by: Carmen Willings
Acuity describes the amount of detail the individual sees compared to what a person with normal vision sees. It refers to the sharpness of vision in relation to discriminating detail. It is usually given in Snellen notation. 20/20 is considered 100% or "normal" vision and means that the individual being checked sees at 20 feet what should be seen at 20 feet.
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Hi! I'm Carmen Willings. Welcome to my website! I developed this web resource to support fellow TVI's and to educate those new to the field of visual impairments in how to best support students who are blind or visually impaired. Read More
The common phrase “20/20 vision” refers to how acuity is measured; the top number, 20, refers to the distance at which the measurement is taken, and the bottom number refers to the distance at which a “normal” eye can see a specific size image. If a person has 20/40 acuity, it means he can see at 20 feet that which someone with “normal” vision can see at 40 feet. The smaller the second number, the more sight a person has. Low vision means a person’s visual acuity is 20/50 or worse. The larger the second number, the less vision a person has.
How Acuity is Written on Reports:
An acuity provides information on how much the cones and the rods in the eyes are capable of seeing under certain lighting conditions and what their threshold is. Keep in mind that there is no correlation between acuity and visual functioning. Two people with the same visual acuity may have very different visual functioning. Also be aware that vision is a learned response.
Visual acuity is one of the main factors in determining eligibility for school based vision services and whether the student is identified as visually impaired or blind. The clinical eye exam should include formal testing of the student's near and distance acuity when a student is able to participate in formal testing. Visual acuity testing should be conducted with correction (cc) if the student has been prescribed glasses or contacts and also without correction (sc). Having both acuities can be very helpful in understanding the student's vision and whether or not they tolerate wearing the classes or don't as they don't see much improvement.
Near Visual Acuity
Near visual acuity indicates the smallest letter, picture or symbol size the student is able to discriminate at a near distance, typically measured at 12 to 14 inches. When an acuity is provided, there should also be a note about the distance the student used in order to identify the symbols. For educational purposes, this impacts a students ability to read various size materials within the classroom, menus, brochures, etc.
Distance Visual Acuity
Distance visual acuity indicates the smallest letter, picture or symbol size the student is able to discriminate at a distance, typically measured at 20 feet. For educational purposes, this will indicate whether or not a student can see information presented in the classroom, during assemblies, or demonstration lessons. This will affect the student in extracurricular activities including ability to see a fast food menu, reading street signs, seeing a scoreboard or jerseys at sports games, or locating people at distances. The Snellen chart is the most common eye chart used when assessing the person's visual acuity.
Functional Acuity Estimates
When a student is unable to participate in formal testing, an ophthalmologist may provide an estimate of the child's acuity or they may indicate the student Functions at the Definition of Blindness.