Farhana Adnin

B.optom,4th batch

ICO,CU

Visual acuity

VISION…….?

VISUAL ACUITY…?

Resolving power of the eye or the ability to see two separate objects as separate.

Visual Angle…

The angle subtended at the nodal point of the eye by the physical dimensions of an object in the visual field.

Theories of visual acuity…

Receptor theory:

If the images fell on two cones separated by an unilluminated cone, then the points of light would be perceived as two distinct sources.

Means that two distinct points only can be recognized as separate when they subtend an angle of 1’ of arc.

3 types of Cone:

1. Green- Chlorolabe

2. Red- Erythrolabe

3. Blue- Cyanolabe

Diameter of each cone : 0.002µm

Rayleigh Criterion for resolution

Two “point” (unresolved) sources are resolved from each other when separated by at least the radius of the airy disk.

The central disk is surrounded by less intense concentric rings. (airy disk)

Types of Visual Acuity

Minimum Detectable ( visible)

Minimum Separable (resolution)

Minimum cognizable (recognition)

Minimum Discriminable ( hyperacuity)

ACUITY TASKS

MINIMUM DETECTABLE:

Ability to determine whether or not an object is present in a visual field.The limit for this kind of acuity is ~ 1 arc second.

Minimum Separable (resolution)

Discrimination of 2 spatially separated targets.

Normal angular threshold of discrimination for resolution 30-60 seconds of an arc. (minimum angle of resolution).

MINIMUM COGNIZIABLE: not only discrimination of the spatial characteristics of the test pattern but also the pattern with which he has previous experience

(Identification of faces, letters, symbols, pictures etc.)

Minimum Discriminable

Ability to determine whether or not two parallel and straight lines are aligned in the frontal plane.

CLINICAL MEASUREMENT OF

VISUAL ACUITY

Criteria:

Normal testing distance (distance VA): 6m

For near : 33cm/ 40cm.

Illuminated visual acuity chart.

Proper room illumination.

Steps:

Unaided VA

Aided VA

Pinhole VA

6m distance

Factors affecting visual acuity

Overall

Illumination of test object.

Contrast of letters.

Pupil size

Cognitive ability

Ocular health

Refractive Status

Physical factor

Those which influence the light characteristics of the distribution & hence influence the nature of retinal image.

Physiological factor

Stimulus related

Luminance of test object

Geometrical configuration

Contrast difference of stimulus from background

Influence of wavelength

Exposure duration of stimulus

Interaction effects of 2 targets

Observer related

Retinal locus of stimulation

Pupil size

Accommodation

Effect of eye movement

Optical elements of eye

Development of aspects

Distance Acuity chart

Snellen’s distance acuity chart

Bailey-Lovie charts

Landolt ring (or C) chart

Feinbloom Distance Test Chart

Chronister Pocket Acuity Chart

NEAR ACUITY CHARTS

Single letter chartReduced Snellen’s chart:lighthouse near visual acuity testDesigns for Vision Number chart Reduced Ferris-Bailey ETDRS Chart

Word & Continuous Text chartsJaeger’s chartRoman test typesLighthouse game cardLighthouse continuous text card for adultsMNREAD card

Assessment in Children Forced choice preferential looking Optokinetic nystagmus test Stycar ball test Static test Catford drum test Marble game test Tumbling E test Sheridan gardiner test Pictorial test Cardiff acuity test (0-1year) Dot acuity test Coin test (2-3year) Miniature toy test

Optotypes

Landolt Rings (Landolt ‘C’)

Numbers

Figures

Tumbling E

Letters (snellen)

LETTER STYLE VARIATION

SERIF: ornamental cross stroke at the end of the limb

SANS SERIF; bold type faces and appear less cluttered Easier to read than serif letters

Distance test charts & Acuity

Basics of snellen acuity chart…

Snellen Fraction is the most common notation of acuity.

The distant acuity is usually tested by snellen chart.

Consist of a series of black capital letters on a white board,arranged in lines,each progressively diminishing in size.

Each letters fits in a square.

Assume 5x5 grid for letters with detail separation of 1/5 of letter size.

..

Formula

Testing distance

VA=

distance at which letter subtends 5min of

arc

…

The number used to indicate the “size” of the letter is the distance at which the letter height subtends 5 min of arc.A “20 foot letter” is one that subtends 5 min

of arc at 20 feet.A “40 foot letter” is one that subtends 5 min

of arc at 40 feet.A “100 foot letter’ is one that subtends 5

min of arc at 100 feet.

Snellen chart…6/60, 6/36, 6/24, 6/18, 6/12, 6/9,

6/6

θ

tan θ = h / d

Testing & Notation One eye at a time, OD then OS Line by line scoring system Count a line as “seen” if over 50% of the Optotypes of line

are correctly identified.

Using (-) and (+) signs in VA recording T Z V E C L (20/20) – read as “T Z V E C K” 20/20 -1

read as “T W W E C M” but all of the 20/25 line was correctly identified 20/25+3

TYPES OF NOTATION

Decimal Notation

Snellen Fraction notated as a decimal 20/20 = 1.00 20/40 = 0.50 20/100 = 0.20

Percentage Acuity

Represent in %

20/20 = 100%

20/200 = 10%

20/2000 = 1%

Minimum Angle of Resolution (MAR) The MAR is expressed in minutes of arc

The angular size of the just-resolvable optotype.

Normally 1/5 of the letter height

Reciprocal of the Snellen Fraction or decimal acuity 20/20: MAR = 1.0 min of arc 20/40: MAR = 2.0 min of arc 20/200: MAR = 10 min of arc

Log MAR(LOGarithm of the minimal

angle of resoluation. )

Log of the MAR used to notate the acuity

Used in research and low vision

Bailey-Lovie ChartTheir chart and its derivatives follow the

principles: Geometric progression of size & spacing by

0.1 Log unit or 4/5 Found that the relative spacing between letters

and the number of letters per row can cause substantial variation in acuity scores

… Acuity becomes worse as the log MAR value

increases

Decrease brightness line by line

Characteristics of LogMAR

It is flat and typical “V” shape.

Testing distance 4m.

The letter is sanserif and sollan letters. ex: s,o,c,v,z

Every line has 5 letters .

Point one (0.1) logarithmic progression. (5:4)

Between two letter space is the width of a single letter.

Between two line space is the height of the lower line letter.

..

..

Every letter read counts as 0.02 of each line (every line has 5 letters)

Letter to Letter scoring system

Each line = 0.1 log unit The log MAR chart is available in Landolt C, Tumbling E as well as in numeric and alphabet optotypes

Log MAR Snellen Equivalents

20/10 = -0.30

20/15 = -0.13

20/20 = 0.0020/80 = 0.60

20/100 = 0.70

20/125 = 0.80

20/160 = 0.90

20/200 = 1.00

20/317 = 1.20

20/502 = 1.40

20/25 = 0.10

20/32 = 0.20

20/40 = 0.30

20/50 = 0.40

20/63 = 0.50

Exam… Letter to letter Acuity assessment

Every letter that is correctly read deducts 0.02 from that line

Example:

If the patient reads all the letters of the logMAR 0.40 line (20/50) and two letters from the 0.30 line (20/40), it is scored as 0.36

Snellen Equivalent to this would be 20/50+2

Position/LIMITATION OF V/A Standard visual acuity depends on two limitation .

1.Anatomical

two cones stimulate but one cone separated by one minute of arc angle two separate object as separate

2.Optical Airy disc form by diffraction Diffraction form in smaller in size of pupil Aberration

References….. THEORY AND PRACTICE OF OPTICS &

REFRACTION…A.K.KHURANA

CLINICAL PROCEDURE OF OPTOMETRY

INTERNET