visual acuity and ophthalmoscopy

DIFINITION OF VISUAL ACUITY
•is acuteness or clearness of vision ,
it measured according to the size of letters viewed on a Snellen chart .
VISUAL ACUITY TEST
?The visual acuity test is used to determine the smallest letters a person can read on a standardized chart (Snellen chart)
V = d / D
•V = Visual acuity.
•d = distance of from the chart (Distance at which test is made (6 meters=20 feets).
•D = distance at which the eye should be capable to read it clearly.
?Using the foot as a unit of measurement, (fractional) visual acuity is expressed relative to 20/20. Otherwise, using the meter, visual acuity is expressed relative to 6/6.
?So 6/6 vision is equivalent to 20/20.
To examine the visual acuity of eyes.
Subjects and instruments:
1- Subjects.
2- Snellen chart.
Objective:
The Method:
1.The subject is placed at a distance of six meters from the test chart.
2.Each eye is tested separately.
3.If the subject wears glasses, test eyes with and without them.
4.Normal person should be able to read lines (6/6).
5.If visual acuity of person is less than 6/60, it is recorded as:
a- Counting finger (CF).
b- Hand movement (HM).
c- Perception of light (PL).
d- No perception of light (no PL).
MYOPIA (NEARSIGHTEDNESS)
?The AP diameter of
eyeball is too long.
?The focus of the image would have been in front the retina.
?Myopia is corrected by glasses with concave lenses that cause the light rays to diverge.