Visual acuity (VA) is a measure of the ability of the eye to distinguish shapes and the details of objects at a given distance. It is important to assess VA in a consistent way in order to detect any changes in vision. One eye is tested at a time. Go to: Indications To provide a baseline recording of VA To aid examination and diagnosis of eye disease or refractive error To assess any changes in vision To measure the outcomes of cataract or other surgery. Go to: Equipment Multi-letter Snellen or E chart Plain occluder, card or tissue Pinhole occluder Torch or flashlight Patient's documentation. Go to: Procedure Ensure good natural light or illumination on the chart. It is important to ensure that the person has the best possible chance of seeing and reading the test chart as treatment decisions are made based on the results of VA testing. If the test is done outdoors, the chart should be in bright light and the patient in the shade, with enough light to illuminate the patient's face during the test. Explain the procedure to the patient. Tell patients that it is not a test that they have to pass, but a test to help us know how their eyes are working. Tell them not to guess if they cannot see. Ensure that any equipment that the patient touches is clean and is cleaned between patients. Infections can be passed between patients if equipment – or the testers' hands – are not clean. Position the patient, sitting or standing, at a distance of 6 metres from the chart. The patient can hold one end of a cord or rope of 6 metres long to ensure that the distance is maintained Test the eyes one at a time, at first without any spectacles (if worn). Note: Some people prefer to always test the right eye first. Others prefer to test the ‘worse’ eye first (ask the patient out of which eye they see best). This ensures that the minimum is read with the ‘worse’ eye, and more will be read with the ‘good’ eye. This means that no letters are remembered, which could make the second visual acuity appear better than it is. An external file that holds a picture, illustration, etc. Object name is jceh_27_85_016_f04.jpg Visual acuity should be measured from a standard distance, using a standard chart with a white background Ask the patient to cover one eye with a plain occluder, card or tissue. They should not press on the eye; this is not good for an eye that has undergone surgery. It can also make any subsequent intraocular pressure reading inaccurate and it will distort vision when the occluded eye is tested. Ask the patient to read from the top of the chart and from left to right. If the patient cannot read the letters due to language difficulties, use an E chart. The patient is asked to point in the direction the ‘legs’ of the E are facing. Note: there is a one in four chance that the patient can guess the direction; therefore it is recommended that the patient should correctly indicate the orientation of most letters of the same size, e.g. four out of five or five out

1. VISUAL ACUITY
Presented by :
Dr Mohit Kumar Jha
Resident Ophthalmology

2. BASICS
VISION VISUAL ACUITY

3. • Visual Acuity can be measured easily it only
quantifies high contrast vision loss and therefore
doesn't tell you about quality of vision
• It is the spatial resolving capacity of the visual
system which helps the eye to see the fine details

4. TYPES OF VISUAL ACUITY TASKS
1) TARGET DETECTION
2) Target resolution
3) Target localization
4) Target recognition

5. TARGET DETECTION TASKS
+ presence or absence of an aspect of the stimuli
+ Test objects and backgrounds are contrasting
+ Landolt C & the illiterate E test
+ task is too locate the gap.

6. TARGET RESOLUTION
It is the smallest angular size where the separation between
critical elements of a stimulus pattern can be discriminated.
Eg. Pair of dots , A grating

7. TARGET LOCALISATION
+Involves detecting discontinuity in contour , due to the
differences in the spatial position of segments of a test object
called VERNIER ACUITY

8. TARGET RECOGNITION
+Commonly used clinically to measure VA
+Requires the recognition or naming of a target
. Eg – Snellen`s letters
COGNITION IS
INVOLVED

9. CONVERSION OF SNELLEN`S ACUITY TO SPATIAL
FREQUENCY
Gratings can be used as another
method of measuring visual
acuity. Visual acuity in Snellen`s
notation can be expressed in
spatial frequency and vice versa.

10. PHYSIOLOGIC BASIS OF VISION
 Two points can be recognized as separate when they subtend an
angle of one minute of an arc at the nodal point of the eye
 visual acuity is a measure of the finest detail that can be
resolved or recognized by the visual system
 visual acuity tests in infants check resolution and not
recognition therefore amblyopia can go undetected.

11. SNELLEN`S ACUITY CHART
 Measures distance central acuity
 designed by Dr Herman snellen in1862
 symbols and letters are called optotypes
 8 to 9 lines on the chart
 BLACK capital letters against white background(high contrast)
 Sans-serif font , equally legible
 number of letters varies in different lines
 non uniform degrees in the size of the letters from top to
bottom
 fixed spacing between lines
 evenly illuminated (34-51 cd/m )

12. SNELLEN`S OPTOTYPES
+Each letter is designed to fit a 5X5 square
+the thickness of the dark lines and white
spaces between them is equal
+the height and the width of the letter is five
times the thickness of the constituent line
+at a given distance each letter subtended at
an angle of 5 minute at the nodal point
+and each small square or the thickness of
the constituent line subtends at an angle of
1 min at the nodal point

13. METHOD OF TESTING
+ Why is vision recorded at six meter or 20 feet?
Because the rays are parallel and no accommodation is exerted
and it's simulates Infinity
+ Illumination
• illuminated chart- dimly lit room
• non illuminated -chart in the sun, patient in shade
+ one eye at a time don't press, both eyes together
+ repeat with pinhole and spectacle correction

14. How is visual acuity quantified when the patient
is unable to see a standard snellen chart ?
• the best way is to move the patient and chart closer together.
• note the size of the optotype seen at a particular distance.
• move 1m closer to chart ( 5/60, 4/60, 1/60 )
• hand motions or light perceptions with or without projection.
• If Topmost Letter is not Visible , move closer 1m at a time ,and
if not visible at 1m then we tell patient to count fingers ,
further hand movement and lastly light perception

15. PERCEPTION OF
LIGHT
Light source is inferior. Illuminates
the superior retina. If patient cannot
localize the light inferiorly, PR
inaccurate in the superior quadrant .

16. SNELLEN`S FRACTION
 A visual equity of 6/6- represents the ability of the eye to see one minute of arc
 VA is equal to distance of the test BY distance at which a person with normal vision can see
the optotype
EXAMPLE
6/24 = 6 is the distance of the subject in metres from the vision chart
24 is the distance in meters at which a person with normal vision sees
the same autotype
at least 5 out of 6 letters have to be read to denote is as a snellen`s fraction
Reads a line partially
record vision of the previous full line
add the number of letters read in the subsequent line
example a reads full 6/12 line, two letters of 6/9 then visual equity equals to 6/12+2

17. Log MAR ACUITY
+ Logarithm of minimum angle of resolution
+ the logmar scale converts the geometric sequence of a traditional chart to a linear
scale
+ logmar charts designed by Bailey and Lovie in 1980
+ Sans serified Sloan optotypes
+ equal viewing difficulty each line has five letters and space between the letters is
equal to the letter size on that line
+ the distance between letters of a line as well as the space between the lines
gradually changes
+ the progression of optotype heights- is geometric, decreasing in 0.1 log unit
increments, similarly the space between rows
+ any three line (15 letters )decreases or increases results doubling or half
respectively of the visual angle .

18. BAILEY & LOVIE CHART

19. Recording VA in LogMAR Chart
• Each Letter – 0.02log units
• Total score per line+ 5x0.02 represents a change of
0.1 log units
SCORING METHODS
+ Score of last row where all 5 letters are read
+ Subtract 0.02 log units for every letter read beyond
the last row
- 20/30 row = 0.2
- 20/25 row 2x 0.2log/letter = -0.04
Acuity log score = 0.16

21. PIN HOLE TESTING
 A test performed on a person who has diminished visual equity to distinguish a
refractive error from organic diseases
 the patient looks through it with one eye at a time without wearing corrective lenses
 light passes only through the center of eyes lens and errors of refraction have no effect
while the occluder is used
 pinhole blocks the peripheral rays only letting those rays which pass through the central
portion of the pupil
 if visual acuity is improved the defect is refractive , if not its organic

22. NEAR VISUAL ACUITY
+The patient is comfortably seat in a chair and asked to read the near
vision chart kept in a distance of 33- 40 cm
+good illumination drawn over from his or 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 30 cm

23. NEAR VISION CHARTS
1. Roman Test Types
2. Snellen`s Near Vision Test Type
3. Jager`s Chart
The purpose is to detect people with
near vision difficulties
e.g (uncorrected high hyperopia ,
accommodative dysfunction)
In patients over 40 year old the
reduce near visual acuity is one of the
symptoms of presbyopia.

24. Factors other than diseases that reduce
visual activity
1. Uncorrected ametropia
2. Eccentric viewing
3. Decreased contrast
4. Large ( >6mm ) or small ( <2.5mm ) pupil size
5. Young or old age
6. large pupils reduce visual equity due to increased spherical
abrasion
7. smaller pupils reduce these optical aberrations but at the expense
of increased diffraction optimal 3- 5 millimeter

25. THANK YOU