2. Light :
It may be defined as an energy to which
human eye is sensitive.
In Electromagnetic spectrum visual light
occupy a small area of wavelengths
between 400nm – 780nm.
4. We are Lucky !
UV-B
UV-C Cornea & Sclera
IR-B
IR-C
-------------------------------------------------------
UV-A Lens
-------------------------------------------------------------
Visible Lights Pass through to Retina
Near IR
5. Visible Radiation (400 – 780 nm) pass through
the ocular media → fall on the retina →
stimulate retinal photoreceptor cells → giving
the sensation of light or vision.
While the near IR (781 – 1400 nm) cause
thermal effect. So if we focus to the incident of
IR it can cause retinal damage (Eclipse burn).
6. 400nm Violet (400 – 420nm)
Indigo (420 – 440nm)
Blue (440 – 490nm)
Green (490 – 570nm)
Yellow (570 – 585nm)
Orange (585 – 620nm)
780nm Red (620 – 780nm)
The normal eye is able to discriminate
between light of shorter or longer wavelength
within the visible spectrum
14. Definition:
The resolving power of the eyes by
which objects are distinguished clearly from
the others.
In other words, an objective measure of what
the person can see.
15.
16. Why needed?
To test visual function
Refractive status of the eye
Outcome measure for a treatment
Medico legal purposes
Criteria for –
person’s fitness to drive
eligibility for entrance into a profession
17. Factors affecting VA :
Refractive error
Size of the pupil
Accommodation
The health and integrity of the eye
Illumination of the test object
The test target used
Area of retina stimulated
State of adaptation of eye
Eye movement
Cognitive status
18. Today’s discussion about visual acuity will be
limited only within school going children to adults
22. Near acuity chart :
Roman test types
Snellen’s near vision test type
Jager’s chart
Etc…
23. Herman Snellen : A Dutch ophthalmologist
who introduced the Snellen chart to study
visual acuity in 1862.
24. Principles of snellen acuity :
The 6/6 line is normal vision
The number above the line describe the
distance of the patient from the chart
The number below the line denotes
which line is seen
Each letter is designed in a square with
sides 5 times the width of letter strokes
The breath of black strokes and white
spaces are equal
25. Cont…:
The breath of line and spaces produce
1’ min of arc at nodal point when viewed
from a certain distance
Each letter subtends an of 5’ of arc at
nodal point when seen at a certain
distance
26. Cont…:
But in order to analyses its form completely
and see its constituent parts, the eye must be
able to resolve them down to the standard
limit of 1’
So,
On the 6/6 line each letter is constructed
to subtend an angle of 1’ of arc at a
distance of 6 meter
Other lines are constructed in a similar way, so
that letters on the 6/18 line or 6/60 line
subtend an angle of 1’ of arc if tested at 18m
or 60m from the chart
27.
28. Procedure to test distance VA :
Patient seated at 6m distant from the chart
So light rays are parallel and pt exerts minimal
accommodation
Chart should be properly illuminated
The pt is asked to read the chart each eye
separately and VA is recorded
Depending upon the smallest line that the
pt can read from 6m distance, his/her VA is
as 6/6, 6/9, 6/12, 6/18, 6/24, 6/36 and 6/60
If one cannot see the top line from 6m, pt
is asked to slowly walk towards the chart till
can read the top line
29. Cont… :
Depending upon the distance at which one
can read the top line, the vision is recorded
as 5/60, 4/60, 3/60, 2/60, 1/60
If the patient is unable to read the top line
even from 1m, he/she is asked to count
fingers (CF) of the examiner
His/her VA is recorded as CF-3’, CF-2’, CF-1’
or CF close to face depending upon the
distance at which the pt is able to count
fingers
30. Cont… :
When the pt fails to count fingers, the examiner
should move his/her hand close to pt’s face
If pt can appreciate the hand movements, the
VA is recorded as HM+
If pt cannot distinguish hand movements, the
examiner should test whether the pt can
perceive light or not
If yes, vision is recorded as PL+ and if not it is
recorded as PL-/NPL
PL+ pts must be tested with projection of rays
to represent quadrants - nasal, superior,
temporal and inferior
Then VA is recorded as PL+, PR ±
31. Procedure to test near VA :
The pt is comfortably seat in a chair and asked
to read the near vision chart kept in a distance
of 33 – 40 cm
Good illumination thrown over from his/her
back preferably over left shoulder
Each eye should be tested separately
The near vision is recorded as the smallest
type that can be read comfortably by the
patient as N5, N6, N8, N10, N12, N14, N18,
N24, N36 and N48
Notation is made as NV = N5 at 30cm
TIP – N8 is the most common size print in most books
32. Steps :
Unaided VA ../..
Aided VA ../..
Pinhole VA ../..
NV N.. at ..cm
33.
34. KEY POINTS :
Test each eye separately
Test before shining a bright light into eyes
Test before dilating the pupil
Test every cases of VA 6/9 or less with
pinhole to detect refractive error
Always test Rt. eye first
Make sure pt isn’t peeking through his/her
fingers