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What is Visual acuity?

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Visual Acuity is defined as the smallest retinal image that can be appreciated and it is measured by the smallest object that can be seen at a distance.

 The vision or visual perception is a complex integration of light sense, form sense, contrast sense, and color sense. visual acuity is considered a measure of form sense so it refers  to the spatial limit of visual discrimination


 To get a firm grip on  understanding visual acuity we must learn about the visual angle

 It is the angle is subtended at the nodal point of the eyes by the physical dimensions of the object in the visual field, it is a very useful and convenient mode of specifying the spatial extent of objects or elements in the visual field.

So now let us define visual acuity in terms of visual  angle 

The visual acuity is defined as the reciprocal of the minimum resolvable visual angle measured in minutes of arc.

There is a clinical definition of visual acuity as well, it states that visual acuity is the measurement of discrimination of  2 spatially separated targets.

 there are a few components of visual acuity:

  • Minimum visible

  •  Resolution

  •  Recognition and

  •  minimum discriminable


Let us understand the components of visual acuity in detail:-

Minimum visible

This component Is their ability to determine whether or not the object is present. This component can also be termed as visibility or detection. The patient has to appreciate the presence of an object and if he is successful in doing so it may be said that this component is present


Resolution 

Also called as ordinary visual acuity this component is defined as the ability to discriminate  2 spatially separated targets. The minimum  separation between the two points which can be distributed is known as minimum resolvable and the distance between the two targets is specified by the angle subtended at the nodal point of the eye which is called the minimum angle of resolution


 Recognition

It is the ability by which an individual not only discriminates the spatial characteristics of the test but also identifies the patterns which one has had some experience. Identification of faces and be a good example of this component


Minimum discriminable

It refers to spatial distinction by an observer when the threshold is much lower than the ordinary acuity. When this distinction by an individual is possible it may be also be termed hyperacuity.



 As we have finished learning about visual acuity and its components  let us learn about some factors affecting visual acuity: 

Physical factors:

These include light characteristics of the distribution and thereby influencing the nature of the retinal image

 Physiological factors:

These are the factors that influence the processing of stimulus and thus observer related, for example, lens is a physical factor but the related accommodation process is physiological.

 Stimulus-related factors:

  1. The luminance of the test object

  2.  The geometrical configuration of the stimulus

  3.  Contrast etc

 Observer related factors:

  1. Retinal locus of stimulation

  2.  Pupil size

  3.  accommodation etc



Measurement of visual acuity

This is much more simple in adults compared to children the cooperation we get from adults and generally hai and testing   visual acuity in adults is comparatively easier


 Some charts that we use for measuring visual acuity  in adults are: 


Logmar Chart

Snellen’s Chart



Some methods we take in measuring visual acuity in children are as follows:

     

OKN Drum                                                               Teller Acuity Card Testing

                                                                                                

                                                                                     

                                            

HOTV Chart(Consisting of H,O,T,V

Alphabets only)                                          

Tumbling E chart


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