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Theme 4: Visual Acuity Concept of Visual Acuity. Visual Discrimination Tasks. Types of Visual Acuity Scales of Visual Acuity Optotypes for measuring Visual.

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Presentation on theme: "Theme 4: Visual Acuity Concept of Visual Acuity. Visual Discrimination Tasks. Types of Visual Acuity Scales of Visual Acuity Optotypes for measuring Visual."— Presentation transcript:

1 Theme 4: Visual Acuity Concept of Visual Acuity. Visual Discrimination Tasks. Types of Visual Acuity Scales of Visual Acuity Optotypes for measuring Visual Acuity Factors that affect Visual Acuity

2 Concept of visual acuity Visual Acuity is the capacity to detect, resolve or recognize the details of objects It is quantified by the minimum dimension that an object must have in order for the observer to be able to identify, distinguish or simply detect it.

3 Concept of visual acuity Is normally expressed as the inverse of the angle in minutes subtended by the minimum detail of the object.

4 Tasks of Visual Discrimination. Types of Visual Acuity Task of detection or minimum visible Task of resolution or minimum separable Task of recognition or minimum recognizable Task of localization Tasks of visual discrimination. Types of visual acuity

5 Tasks of Visual Discrimination. Types of Visual Acuity The observer should be able to decide on the presence or absence of an object in their field of vision This is often quantified by the angle that is subtended by the object that is perceived by the observer. Task of detection or minimum visible

6 Tasks of Visual Discrimination. Types of Visual Acuity Task of detection of the minimum visible Types of test: - Luminous points on black background - Black point on luminous background

7 Tasks of Visual Discrimination. Types of Visual Acuity Task of detection or minimum visible TestLimit Luminous ObjectPoints1” Dark ObjectPoints15” a 30” LinesUntil 1’ Standard values for minimum visible

8 Tasks of Visual Discrimination. Types of Visual Acuity Task of resolution or minimum separable The observer should be able to decide if two objects that are very close together are separated or not. It is usually quantified by the minimum angle of separation that should exist between the two objects so that they can be perceived as separate by the observer.

9 Tasks of Visual Discrimination. Types of Visual Acuity Type of test: - Points or lines saperated by a variable distance, which are clearly distinguishable if they are presented individually. Task of resolution or minimum separable

10 Tasks of Visual Discrimination. Types of Visual Acuity TestLimit Points1’ to 3’ Lines40” to 1’ Foucault grattingUntil 1’ Standard values of minimum separable Task of resolution or minimum separable

11 Tasks of Visual Discrimination. Types of Visual Acuity Task of recognition or minimum recognizable The observer should be able to recognize the details of the object Usually quantified by the inverse of the angle that subtends the object to be recognized. The minimum recognizable is the Visual Acuity normally used by an optometry clinic

12 Tasks of Visual Discrimination. Types of Visual Acuity Types of test (Optotypes): - Letters - Numbers - Snellen “E” - Landolt Rings Task of recognition or minimum recognizable

13 Tasks of Visual Discrimination. Types of Visual Acuity Size of the test (Optotypes): The size of the optotypes used is often a 5sx5s box or a 4sx5s rectangle, with s being the minimum detail of the object to recognize. Recognition or minimum recognizable

14 Tasks of Visual Discrimination. Types of Visual Acuity TestLimitVA Optotypes1’1 Task of recognition or minimum recognizable Standard values of the minimum recognizable

15 Tasks of Visual Discrimination. Types of Visual Acuity Task of localization. The observer should be able to discriminate small displacements within the test It is usually quantified by the minimum angle of displacement that the observer can perceive. The classic measurement of localization is called Vernier acuity

16 Tasks of Visual Discrimination. Types of Visual Acuity Types of test: - Lines that shift until the observer sees them aligned Task of localization. u

17 Tasks of Visual Discrimination. Types of Visual Acuity TestLimit Alignment (Vernier)<1” Standard values of minimum displacement Task of localization.

18 Scales of visual acuity DECIMAL OR MONOYER SCALE Visual Acuity values are expressed as the inverse of the angle in minutes of arc that subtends the minimum detail of the test The standard value is considered to be AV = 1 The rest of the acuities are expressed as a decimal Example: 0.8, 1, 1.2

19 Relation of visual acuity with the details size and distance of observation. Concept of Visual Acuity

20 Relation of visual acuity and observation distance.

21 Concept of Visual Acuity Relation of visual acuity with size of object.

22 Scales of Visual Acuity SNELLEN SCALE Visual acuity values are expressed as a fraction between the distance that the observation is made (d’) and the distance that the detail of the test subtends an angle of 1 minute (d) Usually, the distance d’ is 6 meters or 20 feet. The standard value for VA is considered to be = 6/6 or 20/20. Example: 6/9, 6/6, 6/5; 20/30, 20/20, 20/15

23 Scales of Visual Acuity SNELLEN SCALE AND DECIMAL SCALE RELATIONSHIP If we have a visual acuity in the Snellen Scale we obtain its decimal value by dividing the two distances of the Snellen Scale Example:

24 Scales of Visual Acuity SNELLEN SCALE AND DECIMAL SCALE RELATIONSHIP If the visual acuity is in the decimal scale, we can obtain the Snellen scale value by deducing the value of d, once d’ is fixed at a value of either 6 or 20. Example:

25 Scales of Visual Acuity LogMAR Scale In the logarithmic scale of the minimum angle of resolution (MAR), as the name indicates, visual acuity is expressed by the logarithm of the angle subtended by the recognizable detail of the test. The standard value is VA MAR = log MAR = 0 Example: 0.2, 0, -0.1 It is the only scale where the value for visual acuity can be negative. In addition, a negative value is considered to be a good value.

26 Scales of Visual Acuity DECIMAL SCALE- logMAR SCALE RELATION If we have a visual acuity in the decimal scale we can obtain the logMAR value by calculating the logarithm of the angle expressed in minutes, which is the inverse of the decimal visual acuity Example:

27 Scales of Visual Acuity DECIMAL SCALE-LogMAR SCALE RELATIONSHIP With a visual acuity in the logMAR scale we obtain its value in the decimal scale calculating the antilogarithm of the LogMAR and then calculating the inverse. Example:

28 Scales of Visual Acuity Minimum angle of resolution Acuity decimal Acuity Snellen LogMAR / / / / / / / / / /601.00

29 Optotypes for measuring visual acuity In a clinical practice it is common to use Optotype cards An optotype card is formed by rows of optotypes of progressively smaller sizes. They are designed to be presented at a fixed distance and on one side they usually show the visual acuity value belonging to each row.

30 Optotypes for measuring visual acuity Optotype cards Letter Cards Snellen

31 Optotypes for measuring visual acuity Optotype Cards Letter Cards Bailey-Lovie

32 Optotype Cards Letter Cards Optotype Projectors Optotypes for measuring visual acuity

33 Optotype Cards Number Cards Optotype Projectors Optotypes for measuring visual acuity

34 Optotype Cards One Character Optotype Cards Landolt Rings

35 Optotypes for measuring visual acuity Optotype Cards One Character Optotype Cards Snellen “E”

36 Optotypes for measuring visual acuity Optotype Cards Other Cards Infant Visual Acuity

37 Factors that affect Visual Acuity PHYSICAL FACTORS (Observation conditions) Type of test used Luminance level of test Contrast of test INTERNAL FACTORS (Ocular Optics) Defocus Pupil diameter

38 Factors that affect visual acuity Type of test used -Foucault gratting provide a greater acuity than Landolt rings or Snellen E’s - Easy letters (Z,U,E,F,T,L) or difficult letters (V,R,D,B,G)

39 Factors that affect visual acuity Luminance level It is very important to always measure Visual Acuity with luminance values of greater than or equal to 100 cd/m 2

40 Factors that affect visual acuity Contrast It is very important to always measure Visual Acuity with contrast values greater than 0.9 (regular situation in a clinical practice)

41 Factors that affect visual acuity Defocus Grade of defocus (D) Visual Acuity 1.25>D  2.5 < >D  1.25 <0.20 0>D  0.5 <0.5

42 Factors that affect visual acuity Pupil Diameter - Maximum visual acuity for pupil diameters between 2mm and 4mm. - For larger diameters the aberrations are more important. - For smaller diameters diffraction is important.

43 Factors that affect visual acuity Pupil Diameter - If the diameter is small the size of the circle of defocus is reduced. Stenopeic glasses Small circular diaphragm of 1 to 2 mm on a black background. It improves vision when defocus exists.


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