What is the meaning of visual acuity?
Visual acuity refers to the ability of the eye to distinguish shapes and object details at a given distance. When visual acuity is high, the contours of an object are crisp and readily identifiable.
Many people refer to perfect vision as 6/6 vision, or 20/20 vision (US notation measured in feet), but this isn't strictly true. These terms refer to 'average' vision which a person with normal eyesight would expect to have.
A person with 20/20 vision can see at 20 feet what a person with normal vision can also see at 20 feet. Someone with 20/40 vision can see at 20 feet what a person with normal vision can see at 40 feet. In essence, the larger the second number in the ratio, the poorer the visual acuity.
Types of visual acuity
| Minimum visible acuity |
Minimum separable acuity |
|
Also know as detection acuity; it is the smallest object or detail a person can see. It means a person can detect the presence of an object against a contrasting background, but may not be able to recognise it.
|
Also known as resolvable acuity, this is the ability to distinguish two seperate objects or details with the smallest separation. This type of acuity is the visual system's ability to resolve fine spatial details.
|
| Minimum recognisable acuity |
Minimum discriminable acuity |
| Refers to the smallest detail a person can identify or recognise. This is also referred to as recognition acuity, and is the ability to recognise and name the object or item seen. |
Minimum discriminable acuity is the smallest difference in visual detail that can be seen between two nearby objects or features. This is closely related to the concept of just noticeable difference (JND) in sensory perception. |
How to measure visual acuity?
A visual acuity test is usually performed by a healthcare professional to screen for issues. An orthoptist may also conduct the test to diagnose refractive errors and determine necessary measurements.
There are several types of visual acuity tests, each tailored for various ages and stages. For example, a visual sharpness test for a non-speaking toddler would obviously not be the same as that for an adult.
Typically, we express visual acuity as a fraction. The first number indicates the testing distance, while the second represents the distance from which a person with 'normal' vision can discern the same details. For example:
- Visual acuity 6/6 is a term used to define 'normal' clarity and sharpness of an object at a distance of 6 metres
- If you have 6/12 vision it means that at 6 metres you can only see what a person with 'normal' vision can see at 12 metres, so your eyesight would be poor
- 6/3, on the other hand, would mean you have exceptionally good vision. Being able to read at 6 metres what a person with 'normal' vision could only read at 3 metres.
Visual acuity tests
The most commonly used test for measuring visual acuity are:
| The Snellen Chart |
The Random E Chart |
|
The Snellen test is the most commonly used test for visual acuity.
It's a chart that consists of letters, numbers, or symbols in different sizes, commonly used for screening older children in school and adults in the workplace.
In addition to diagnostic settings such as an opticians and eye care centres.
|
The Random E Chart contains rows of a “tumbling” E facing different directions. Each row decreases in size, similar to the Snellen chart.
This is typically used for young children who can point in the direction that the E is facing.
It can also be used to test employees in specific manufacturing industries such as aerospace and aviation (NDT vision testing).
|
| Landolt rings |
Simplified testing for children |
The Landolt C, also known as a Landolt ring replaces numbers and letters with four rings, each with a single gap, thus looking similar to the letter C.
The gap in each ring can be at various positions (usually left, right, bottom, top).
The task of the subject is to identify which side the gap is. The advantage of this test is that it overcomes any language or letter identification issues. The size of the rings relates to Snellen sizes. |
This is a non-invasive test that takes less than 10 minutes and is a fun experience for most children.
The orthoptist will assess the child’s vision using a letter matching or picture naming game, and by getting them to answer simple questions.
These tests are child friendly and the child won't need to know their letters. |
Factors which affect visual acuity
Visual acuity less than 20/20 is common. In most cases, blurry vision from a refractive error such as nearsightedness (myopia), farsightedness (hyperopia) or astigmatism is the cause.
In myopia, distant objects appear blurry, while in hyperopia, close-up objects may be more challenging to see clearly. Astigmatism can cause distorted or blurred vision due to an irregular curvature of the cornea or lens.
Why visual acuity tests are important
Regular eye examinations are essential for assessing visual acuity and detecting any refractive errors or eye conditions. Many occupations have specific visual acuity entrance standards that have to be achieved. In some instances aided (with glasses etc.) or unaided. Within an occupational health setting, visual acuity tests can:
- Detect individuals with a refractive error or ocular pathology
- Detect individuals wearing visual aids who may require a new prescription
- Detect individuals who have defective vision but have not demonstrated any symptoms
- Ensure individuals are able to carry out the task safely and efficiently
Visual acuity tests can also determine whether:
- An infant or child has proper visual development
- An individual possesses the necessary visual sharpness to obtain a driver's license
- An individual is eligible for specific services because of reduced visual sharpness
- A medical condition has advanced and resulted in vision changes
- Medication is effective in stabilising vision or preventing vision loss
Eye care
After receiving a comprehensive eye exam, corrective measures such as glasses or contact lenses can be prescribed, to optimise visual acuity and enhance the overall quality of vision.
Eye specialists may also recommend LASIK (laser-assisted in situ keratomileusis) surgery to correct vision issues. This leads to improved visual acuity without the need for external lenses.
Understanding and monitoring visual acuity is fundamental to maintaining eye health and addressing vision-related issues effectively.
Industry-leading vision testers
At Amplivox, we supply innovative PC-based and desktop vision testers. Each designed to provide occupational health experts and general healthcare practitioners with a class-leading vision screening programme.
Our vision testers offer a wide range of testing options for all occupational vision screening needs, as well as a compact and portable solution for mobile screening. They also provide significant improvements in workflow and time management through OH EMR connectivity.
For more information on our vision equipment please visit our vision testers webpage, contact our customer support team on +44 (0)1865 880 846 or email.
References
American Academy of Ophthalmology, Visual Acuity (Jan 2022). Accessed at: https://www.aao.org/eye-health/tips-prevention/visual-acuity
National Library of Medicine, Evaluation of Visual Acuity (May 2024). Accessed at: https://www.ncbi.nlm.nih.gov/books/NBK564307/