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Figure 1 - Humphrey field analyser. Humphrey field analyser (HFA) is a tool for measuring the human visual field that is commonly used by optometrists, orthoptists and ophthalmologists, particularly for detecting monocular visual field. [1] The results of the analyser identify the type of vision defect.
An example of the Landolt C eye chart (also known as the Japanese eye chart). Numerous types of eye charts exist and are used in various situations. For example, the Snellen chart is designed for use at 6 meters or 20 feet, and is thus appropriate for testing distance vision, while the ETDRS chart is designed for use at 4 meters. [16]
If the analogy of the eye's retina working as a sensor is drawn upon, the corresponding concept in human (and much of animal vision) is the visual field. [2] It is defined as "the number of degrees of visual angle during stable fixation of the eyes". [3] Note that eye movements are excluded in the visual field's definition.
Near visual acuity or near vision is a measure of how clearly a person can see nearby small objects or letters.Visual acuity in general usually refers clarity of distance vision, and is measured using eye charts like Snellen chart, LogMAR chart etc. Near vision is usually measured and recorded using a printed hand-held card containing different sized paragraphs, words, letters or symbols.
Visual acuity is a measure of how well small details are resolved in the very center of the visual field; it therefore does not indicate how larger patterns are recognized. Visual acuity alone thus cannot determine the overall quality of visual function. [8]
It is a diagnostic tool that aids in the detection of visual disturbances caused by changes in the retina, particularly the macula (e.g. macular degeneration, Epiretinal membrane), as well as the optic nerve and the visual pathway to the brain. Amsler grid usually help detecting defects in central 20 degrees of the visual field. [2]
In human vision, the useful field of view (or UFOV) is the visual area from which information can be extracted without eye or head movements. [1] UFOV size generally decreases with age, [ 2 ] most likely due to decreases in visual processing speed, reduced perception, and increased susceptibility to distraction.
The large body of evidence confirms that the VVIQ is a valid and reliable psychometric measure of visual image vividness. In 1995 Marks published a new version of the VVIQ, the VVIQ2. [2] This questionnaire consists of twice the number of items and reverses the rating scale so that higher scores reflect higher vividness.