1. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science The Human Visual System The Eye

2. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science In this section... u Anatomy of human eye u Image formation by human eye u Method of light detection u Retinal processing u Eye optical defects and diseases

3. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Human Visual System Image formation Exposure Control DetectionProcessing Cornea Lens Iris/pupil Photoreceptor sensitivity Retina Rods Cones Brain

4. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Human Eye u Human eye is a complete imaging system. Ear side (Temporal) Nose side (Nasal) Cornea Aqueous Humor Pupil Iris Ciliary Muscle Sclera Fovea Retina Optic Nerve Vitreous Humor Eyelens Choroid Suspensory ligament

5. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Image Formation u The curved surfaces of the eye focus the image onto the back surface of the eye. Object Image

6. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Cornea u The outer wall of the eye is formed by the hard, white sclera. u Cornea is the clear portion of the sclera. u 2/3 of the refraction takes place at the cornea. Sclera Cornea

7. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Iris and Pupil u Colored iris controls the size of the opening ( pupil ) where the light enters. u Pupil determines the amount of light, like the aperture of a camera. Iris Iris open Dilated pupil Iris closed Constricted pupil Pupil

8. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Lens u Eye lens is made of transparent fibers in a clear membrane. u Suspended by suspensory ligament. u Used as a fine focusing mechanism by the eye; provides 1/3 of eye’s total refracting power. u Non-uniform index of refraction. Lens Suspensory Ligament Ciliary muscle Transparent Fibers Cross section of the eye lens

9. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Accommodation u The suspensory ligaments attach the lens to the ciliary muscle. u When the muscle contracts, the lens bulges out in the back, decreasing its focal length. u The process by which the lens changes shape to focus is called accommodation. Relaxed muscle Taut ligaments Distant object Near object Contracted muscle Slack ligaments

10. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Aqueous Humor and Vitreous Humor u Transparent gelatinous liquid filling the eye. u Provides nutrients to the cornea and eye lens. u Also helps maintain the eyeball shape with its pressure. Vitreous Humor Aqueous Humor

11. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Retina u Retina is the photosensitive “detector” for the eye. u Two types of receptors in the retina: rods for low light level, and cones for color. u Located at the center of the retina, fovea contains a greater concentration of cones. u Signals from the receptors leave through the optic nerve to the brain. Retina Optic Nerve Fovea

12. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Plexiform Layer u The retina is made of three layers: u Plexiform layer is a network of nerves which carry the signals from the photo receptors. u Photo receptors. u Choroid provides nourishment to the receptors, as well as absorb any light that didn’t get absorbed by the photo receptors, like a antihalation backing in film. Fovea Plexiform Layer Photo receptors Choroid Optic Nerve Light

13. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Rods and Cones u Highly sensitive to low light level or scotopic conditions. u Black and white. u Dispersed in the periphery of the retina. Synaptic endings Cell nucleus Inner segments Outer segments Rod Cone u Sensitive to high light level or photopic conditions. u Three types of cones responsible for color vision. u Concentrated in the fovea.

14. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Adaptation u Why can’t you see immediately after you enter a movie theater from daylight? u The threshold of detection changes with overall light level. u The switch is quite gradual, until the sensitivities of cones and rods cross over at about 7 minutes in the dark. Photopic (cones) Scotopic (rods) 0 51015202530 Time in dark (minutes) Threshold of detection (log scale)

15. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Distribution of Photoreceptors u Cones are concentrated in the fovea. u Rods predominate the periphery. u There is a blind spot where there are no photoreceptors, at the point where the nerves exit the eye (optic nerve). 20 º 40 º 60 º 80 º 20 º 40 º 60 º 80º 0 º Angle 0 º 20 º 40 º 60 º 80 º 60 º 40 º 20 º 20 40 60 80 100 120 140 160 Number of receptors per mm 2 Rods Cones Blind spot Visual Axis Nasal Temporal

16. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Human Vision u Human Cone Response to Color u three cone types (S,I,L) correspond to B,G,R 400460530650600700500 Wavelength (nm) Relative response BlueCyanGreenRed 490 ILS

17. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Retina u The retina is made of network of nerve cells. u The network works together to reduce the amount of information in a process called lateral inhibition. To optic nerve Light Cones Rods Bipolar cells Amicrine cells Ganglion cells Horizontal cells

18. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Hermann Grid u Illustrates lateral inhibition.

19. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Hermann Grid u Point A looks darker because there are 4 inhibitory inputs u Point B looks lighter because there are only 2 inhibitory inputs A B

20. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Mach Bands Actual brightness Perceived by you

21. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Eye Defects u Image focuses on the retina for a normal eye. u Distant objects look blurry for a myopic (near sighted) eye. u Near objects look blurry for a hyperopic (far sighted) eye. Normal Myopic Hyperopic Object at infinity Eyes at relax state.

22. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Myopia - Near sightedness u Distant objects look blurry because the eye cannot relax any farther so that the image is focused before the retina. u Near object in focus without accommodation. u Corrected with a negative lens. Myopic eye relaxed Blurry Myopia corrected with a negative lens Far object The virtual image from the diverging lens appears to be closer. Near object Myopic eye relaxed In focus Far object

23. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Hyperopia - Far sightedness u Near objects look blurry because the eye cannot accommodate enough for near objects. u Far object in focus. u Corrected with a positive lens. Hyperopic eye Partially accommodated In focus Hyperopia corrected with a positive lens Far object Light from the converging lens looks as though it is coming from the distance. Hyperopic eye Fully accommodated Blurry Near object

24. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Contact Lens u Contact lens is an alternative to corrective lenses. u Changes the curvature of the cornea by adhering to the surface with some fluid. Cornea Contact lens Fluid

25. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Presbyopia - “Old eye” u Lens hardens with age. u Eye cannot adequately accommodate near objects. u Bifocals (lens with two focal lengths) contains a concave lens for distance (if needed) and a convex lens for near objects. Near objects magnified Far objects Concave for distance correction (if needed) Convex for near object correction

26. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Astigmatism u The cornea is not spherical; Focal length different from one plane to a perpendicular plane. F’ horizontal F’ Vertical Object Image at F’ Vertical Image at F’ Horizontal Cornea Direction of blur

27. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Astigmatism u Correction of astigmatism is done through the use of a cylindrical lens. u Cylindrical lens converge rays in one plane but not the perpendicular plane. Cylindrical lens Rays in the vertical plane are undeviated Rays in the horizontal plane are focused

28. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Common Eye Diseases u Cataract - Clouding of the lens. u Symptom: Loss of vision u Cure: Lens replacement u Glaucoma - Pressure buildup in the eye, damaging the retina. u Symptom: Loss of vision first in the periphery. u Cure: Surgery to drain fluid from the eye. u Loss of vision is usually permanent

29. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Common Eye Diseases u Detached retina - portion of the retina detaches from the back of the eye. u Symptom: Perception of flashes, Loss of vision u Cure: Laser surgery to reattach retina u Pink eye - Infection of the surface of the eye. u Symptom: Irritation u Cure: Antibiotics Ow!

30. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Your eye care Go see a doctor if you think there is something wrong with your eye- Early detection is essential to keeping damage low and preventing permanent loss of your vision.