Brightness

Brightness is an umbrella term impressions of subjective and objective measures of the strength of a visual perception of - visible - light.

Brightness sensation

The words light and dark ( even the darkness ) are mostly used for the subjective sensation of light, as it acts on the observer's eye - Darkness is in this sense, the lower "measured value" of brightness.

The term includes two concepts.

• The entire light incident to the eye, which on the other hand referred to as lighting or illumination
• The reflected light emitted from a light source amount, regardless of whether it is self-luminous ( light emitted ), or only one lighting

Most higher organisms have two - generally working separately - components of the visual system: The normal lighting conditions are referred to as photopic range (tags marriages), seeing in dim light as mesopic vision, night vision than scotopic range. Below the visual threshold of scotopic range takes an eye nothing more true, and there's darkness - even if the illumination or light intensity is not zero ( low light ). Excessive brightness causes glare, denied the sense of sight. The color perception only works with sufficient light ( photopic vision area), including only gray tones are perceived, about just " white". The physiological thresholds of the visual system are extremely different in all living creatures, including humans, they vary quite significantly.

Objectification of brightness

The sensation of brightness is roughly proportional to the logarithm of the stimulus - it follows as many other neurological processes the Weber- Fechner law. Here, the perception of brightness may be slightly different in different persons or in both eyes. In particular, it depends on the spectral sensitivity for daylight vision ( photopic vision ) of the visual cells for medium wavelengths from the (green) is for most people in the region around 555 nm wavelength at the highest, approximately corresponding to the maximum of the solar radiation. In many animals, this maximum of the perception of brightness at different wavelengths, was found to go with other colors, such as in cats or bees. The precise distribution of the brightness sensitivity of the human visual system in response to the spectral light color describes the V lambda curve.

The human eye works in a very large range of brightness of the light intensities of 1: equivalent to 10 billion. The visual threshold is 10-13 lumens. However, we can perceive different brightness as different as soon as their light amount differs by more than 10 %. This is the basis photometric step method for apparent magnitudes, the astronomer Friedrich Argelander has developed around 1840.

If you want to determine brightness of objective, two effects have to be considered particularly.

The term is to be understood generally as brightness intensity of the forces acting on an observer or a sensor radiation and spatially averaged over a frequency band with adjacent electromagnetic radiation.

• In astronomy, this averaging of importance if the relative (apparent) or absolute magnitudes of stars or other astronomical objects are determined. The different depending upon the prevailing light color visual or photographic brightness will therefore complemented by a color index.
• In color theory, the colorimetric brightness on a comparison color, such as a reference white or black or gray is related to the effects of background illumination (ambient contrast) and total light (such as the adaptation of the eye to it) off and be able to work in a three dimensional color space.

Physical definition

The brightness of the photometry is replaced by the light intensity as a purely physical measure which indicates the object of an outgoing, spectrally averaged radiation in the unit candela ( cd). The brightness scale can also be defined by the energy of the incident light, so that the o a subjectivity in the perception of faces or the brightnesses of stars invisible. M If the mag and L are the measured light fluxes of two stars, their brightness difference

For Δ m = 1 this corresponds to a ratio of the light energy of 1: 2,512 or a logarithm of 0.4.

• As a reference value of astronomy this relative se scale is 2.1 like the North Star, or zero for Vega (brightest star of the northern sky ), so the usual for 2000 years brightness scale of Hipparchus for modern measurement instruments and also for light objects ( such as the sun ) is adapted.
• Colorimetry is used for this purpose the reference white, or black in the visual threshold.

The distinction between the illumination and brightness of a light source is quantified in emissivity or reflectance with respect to light than hemispherical, in light sources directed degrees, all with respect to each case as a section of the spectrum or the entire spectrum. These then lead, based on a time unit or a unit solid angle field of view, or both, the photometric basic sizes. Depending on the context one also speaks of a physical quantity of, lighting ' (eg luminance ) or, light ' (eg light intensity).

The sizes of Light / lighting and the radiation differ by the weighted by the V- lambda curve spectral components, as they can handle the sense of sight because light - as perceptible optical radiation ' - not a ( pure) physical ', but a physiological phenomenon. Referring specifically with respect to the visual system - whether a biogenic eye or a technical sensor - puts you, the radiation quantities based (such as the radiant flux instead of the luminous flux ).