Blindness, low vision, and color vision deficiency

6 min read · BoK 1B· p. 10–11

Learning objectives (3)

Distinguish blindness, low vision, and color vision deficiency
Recall key WHO and NIH prevalence figures for visual conditions
Identify the most common cause of color vision deficiency

Why it matters

Visual conditions are among the most common disabilities worldwide and the ones most directly addressed by web accessibility standards. Knowing the clinical distinctions — and the scale of the population — helps you communicate the urgency of visual accessibility to stakeholders and understand why different design decisions serve different visual needs. (BoK p. 10)

Scale of visual impairment

The World Health Organization estimates that approximately 2.2 billion people live with some form of vision impairment. Of these, at least 1 billion cases are preventable or have not yet been addressed — primarily through correction of refractive errors or treatment of cataracts. (BoK p. 10)

Of that 2.2 billion:

Roughly 43 million people are classified as blind.
Approximately 246 million people have low vision. (BoK p. 11)

Blindness

Blindness is defined by the WHO as visual acuity below 3/60 in the better eye — meaning a person can only see at 3 meters what someone with typical vision sees at 60 meters. It also covers complete absence of light perception. People who are blind rely on non-visual information channels: synthesized speech, braille, tactile cues, and audio descriptions. (BoK p. 10)

Low vision

Low vision refers to a significant reduction in visual function that cannot be fully corrected with lenses. The person retains some usable vision but may struggle with fine print, low contrast, or peripheral field loss. Low vision is far more prevalent than blindness and serves a large portion of the population that benefits from magnification, high-contrast design, and large-print formats. (BoK p. 11)

Low vision is caused by:

Age-related macular degeneration (the leading cause in older adults).
Cataracts (often treatable; the leading preventable cause globally).
Glaucoma, diabetic retinopathy, and other conditions.

Color vision deficiency

Color vision deficiency (often called “color blindness”) is the reduced ability to distinguish between certain colors. It results from variation in the cone cells of the retina. (BoK p. 11)

Key facts:

The most common form is red-green deficiency, affecting the ability to distinguish red from green.
Approximately 8% of males and 0.5% of females of Northern European descent have some degree of color vision deficiency.
It is largely genetic and does not progress or cause total color blindness in most cases.

Real-world example

A web analytics dashboard uses red bars for “below target” and green bars for “above target.” No text labels are attached. An estimated 8% of the male user base cannot reliably distinguish the two. Adding a text label (“Below” / “Above”) and distinct bar patterns removes the barrier without changing the visual design for other users.

Key terms

Blindness: Visual acuity of less than 3/60 in the better eye, or visual field loss to less than 10 degrees — the WHO's threshold for severe visual impairment. (BoK p. 10)
Low vision: Visual impairment that cannot be fully corrected by spectacles or contact lenses, yet retains some usable vision. (BoK p. 11)
Visual acuity: The clarity or sharpness of vision measured by the smallest detail a person can resolve at a standard distance. (BoK p. 10)
Color vision deficiency: Reduced ability to distinguish certain colors, most commonly red from green; affects roughly 8% of males and 0.5% of females. (BoK p. 11)
Refractive error: A visual condition — myopia, hyperopia, or astigmatism — correctable by lenses and therefore not classed as disability by the WHO. (BoK p. 10)

Check yourself

.cy-island-wrapper { display: none; }JavaScript is required for the interactive quiz. Here are the questions for reference:

According to the BoK, approximately how many people worldwide have low vision — defined as visual impairment that cannot be fully corrected by lenses but retains some usable sight?

a) 43 million
b) 246 million — correct answer
c) 1.3 billion
d) 2.2 billion

Explanation: The BoK records roughly 246 million people with low vision and 43 million people who are blind. These sit within the larger WHO estimate of 2.2 billion with any vision impairment. Keeping these figures distinct is a high-yield area on the exam (BoK p. 11).

A color-blind user reports difficulty distinguishing between the "pass" (green) and "fail" (red) indicators in a quality-control dashboard. Which form of color vision deficiency does the BoK identify as the most common, and what percentage of males does it affect?

a) Blue-yellow deficiency; affects approximately 8% of males
b) Red-green deficiency; affects approximately 0.5% of males
c) Red-green deficiency; affects approximately 8% of males — correct answer
d) Total color blindness; affects approximately 8% of males

Explanation: Red-green color vision deficiency is the most common form, arising from variation in red or green cone cells. It affects approximately 8% of males and 0.5% of females, making it one of the most widespread disabilities relevant to web design. The design implication is that color alone must never be the sole indicator of status or meaning (BoK p. 11).

A developer argues that most vision impairment can be addressed by telling users to get glasses. An accessibility professional responds that correctable refractive errors represent only part of the 2.2 billion figure, and that many people have impairment that spectacles cannot fix. Which condition does the BoK identify as the leading cause of low vision in older adults?

a) Glaucoma
b) Age-related macular degeneration — correct answer
c) Cataracts
d) Diabetic retinopathy

Explanation: The BoK identifies age-related macular degeneration as the primary cause of low vision in older adults. Unlike cataracts (treatable) or refractive errors (correctable), AMD causes progressive, irreversible central vision loss. This is why low-vision accessibility — high contrast, magnification support, and clear layout — matters increasingly as populations age (BoK p. 11).

Sources

BoK references

Section 1B — IAAP CPACC Body of Knowledge v4.0 (Oct 2023), p. 10–11