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Eye and the Process of Vision
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Eye and the process of vision
VISION - 1. vision sensor (eye) receive
information brought by the light stimulus
2. processing, selection and encoding
information (opt. stimuli in nerve impulses)
3. transfer to the vision center of
the brain arise the visual sensation
4. synthesis of sensations creates a
visual perception 5.
classification of the perception in the mind a)
for immediate use
b) to
store in memory later application
EYE a) optical part conveys receiving the
information cornea, anterior chamber,
iris, pupil, lensb) nerve part retina
(photoreceptors, ganglion and other nerve cells,
reciprocal links), optic nerve, vision
centers in the brain, links with other centers
Retina translucent thin (0,2 mm) membrane 11
layers complicated regular
cellular structure especially
the ganglion cells, bipolar and others,
receptors (6,5 mil. cones 125 mil. rods
C) ? first processing of received
information, ? coding of info
(frequency-modulated pulses) ? sorting
and selecting of the information
Macula bright brown region, without any vessel
its central raised section (diameter
approximately 1,5 mm) is called fovea
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Visual system
The visual system consists of a set of human
organs responsible for receiving, transmitting
and processing of the information brought in by
the light stimulus into the complex nerve
irritation, that results in the visual perception.
Human visual systém consists approximately from
three main parts peripheral (eyes),
connecting (zrakové nervy), central
(podkorové a korové cásti mozku).
Simplified diagram of the visual system SPO, SLO
retina of the right and left eye PZN, LZN
right and left optic nerve CH place of the
partial crossing of nerve fibers
(chiasma) PZT, LZT right and left optic tract
(tractus opticus) LG lateral
geniculate nukleus (primary brain center) HH
optic radiations (colliculi superiores) ZK
primary visual cortex
At all levels there are numerous links with
centers of other sensory organs.
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Perceptual field
Basic functional unit of the retina
part (approximately circular) of the retina
area, from which can irritate one ganglion cell
associated with a single fiber optic nerve.
The basic functional unit of the retina is not
one photoreceptor
Size of the perceptual field varies depending
on ? luminance of the light
initiative ? spectral
composition of the initiative
? condition of adaptation of the retina
In the human retina there are many kinds and
types of functional perceptual fields. Fields may
partly overlap.
Reaction of the field is dependent on -
illuminance level - spectral composition of the
initiative - duration of the initiative/stimulus
- spatial distribution of flux - time
distribution of flux
E.g. in some fields reacts field center to the
start initiative, margins to its end.In other
fields, the opposite is true. Another field
exhibit both types of reactions.
Perceptual field react a) Either throughout the
duration of the stimulus. Than convey information
about luminance contrast or color and small
details. important for resolution ability
b) Or it is temporary, short response to
illumination changes and information about the
time changes of the stimulus important for
the adaptation process
With one ganglion cell it is connected - few
thousand receptors at the edge of the retina -
in the central hole area (densely located cones)
is 1 receptor (cone) connected to the one
ganglion cell this makes the highest resolution
in this area
The resulting evaluation of the information in
significantly affected by many connections
between different nerve cells and centers of
other sensory organs and numerous feedback.
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Eye accommodation
Ability of the eye to accommodate Fragility to
the eye for a near vision Changing curvature of
the front and rear wall of the lens (change of
the focal distance of the eye) in a way that even
nearby objects appear sharply on the retina.
Normal eye looking into the distance displays on
retina sharp objects which are placed at the
theoretically infinitely distant (almost more
than 6m) from the eye. Rays which are bringing
information about objects are placed in this way
than fall into the eye parallel.
The reciprocal of the focal length/distance
optical power
is measured in diopters (D)
Near point closest point where can fully
accommodated eye see sharply
r1 - near point distance from the
eye (m)
( r1 age 15 9 to 10 cm age 30 13 cm
age 50 50 cm) Far point furthest point where
can fully accommodated eye see sharply
r2 - far point distance
from the eye (m)
Note Age 15 10 D age 50 only 2
D Short-sighted elderly may also have 10D, but
ranging between 10-5cm in front of the eye
Accommodation range
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Adaptation
adaptation of the eye to different levels of
illuminance
The eye is capable of adapting the illuminance of
the vertical plane longitudinal to the pupil from
about 0,25 lx to 105 lx
Adaptation mechanisms
? change in pupil diameter (1,8 to 7,5 mm) ?
change in the pupil opening area in ratio 116 to
120, time of the change 360 ms ? change in
sensitivity of the photoreceptors
(decomposition or synthesis of visual pigments
photochemical action) minutes ? change in
size of perceptual fields (smaller diameter at
higher levels and vice versa) ? adaptation to
even large changes in the spectral composition of
the stimulus (stability of color tones
perception) ? compensatory mechanisms - cancels
information about changes caused by movements of
the eye, head or body (image on the retina
varies from about 5 images per second) ? visual
perception arises simultaneously with impulse,
but with a time lag (luminance
over 1 cd.m-2 approx.. 0,5 s low luminance
levels approx. 1 s) ? adaptation of the visual
organ due to the reflex responses of the brain
center for radiation ? physiological adaptation
mechanisms (memory and attention mechanisms also
determine the final position and response to
human visual perception)

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part of the space perceived by the observer in
gaze without eye and head movement
FIELD OF VISION
Precisely one sees in the range of about 8
horizontally and about 6 in the vertical plane.
The greatest sharpness is in the range of about
1,5 - macula region
Distinguishing detail (critical detail) is placed
by eye by reflective movement to the center of
the visual field. The detail is then displayed on
the retina in the center of the macula.
Immediate surrounding area details Area of the
field of vision about the peak angle 20
(important for direct resolution of detail)
Observed object detail immediate
surrounding area
Surrounding area visual field from about 20 to
60
Binocular and monocular fields of vision for
white light (eye position indicated by circles)
Far surrounding area from 60 to the edges of
the field
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Dependency of the visual field on the correlated
color
Monocular field of vision of the right eye at
different colors of light stimuli. Hatched
circle marks the region which is projected into a
blind spot. ? continuous line yellow and blue
light ? dashed line red light ? dotted line
green light
90
60
0
30
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RESOLUTION ABILITY
differently bright objects (luminance difference)
Observer differentiates details in the visual
field (objects) from which are coming
sufficiently different light stimuli
color difference
The degree of recognisability of variously clear
details is characterized by luminance contrast C
(- cdm-2, cdm-2)
La luminance of differenciate detail (task
area) Lb luminance of immediate surrounding
area of detail (luminance of surrounding area
adaptive luinance)
Probability of the resolution of detail with the
incerasing growth of contrast C
The smallest distinguishable luminance difference
La Lbmin ?Lmin is called luminance
operating threshold
Threshold contrast Cmin
The ability of resolution is generally determined
by a sinusoidal component of the image, for which
frequency is the eye most sensitive
For a normal individuals is the best resolution
at a frequency of about 6-9 cycles per 1viewing
angle.
Human vision is able to distinguish the lines of
very high frequency or very low frequency.
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CONTRAST SENSITIVITY

Reciprocal value of threshold contrast Cmin
Contrast sensitivity depends on size of the
diferentiated detail characterized by the
luminance La , is inversely proportional to
the treshold of luminance determination La
Lbmin ?Lmin increases with the value of the
adaptation luminance Lb
Contrast sensitivity decreases with decreasing
levels of illuminance. (to capture the small
number of quanta a large number of receptors are
coombined to a perceptual field of large diameter
decreases probability of finding adifference of
few quant)
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The criterion for valuing the eye's ability to
recognize at the background two small details
(points, lines, etc.) that are very close to each
other.
VISUAL ACUITY
visual acuity
amin smallest angle in minutes at which the
eye is unable to distinguish two small details as
separate
Eye with normal acuity detects two points, whose
distance is seen at an angle of 1 min

Eye with a visual acuity 1
?
The smaller is the distance of observed details
by the eye, the greater is visual acuity.
Visual acuity drops sharply from the central hole
to the edges of the retina.
Distribution of visual acuity on the
retina Continuous line for photopic
vision Dotted line for scotopic vision
(between 10 and 20 degrees of nasal direction
is marked the area of blind spot)
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SPECTRAL VISION SENSITIVITY
Each indivisual has a different course of a
vision sensitivity to radiation of different
wavelengths. To make photometric calculations
consistent was adopted an agreement by the
International Commission on Illumination (CIE) on
values of the relative spectral sensitivity of so
called normal photometric observer (standard) in
daylight (photopic) vision curve V(l) and at
night (scotopic) vision curve V?(?).
Spectral sensitivity is most often expressed in
values relative to the maximumabsolute value of
the sensitivity, respectively the maximum
absolute value of the luminous effect of
radiation.
1 La 10-5 cd.m-2 curve V?(?) according
to CIE for scotopic (night) vision 2
La 10-4 cd.m-2 3
La 10-3 cd.m-2
4 La 10-2 cd.m-2
5 La 0,1 cd.m-2 6 La 1
cd.m-2 7 La 10 cd.m-2 8
La 100 cd.m-2 curve V(?) according to
CIE for photopic (day) vision
Relative spectral sensitivity curves of CIE
standard photometric observer to radiation of
different wavelengths for different adaptation
luminance La
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FAILURES OF VISION
Emmetropic eye - correct display parallel
rays incident on the cornea and converge at
the retina to one point
Rays converge behind retina ? hyperopia
Refractive failures (ametropia)
Rays converge before retina ? myopia
Almost everyone older than 45 years have reading
difficulties in the near distance elderly
farsightedness (presbyopia)
Other common variations (aberrations)
Spherical error different refraction of
central and edge parts of the lens
beams farther from the
optical axis is refracted closer to the lens
can not be
removed, mitigation by the quality lighting
Chromatic error closer to the axis are
refracted rays of light of shorter wavelengths
the
distance between the extremities of the focal
range about 0,6mm small error for yellow
light
(focus of violet closer to range, focus of red
farther from lens)
can not be removed, mitigation by the
quality lighting
Physiological astigmatism unequal curvature
of the light-fracture surface of the
lens in different meridians More often are rays
in the vertical plane refracted more than in the
horizontal plane. Certain
corrections Cylindrical spectacle lenses
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HYPEROPIA
Correction
converging lens
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MYOPIA
Correction
diffusing lens
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PHYSIOLOGICAL ASTIGMATISM
Certain correction cylindrical spectacle
lenses processing the front surface of the
cornea with laser
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• Color perception failures
• The inability to see colors across the spectrum
  complete color blindness very rare, more often
  is disturbance in the perception of certain
  colors.
• Usually inherited disorders, nonprogressive (very
  important whether the client knows). Acquired
  disorders may than occur in the elderly, at
  neuropathies, retinal inflammation, glaucoma, and
  after the administration of certain drugs,
  especially cardiac.
• Cones and their function is violated cones
  except for color perception also providing visual
  accuracy (VA), the VA is also reduced (see also
  note Evaluation of vision).
• Physiological state of the correct color vision
  is called trichromacy in the eye are three
  groups of retinal cone pigments reacting to blue,
  green and red. Anomalies are called
• protanomaly (sees worse red)
• deuteranomaly (sees worse green)
• By the complete absence of one group of pigment
  we can talk about dichromacy
• protanopy (can not see red)
• deuteranopy (can not see green)
• tritanopy (can not see blue) - rare
• People with very rare monochromacy have got only
  one cone pigment. The population frequency of
  color blindness is estimated at 8,5 (8 of men
  and 0,5 of women). Most common is deuteranomaly.

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Color perception failures
Normal
Protanop
Deuteranop
KURZ OSVETLOVACÍ TECHNIKY 30.9-2. 10. 2013
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