visual acuity (2)
TRANSCRIPT
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