Optics and Optical Devices

1. Optics and Optical Devices

2. Learning Objectives Understand basic optical principles of lenses How lenses manipulate light Distinguish between types Determine focal length and power of lenses Understand correction of refractive errors Identify the appropriate lens for each type Use magnifiers appropriately Describe properties of each type Describe correct technique for using each type

3. Optical Devices Use lenses or prisms (or a combination) to focus light more precisely onto the fovea Enable retina to capture the highest quality visual data Enable CNS to �see� accurate and complete images

4. Types of Optical Devices

5. Types of Optical Devices Contact lenses Eyeglasses Magnifiers Binoculars Telescopes Microscopes Electronic magnifiers

6. Optical devices correct for deficiencies in the ocular system Inherited imperfections Acquired imperfections Imperfections caused by disease Imperfections caused by trauma Assist in situations where �super� vision is needed

7. Differ in Price and Complexity Free magnifier in cereal box Basic $10 handheld from Walmart Precision made $200 magnifier $300 bifocals $1,000 plus electronic magnifiers Million dollar Hubble telescope

8. Each device is only as good as the human using it and the CNS receiving the image.

9. Basic Optical Principles

10. Measurement of Light Moves in electromagnetic waves See peaks and valleys Distance between peaks is a wavelength

11. White Light Only component of electromagnetic waves that is visible Mixture of all the colors of the rainbow Each color has its own wavelength Red is the longest wavelength Violet is the shortest Beyond ends of this visible spectrum is invisible light Ultraviolet, x rays, gamma rays on short end Infra red radiation, radio waves on the long end

12. Principles of Refraction A light ray striking the flat surface of a glass lens at a 90 degree angle will pass through without bending

13. Principles of Refraction cont� A light ray striking the surface of a lens which is curved will bend toward an imaginary line perpendicular to the edge of the surface of the glass

14. Refractive Index Measurement reflecting the density of the lens Speed of light as it passes through the medium ( the lens) will vary with the density of the medium The denser the medium, the slower the light passes through The slower the light passes through, the greater the refractive index

15. The greater the angle of the surface and the slower the index of refraction, the greater the refractive power of the lens

16. Why do we need refraction? At a distance of 20 feet or greater Light rays travel in parallel lines No refraction is needed to focus on fovea Closer than 20 feet Light rays begin to diverge And must be refracted to focus on the fovea

17. Role of Ocular System in Refraction Ocular system ensures that light rays are focused on the fovea Light passes through 4 transparent structures before reaching retina

18. Role of Ocular System in Refraction Ocular system ensures that light rays are focused on the fovea Light passes through 4 transparent structures before reaching retina Cornea Aqueous Lens Vitreous

19. As light passes through each structure, it is slowed down and bent

20. Cornea and lens provide the greatest refractive power of the system Cornea High because air is less dense than cornea Lens High because it can change shape and be made very thick

23. Moving an image closer or farther away from the lens will cause the focal point to move closer or farther to the lens

24. Reason for Accommodation When an object comes closer to the lens, the distance between the focal point and the lens will increase, causing the image to go out of focus Accommodation counters this by increasing refraction of the lens to shorten the focal point distance

26. A variety of lenses have been developed to help persons achieve better focus and see larger images

27. Optical Spherical Lenses Transparent, made of glass or plastic Have two sides, at least one of which is curved

30. Aspherical lens There is greater light scatter at the edges of all lenses Causes chromatic aberration Image looks fuzzy on edges Aspherical lens have slight alterations made to curvature to reduce aberrations Person will experience fewer aberrations if he/she views directly through the CENTER of the lens Aka the optical center

32. Prism Used to shift images on the retina Displaces image towards the apex of the prism Used in correction of strabismus Person wearing very strong plus lenses will need prism to assist in convergence

33. Lenses have varying refractive strengths Refractive strength of lens is measured in diopters Magnifiers range between 2-60 Stronger the lens, sharper the curvature and density of the lens material Also the shorter the focal distance between the rear of the lens and the focal point

35. Formula for determining dioptic strength of a magnifier Divide 100 by the focal length of the magnifier (measured in centimeters) Example Lens brings image into focus at 5 cm 100 = 20 diopters of strength 5

36. If you know the focal length of the magnifier you can determine the diopters AND VICE VERSA If you know the diopters of the magnifier you can determine the correct focal length

37. Important to Remember: Strength of magnifiers (especially low power ones) is sometimes given in X units instead of diopters There is no standardization of X units but 1 X = approximately 4 diopters

38. Why is it important to know these formulas?

39. Why is it important to know these formulas?

40. Optical Correction of Refractive Errors With an emmetropic (perfect) optical system, the image comes into focus precisely on the retina If the optical system is imperfect, the image does not come into focus exactly on the fovea Person experiences a refractive error Objects appear blurry

41. Myopia

42. Correction with Concave Lens

43. Hyperopia

44. Correction with Convex Lens

45. Astigmatism Image is blurred in one direction more than another WHICH TYPE OF LENS IS APPLIED?

46. Astigmatism Image is blurred in one direction more than another WHICH TYPE OF LENS IS APPLIED? cylindrical

47. Presbyopia Lens loses refractive power as it loses ability to change shape WHICH TYPE OF LENS IS APPLIED?

48. Presbyopia Lens loses refractive power as it loses ability to change shape WHICH TYPE OF LENS IS APPLIED? Convex Applied as a reading add usually in a bifocal

49. Reading Add Always a convex lens May be worn as a single lens Reading glasses found at Walmart When combined with a distance lens, it creates a bifocal Not always readily apparent to observer that patient has a bifocal if wearing progressive lens

50. Bifocal Lens

51. Spectacle Prescription

52. Types of Magnification Relative Distance If you move closer to an object, the object becomes RELATIVELY larger

53. Relative Size Magnification Object is made larger Viewed at the same distance, it appears RELATIVELY larger

54. Angular Magnification Magnification achieved by lenses Makes objects at a distance appear closer to eyes and therefore larger

55. Electronic Magnification A.k.a. Projection magnification Increases size of object through projection

56. Convex Lenses When an object is between the convex lens and its focal point, the object will appear right side up and larger Closer the object is to the focal point, the larger the image will appear

57. Convex Lenses If an object is outside the focal range, it will appear upside down Only a limited area is visible inside the magnifier Called the magnifier�s �field of view�

58. Field of view varies with the strength of magnifier

59. General Rules of Magnifiers The stronger the power�. The shorter the focal distance The smaller the field of view The heavier the magnifier

60. You can hold a magnifier any where in relation to the eye as long as the task material is held within the focal distance of the magnifier

61. Types of Magnifiers Pros and cons of these very important adaptive devices

62. Hand Held Magnifiers Advantages Greater flexibility in placement Portable-can go into community Generally less expensive

63. Hand Held Magnifiers Disadvantages More difficult to maintain proper focal distance Requires more hand strength and steadiness

64. Stand Magnifiers Advantages Maintain a constant focal distance Can use stronger powers Less hand strength is needed Can be illuminated

65. Stand Magnifiers Disadvantages Narrow field of view for stronger powers Has to be viewed monocularly Often awkward to position Can be expensive

66. Head Worn/Hands Free Advantages Spectacles are familiar concept Portable Preserve more field of view Allow binocularity up to 8-10 diopters

67. Head Worn�. Disadvantages Difficult to adjust to and maintain close focal distance Heavy-can be uncomfortable Some types can be very expensive