OPB451 W1 - Visual Acuity

Visual Acuity (VA)

A measure of fine spatial vision and a key clinical measure of the integrity of the optical and physiological state of the eye.

Minimum Visible Acuity

The smallest target that can be detected under ideal conditions, such as bright photopic conditions.

Minimum Resolvable Acuity

The smallest angular separation between neighboring features that can be resolved, usually measured with gratings.

Minimum Recognisable Acuity

The angular size of the smallest feature that can be recognized or identified, typically measured using letter charts like the Snellen chart.

Minimum Discriminable Acuity

The angular size of the smallest change in a feature that can be discriminated, also known as hyperacuity.

Snellen Acuity

Common visual acuity measurement based on the angle that the critical detail of an object subtends at the anterior nodal point of the eye.

Minimum Angle of Resolution (MAR)

The smallest angle which can be resolved by the eye, often used in visual acuity measurements.

Conversion to Contrast Sensitivity

A method of relating visual acuity stimulus to contrast sensitivity using Minimum Angle of Resolution (MAR).

Factors Affecting Visual Acuity

Includes optical blur, retinal illumination, target contrast, pupil size, and motion.

Vernier Acuity

A type of minimum discriminable acuity where small offsets in the position of features are detected.

Crowding Effect

The reduced ability to identify letters or features when they are surrounded by nearby visual stimuli, more pronounced in peripheral vision.

Binocular vs Monocular Acuity

Binocular visual acuity is generally greater than monocular by about 10%, due to probability summation.

logMAR Acuity

A scale used in low vision and research that represents visual acuity in a logarithmic manner.

Nyquist Limit

The concept that two adjacent points can only be resolved if their retinal images fall on separate cones.

Rayleigh’s Criterion

A principle that defines the limit of resolution for two point targets in a diffraction-limited system.

Age-Related Changes in Visual Acuity

Visual acuity tends to decrease with increasing age due to various factors including ocular and neural changes.

Contrast Sensitivity Function

A measure of the ability to discern foreground from background in varying levels of contrast.

Snellen Acuity Ratio

A comparison of a person's visual acuity to the standard vision chart, often expressed as a fraction.

Decimal Acuity

A representation of visual acuity as a decimal, where 1.0 corresponds to normal vision, usually derived from Snellen ratios.

Problems with Snellen Charts

Limitations including dependence on language, familiarity with letters, and variability in lighting conditions.

Bailey-Lovie Charts

A type of visual acuity chart that uses equal-sized letters at different levels of contrast, minimizing crowding effects.

Optical Constraints to Visual Acuity

Factors such as lens quality, corneal clarity, and distortion that limit the resolution of an image by the eye.

Retinal Influence on Visual Acuity

The ability of the retinal structure and health to resolve details and impacts on visual outcomes.

Absolute Threshold

The minimum stimulus intensity required for detection of a visual stimulus.

Increment Threshold

The minimum change in stimulus intensity that can be detected, used to measure sensitivity.

Task Design Effect

The impact of the design of visual tasks on the measured visual performance and acuity.

Optical Blur

A reduction in visual acuity due to out-of-focus images caused by imperfections in optics.

Astigmatic Defocus

A condition where light rays are not focused evenly on the retina, leading to blurred vision.

Retinal Illumination

The level of light reaching the retina, which affects visual function and acuity.

Effect of Retinal Eccentricity

The variation in visual acuity based on the location on the retina, with central vision typically being sharper.

Pupil Size Effect on Visual Acuity

Changes in pupil size influence depth of field and optical quality, impacting visual clarity.

Motion Effects on Visual Acuity

The phenomenon where quick movements can impair the ability to discern visual details.

Binocular vs Monocular Acuity

Comparison where binocular acuity is typically better than monocular due to spatial summation.