9.2 Human Performance And Limitations - Vision Flashcards
what is the basic structure of the eye? (3)
aperture (iris)
lens
light-sensitive surface (retina)
what is the cornea?
clear window at the front of the eye which allows light to enter
what is the cornea responsible for?
70-80% of the total focusing ability of the eye
what is the iris?
the coloured part of your eye,
controls the amount of light that enters the eye
what is the pupil?
the black section of your eye in the centre of the iris.
rapidly changes its size to changing light levels
how many times can the amount of light be adjusted by?
factor of 5:1
what does light pass through after the pupil?
the lens
how is the shape of the lens changed? what is this called?
by ciliary muscles by thickening or flattening the lens according to the distance of focus
accommodation
how does the lens adjust for close and far distances?
close = thickening
far = flattening
how is accommodation affected?
ageing
fatigue
tiredness
what is the retina?
complex layer of nerve cells connected to the optic nerve
what two cells are found in the retina?
cones and rods
what are cones?
function in good light
capable of detecting fine details
colour sensitive (1000 different shades of colour)
what are rods?
cannot detect colour
poor at distinguishing fine detail
good at detecting movement in the edge of the visual field (peripheral vision)
much more sensitive in lower light levels
why do we see only black and white and shades of grey in poor light?
due to the sensing task being passed from the cones to the rods
what is the central area of the retina known as? what receptors does this have?
fovea
all cones
what is typically focused at the fovea?
the visual image
what happens when light stimulates the cells on the retina?
small electrical impulses by the way of the optic nerve to the visual cortex of the brain, where an image is perceived
what about blind spots?
a blind spot occurs at the back of the eye where the optic nerve join,
but rarely affects viewing as it is not possible for the image of an object to fall on the blind spot of both eyes at the same time
how many times are the eyes more sensitive than the ears?
24 times
what is visual acuity? which chart is used?
the ability of the eye to discriminate sharp detail at varying distances
Snellen chart
what is the meaning of 20/40 vision?
the observer can read at 20 feet what a non-impaired person can read at 40 feet
how does angular disatnce affect the resolving power of the fovea?
drops as angular distance increases
5 degrees = drop of 50%
what can affect the visual acuity of the eye? (13)
age
physical imperfections in one or both eyes (myopia and hypermetropia)
drugs
medication
alcohol
cigarettes
available light
clarity of the air
size and contours of the object
the contrast of the object with its surroundings
motion of the object
distance from the object
angle from the object
what is hypermetropia?
known as long sightedness
how is hypermetropia caused? (long sightedness)
shorter than normal eyeball causing the image to be formed behind the retina
blurred vision looking at close object
what is myopia?
known as short sightedness
how is myopia caused? (short sightedness)
a longer than normal eyeball causing an image to be formed in front of the retina
distant object blurred
what visual problems may happen? (6)
-cataracts (clouding of the lens with ageing)
-astigmatism (misshapen cornea causing objects to be irregularly shaped)
-glaucoma (build-up of pressure of the fluid within the eye causing damage to optic nerve)
-migraine
-presbyopia (lens less flexible, unable to accommodate sufficiently)
-fatigue
how does carbon monoxide affect the eye?
builds up in the bloodstream allowing less oxygen to the eye.
rapidly impairs sensitivity of the rods
how may environment affect vision? (4)
lighting level
dust
glare
spectacles
who does colour vision deficiency affect most?
men
men 8%, women 0.5%
what is the most common type of colour vision deficiency?
distinguishing between red and green
how can ageing affect colour vision deficiency?
progressive yellowing of the lens results in a reduction of colour discrimination in the blue-yellow range
what consideration must be taken with eyesight for an engineer?
that a reasonable standard of eyesight is needed to perform their duties
frequency of checks to be maintained
