Visual acuity

With visual acuity (also: visual acuity, visual acuity, Acies visus, minimum separabile ) refers to the measurable extent of the ability of an organism to perceive with his organ of sight patterns and contours in the outside world as such. For disambiguation it is necessary to distinguish further essential characteristics.

• 2.1 Comparison values
• 2.2 Visusangaben in percent

• 3.1 Significant visual range

• 5.1 Vision test 5.1.1 contours interaction

Differentiation

Minimum visibile

Under minimum visibile is defined as the limit of visibility. This is achieved when objects are observed and imaged on the retina not define, as a contour and the contrast of the luminance of surrounding. This means that the visibility of external objects essentially depends on the ability of the visual system to perceive differences in brightness.

Minimum discriminibile

With minimum discriminibile is called the visibility threshold for the smallest differences of external objects. In testing situations, it goes here, for example, to clarify whether it is while looking at a line to a solid line or a so-called Noniusanordnung is ( Noniussehschärfe ) whether a minimum local change ( motion ) or a change in orientation ( tilt ) of two objects to each other occurred has.

Minimum separabile

The minimum separabile (synonyms: anguläre visual acuity, visual acuity ) requires not only a relative localization separation of closely spaced contours that just yet can be perceived as separated by minimal differences in luminance on the retina. Here, the luminance distributions of the neighboring objects overlap more so the closer the pattern lie in the outdoor space next to each other.

Minimum legibile

The minimum legibile is the reading acuity. It differs from the other variants of visual acuity by the involvement of stored forms of optotypes as parts of words. Therefore, their value is generally also higher than the value of the other visual acuity species because words here not only recognized, but also be classified according to the sense of the context.

Units

The angular visual acuity ( angular visual acuity ) is the resolution, in which two visual objects are still perceived as separate (minimum separabile ). The resolution of 1 ' (one minute of angle ) corresponds to a spatial resolution of about 1.5 mm at 5 m distance. The smaller the angle of visual acuity, the visual acuity is better.

The dimensionless characteristic visual acuity is defined by the reference 1 ' is related to the individual angular visual acuity.

Example: if a person can only separate points at an angular distance of 2 ', it has a visual acuity of 0.5.

Instead of angles, distances can be determined. If the distance D is chosen as a reference, in which ' provides two points at an angle of 1, then:

Example: if a person only at a distance of 6 m, the points can be seen separately, which have an angular distance of 1 m have at 12 ', it has a visual acuity of 6/12 = 0.5.

Comparison values

For comparison, the angular acuities ( in arcmin ) for some animals: Lens Eye

• Peregrine Falcon: 0.4 '
• Man ( fovea ): 0.4-1.0 '= 3 cm at 100 m
• Driving license, required less than 1.4 '
• Cat: 5 '
• Frog: 7 '
• Elephant: 10.3 '
• Rat: 40 '

Compound eyes

• Honey Bee: 60 '
• Hermit Crab: 270 '
• Drosophila: 540 '
• Shrimp: 780 '

Visusangaben in percent

In many cases, a percentage is given to describe the visual acuity.

Although such a statement seems easier to understand and is used to supposedly better understanding towards patients generally. On closer inspection, it is misleading; there are four problems here:

The use of this percentage, however, also suggested by the fact that a resolution of 1 minute of arc within the normal range in each age group and therefore is a visual acuity of "100%" as a (still) can be considered normal on a regular basis. In the German Social Law a visual acuity of 1.0 is accordingly, regardless of age, defined as non- restricted.

Importance of visual acuity

Visual acuity is the most important measurable parameters of the visual system.

The normal visual acuity is age-dependent and is at a 20 -year-old man at 1.0 to 1.6, with a 80 -year-old 0.6 to 1.0.

Visual acuity is in the ophthalmology of the target parameter whose improvement or preservation with most ophthalmic measures is sought. Numerous legal rules that determine the suitability for operating dangerous equipment ( eg vehicles ) or for specific professions (eg police officer ), require certain minimum visual acuity. Similarly, services private and statutory accident insurance for damage to the eyes and the payment of legal blind allowance primarily on the visual acuity judge.

Distance of distinct vision

Not to be confused with the visual acuity is called the concept of distinct vision, and eye relief, or Normsehweite. She is set to 250 mm. The reference is necessary, for example, to define the magnification of a magnifying glass.

Physiology of visual acuity

The visual acuity of the people depends on:

• The resolving power of the eye (see below)
• The image quality on the retina caused by the refractive media of the eye - is determined - the cornea, aqueous humor, lens and vitreous body
• The optical properties of the object and its environment (contrast, color, brightness )
• The shape of the object: the retina and central nervous system are capable of certain forms (horizontal and vertical straight lines, right angles ) to resolve higher than that corresponding to the resolution of the eyeball alone

In the practical measurement of the visual acuity also makes the ability to describe the visual impression, a significant role.

The size of the eye pupil is physically limited the resolution of the eyeball, physiologically, it is the density of receptors ( rods and cones ) and the signal processing of the receptive fields of the retina. The resolution further reaches its highest value at maximum pupil and sufficient brightness in the region of the fovea centralis retinae ( " fovea "). The field of view of the fovea by measures less than 1 °. When an eccentric fixation of 2 ° of visual acuity has decreased to 0.5.

Examination and measurement methods

In Germany, the visual acuity is determined usually defined by means of projected optotypes size, brightness, shape and defined contrast. The use of a projector instead of a board has the advantage of being independent of the test distance. For reproducible Visusprüfung exist DIN regulations. After that, the standard of the so-called Landolt ring optotype, a ring with a gap of defined width of the same width may be arranged in eight different directions. By recognizing the direction of the gap shows the examinee that his resolution is at least equal to the width of the gap. In practice, however, most standard illustrations of numbers are used as test figure for ease of understanding. There are other standardized optotypes, the Snellen hooks that resemble a D, and others that are suitable for Visusprüfung of illiterate and preschoolers as well as for the non - verbal communication. On most chart projectors will sign up to a visual acuity of 0.05 (corresponding to about 0.3 ° ) are offered. This is due to the increasing with increasing visual impairment measurement uncertainty. For even lower acuity eye charts are used (see below) or counting fingers, hand movements and light sources used.

In the measurement of visual acuity is between that without correction, such as glasses or contact lenses, and that distinction with correction. It is called visual acuity without correction as a rough visus. Often, the abbreviations are s.c ( sine correctione, Latin for " without correction " ) and cc (cum correctione, Latin for " with correction " ) are used. Those optical correction that yields the highest Visuswert is often referred to as the "best correction". The possibility of the optical correction causes the visual acuity is independent of the refractive properties of the eye. Visual acuity in the sense of the above definition is thus the visual acuity with best correction.

With a visual acuity despite optimal eyeglass or contact lens correction of less than 1/50 = 0.02 ( determined by eye charts ) in the better eye, a person shall, for purposes of the Social Code in Germany as blind, for the World Health Organization (WHO ) from a visual acuity of 3/60 = 0.05 or less.

Vision test

When using eye charts must, because of the constant size of the optotypes, be paid very close attention to compliance with the test distance. Furthermore, the compliance of the right (right- light ) illumination of the panel important because the visual acuity depends essentially on the luminance of the white background. A well-known eye chart is the Snellen index, created by the Dutch ophthalmologist Herman Snellen, must be read in the shrinking letters and numbers. There are various other boards here.

Vision test are calibrated to a standard distance. Visual acuity is measured by a fracture. The numerator is the actual distance, thus the distance from which the examinee recognizes the optotypes. The denominator is the standard distance, the distance at which a person could recognize with a visual acuity of 1.0 the same optotypes. Distance is measured in either the metric system or the English Snellen Foot - measure, which can be converted into each other. Often the specification of visual acuity occurs as a decimal number.

An example: Detects a patient is a character from a distance of 6 meters, for which the standard distance is 30 meters, so is his visual acuity 6/30 or 0.2.

Contours interaction

Optotypes that are close to each other can be recognized less well, as closely spaced contours affect the resolution ( separation difficulties or crowding ). This may account for series optotypes deterioration of up to a visual acuity. In amblyopia, nystagmus or insufficiently corrected ametropia the differences between series and single optotypes can even have a very significant impact. In Landolt rings, therefore, a minimum spacing of optotypes is prescribed to each other from a visual acuity of 0.32 and for all larger optotypes, which should be larger than 35 arcmin. This distance is outside the zone of contour interactions. For smaller optotypes from a visual acuity of 0.4, however, only twice the diameter of the Landolt ring is as the distance of the optotypes among themselves demanded. Thus, they fall into the interaction zone. For this reason, special tests have been developed with Landolt rings (C- test) for clinical trials, which have a clear separation between single and Reihensehzeichen. Here all optotypes are included in the individual optotypes by all acuity through at least 35 minutes of arc apart. The number optotypes, the distance through all acuity through exactly 2.6 minutes of arc. The test distance is in both cases at 40 cm.

Differential diagnosis

To assess whether the visual acuity of a subject refraktionsbedingt, ie due to a not or only inadequately corrected refractive error ( ametropia ) is reduced or whether this is caused by organic or functional problems, the use of a stenopaic gap may be useful. One way to organically caused Sehminderungen be distinguished from functional is the so-called Ammann'sche gray glass test.