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If your child has been diagnosed with a visual impairment, you may feel at first as though you are entering a new world full of frightening possibilities described in unfamiliar language. And, as you begin meeting with doctors and special educators, filing reports, and looking for information about your child’s eye condition, you may encounter many different terms used in reference to your child and to visual impairment. You may find it confusing to discover that many of the terms are used generally and do not have precise definitions and that in many cases, they overlap. But, once you know some basic terminology, phrases such as “low vision” and “legally blind” may no longer seem intimidating and mysterious.

What Is Visual Impairment, Anyway?

“Visual impairment” is a broad term that is used to refer to any degree of vision loss that affects a person’s ability to perform the usual activities of daily life. It does not refer to the kind of vision many of us have who need to wear eyeglasses or contact lenses to read a book comfortably or see highway signs when we’re driving a car. Instead, visual impairment refers to a loss of vision that cannot be corrected to normal vision, even when the person is wearing eyeglasses or contact lenses. Because it is so broad a term, “visual impairment” usually includes blindness as well.

Most visually impaired people have some usable vision. Only a small percentage of people with visual impairments are “blind”—that is, they have very little, if any, vision. Some people who are blind may have light perception or the ability to tell whether or not a light is on in a darkened room. But most will not be able to perceive light.

It’s equally important to know that there is a great range of ability and disability among visually impaired people. Visual impairments range from mild to severe. And no two people see exactly alike. Even among people who have the same diagnosis, for example, retinopathy of prematurity (ROP) or retinitis pigmentosa, people may have very different visual abilities. In addition, a person’s vision can be affected by daily factors and variables, such as fatigue and environmental conditions like lighting. Therefore, someone’s vision can be different from day to day.

Measurements of Visual Impairment

Although vision is variable and influenced by day-to-day factors as well as the progression of a given disease or eye condition, measuring someone’s vision medically and clinically is done for many important purposes. For example, in order to prescribe eyeglasses or other optical devices for a person, his or her vision needs to be measured in a formal way. Two basic factors—visual acuity and visual fields—are often determined through this kind of measurement.

Visual Acuity and Visual Fields

Visual acuity refers to the clarity with which one sees an object and to the sharpness of someone’s ability to see detail. Usually, someone’s near acuity—the ability to see within 16 inches—and distance acuity—beyond 16 inches—are both measured. In the United States, acuity is most typically measured at 20 feet, so you’ll hear references to measurements such as 20/20, 20/70, or 20/200. The first number, 20, is the distance your child is from the eye chart being shown. (In the eye care specialist’s office, mirrors are sometimes used to trick the brain into thinking it is 20 feet from the chart.) The second number, 70, for example, is the distance where a person with “perfect” or “normal” vision would stand to be able to see the chart. So someone with 20/70 vision can see at 20 feet what someone with “normal” vision can see from 70 feet away.

The larger the second number, the more reduced the distance acuity a person has. People with acuities of 20/200, 20/400, or 20/600 have some vision, but they don’t see with the same degree of clarity that a person with an acuity of 20/20 or 20/50 sees. When a person has 20/200 distance acuity during an exam, he is usually only able to see the big “E” on the eye chart in the eye care specialist’s office.

Near and distance acuity can vary for an individual. For example, your child may have near-normal visual acuity for near tasks. Some children have poor near visual acuity and better distance acuity. For most children with visual impairments, visual acuity is less than “normal” both at near and at distance.

Visual field refers to what one can see to the sides, above, and below when looking straight ahead and without moving one’s head. For a “normally” sighted person, the visual field is approximately 190 degrees. It is possible for your child to have “normal” visual acuity and reduced or decreased visual fields. It is also possible for his visual fields to be normal and his acuity to be decreased.

Legal Blindness

Legal blindness is a confusing term because most people who are legally blind are not blind. Their vision may be reduced, but they are able to use it to some extent to function. The definition of legal blindness was developed in 1934 by a group of ophthalmologists. The United States government had called the group together to come up with a definition that could be used to qualify people for government services. Today, the term is applied to both children and adults, and it is used by some agencies in various states as a requirement for qualifying for services. The definition of legal blindness is:

Visual acuity of 20/200 or less in the better eye after best correction (that is, while wearing prescribed eyeglasses or contact lenses), or a visual field of 20 degrees or less with best correction.

Low Vision and Other Terms

The term “low vision” covers a broad range of people who have a vision loss. There is not a single definition of low vision or a legal definition of it either. Low vision refers to a vision loss that is severe enough to interfere with everyday tasks and that cannot be corrected to normal vision by regular eyeglasses or contact lenses. A child with low vision has some functional, usable vision, but that vision is impaired or decreased. Most children who have low vision are encouraged to use their vision for everyday tasks and are often helped to do so by the use of eyeglasses, contact lenses, or other optical devices such as a magnifier or monocular.

Other terms you may hear used to describe your child are “functionally blind” and “partially sighted.” If your child has a severe vision loss for educational purposes, you may hear him being described as “functionally blind,” and he may be provided the same type of instruction as a child who is totally blind—for example, he may be taught how to read braille so that he can read comfortably.

“Partially sighted” is a term that was sometimes used to describe visual acuities of 20/70 to 20/200, but it is now generally used to describe a visual impairment in which the person has some usable vision.

Functional Vision

Although a clinical measurement like 20/40 may tell you that someone sees at a distance of 40 feet what someone else sees at 20, it will not be helpful in providing information about what that person can in fact see. Even people with the same numerical measurements of their vision will have different vision because eye conditions vary greatly. For this reason, knowing that your child has 20/300 vision may not give you an understanding of how he perceives the world, how he can perform certain tasks, and what he might need to function effectively at home, at school, and in your community. This is why the concept of functional vision is so important. Functional vision is the vision that can be used to plan and perform a task, and observing how someone goes about his or her daily activities can reveal the extent of that vision. When your child is given a functional vision assessment, he will be observed doing various activities such as reading, writing, and walking to see how he uses the vision he may have. By observing your child in daily life, you may learn much about the kind of vision he has.

What Term Applies to My Child?

You’ll find that different people will use different terms when referring to your child. And that different people will use different terms when referring to the same thing! For example, “visual impairment” and “low vision” are often used interchangeably, and you can feel comfortable using either term. What’s really important to remember is that a term used is simply a way to describe just one attribute or characteristic of your child. Try not to let the term define your child, for you or for others.