The Human Visual System The Eye Imaging Science

The Human Visual System The Eye Imaging Science Fundamentals Chester 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 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

Human Visual System Image formation • Cornea • Lens Imaging Science Fundamentals Exposure Control Detection Processing • Iris/pupil • Photoreceptor sensitivity • Retina • Rods • Cones • Brain Chester F. Carlson Center for Imaging Science

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

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

Cornea u The Sclera Cornea Imaging Science Fundamentals 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. Chester F. Carlson Center for Imaging Science

Iris and Pupil Iris 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. Pupil Iris open Dilated pupil Imaging Science Fundamentals Iris closed Constricted pupil Chester F. Carlson Center for Imaging Science

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

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

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

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

Plexiform Layer u The retina is made of three layers: Plexiform layer is a network of nerves which carry the signals from the photo receptors. 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. u Fovea Light Plexiform Layer Optic Nerve Imaging Science Fundamentals Choroid Chester F. Carlson Center for Imaging Science

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

Adaptation u Threshold of detection (log scale) Photopic (cones) 0 u Scotopic (rods) u 5 10 15 20 25 Time in dark (minutes) Imaging Science Fundamentals 30 Why can’t you see immediately after you enter a movie theater from daylight? The threshold of detection changes with overall light level. The switch is quite gradual, until the sensitivities of cones and rods cross over at about 7 minutes in the dark. Chester F. Carlson Center for Imaging Science

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

Human Vision u Human u three Cone Response to Color cone types (S, I, L) correspond to B, G, R I L Relative response S 400 460 490 500 530 600 650 700 Wavelength (nm) Blue Imaging Science Fundamentals Cyan Green Red Chester F. Carlson Center for Imaging Science

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

Hermann Grid u Illustrates lateral inhibition. Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

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

Mach Bands Actual brightness Perceived by you Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

Eye Defects Object at infinity 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 Eyes at relax state. Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

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

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

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

Presbyopia - “Old eye” Concave for distance correction (if needed) Convex for near object correction Far objects 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 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

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

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

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

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