Presentation on theme: "Perception Chapter 2 The physics of light: What we see is a small part of the electromagnetic spectrum call visible light -- refer to overhead. Physical."— Presentation transcript:
Perception Chapter 2 The physics of light: What we see is a small part of the electromagnetic spectrum call visible light -- refer to overhead. Physical properties of visible light: wavelength (varies from 400 to 700 nm) -- perception of hue amplitude -- perception of brightness purity – saturation External light falls on receptors within the eye to generate the visual message. Light = electromagnetic radiation Wavelength of light determines color
Perceived Light Energy: Terminology Emitted and Reflected light Emitted: refers to light energy emanating from a light producing source such as a light bulb or the sun -- this type of light is also referred to radiance. Light striking a surface: illuminance Reflected: light bouncing off of a surface (luminance). The surface itself does not produce the light. The reflectance properties of the surface determine the appearance of the surface. The lightness of a surface is determined by the degree of reflectance vs. absorption properties of the surface. These properties determine if the surface appears light or dark, green or blue (or any other hue), glossy or matte. The appearance of an object is jointly determined by the reflectance properties of the surface and the qualities of energy available to be reflected. This is why objects look different under different lighting conditions. We turn on a light so that that range of energy will be available to strike the varied surfaces of the environment, reflecting back to our eye, and therefore, provide is with information about objects and surfaces in the environment.
Three layers of eye: 1) Outermost layer -- the Fibrous Tunic: this layer makes up the very outside surface of the eye, as name implies it is made up of tough, rather tightly interwoven fibers called sclera. The "white" of the eye is the sclera, this surfaces loses its white color at the very front of the eye and becomes the transparent cornea. This layer is the protective outer covering of eye. 2) Middle layer -- Vascular Tunic: As name implies this layer is a densely packed network of blood vessels and capillaries which serve to nourish the structures in the retina of the eye. This layer also contains a spongy darkened structured called Choroid. Choroid serves to absorb light not absorbed by photoreceptors in retina, this reduced visual scatter and blurriness. "Detached" retina is one that is no longer connected to this layer and therefore has lost an important supply of nutrition. 3) Innermost layer -- Retina: thin, fragile layer containing photoreceptor cells which respond to visual light, and initiate visual information processing. More details about retina later. Three layers: one for protection, one for nourishment, one for processing.
Two chambers of the eye: 1) Front or Anterior Chamber: Structures a) Ciliary body: toward front of eye, choriod forms ciliary body which lies immediately adjacent to opening of anterior chamber, this body produces aqueous humor. b) Aqueous humor: clear fluid which provides nutrients to structures of anterior chamber, replaces blood, helps maintain proper pressure and shape of eye. Improper circulation can cause visual disorders, blindness, glaucoma. c) Iris: muscular tissues which control opening of eye, and provide color to eye. d) pupil: opening of eye, constricted in bright light, dilated in dim. pupil tries to maintain appropriate depth of field, or degree of image sharpness vs. amount of necessary incoming light. e) lens: elastic structure which changes shape (accommodates) in order to focus incoming light properly on back of eye (retina). 2) Posterior or Vitreous Chamber: this chamber comprises two thirds of entire eye. Contains a clear jelly-like substance called vitreous humor, which similar function as aqueous human.
The retina: parts 1) Macula: point on the retina where the center of the visual field is focused. 2) Fovea: depression in center of macula, point of highest visual acuity. 3) Optic disk: point in retina where optic nerve leaves back of eye, no light sensitivity here, perceptual blind spot. 4) Photoreceptor cells: cells in retina which respond to the presence of light. There are two types: Photoreceptor cells: rods & cones
Rods and Cones Rods: peripherally more abundant highest wavelength absorption about 505 nm active under low amplitude light conditions, twilight. color blind Cones: centrally more abundant, fovea contains densely packed cones only three types of cones one has highest absorption around 440nm, one at 550, and one 570. these three "families" of cones are important for producing color vision as will be seen later. more active in daylight conditions.
Optics of the eye: Visible light, when reflected off of a surface, has the tendency to diverge, or put another way, the waves of light tend to spread out as they leave the reflected surface. For light to be focused into a sharp visible image, these divergent waves must be refracted or bent back into a converging pattern. This is exactly what a convex lens does (magnifying glass) and this exactly what is accomplished by the convex shape of the cornea, and the accommodation of the lens. As objects get closer to eye, degree of divergence is greater, therefore lens must getter fatter (greater accommodation, increased convexity) in order to focus image on to retina. However the optical power of eye, and the shape of eye do not always match. Emmetropic: optics of eye in balance such that light images are focused properly on retina. Myopic: optics of eye and shape do not match such that eye is too long for refractive power of eye, hence, images are focused in front of retina. Solutions -- move closer to object (nearsightedness), or put a concave lens in front of eye. Hyperopic: optics of eye and shape of eye do not match such that eye is too short for refractive power of eye, hence, images are focused behind (theoretically) retina. Solutions -- move object further from eye or place on convex lens in front of eye. Presbyopia: "old sight", reduction in accommodating ability of lens causes increased hyperopia as one gets older. Astigmatism: deformations of cornea which cause distorted images. Ok, once we get an image back to the retina, what happens?
Presbyopia: “old sight” Near point: closest distance where objects are in focus. Hyperope’s problem: Near point is too far. Increases with age due to accommodation failure
Photoreceptor response: Each photoreceptor cell contains a light sensitive molecule called a photopigment. The photopigment contains tight bound parts. One part is a protein called opsin and the second part is a derivative substance of Vitamin A called retinal. (Hence the importance of Vit. A in vision). When light strikes the cell, the retinal absorbs the light energy and changes its shape, called: Isomerization. It then splits off of the opsin. The splitting of the photopigment causes changes in the electrical current of the cell, a change which is then transmitted to adjoin cells and eventually down the optic nerve to the brain.