Chapter 2

Perception Chapter 2
The physics of light:
What we see is a small part of the electromagnetic spectrum call visible light -- refer to
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
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.
Properties of Light
wavelength: distance between peaks
• perceived as hue
• some wavelengths beyond human sensation
amplitude: height of wave
• perceived as brightness
purity: mixture of wavelengths
• perceived as saturation
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Why use visible light?
Fast, straight, abundant, interacts with surfaces
Physical/Perceptual properties of light
© 2004 John Wiley & Sons, Inc.
Reflectance curves
Eyes: Visual receptive organs
• Where to put them? Frontal vs.
• Mobile vs. Immobile
Three layers of eye
Fibrous tunic, Vascular tunic, and Retina
© 2004 John Wiley & Sons, Inc.
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
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.
Anatomy of the Eye
© 2004 John Wiley & Sons, Inc.
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.
Glaucoma: Open and Closed Angle
Tonometry: test for frontal eye pressure
© 2004 John Wiley & Sons, Inc.
Structure of the Eye: Retina
© 2011 The McGraw-Hill Companies, Inc.
The retina: parts
1) Macula: point on the retina where the center of the visual field is
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.
Structure of the Eye: Retina
– sensitive to even dim light, but not color
– function well in low illumination
– humans have ≈ 120 million rods
– respond to color
– operate best under high illumination
– humans have ≈ 6 million cones
© 2011 The McGraw-Hill Companies, Inc.
Eye as optical instrument
Light refraction
Far point: farthest point out where objects are in
focus. Problem for myope is that far point is too
near. Past far point, things out-of-focus
© 2004 John Wiley & Sons, Inc.
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
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.
problem: Near
point is too far.
Increases with age
due to
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

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