Eye and the Process of Vision

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

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.

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.

Eye accommodation Accommodation range 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

Adaptation Adaptation mechanisms: – 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)

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

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°

RESOLUTION ABILITY luminance operating threshold color difference 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 (-; cd·m-2, cd·m-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 – Lb|min = ΔLmin 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 1°viewing angle. Human vision is able to distinguish the lines of very high frequency or very low frequency.

Contrast sensitivity decreases with decreasing levels of illuminance. = 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)

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)

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

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

HYPEROPIA Correction converging lens

MYOPIA Correction diffusing lens

PHYSIOLOGICAL ASTIGMATISM Certain correction: cylindrical spectacle lenses processing the front surface of the cornea with laser

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.

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

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