2 most complex

17. Gullstrand #1 Exact Eye Six refracting surfaces, 4 different refractive indices,separate anterior and posterior corneal surfaces,separate crystalline lens cortex and nucleus Page 2.2 Figure 2.1

18. 1.376 1.386 1.406 Exact Eye – Thick Lens 1.00 1.336 1.336

19. Exact Eye – Equivalent Lens

20. Exact Eye – Thick Lens/Equivalent Lens

21. Gullstrand #1 Exact Eye Page 2.2 Figure 2.1

22. Gullstrand #1 Exact Eye (Thick lens parameters) Page 2.2

23. Gullstrand #2 Simplified Schematic Eye Page 2.3

24. Gullstrand #2 Simplified Schematic Eye Page 2.3 Three refracting surfaces, 2 different indices, single corneal surface, single homogeneous crystalline lens medium Figure 2.2

25. Gullstrand #2 Simplified Schematic Eye (Thick lens parameters) Page 2.2

26. The Reduced Eye Total ocular power “reduced” to a single refracting surface Page 2.5 Figure 2.3

27. Reduced Eye Simplifications Page 2.5 Figure 2.3 • Reduced surface represents the “balance of power” between cornea and crystalline lens (balance favors the cornea). • Because the reduced surface represents a power balance, it sits about 1.67 mm behind the (anterior) corneal plane • Pupil considered to coincide with the reduced surface (in reality, it is ~ 2 mm behind the reduced surface)

28. “Reducing” the Simplified Schematic Eye Page 2.6 SS Eye Reduced Eye Figure 2.4

29. Q4. In Visual Optics, three different schematic eyes are used, varying in complexity from a single refracting surface (reduced eye) to six surfaces (exact eye). The reason for using three schematic eyes instead of just one is: • Accuracy. Some situations require more accurate representation of the optics of the eye than others • Necessity. Emmetropic eyes can be accurately represented by a single refracting surface, but ametropic eyes can only be accurately represented by multiple surfaces • Simplicity. It allows selection of the simplest schematic eye to accurately represent each situation • Complexity. Sometimes it is better to make an optics problem more complex than it needs to be

30. Q5. Phakometry (the study of the crystalline lens and accommodation) makes use of the small fraction of incident light that reflects from each ocular surface. Much of what we know today about accommodation comes from early phakometry studies. The most appropriate schematic eye to use to measure size and brightness of the reflected images in phakometry is: • the Exact Eye • the Simplified Schematic Eye (SSE) • The Reduced Eye • None of the above

31. Gullstrand #1 Exact Eye Page 2.2 Figure 2.1

32. Visual Optics The Human Eye: Axes and Angles

33. Why Define Axes and Angles? 1: Strabismus (ocular misalignment) Which eye is deviating? Which eye is deviating?

34. Why Define Axes and Angles? Where are the eyes looking? Where are the light “reflexes” relative to the centers of the pupils?

35. A note on Entrance and Exit Pupils Significance? When you look at someone’s eye, do you see their actual pupil?

36. Entrance and Exit Pupils

37. Path of the Chief (Pupil) Ray through the Eye

38. The Human Eye: Axes (and angles) – Near Vision Page 2.7

39. The Human Eye: Axes (and angles) – Near Vision Page 2.7 Optic Axis: a line through the optical centers of the eye’s refracting surfaces (ONN′O′) Figure 2.5

40. Visual Axis: a line from the object being viewed through the nodal points to the fovea The Human Eye: Axes (and angles) – Near Vision Page 2.8 Figure 2.5

41. PLS: a chief (pupil) ray from the object being viewed through the EnP and ExP to the fovea The Human Eye: Axes (and angles) – Near Vision Page 2.8 Figure 2.5

42. Fixation Axis: a line connecting the object of regard to the center of rotation of the eye The Human Eye: Axes (and angles) – Near Vision Page 2.9 Figure 2.5

43. Fixation Axis: used in BV: Eye Movements Where is the eye looking as it rotates?

44. The Human Eye: Axes – Distance Vision Page 2.8 The visual axis, PLS and fixation axis are all parallel outside the eye in distance vision (parallel incident ray paths from the distant object of regard) Figure 2.6

45. The Human Eye: Pupillary Axis Page 2.9 Line traveling into object space through the center of the pupil (EnP) normal to the cornea Standing in front of a patient viewing “normal” to their cornea at the center of the (entrance) pupil, you are aligned with their pupillary axis Figure 2.7

46. The Human Eye: Angles – Near Vision Page 2.8 • - angle between optic and visual axis (~5) • - angle between optic and fixation axis • - angle between pupillary axis and PLS  - angle between pupillary and visual axis Figure 2.5

47.   The Human Eye: Axes – Distance Vision Page 2.8 In distance vision, angle  =  = angle between optic axis and PLS Figure 2.6

48. Page 2.11 Introduction to Ametropia

49. Demographic question (no wrong answer) What is your refractive error (ametropia)? • Hyperopia (far-sighted) • Emmetropia (no error) • Low myopia (< 5 D) • High myopia (> 5 D)

50. Introduction to Ametropia • Are you near-sighted or far-sighted? • Near-sighted (myopic) • high or low? • high • How do you know how myopic you are? • What’s the difference between a 2 D myope and an 8 D myope? • Before we can quantify ametropia, we have to set a standard for emmetropia