Visual acuity

Visible light and Visual acuity

1. Visible Light
Visual Acuity
Dr. Md. Nurul Islam Sabuj
Sr. Medical Officer
Grameen GC Eye Hospital, Barisal.

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.

3. Optical Radiation :
UV-C - 200 – 280 nm
UV-B - 280 – 315 nm
UV-A - 315 – 400 nm
Visible radiation 400 – 780 nm
IR-A - 780 – 1400 nm
IR-B - 1400 – 3000 nm
IR-C - 3000 – 10000 nm

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

7. How?

8. By means of three different cone cells –
ERYTHROLABE – Red sensitive
CHOLOROLABE – Green sensitive
CYANOLABE – Blue Sensitive

9. “ Everything looks bluer than before.”

A common complain of newly aphakic or
IOL implanted (without UV filter) patient.

Do you know, why?

11. Vision Assessment :
 Visual acuity
 Visual field exam
 Color vision
 Contrast sensitivity
 Vernier acuity
 Dark adaptation
 Amsler grid test
 Pupillary reaction
 Etc..

12. Visual Acuity

13. What is visual acuity?

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.

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

20. Distance acuity chart :

 Snellen’s distance acuity chart
 Letters
 E
 Numbers
 Landolt broken ring (or, C chart)
 Bailey – Lovie chart
 LogMAR chart
 Etc..

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

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

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

35. THANK YOU