visual acuity measurement, contrast sensitivity

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The visual acuity is determined by the smallest retinal image the form of which can be appreciated

For discriminating the form of an object its parts must be differentiated

It is necessary that two individual cones must be stimulated with one between them remaining unstimulated

VISUAL ANGLEIt is found that the object must subtend a visual angle of 1 minute at the nodal point of the eye

VA test types consists of a series of letters of diminishing size. Each letter is shaped such that it can be placed in a square, the sides of which are five times the breadth of the constituent lines. Hence the whole letter will subtend an angle of 5 min. at the nodal point of the eye at the given distance.


Detection or visibility



1. Detection or Visibility

Ability to determine whether or not an object is present in an otherwise empty visual field is termed visibility.

This depends upon the specification of stimulus such as size, shape, & illumination.

A black dot against a white background can be detected if its diameter is of the order of 30 sec or more

2. Resolution (Ordinary VA)

Discrimination of two spatially separated targets is termed resolution

It is essentially an assessment of function of the fovea centralis

This component of VA is measured clinically using Snellens or other test types

3. Recognition

Virtue by which an individual identifies the test patterns with which he had some experience

It involves cognitive component in addition to spatial resolution

E.g. Identification of faces

MEASUREMENT OF VISUAL ACUITYThe visual acuity is a highly complex function

In clinical practice, VA is considered synonymous with the measurement of minimum resolvable only

Hence, examination with various VA charts is quite satisfactory, although incomplete

MEASUREMENT OF VISUAL ACUITY IN ADULTSThe distant central VA in adults is tested by :

Snellens test types Landolts C test types E chart

SNELLENS TEST TYPEBasis of the test : Two distant points are visible as separate only when they subtend an angle of 1 min at the nodal point of eye

Black capital letters on white board, arranged in lines, each progressively diminishing in size

Each letter of the chart is so designed that it fits in a square,the sides of which are 5 times the breadth of constituent lines

The line comprising the letters have such a breadth that they will subtend an angle of 1 min at the nodal point

METHOD Patient seated at 6 meters from the chartIllumination is 20 foot candles Each eye is tested separately.VA is recorded as a fraction (6/60, 6/36, 6/24,6/12,6/9, 6/6) Numerator: distance of the pt. from the chart Denominator: smallest letters accurately read

If pt. cannot see the top line from 6m, he is asked to slowly move towards the chart till he can read the top line(5/60, 4/60, 3/60, 2/60, 1/60)

If the pt. is unable to see even from 1m, he is asked to count fingers of the examiner

When the pt. fails to count fingers, the examiner moves his hands close to the pts face & asks whether he could appreciate the movements or not. (HM +/-)

When the patient cannot appreciate hand movements, perception to light is noted. (PL +/-) with projection of rays in four quadrants.


Similar to Snellens test types

Instead of letters, broken rings are used & the patient is asked to detect the direction of the break in the circle

Each broken ring subtends an angle of 5 min. at nodal point


Similar to snellens and landolts the difference is that in this chart E of different sizes are arranged

Pt. is asked to tell the direction towards which the arms of the E are pointing

SNELLENS EQUIVALENT In U.S., the metric system is not usually employed & the values are converted to feet ( 6m = 20 feet)

VA 6/6 = 20/20 VA 6/60 = 20/200 VA 3/60 = 20/400

Decimal acuity:-

In this system, the Snellens fraction is reduced to a decimal no.

Higher VA is represented by a numerically larger number, which is reverse in the Snellens grading

Ex- 6/6 = 1.0 6/9 = 0.67 6/60 = 0.10

The Bailey-Lovie logMAR chartPrinciple:-

Used logarithmic scale.

Each step indicates increase of 25% in letter size.

Letter sizes ratio as we move up is a constant value of 1.26(0.1log unit steps).

Incorporated 5 letters in every row.

Spacing b/w 2 adjacent letters = width of 1 letter.

Spacing b/w 2 hz rows = height of the letter on lower row.

Results of this chart were obtained in terms of logMAR score i.e log of minimum angle of resolution

As each letter size changes by 0.1logMAR units per row & there are 5 letters on each row ,therefore each letter can be assigned value of 0.02

Thus final logMAR takes account of every letter that has been correctly read

Snellens ChartIrregular progression of letter size Variable number of letters in each line

Variable legibility (difficulty) of test letters

Distance between each letter is not uniform

logMAR ChartUniform progression of letter size

Same number of letters in each line

All letters with similar legibility

The distance b/w each letter is equal to the width of the letter

Snellen fraction(20ft)Snellen fraction(6m)Snellen Acuity(Decimal)Minimal Angle of Resolution(inverse of sn.frac)Log-MAR20/2006/600.1010120/1600.1280.920/125~6/360.820/1000.2050.720/806/240.2540.620/63~6/180.520/500.400.420/406/120.5020.320/320.670.220/250.800.120/206/611020/16-0.120/12.5-0.220/10-0.3

Measurement of VA for NearNear vision is tested by asking the patient to read a near vision chart kept at a distance of 25cm to 35cm

Each eye should be tested separately

The near vision is recorded as the smallest type which the patient can read comfortably

1) Jaegers charts :- Consist of ordinary printers fonts of varying sizes

Prints are marked from 1 to 7 and accordingly pts acuity is labeled as J1 to J7 depending upon the print he can read

(2) Roman test types :-Consists of Times Roman fonts with standard spacing.

-The near vision is recorded as N5, N6, N8, N10, N12, N18, N36 and N48.

(3) Snellens near vision test types :-Constructed on the same principles as of the distant types.

The graded thickness of the letters is about 1/17 of the distant vision chart letter.

The letter equivalent to 6/6 line subtend an angle of 5 min. at he average reading distance.



OBJECTIVE TESTS Preverbal children ( < 2 yrs )

SUBJECTIVE TESTS Verbal children ( > 2 yrs )


Fixation & following behaviour

Preferential looking test ( PLT)

Optokinetic Nystagmus (OKN)

Visual evoked potential (VEP)

1) FIXATION & FOLLOWINGBright colored objects with high contrast edges are used.Best target however is the human face.Binocular fixation is assesed first.Monocular fixation reveals the defective vision in one eye.Infant may not fix with the defective eye and objects to occlusion of the better eye

Kanski fixation and following in squint chapter29

Quality of fixation behaviour C S M

C Central foveal fixation S - Steady no nystagmus M Maintained fixation after a blink


Methods to detect the resolution acuityMore sophisticated method of visual assesment than mere fixation assesment.Include -


(2) Preferential looking test

Assumes that the child will prefer to look at an area of higher visual interest, rather than a neutral grey field

Child presented with two adjacent stimulus fields,one which is striped and other homogenous

Method suitable for infants upto 4 months of age Ex- Leas paddles, Tellers acuity cards

ProcedureOn a screen homogenous surface is projected on one side & black and white strips on the other

These two stimuli are alternated randomly

The eyes of the infant are observed and the movements recorded


Gradually the fineness of stripes is reduced unless there is no longer correlation between direction of gaze & location of the striped pattern.

Visual acuity ranges from 6/240 in newborn,6/60 at 3 months and 6/6 at 36 months

(3) Optokinetic Nystagmus Test (OKN) Nystagmus is elicited by passing a succession of black and white stripes through the patients field of vision

The visual angle subtended by the smallest strip which elicits an eye movement is a measure of VA

OKN acuity is 6/120 in newborns,6/20 at 2 months,6/6 by 20-30 months

(4) Visual evoked response (VER)Refers to EEG recording made from the occipital lobe in response to visual stimuli. It is useful in assessing visual function in infants.Only clinically objective technique available to assess the functional state of visual system beyond the retinal ganglion cells.Two types flash & pattern reversal VER

Visual evoked response (VER)Flash VER tells about the integrity of macular and visual pathwayPattern reversal VER uses some patt


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