visual acuity is very important for us . its the spatial resolving capacity of the visual system . visual perception (sensation) from stimulation of the retina by light and its of four type . 1- light sense 2- form sense 3- sense of contrast 4- colour sense

2. Visual sensation from stimulation of the retina by light and is of
four varieties.
 Light sense
 Form sense
 Sense of contrast
 Colour sense
1- Light sense- light sense is defined as the faculty /ability
which permits us to perceive light in all its gradation of
intensity.
 Light minimum is at a point when light is just no longer
perceived if light falling upon the retina gradually decreased
in intensity. The light minimum for fovea is higher then the
other areas of the retina.
2- Form sense- form sense is defined as the faculty /ability
which enables us to perceive the shape of an object.

3. •Cones are responsible for form sense. So form sense is most delicate and
acute at the fovea and falls off very rapidly towards the periphery.
•Form sense has also psychological component to perceive composite
forms e.g - letter in addition to the cone function.
3- Sense of contrast - it is defined as the faculty /ability enables us to
perceive contrast in luminanace between region which are not separated
by definite margin.
•Contrast sensitivity is reduced in many disease. Eg - cataract, optic
neuritis, glaucoma, macular disease etc.
•Person requiring lot of contrast to see a target has lower contrast
sensitivity.
4- Colour sense - this is defined as the faculty /ability which enables us
to distinguish between different colour.
Cones are reponsible for colour sense and occurs only in photopic
condition.
•There is existence of 3 type of pigment in different cones which are
responsible for preferential absorption of wave lengths of light
corresponding to red green and blue colour.
•All other colours and white colour can be formed by their suitable
proportional combination.

4. •An object is perceived as a coloured one when light rays of a particular
wavelength are reflected from it to reach the retina.
•Normal colour vision is turned trichromatic and people with normal
colour vision are called normal trichromals.

5.  it is the measurement of spatial resolving
capacity of the eye and is applied to central
vision. There must be an unstimulated cone
in between the two stimulated cones to allow
for the resolution of two objects. The
distance between two cones at the macular
region is 0.004 mm and the object must
subtend a visual angle of 1 minute at the
nodal point of the eye to produce an image
size of 0.004 mm. it based on this principal
distance visual acuity test are developed.

11. 
Age group Visual acuity Method of determination
Neonate (birth - 12
month)
6/30 OKN (optokinetic
nystagmus)
Infant (1month - 12
month)
6/15 OKN , VER (visual evoked
response)
Toddler (1year to
3year)
6/12 E chart OKN, VER
Preschool (3year to
5year)
6/9 E chart , allen test, OKN
School age (5year to
16year)
6/4.8 Snellen chart

13.  it is the most commonly used test for visual acuity. It
consists of row of letters of diminishing size. Each
snellen's letter is constructed in such way that it can
be perfectly placed in a square which is further
subdivided in to 25 small squares. Each component
part of the letter subtends an angle of 1minute (1/60)
and the whole letter subtends an angle of five minute
of arc at the nodal point of the eye from a particular
distance. The largest letter in a top row will subtend
an angle of 5 minute at the nodal point if it is 60
meters from the eye . Hence , each row is assigned a
specific number which indicates the distance in
meters at which a person with normal visual acuity
will be able to identify properly the letters.

14. Snellen's chart have a single letter at top row and increasingly more
letters of smaller sizes in lower rows the test chart is illuminated by a
lamp of 100 ft cs (foot candles). The snellen's chart is read from 6
meter or feet distance.
Interpretation of visual acuity -
First note the lowest line which subject can properly identify.
Visual acuity= viewing distance
lowest identifiable line notation
If a subject is able to identify up to row of letters on the ''24'' line for
six meter distance he has a visual acuity of 6/24. a visual acuity 6/6 is
accepted as normal universally.
1. If a patient can not identify the letter on the top row his vision is
<6/60 and he is told to walk toward the acuity chart
2. If the patient can not identify the letter of the top row at 3 meter
distance he visual acuity will be 3/60 and so on.
3. If the patient can not identify the letter even at 1 meter distance he
is asked to count the fingers of the examiner at 1 meter distance. If
he is able to count fingers his visual acuity is counting figures at 1
meter (CFIM)
4. If he can only identify the hand movement by the examiner his vision
is recorder as hand movements only(HM)

15. • If he is still unable to detect only hand movement he is shifted to a dark
room and light shown on that eye (other eye is covered with palm of the
hand) form 4 direction (UP,DOWN,RIGHT,LEFT).he is asked to specify
when the light is present and when it is absent .if he is able to perceive
light in all direction his visual acuity is recorded as perception of light
present projection of rays good
• If his light perception is absent in any quadrant it is recorded as PL+
inaccurate projection of rays in that quadrant
• If he is able only to perceive light his visual acuity is perception of light
only (PL only)

16.  it consists of a graduated series of ring (A circle with a break
or gap) conventionally the break is oriented in four direction
,up, down, right, left in different optotypes. The subject is
asked to identify the location of the break in each landolt ring

17.  it consist of a chart with the graduated series of sneller's
letter "E" Oriented in various direction which the limbs of the
"E" point (up, down, right, left)

18.  this test can be successfully use to small children of age
group 4 to 6 years in all test the size of simple such as it
suitable correspond to snellen letter and visual acuity is noted
in the same manner.

19.  this test is actually a part of screening test for young children. It
consist of 5 letter set , 7 letter set and 9 letter set. The test letter
chosen are on the basis of their symmetry H,O,T,V,X,A,U,C,L. each
letter is printed out on 5*5inch card and the test is done from 10
feet distance and the test is done from 10 feet distance and results
are correlated to snellen's type.

20.  this test is use 1 year of age. 5 ivory ball of 1/2 inch to one
and half inch diameter sizes are rolled and spinned to a
distance of 20 feet. The child is expected to fetch the balls of
size which he can easily seen. Balls are rolled infront of the
child and examiner noted the quality of fixation reflexes in
relation to size of balls.

21.  after examination of visual acuity it is important to do a general torch
examination to record and not position of the orbit eyeballs relative position
of ocular structure any gross abnormality in these should be noted with
closer attention following point are recorded.
 Position and size of the eyeballs and orbital socket.
 Position of eyelid , shape , size ,contour margins , palpebral fissure and
eyelashes.
 Position of the punctum in upper and lower lid near nasal canthus of both
eyes.
 Position and texture of conjunctiva , presence of abnormal redness
,discharge at canthus follicle.
 Surface and texture of cornea its shape , size, presence of surface opacities
and irregularities etc.
 Depth of anterior chamber , floaters in aqueous.
 Iris position , colour , pattern, texture.
 Pupil position , shape and pupillary reflex.
 Reflex from lens in pupillary area presence of any opacity in pupillary area.

22.  a chart containing an apple , a house , and an umbrella arranged in
snellen equivalents of 20/200 - 20/20 is used and the child is asked
to identify the picture along the lines. The test is carried out at 10 ft.

23.  in this test the child is shown a miniature toy from a distance
of 10 ft and is asked to name or pick the pair from the
assortment.

24.  in this test child is asked to identify the two faces of coins of
different sizes held at different distance.

25.  this test is use of 6-12 month of age. Placing games can be used to
estimate visual function one such game is the marble game the child
is asked to place marbles in the holes of a card. This test is not
intended to measure visual acuity of each eye but rather to compare
the functioning of the child's eye when one or the other is closed.
The vision of an eye is then noted as being useful or less useful.

26.  it is a direction acuity test useful in infants and preschool
children. The child is made to observe an oscillating drum
with black dots of varying sizes. The smallest dot that evokes
pendular eye movements denotes the level of visual acuity.
This test is unreliable since it over estimates the vision.

27.  near visual acuity is tested at a standard distance of 25
- 35 cm and the size of the smallest print that can be
read is interpreted as the near visual acuity.
 Jaeger's chart - in 1867 there is lack of uniformity in
jaeger's test print sizes progression. It is indicated by
the letter J followed by a print size J6 ,J8 etc.
 Procedure of testing -the patients is seated in chair and
asked to read the near vision chart kept at a distance of
25-35 cm with good illumination thrown over his left
should each eye should be test separately. The near
vision is recorded as the smallest type that can be read
comfortably by the patient.