The brightness

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Brightness affects the resolving power of the visual system in several ways. First, it affects pupil size. This is the free aperture of the iris muscle, which is situated immediately in front of the eye’s lens. Since the retina, which is located at the very back of the eye and on which the seen image is projected, adapts only slowly to changes in luminance, the pupil has the protective function of a fast-closing diaphragm. It can regulate the size of its aperture between 2 mm and 8 mm and thus reduce or increase the incident light quantity by a factor of 16 (for comparison, the ambient luminances can differ by a factor of about 10¹⁰ during the day – maximum 10⁵ candela/m² – and at night – minimum 10^-5 candela/m²⁾. Only after the immediate adjustment through the pupil do the sensory cells of the retina become accustomed to the changed luminance. In addition to regulating the amount of light, the iris diaphragm shares another analogy with the camera aperture. Its narrowing increases the depth of field during near vision. This results in significantly sharper retinal images, and this is particularly important in daytime vision. The aperture size is the crucial point because, as mentioned in the section on diffraction, the theoretically maximum achievable resolving power depends on it. In practice, however, the relationship „larger pupil equals greater resolving power“, which can be derived from this section, does not apply because the decreasing pupil diameter with increasing brightness reduces the optical aberrations inherent in the eye. Much like stopping down the lens in photography. This double effect necessitates a „mixed calculation“ to balance the diffraction errors at small apertures and the aberration errors at large apertures. For a large average of normal-sighted eyes, it has been found that an average pupil size of 3 mm to 5 mm diameter (corresponding to 7 mm to 20 mm pupil area) has the least disadvantages for resolving power. Depending on our age, we achieve these values at luminances between 150 and 300 cd/m², which roughly corresponds to the brightness required for comfortable reading or precise work indoors.

As described in detail in the section „The light/dark adaptation“ in the chapter about Contrast, the ambient brightness also determines the adaptation state of the visual system. Thus, whether we see with the rod or the cone receptors. The cones, which are responsible for color vision and the highest resolution, are active in mesopic vision at dusk and in photopic vision during the day, i.e., at luminances between 0.01 cd/m² and 100,000,000 cd/m². Below that, the much lower-resolution rods operate. With regard to resolution, the wide range of the cone adaptation level is optimal only up to 10,000 cd/m². Above that value, the resolving power drops again due to glare.

Between these two points, in the medium brightness range, the resolving power behaves almost linearly to the light intensity, i.e., the visual acuity decreases proportionally with the brightness, as figure 10 (visual acuity and brightness) shows. There are two different explanations for this behavior. The first explanation suggests that different sensitivities, randomly distributed over the retina, occur within the receptor population. At low ambient brightness levels, only the receptors sensitive to these levels should be active, while higher brightness levels should affect all photoreceptors and thus provide the observed high resolution. The second approach assumes that the probability of capturing a light quantum is highest at low brightness in the retinal periphery due to its larger area and greater spatial summation. However, because photoreceptors are sparse in this area, resolution is low. As the brightness increases, the receptors located in the comparatively small point of sharpest vision also capture more light particles, and their high density contributes to their high resolving power.

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