Glossary

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Adaptive Optics(also: AO)

A technology that measures and corrects optical distortions in real time, originally developed for astronomical telescopes and now applied in ophthalmology and vision science. Adaptive optics systems use a wavefront sensor to detect aberrations and a deformable mirror or other…

Anomalous Trichromat(also: Anomalous Trichromacy, Colour Anomaly)

A person who has all three types of cone cells in the retina but one type has a shifted spectral sensitivity, causing altered colour perception that is less severe than dichromacy. Anomalous trichromats include protanomalous individuals (shifted L-cones, reduced red…

Artificial Eye(also: Eye Model, Optical Eye Model)

A physical device constructed from optical components to simulate the optical properties of a human eye. Typically consisting of a lens, adjustable aperture (simulating the iris), and an image sensor (simulating the retina), artificial eyes are used in vision research to test…

Astigmatism

A common refractive error caused by an irregularly shaped cornea or lens, resulting in blurred or distorted vision at all distances. The cornea is shaped more like a rugby ball than a sphere, causing light to focus on multiple points rather than one. Astigmatism frequently…

Central Vision(also: Foveal Vision)

Central vision is the area of sharpest sight in the visual field, corresponding to the fovea at the centre of the retina. It is responsible for detailed tasks such as reading, recognizing faces, and distinguishing fine detail and colour. Loss of central vision, commonly caused…

Cerebral Visual Impairment(also: CVI, Cortical Visual Impairment, Brain-Based Visual Impairment)

A form of visual impairment caused by damage to or dysfunction in the brain's visual processing centres, rather than problems with the eyes themselves. CVI is the leading cause of childhood vision impairment in developed countries and is projected to become a leading cause of…

Color Perception(also: Color Vision, Chromatic Vision)

Color perception is the ability to detect, distinguish, and identify colors. Impairments in color perception range from complete color blindness (achromatopsia) to partial deficiencies in distinguishing specific color ranges, such as red-green or blue-yellow color vision…

Color Vision Deficiency(also: Color Blindness, CVD, Colour Blindness)

A condition in which a person has difficulty distinguishing between certain colors, most commonly red and green (deuteranopia/protanopia), but also blue and yellow (tritanopia) or all colors (achromatopsia). Color vision deficiency affects approximately 8% of males and 0.5% of…

Convergence Insufficiency(also: CI)

A binocular vision disorder in which the eyes have difficulty turning inward (converging) to focus on nearby objects, causing symptoms such as double vision, eye strain, headaches, difficulty reading, and blurred vision during close work. Convergence insufficiency affects an…

Dichromat(also: Dichromacy, Dichromatic Vision)

A person with colour vision based on only two types of functional cone cells in the retina instead of the typical three, resulting in a reduced ability to distinguish certain colours. Dichromats perceive colour in a two-dimensional colour space rather than the three-dimensional…

Farnsworth D-15 Test(also: D-15 Color Test, Farnsworth Dichotomous Test)

The Farnsworth D-15 test is a clinical assessment used to evaluate color perception by asking a person to arrange 15 colored caps in order of hue. The pattern of errors reveals the type and severity of color vision deficiency, distinguishing between protan (red), deutan (green),…

Fixation Problem(also: Reading Fixation Difficulty, Visual Fixation Issue)

A difficulty in maintaining stable visual focus on a specific point or line of text during reading. People with fixation problems may lose their place frequently, skip lines, or re-read the same passage unintentionally. This is a common challenge for dyslexic readers and can be…

Fovea(also: Foveal Vision, Fovea Centralis)

The small, central area of the retina responsible for sharp, detailed vision, covering approximately 1 degree of visual angle. The fovea contains the highest density of cone photoreceptors and provides the clearest visual acuity, which is why people move their eyes to point the…

Glare Sensitivity(also: Photophobia, Light Sensitivity)

An increased sensitivity to light that causes discomfort, visual difficulty, or pain, commonly experienced by people with various eye conditions including cataracts, macular degeneration, and albinism. Glare sensitivity significantly affects the usability of digital displays and…

Goldmann Perimetry(also: Goldmann Visual Field Test, Kinetic Perimetry)

Goldmann perimetry is a clinical technique for mapping the visual field — the total area a person can see while fixating on a central point. The test uses a moving stimulus of controlled size and brightness projected onto a white hemispherical bowl, with the examiner tracking…

Hemi-Attention(also: Hemispatial Neglect, Unilateral Neglect, Visual Neglect)

A neurological condition in which a person has reduced attention or awareness to one side of space, typically the side opposite to a brain injury. Unlike hemianopsia (where visual input is lost), hemi-attention involves a failure to attend to or process stimuli on the affected…

Hemianopsia(also: Hemianopia, Half-Field Vision Loss)

Loss of vision in one half of the visual field in one or both eyes, typically caused by damage to the brain's visual pathways rather than the eyes themselves. The most common form is homonymous hemianopsia, where the same half of the visual field is lost in both eyes — for…

Image Enhancement(also: Image Pre-Compensation, Visual Enhancement)

Image enhancement in the context of accessibility refers to techniques that modify digital images or on-screen content to improve their visibility and usability for people with visual impairments. Methods include contrast adjustment, edge highlighting, color remapping,…

Ishihara Test(also: Ishihara Colour Test, Ishihara Plates)

A widely used clinical screening test for red-green colour vision deficiencies, consisting of a series of circular plates made up of coloured dots arranged in a mosaic pattern. Each plate contains a number or shape formed by dots of certain colours set against a background of…

Monochromacy(also: Achromatopsia, Total Color Blindness)

The rarest and most severe form of color vision deficiency, in which an individual has complete or near-complete absence of color perception, seeing the world only in shades of gray. Monochromacy occurs when two or all three types of cone cells in the retina are absent or…

Near Visual Acuity(also: NVA, Near Acuity)

A measure of the eye's ability to resolve fine detail at a close distance, typically tested at 40 centimetres. Near visual acuity is particularly important for tasks involving reading, handheld device use, and close-up work. It is commonly measured using Snellen notation (e.g.,…

Ocular Aberration(also: Optical Aberration, Wavefront Aberration)

Ocular aberration refers to deviations in the way light is focused by the optical components of the eye (cornea and lens) compared to an ideal optical system. These aberrations distort the image formed on the retina and contribute to blurred or distorted vision. Lower-order…

Optic Ataxia(also: Visuomotor Ataxia)

A neurological condition characterised by impaired reaching for objects using visual guidance, despite having adequate vision and motor strength. People with optic ataxia can see an object and describe its location but struggle to accurately direct their hand toward it. This…

Optic Flow(also: Visual Flow, Optical Flow)

The pattern of apparent motion of objects, surfaces, and edges in a visual scene caused by relative movement between the observer and the scene. Optic flow provides critical information about self-motion, speed, direction, and the structure of the environment. It plays a key…

Optic Nerve Hypoplasia(also: ONH)

A congenital condition in which the optic nerve is underdeveloped, resulting in varying degrees of vision loss from mild visual impairment to complete blindness. It is one of the most common causes of visual impairment in children. People with optic nerve hypoplasia may…

Parallax(also: Visual Parallax, Binocular Parallax)

Parallax is the apparent displacement or difference in position of an object when viewed from two different vantage points. In human vision, binocular parallax — the slight difference between the images seen by each eye due to their spatial separation — is a primary cue for…

Pelli-Robson Chart(also: Pelli-Robson Contrast Sensitivity Chart)

The Pelli-Robson chart is a clinical tool used to measure contrast sensitivity — the ability to detect objects at low-to-moderate contrast levels. The chart consists of a series of letter-charts composed of different contrasts, mapping a contrast-sensitivity function for the…

Point Spread Function(also: PSF)

A mathematical description of how a single point of light is spread or blurred by an optical system such as the human eye. The point spread function characterizes the degree and pattern of distortion introduced by optical aberrations. In accessibility research, PSFs are used to…

Presbyopia(also: Age-Related Farsightedness, Loss of Accommodation)

An age-related vision condition in which the eye gradually loses the ability to focus on nearby objects, typically becoming noticeable after age 40. Presbyopia affects the vast majority of older adults and is caused by the hardening of the eye's lens, reducing its flexibility.…

Prosopagnosia(also: Face Blindness)

A neurological condition characterised by the inability to recognise familiar faces, despite otherwise intact visual and cognitive abilities. People with prosopagnosia may fail to recognise family members, friends, or colleagues by face alone, instead relying on alternative cues…

Pupil Diameter(also: Pupil Size, Aperture Size)

The width of the opening in the iris through which light enters the eye, typically ranging from about 2mm in bright light to 8mm in darkness. Pupil diameter significantly affects visual quality because it determines how much of the eye's optical aberrations influence the image…

Refractive Error(also: Refractive Disorder, Ametropia)

Refractive error is a common vision condition in which the shape of the eye prevents light from focusing correctly on the retina, resulting in blurred vision. Types include myopia (nearsightedness), hyperopia (farsightedness), astigmatism (irregular corneal curvature), and…

Scotopic Sensitivity(also: Irlen Syndrome, Visual Stress, Meares-Irlen Syndrome)

A visual-perceptual condition in which certain wavelengths of light cause discomfort, distortion, or difficulty when reading. People with scotopic sensitivity may experience text appearing to shimmer, move, or blur on the page, particularly with high-contrast black text on white…

Sensory Overload(also: Sensory Overwhelm, Overstimulation)

A state in which the brain receives more sensory input than it can effectively process, leading to stress, anxiety, difficulty concentrating, and in severe cases, meltdowns or shutdowns. Sensory overload can be triggered by excessive visual complexity, noise, crowds, bright or…

Simultaneous Color Contrast(also: Simultaneous Contrast)

A perceptual phenomenon where the appearance of a colour is influenced by the colours surrounding it, causing the same colour to look different when placed against different backgrounds. For example, a grey square appears lighter against a dark background and darker against a…

Trichromat(also: Trichromacy, Trichromatic Vision)

A person with typical colour vision based on three types of functional cone cells (L, M, and S cones) in the retina, each sensitive to different wavelengths of light corresponding roughly to long (red), medium (green), and short (blue) wavelengths. Trichromatic vision allows…

Vision Multiplexing(also: Visual Multiplexing)

An optical engineering concept for low-vision aids that involves presenting multiple views or visual channels simultaneously to compensate for reduced visual function. In the context of assistive technology, vision multiplexing allows users to see both a wide field of view and a…

Vision Therapy(also: Visual Training, Orthoptics, Eye Training)

A program of visual activities and exercises prescribed to improve visual skills and processing, often used to treat conditions like amblyopia, strabismus, and convergence insufficiency. Vision therapy can include in-office sessions and at-home exercises, and increasingly…

Visual Aberration(also: Optical Aberration, Refractive Error)

A deviation from ideal optical performance in the eye that causes images to appear blurred, distorted, or otherwise degraded on the retina. Visual aberrations include both lower-order aberrations (such as myopia, hyperopia, and astigmatism) and higher-order aberrations (such as…

Visual Acuity

A measure of the sharpness or clarity of vision, typically expressed as a fraction (e.g., 20/20) or percentage indicating the smallest detail a person can resolve at a standard distance. Visual acuity is one of the primary metrics used to classify the degree of visual…

Visual Clutter(also: Visual Noise, Visual Complexity)

An excess of visual elements in an environment or interface that makes it difficult to locate, identify, or focus on relevant information. Visual clutter is a significant barrier for people with cerebral visual impairment, simultanagnosia, and other visual processing conditions,…

Visual Crowding(also: Crowding)

A perceptual phenomenon in which the presence of nearby flanking characters or objects makes it harder to recognise a target character, especially in peripheral vision or when the target is small, low-contrast, or briefly viewed. Crowding jointly with limited visual span sets an…

Visual Field Loss(also: Visual Field Deficit, VFL, Field Loss)

A reduction in the area of vision that a person can see without moving their eyes or head. Visual field loss can affect any portion of the visual field — central, peripheral, or specific quadrants — and can result from conditions affecting the eyes (glaucoma, retinal detachment)…

Wavefront Analyzer(also: Aberrometer, Wavefront Sensor)

An optical instrument that measures how light waves are distorted as they pass through the eye's optical system, producing a detailed map of the eye's refractive characteristics called a wavefront aberration function. Unlike standard eye exams that measure only basic refractive…

Zernike Polynomials(also: Zernike Coefficients, Zernike Modes)

A set of mathematical functions used to describe the shape of optical wavefronts, commonly employed in ophthalmology and optometry to characterise the optical aberrations of the human eye. Each Zernike polynomial represents a specific type of optical distortion — for example,…

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