Transcript Visual acuity and color vision

Visual acuity and color vision
Aims and Objectives
•Understand the principles behind vision testing
•Perform an accurate visual acuity
• To differentiate between subjects with normal
& abnormal color vision
1.Visual Acuity
•Visual acuity is a measurement of central vision
only
•Assessment of total visual system from cornea
to occipital cortex
Why do a visual acuity test?
•Diagnostic tool
•Measures progression of disease
•Evaluates treatment
•Legal requirement
Vision Testing Tools
• Vision
charts
are
standardized for size and
contrast
• The Snellen Chart is used for
testing distance vision They
are designed to be read at 6
metres
• test types, a series of letters
of different sizes constructed
so that the top letter is
visible to the normal eye by
60 meters& the subsequent
lines at 36, 24, 18, 12,9,6&5
meters respectively
E chart
For use in illiterate patients
Patient matches direction of E with fingers
Distance Correction
•People who are short-sighted and normally
wear glasses or contact lenses should have their
visual acuity tested wearing their glasses
•Reading glasses (magnifiers) should not be
worn during distance testing
Note
If the subject wears glasses, repeat these
observations with & without glasses
Occlusion
•Each eye needs to be tested
separately
• cover the eye that is not being
tested
•Avoid pressure on the eye
haw to do the test?
Using the Snellen Chart
•Place patient at 6 metres
from chart
•Use adequate illumination
•Cover left eye
•Ask patient to read from the
top letter
• Keep going until they cannot
read the line clearly and start
to make multiple errors.
Visual acuity (V) is recorded according to the
ratio: V= d/D
V= Visual acuity.
d= Distance of the eye from the Snellen’s test.
D= Distance at which the eye should be able to
read clearly
Normal person should be able to read at least the
7th line (i.e.) V = 6 / 6.
If the V is less than 6 /60, walk patient towards the
chart about 1 meters.
•Still can’t read the chart?
walk patient towards the chart about anther 1
meters and so on
until he can read the top letter. If the top letter is
visible at 2 meters; so the V = 2 / 60.
If the Visual acuity of less than 1 / 60 is recorded as:
Still can’t read the chart?
Ask patient to count how many fingers you are
holding up at 1 metre.
Keep fingers still. Recorded as Count Fingers (CF
@1m)
•If they cannot count fingers see if they can see a
moving hand. Recorded as Hand Movements (HM
@1m)
•Still no result: can they see a pen torch light : Light
perception (LP)
•Unable to perceive light: No Light Perception (NLP)
Emmetropia
Eye considered to be normal “emmetropic” if •
a parallel light rays from distant object (6
meters or more) in front of the eye, are in
sharp focus on the retina when the ciliary
muscles is completely relaxed
Myopia (Near sightedness(
When the ciliary muscle is completely relaxed, the
light rays coming from distant objects are focused in
front of the retina due to:
a- Usually due to too long eyeball.
b- Occasionally result from too much refractive
power of the lens system of the eye.
Correction
BY placing in front the eye a concave lens which
diverge the light rays.
2*Hyperopia “far- sightedness”
Eye ball that is too short or.
Occasionally the lens system too weak
when the ciliary muscle is relaxed parallel light
rays are not bent sufficiently by the lens system
to come to a focus by the time they reach
behind the retina.
Correction: use convex lenses.
3*Astigmatism
the curvature of the cornea is not uniform.
light rays are refracted to a different focus, so
that part of the retinal image is blurred.
Correction:
by cylindrical lens
Color Vision
Subjects & methods
1. Subjects.
2.Ishihara Pseudo-Isochromatic palates
White light sensation: due to equal stimulation of all red,
green & blue cones. Yet there is no wave length for white
light .
Yellow color sensation: no wave length for yellow but
equal stimulation of red & green cones at the same time.
An object appears red when all wave lengths, except red
wave length, are absorbed by the object.
An object appears white when it reflects all wave lengths.
An object appear black when the object absorbs all the
wave lengths of light.
color blindness
When a single group of color receptive cones is missing from
the eye, the person is unable to distinguish some colors from
others; the person is especially unable to distinguish red from
green and is therefore said to have red-green color blindness.
2% red color blindness ( protanope).(
6%green color blindness(deuteranope)
8% red- green color blindness.(
blue color blindness very rare, but may have color weakness.
The person with loss of Blue cones is called Tritanope
Red-green color blindness
It is a genetic disorder that occurs almost
exclusively in males. That is, genes in the female X
chromosome code for the respective cones
Yet color blindness almost never occurs in females
because at least one of the two X chromosomes
almost always has a normal gene for each type of
cone. Because the male has only one X
chromosome, a missing gene can lead to color
blindness. Because the X chromosome in the male
is always inherited from the mother, never from the
father
People with defective color vision will read a
different number from a normal person on the
same plate; in other word color blindness is a
defect in the ability to discriminate between
light of different wave lengths, it is usually
explained by a deficiency of one or more of the
cone pigments
procedure
1.The subject is asked to read figures in the
available Ishihara Pseudo-Isochromatic palates.
2.In the top chart; the normal person reads (74),
while the red green color blind person reads
(21).
3.In the bottom chart; the normal person reads
(42), while the red blind person reads (2), & the
green blind person reads (4).
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