Brightness as a sensation
The words brightness and darkness as well as darkness are mostly used for the subjective sensation of light as it affects the eye of the observer - in this sense, darkness is a lower degree of brightness.
Conceptually, a degree of brightness can be grasped by two concepts,
- as all the light that falls into the eye and illuminates the retina, also known as illumination or light conditions , and
- as the amount of light emitted by a light source , regardless of whether it emits self-luminous light or only reflects light when it is illuminated .
Many animal creatures have developed a visual system in which two sub-systems can be distinguished, which are activated in different brightness ranges : In the daytime lighting conditions, in the photopic range , the sensory cells ( cone cells ) of one component are addressed (day vision) , in the During the night, in the scotopic area , only the sensory cells ( rod cells ) of the other component (night vision) prevail, in the mesopic transition area in twilight still both.
Below the visual threshold of the scotopic area, nothing is perceived with the eye, there is darkness - even if the light or illuminance is not yet zero ( residual light ). Color perception only works with enough light, below only gray tones are perceived, above only “white”. Excessive brightness leads to glare , the sense of sight fails. The physiological thresholds of the sense of sight are quite different in all living beings, and in humans they fluctuate relatively significantly.
Objectification of brightness
The sensation of brightness is roughly proportional to the logarithm of the stimulus - like many other neurological processes, it follows the Weber-Fechner law . The perception of brightness can be slightly different for different people or for both eyes. In particular, it depends on the spectral sensitivity of the visual cells for daytime vision (photopic vision) at medium wavelengths, which for most people is highest in the range around 555 nm wavelength (yellow-green), roughly corresponding to the maximum of solar radiation . In many animals, this maximum of perception for brightness is at other wavelengths, and therefore for other colors, as was found in cats or bees . The more precise distribution of the brightness sensitivity of the human visual apparatus as a function of the spectral light color describes the V-lambda curve .
The human eye works in a very large brightness range, which corresponds to light intensities of 1: 10 billion. The visual threshold is 10 -13 lumen . Nevertheless, we can perceive different brightnesses as different as soon as their amount of light differs by more than 10%. The photometric step method for apparent brightnesses , which the astronomer Friedrich Argelander developed around 1840, is based on this.
If brightness is to be determined objectively, two effects must be taken into account.
- The individual characteristics of the eye.
- Simultaneous radiation in the visible wavelength range and in the subsequent infrared - possibly ultraviolet range , which can also lead to fluorescence , for example .
The term brightness is understood more generally as the intensity of the radiation acting on an observer or sensor , which is averaged spatially and over a frequency band with neighboring electromagnetic radiation.
- In astronomy , this averaging is important when determining the relative ( apparent ) or absolute brightnesses of stars or other astronomical objects. The visual or photographic brightness , which varies depending on the prevailing light color , is therefore supplemented by a color index .
- In color theory , the colorimetric brightness is related to a comparison color , such as a reference white or a black or gray, in order to switch off the effects of background lighting (ambient contrast ) and total light incidence (such as the adaptation of the eye to it) and to be able to work in a three-dimensional color space .
Physical and Physiological Definition
For a definition of the brightness you have to determine whether the transmitted electromagnetic energy should be decisive ( radiometry ) or whether you want to take into account the spectral sensitivity of the human eye ( photometry ). The radiant power is the energy per unit of time, measured in watts . It plays a role in technical sensors; in astronomy it is called bolometric brightness . The spectral sensitivity of the human eye (V-lambda curve) must be taken into account for the visual impression. The corresponding quantity is called luminous flux , measured in lumens . In astronomy one speaks of visual brightness . “Luminosity” can mean both.
In everyday life, the physiological (photometric) view is decisive. A distinction must be made between the brightness of a light source and the brightness of the lighting.
For a light source, the luminous intensity indicates which luminous flux it emits in a given solid angle . The light intensity is measured in the unit candela (cd): 1 cd = 1 lm / sr . For the impression of brightness, however, it is also decisive how much the light intensity is concentrated in a spatial area in the light source: A small lightbulb, which has the same light intensity as a large-area light source, appears brighter (“glistening”). The decisive variable here is the luminance (luminous intensity through the emitting surface), measured in cd / m 2 .
For the impression of brightness on an illuminated surface, the area over which the luminous flux is distributed is decisive. The ratio of luminous flux to illuminated area is the illuminance , measured in lux (lx): 1 lx = 1 lm / m 2 . In addition, the proportion of light that the illuminated area reflects back has an effect - whether z. B. white, gray, colored or black.
- Herbert Schober : The Watch . Volume I (partly Volume II). VEB Fachbuchverlag Leipzig , 1957 and 1964.
- Wilhelm Westphal: Physics. A textbook . 22.-24. Edition, 713 p., Chapter IX (optics, light measurement, § 262 ff) and § 394 (brightness and luminosity of stars), Springer-Verlag, Berlin-Göttingen-Heidelberg 1963.
- Gottfried Gerstbach : Eye and sight - the long way to digital recognition . Sternenbote booklet 43/8, p. 142–157, Vienna 2000.
- A. Schödlbauer : Geodetic Astronomy - Basics and Concepts . 634 p., De Gruyter , Berlin 2000.
- J.Bennett, M.Donahue, N.Schneider, M.Voith: Astronomy (Chapters 5 and 6) . Editor Harald Lesch, 5th edition (1170 pages), Pearson-Studienverlag, Munich-Boston-Harlow-Sydney-Madrid 2010.
- A. Roger Ekirch : In the hour of the night. A story of darkness. Lübbe, Bergisch Gladbach 2006, ISBN 978-3785722466 , cultural-historical treatise on darkness.
- see Lit .: Schober, 1957/1964
- Gottfried Gerstbach: Eye and Seeing - the long road to digital recognition . In: Star Messenger . Issue 11/99, p.142-157, Vienna 1999.
- Photometry: numerical description of light