Glare (overexposure)

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Glare from an interfering light source

Glare describes an optical or visual disturbance caused by excessive brightness , which leads to excessive demands on the visual system . Sources of interference that lead to overexposure of optical technical devices, called glare , are also transmitted and simplified .

Basics

Dazzling headlights are also a problem in road traffic ( flare effects can be seen in the photo ).

If too many photons hit a receptor in too short a time , it becomes oversaturated . The light input is measured as brightness per unit area times the duration. This allows the illuminance (in lm / m²) of point light sources to be displayed or the luminance (in cd / m²) of flat sources as the area sum of point sources. Depending on whether the oversaturation occurs only in a small area or for the entire field of view, one speaks of local or total glare. The local glare comes mainly by differences in the luminance of the environment established (relative brightness), while the total glare is triggered by an overall high incidence (absolute magnitude).

Is commonly glare as "white" represented as an increasing light intensity in the relevant for the performance or measurement of light model of additive color mixing in color impressive white ends - and focus on the positive of photographic images depicting also white. But glare can also be caused by monochromatic light and perceived in color.

Disturbance of visual function

In the glare, the respective boundaries playing of the performance for the adaptation of the eye to changing luminance conditions - and the duration and time course of these - by oculomotor reactions such Lidspiel and pupil game for the optical apparatus and retinal adaptation of receptors, such as neurons of the visual system adaptation to Light plays an essential role ( light blindness ).

The healthy eye is able to adapt to the changing luminance of the surroundings from moonless night to glaring sunshine and thus within the wide range of 1:10 12 ; With small fluctuations it only takes a fraction of a second and from one extreme to the other usually just under 30 minutes, faster than twilight. Apart from the eyelid closure or the width of the eyelashes, four different processes or factors are responsible for this:

  1. Reflective constriction or dilation of the pupil. When the surrounding luminance decreases, the pupil opens wider, allowing more light to enter the eye . When the luminance increases, such as sudden additional exposure, it narrows. (Compensation factor about 1:16).
  2. Concentration of the visual pigment: If a lot of light reaches the receptors, many visual pigment molecules break down and the concentration of the visual pigment (and thus the light sensitivity) in the respective cells decreases. In low light, the structure outweighs the decay, which increases the concentration. With constant lighting conditions, there is a dynamic equilibrium between decay and build-up.
  3. Combination of several rods to form a sensory element: As part of the adaptation, the number of receptors for the field from which a downstream optic nerve cell receives its information can also change. This phenomenon is also called "spatial adaptation".
  4. Fixation duration: By staring at the same spot for a long time, otherwise subliminal stimuli can become supra-threshold, which is called "temporal summation" (but does not protect against glare).

Creation of glare

The visual system optimizes and adapts to a certain ambient luminance. Often, however, the surrounding luminance is not homogeneous and shows differences, sometimes considerable. If, for example, there is a bright (disruptive) light source in the field of vision, a light curtain can literally be placed over large areas of the retina by the scattered light generated inside the eye . In this case, the eye adapts to a higher luminance level, higher than would actually be appropriate for the surrounding luminance, due to the intraocular luminous veil. This veil luminance can already impair the function of vision if it only accounts for about 1 to 2 percent of the luminance that is present at the location of the visual field to which the current visual information is currently referenced.

The generation of scattered light inside the eye can often be traced back to scattering centers that are located in different numbers in the refractive media of the eye, be it in the cornea, lens or vitreous humor. These scattering centers can be, for example, opacifying inclusions or also transparent areas with a different refractive index , which now diffusely deflect the light and contribute to the aforementioned veil luminance. Since the transparency of the lens of the eye in particular strongly depends on age, the glare effect of a comparable source of interfering light can also vary depending on age.

Forms of glare

If there is a measurable impairment of visual performance, one speaks of physiological glare ; If the glare is subjective, but impaired visual performance cannot be proven by external measurements, psychological glare is assumed.

If the luminance levels of the surroundings are so high that the visual system is no longer able to adapt to this level at all, this form of glare is referred to as absolute glare , in other cases as adaptation glare . Furthermore, a distinction is made between direct glare , which is triggered by a light source itself, and indirect glare , which is caused by the reflected image of a light source.

In summary, it can be stated that there is absolute glare if the eye can no longer adequately adapt to the luminance of the remaining field of view due to an interfering light source and visual recognition in the remaining field of view is restricted due to the lack of adaptation. For a visual system, the limit of its adaptability to differences in luminance intensity within the visual field is reached.

Glare from technical devices

Similar to the eye, glare can also occur in technical optical systems. In photography, glare or overexposure can usually be largely controlled by controlling the exposure or setting the aperture - which in terms of opening width and opening time roughly corresponds to the pupillomotor adaptation of the eye ( stopping down ).

Individual evidence

  1. ↑ a b S. Silbernagl, A. Despopoulos: Pocket Atlas of Physiology. 5., completely revised. and redesigned edition. dtv-Verlag, 2001, ISBN 3-423-03182-4 , p. 280 ff.