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, lens
b) 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 části 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
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 ]
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
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
Far 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)
– furthest point where can fully accommodated eye see sharply;
[ r2 - far point distance from the eye (m) ]
Accommodation range 
1
1

r1
r2
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
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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 1:16 to 1:20, 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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FIELD OF VISION
part of the space perceived by the observer
in gaze without eye and head movement
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.
90°
Hatched circle marks the region which is
projected into a blind spot.
▪ continuous line
▪ dashed line
▪ dotted line
yellow and blue light
red light
green light
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
C 
La  Lb
Lb

L
(-; cd·m-2, cd·m-2)
Lb
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 – Lb|min = ΔLmin is called
luminance operating threshold
Threshold contrast Cmin
Cmin 
Lmin
Lb
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 1°viewing angle.
Human vision is able to distinguish the lines of very high frequency or very low frequency.
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CONTRAST SENSITIVITY
=
 1

 Cmin

Lb
Lb
 

Lmin
La  Lb

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 – Lb|min = ΔLmin
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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VISUAL ACUITY
visual acuity =
1
 min
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.
min 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
1
 min
1

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()] 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:
Refractive failures
(ametropia)
parallel rays incident on the cornea and
converge at the retina to one point
Rays converge behind retina  hyperopia
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 OSVĚTLOVACÍ TECHNIKY
| 30.9-2. 10. 2013
Thank you for your attention
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