The information presented within this glossary entry is aimed at website owners seeking to learn the ropes of web accessibility. Technical elements are described in layman’s terms, and, as a rule, all topics pertaining to the legalities of web accessibility are presented in as simplified a manner as possible. This guide has no legal bearing, and cannot be relied on in the case of litigation.

Blindness is a condition that affects a person’s ability to see, perceive, and consume visual information. Various factors, including genetic conditions, injuries, diseases, or other medical conditions can cause blindness during different periods of a person’s life.

Blindness is a spectrum. People with blindness can live with varying degrees of vision loss, from partial sight to complete blindness. Importantly, individuals with certain vision impairments cannot simply overcome their vision limitations by using eyeglasses, contact lenses, eye drops, medical therapy, or surgery. If a person experiences a sudden decrease in vision, they should immediately seek medical assistance.

Blindness categories and classifications

Blindness encompasses a wide variety of vision impairments, ranging in severity from partial to total blindness.

Legal blindness

Legal blindness, or statutory blindness, is a specific definition of vision loss codified in certain legal systems. Under American law, a legally blind person is a person who has a best corrected visual acuity of 20/200 or worse in their better eye. This means that an individual must be ten times closer, or that an object must be ten times larger for it to be seen by an individual with 20/200 visual acuity, as compared to a person with 20/20 vision. 

Legal blindness can also be determined by a field of vision restricted to 20 degrees or less in the best eye. Often called tunnel vision, a person with a 20 degree or less field of vision may have 20/20 acuity remaining, but their field of vision is severely restricted, thereby meeting the definition of legal blindness. People who are legally blind may be eligible for certain government benefits, including financial assistance and educational services. 

Under American law, people with legal blindness also face certain restrictions, such as being prohibited from obtaining a driver's license, in most cases. 

It is important to note that legal or statutory blindness is a specific definition used for legal and medical purposes and does not always accurately reflect a person’s ability to function in daily life. With appropriate accommodations and support, many people with legal blindness can fully participate in society.

Total blindness

Total blindness is classified as complete loss of vision, and is sometimes referred to as NLP, an acronym for “no light perception.” Only an estimated 15% of people with vision impairments have total blindness. Individuals who have total blindness have no perception of light, movement, or color. The causes for total blindness vary and include congenital diseases, incurred injuries, trauma, and other health conditions, such as glaucoma. Additionally, those who suffer from end-stage diabetic retinopathy may also experience total blindness.

People with total blindness may require substantial support and accommodations to independently function within their environments. These typically include the use of assistive technologies, tactile cues, and orientation and mobility training to travel safely.

Total blindness can pose significant challenges. However, despite considerable obstacles, with proper support and accommodations, people with total blindness are able to lead fulfilling lives.

Partial blindness

Partial blindness is a more moderate form of blindness that reduces an individual's ability to see in either the central or peripheral vision. People with partial blindness may experience blurred vision, difficulty reading, difficulty recognizing faces, and trouble seeing in low light. The causes for partial blindness are varied, and include conditions such as glaucoma, diabetic retinopathy, macular degeneration, and, more commonly, cataracts. 

People with partial blindness may experience challenges in daily life, such as performing certain everyday tasks like reading, using a computer, or driving, as well as properly navigating their environment inside and outside their home. Treatment for their condition (in the form of contact lenses, glasses, and/or laser surgery), along with vision rehabilitation services can help these individuals lead full, active lives even when their vision cannot be restored to 20/20.

Color blindness

Color blindness, also known as color vision deficiency, is a genetic condition that limits one’s ability (either partially or completely) to distinguish between certain colors. Those who have color blindness typically have normal vision in other aspects of the visual world that does not require color discrimination. 

The most common color pairings people with color blindness have difficulty distinguishing between are red and green, along with yellow and blue.

Color blindness is more common among men than women, with an estimated 8% of men of Northern European descent having some form of color vision deficiency.

People with color blindness may face certain challenges in daily life, including when engaging with a website or app. 

Congenital blindness

Congenital blindness is a severe form of vision loss that is present at birth and is a result of damage to one or both eyes, the visual cortex of the brain, and/or the optic nerve before a person is born. 

In some cases, vision impairment can be caused by genetic mutations. In others, vision loss can be a result of environmental factors such as infection, drugs or toxins, or a lack of oxygen during birth. 

People with congenital blindness face unique challenges that impact their day-to-day lives. However, with proper education, accommodations, and relevant training and tools, people with congenital blindness can still lead full and fulfilling lives. 

Examples of relevant tools include, but aren’t limited to, auditory or tactile learning tools, and assistive technologies like screen magnifiers or screen readers (such as NVDA and JAWS). Both methods of accessing information using a computer are instrumental in allowing people with partial or complete blindness to properly access websites.

Night blindness

Night blindness is a condition where an individual has difficulty seeing in low light, such as in dimly lit environments or at night (hence the naming convention). Night blindness can be caused by inherited retinal diseases like Retinitis Pigmentosa. However, it can also be incurred due to complications with various health conditions such as cataracts and glaucoma.

People who have night blindness can find it difficult to perform certain tasks at night or in dimly lit environments, such as driving. Additionally, people with night blindness may also experience other symptoms within these environments, such as a general reduction in visual acuity, delayed dark adaptation, and decreased peripheral vision.

Treatment for night blindness will largely vary depending on its underlying cause. If the latter includes an existing health condition, this will typically be addressed first. Additional remedial approaches include vitamin A supplements, wearing corrective lenses, or using assistive technologies such as night vision devices.

Snow blindness

Snow blindness is a temporary vision impairment caused by exposure to bright UV light. Such exposure can occur in snowy or high-altitude environments and results in one or both corneas’ surfaces experiencing cellular swelling.  

People with snow blindness can experience vision loss, blurred vision, eye discomfort, and sensitivity to light. However, individuals with snow blindness can still see movement and shapes.

Those suffering from snow blindness typically should rest their eyes in a darkened room and use artificial tears to relieve discomfort. 

When ‌skiing or ascending to high altitudes, individuals can protect themselves from snow blindness by wearing proper eye protection, such as sunglasses or goggles with UV protection.

Primary causes of blindness

Blindness can occur due to a variety of genetic and environmental circumstances. Here are the most common causes of blindness:


A person can be blind due to their genetic makeup. Examples of Inherited Retinal Diseases (i.e., genetic diseases that cause blindness) include, but aren’t limited to:

  • Retinitis pigmentosa, which causes the degeneration of cells in the retina and usually develops during adolescence or early adulthood
  • Stargardt disease, or stargardt macular dystrophy, damages the macula (a section within the retina). The disease typically causes central vision loss beginning in childhood or adolescence
  • Cone-rod dystrophy (CRD) affects the cones and rods, the light cells found in the retina, leading them to deteriorate over time. Initial symptoms are usually felt during childhood, and may include blurred vision and intense sensitivity to light
  • Choroideremia is a visual condition that mostly affects males, with an initial symptom typically being night blindness. As time progresses, those with the condition will develop tunnel vision and continue to lose the ability to see
  • Leber congenital amaurosis (LCA) is one of the earliest onset forms of an inherited retinal disease, and primarily affects the retina. People who have LCA, typically have severe vision impairment beginning in infancy.

Infectious diseases

Certain infectious diseases, such as viruses, bacteria, parasites, and fungi can cause permanent or temporary blindness. Examples of infectious diseases that can cause blindness include, but aren't limited to:

  • Trachoma is a bacterial infection that can cause scarring of the cornea and lead to blindness. Most common in areas with poor sanitation, Trachoma is a leading cause of preventable blindness worldwide
  • Cytomegalovirus (CMV) is a common virus that can cause serious eye infections, especially in people with weakened immune systems, and can eventually lead to vision loss and blindness
  • Herpes simplex virus (HSV) can cause a range of eye infections, from conjunctivitis (pink eye) to more severe infections. These, in turn, can lead to scarring of the cornea, which can result in vision loss and blindness
  • Onchocerciasis, commonly referred to as river blindness, is caused by a parasitic worm transmitted to humans when bit by blackflies. Along with severe itching and skin lesions, Onchocerciasis can cause blindness due to damage to the optic nerve

Macular degeneration

Macular degeneration is a progressive eye disease that affects the macula, the part of the retina responsible for central vision. Affecting one or both eyes, dry macular degeneration can cause a gradual loss of vision or wet macular degeneration can cause a rapid loss of vision, and, if not treated, blindness. 

The different types of macular degeneration are:

  • Dry age-related macular degeneration (AMD) is the most common type of macular degeneration and is caused by the gradual breakdown of cells in the macula. This can lead to a loss of vision, including blurred or distorted vision, and to difficulty seeing in low light
  • Wet age-related macular degeneration (AMD) is less common than dry AMD. However, it can cause more rapid and severe vision loss. Wet AMD is caused by the growth of abnormal blood vessels beneath the macula. This can lead to a leaking of blood and fluid that can cause scarring and vision loss

Other forms of macular degeneration include, but aren’t limited to, pattern dystrophy, and myopic macular degeneration.


A cataract is a clouding of the eye's natural lens, which is located behind the iris and the pupil, and plays a critical role in focusing light onto the retina. When the lens becomes cloudy, it can cause a person’s vision to become dim or blurry.

Cataracts are not typically caused by genetics. However, certain factors can increase a person's risk of developing cataracts, including, but not limited to:

  • Age: Cataracts are more common in older adults, and the risk of developing cataracts increases as a person gets older
  • Prolonged exposure to ultraviolet (UV) radiation
  • Diabetes: People with diabetes are more likely to develop cataracts
  • Smoking: Smoking increases the risk of developing cataracts, as well as other eye conditions
  • Long-term use of certain medications, (e.g., steroids), can also increase the risk of cataracts

Treating cataracts typically entails surgery to remove the cloudy lens and replacing it with an artificial lens.

Diabetic retinopathy

Diabetic retinopathy is an eye disease that affects people with diabetes. Caused by narrowing of the blood vessels in the retina, resulting in the creation of new vessels prone to leaking, diabetic retinopathy can lead to vision loss and blindness if left untreated.

There are two main types of diabetic retinopathy:

  • Non-proliferative diabetic retinopathy (NPDR) is the early stage of the disease, characterized by the weakening or swelling of the blood vessels in the retina. The latter can lead to swelling of the macula, small areas of bleeding and/or fluid buildup in the retina, which, in turn, can cause blurred or distorted vision
  • Proliferative diabetic retinopathy (PDR) is the advanced stage of the disease, characterized by the growth of new blood vessels in the retina. The latter are fragile and can leak blood and other fluids which can cause detached retina. The laser treatments often used to stop these vessels from leaking results in scar tissue on the retina and has been characterized as viewing the world through a piece of Swiss cheese. Continued leakage and treatment can lead to severe vision loss or blindness

Treating diabetic retinopathy will depend on the severity of the disease, and may include medication, laser treatment, or surgery. 


Glaucoma is a group of eye diseases that cause damage to the optic nerve, which transmits visual information from the eye to the brain. Typically, this damage is caused by increased pressure in the eye, and, if left untreated, it can result in vision loss or blindness. It is often painless and substantial peripheral vision loss can occur before the patient is aware they have glaucoma. Annual eye exams are recommended to monitor intraocular eye pressure.

There are a few types of glaucoma, including, but not limited to:

  • Open-angle glaucoma: The most common form of glaucoma, open-angle glaucoma occurs when the drainage angle within the eye becomes clogged. This leads to increased pressure in the eye
  • Angle-closure glaucoma: This form of glaucoma occurs when the iris (i.e., the colored part of the eye) is too close to the drainage angle in the eye. This can cause the angle to become blocked and lead to increased eye pressure
  • Normal-tension glaucoma, which entails damage to the optic nerve even when eye pressure is within the normal range

There are several risk factors that can lead to glaucoma, including, but not limited to age, family history, high eye pressure, ethnicity, and thin corneas.

Treatment for glaucoma can include eye drops, oral medication, laser therapy, and/or surgery.

Living with blindness

An estimated 2.2 billion people live with near or distance vision impairments globally
Out of this group, approximately 43 million people are completely blind, with an additional estimated 295 million people having moderate to severe vision impairments.

The way blindness affects an individual’s life will depend on a variety of factors, such as age, geographic location, and socioeconomic status. 

Blindness among children

90 million children and adolescents (aged 0 to 19) live with blindness or a vision impairment worldwide. Being born with blindness, or developing it during childhood and adolescence, can have significant ramifications on a person’s life. Blindness may cause delayed development in motor, language, emotional, social, and cognitive areas. 

Additionally, blindness can decrease school-aged children’s academic success, which can affect their overall educational and career trajectories. 

Early detection, diagnosis, and medical intervention are instrumental in mitigating the negative effects of childhood blindness.

Blindness among adults

Blindness among adults can have significant impacts on employment opportunities, mental health, and essential life activities. Over 70% of Americans who are blind or have vision impairments are unemployed

To help remedy this situation, organizations are encouraged (and, at times, legally mandated) to comply with anti-discriminatory laws, such as the Americans with Disabilities Act (ADA)

Under the ADA, businesses should provide reasonable accommodations to people who are blind or have vision impairments so that they can join their workforce and properly execute their jobs. Such reasonable accommodations should be extended to people who are blind during the interviewing and hiring process, as well.

Reasonable accommodations include:

  • Providing accessible technology such as screen readers, magnification software, Braille displays, and other assistive technology that help people who are blind or have low vision perform their job duties that require reading print or using a computer
  • Flexible scheduling so that employees who are blind or have low vision can properly navigate public transportation and other logistical challenges.
  • Alternative formats to documents and training materials, such as large print, Braille, and audio
  • Workplace modifications such as installing ramps, handrails, or tactile markers and braille signs

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