Meridian (perimetry, visual field)
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:''This article is about the [[perimetry]] concept. For other uses of the word, see [[meridian (disambiguation)|Meridian]].''
'''Meridian''' is used in '''perimetry''' and in specifying '''visual fields'''. According to IPS Perimetry Standards 1978 (2002): ''"[[Perimetry]] is the measurement of [an observer's] visual functions ... at topographically defined loci in the visual field. The [[visual field]] is that portion of the external environment of the observer [in which when he or she is] steadily fixating ...[he or she] can detect visual stimuli."''
In perimetry, the observer's [[eye]] is considered to be at the centre of an imaginary [[sphere]]. More precisely, the centre of the sphere is in the centre of the [[pupil]] of the observer's eye. The observer is looking at a point, the [[fixation point]], on the interior of the sphere. The [[visual field]] can be considered to be all parts of the sphere for which the observer can see a particular test stimulus. If we consider this surface to be that on which an observer can see anything, then it is a section of the sphere somewhat larger than a [[Sphere|hemisphere]]. In reality it is smaller than this, and irregular, because when the observer is looking straight ahead, his or her nose blocks vision of some possible parts of the surface. In [[perimetric testing]], a section of the imaginary sphere is realized as a hemisphere in the centre of which is a fixation point. Test stimuli can be displayed on the hemisphere.
To specify loci in the visual field, a [[polar coordinate]] system is used, all expressed from the observer's perspective. The [[Origin (mathematics)|origin]] corresponds to the point on which the observer is fixating. The [[polar angle]] is considered to be zero [[degree (angle)|degree]]s when a locus is horizontally to the right of the fixation point and to increase to a maximum of 360 degrees going [[counterclockwise|anticlockwise]]. Distance from the origin is given in degrees of [[visual angle]]; it's a measure of [[Eccentricity (mathematics)|eccentricity]]. Each [[polar axis]] is a '''meridian''' of the visual field. For example, the horizontal meridian runs from the observer's left, through the fixation point, and to the observer's right. The vertical meridian runs from above the observer's line of sight, through the fixation point, and to below the observer's line of sight.
Another way of thinking of the maximum visual field is to think of all of the [[retina]] that can be reached by light from the external environment. The visual field in this case is all of the external environment that can project light onto the retina. Meridians correspond to sections of [[great circles]] passing though the centre of the [[fovea]]. In an analogy to [[Meridian (geography)]], in which meridians are lines of [[longitude]], the [[North pole]] might correspond to the fovea, Greenwich would correspond to a retinal location about 39 degrees to the left of the fovea (because the retinal image is inverted, this corresponds to a location in the visual field to the observer's right), and the [[South pole]] would correspond to the centre of the pupil.
==See also==
*[[Angle of view]]
*[[Central vision]]
*[[Field of view]]
*[[Peripheral vision]]
==References==
IPS Perimetry Standards 1978. (2002). Author. Department of Ophthalmology & Visual Sciences, The University of Iowa. Retrieved March 4, 2005, from http://webeye.ophth.uiowa.edu/ips/GEN-INFO/standards/STANDARD.HTM huuh? what? ok, w/e
[[Category:Vision]]