Contrast vision
The term contrast vision is used in ophthalmology and optics . In order to be able to perceive an object, it must be sufficiently large and have a sufficiently high contrast to its surroundings. This is understood to mean the limit of visibility, also called the minimum visibile . This is achieved when objects that are viewed and imaged on the retina are no longer delimited from the surrounding luminance as a contour and contrast. This means that the recognizability of external objects essentially depends on the ability of the visual system to perceive differences in brightness. You can determine how high the minimum contrast must be for an object to stand out from its surroundings.
A distinction must be made between contrast vision and angular visual acuity , also called visual acuity or minimum separable . Here, the ability of the visual sense to just perceive two adjacent points as such and to distinguish them from one another is determined.
In eye tests , both contrast and visual acuity are de facto involved - for example in the clear-eyed separation of double stars , the so-called eye testers . Even with a good visual acuity of 1 to 2 ', the selectivity is limited to about 3 to 10' due to differences in luminance on the retina.
Verification of contrast vision
If you check the visual acuity ( visual acuity ) of a person with the same optotype but different levels of contrast, you can see that the visual acuity values determined with low contrast are lower than those determined with high contrast. The optotypes of a horizontal row belong to the same visus level, their contrast decreases in continuous steps from the maximum value to a minimum value. If you now look at the optotypes, it becomes clear that the smallest recognizable optotype becomes larger and larger as the contrast decreases. The curve illustrates the relationship between the achieved visual acuity level and the optotype contrast. If the stimulus size is changed with the contrast unchanged, the visual acuity is determined at different contrast levels. However, if the size of the optotype is left unchanged and the contrast values are reduced, the contrast sensitivity is examined.
The conventional visual acuity test tests eyesight based on the ability to see high-contrast objects of decreasing size. Here, the person who can see the smallest objects has the best eyesight. Alternatively, objects of any size can also be presented for which the lowest perceptible contrast is determined. The person who has the highest contrast sensitivity sees best here. The importance of this examination lies in the fact that in daily life one has to recognize not only small objects and fine details, but also large objects with low contrast. The determination of the contrast sensitivity, in particular the examination of the low spatial frequencies, is therefore more suitable than the visual acuity test in order to assess the mobility of the patient in space. It is also suitable for assessing the ability to recognize faces or objects with reduced contrast, for example in fog or at dusk.
See also
Individual evidence
- ↑ Paliaga GP (1993) The determination of the visual acuity, 1st edition, published by Quintessence, Munich