Opacity (optics)

Opacity (Latin opacitas "cloud ", " shading ", . Engl opacity ) generally refers to the opposite of transparency, ie lack of transparency or transmittance. The corresponding adjective is opaque (Latin opacus " cloudy, dark, blurry "; engl opaque. ).

In particular the term is used in the optical system and with regard to materials, but also in the philosophy.


In physics, the opacity is a measure of the opacity (haze) of translucent ( scattering translucent ) materials and layers. Clear materials ( with low variance ), however, are called transparent. Your light (un) is permeability expressed as optical density ( logarithmic) or transmission ( linear).

An obsolete measure of the gray value of plumes is Bacharach.

Opacity and transmission

Opacity is the reciprocal of the transmission T.

With the incident light power and the transmitted light power.

It should be noted that the outlet side is in principle to incorporate the entire half-space, for instance with an integrating sphere. If is actually made, depends on the application. To measure opacity meter for sooty exhaust only in a straight line, because the soot light mainly absorbed instead scatters.


  • Flue gas with high transmission T = 0.97 ( over the given measurement path ) has an opacity O = 1.03.
  • Garment fabric with transmission T = 0.2 has an opacity O = 5
  • The paper greeting card with transmission T = 0.01 has an opacity O = 100

The decadic logarithm of the opacity is called extinction E:

Opacity and opalescence

In finely dispersed media, a transition from the opalescence to the opacity results depending on the size of the scattering particles. The spreader is less than the wavelength, it is wavelength-dependent for Rayleigh scattering. Then the scattered light, the transmitted light, however, this case is bluish, reddish. On the basis of this coloring is called opalescence. You can for example be seen in the sky blue and the red of a sunset. If the particles are larger than the wavelength of the light, instead occurs in the wavelength-independent Mie scattering; the scattered light is white, as you can for example be observed in the clouds. Here one speaks of opacity.

Opaque materials

Opacity, among other physical properties of:

  • Minerals
  • Paper, with a higher opacity by adding fillers or by a higher proportion of wood (lignin ) is generated
  • Printing inks, wherein between a translucent ( translucent ) color and an opaque color ( body color ), a distinction
  • Coatings, with a distinction between wet and Trockenopazität
  • Dispersion paints and varnishes, see also opacity
  • Gases, gas mixtures or loose matter accumulations in astronomy, such as the atmosphere of stars (see main series ), planets and moons of interstellar matter such as dust clouds or the tail of a comet
  • Intelligent glass
  • Sun protective clothing, in general, reduced wetting the opacity of bath textiles, where higher opacity is desired, therefore they are with a second layer of white substance deposited
  • Latex balloons are available in opaque or transparent but coloring start, all during inflation by stretching clear translucent.

Milky substances

Milky white opaque substances or phenomena are often referred to as " milk ":

  • White low-viscosity dispersions, such as soy milk, cereal milk, almond milk, coconut milk Emulsions, such as suntan lotion, leather milk, Bohrmilch
  • Slurries of fine flour bright solids in liquids, such as glacier milk, milk of lime, scouring

However, such designations do not always indicate a milky appearance, eg Liebfrauenmilch.

Opacity in computer graphics

In the field of computer graphics, the opacity means the opposite of transparency.

Opacity in philosophy

In philosophy, the concept of opacity around the middle of the 20th century comes simultaneously in such diverse movements such as critical theory and analytic philosophy and refers to the impenetrability of facts or linguistic content.

In critical theory, the opacity refers to the inaccessibility of the immediately given. That which appears as an opaque immediacy or is issued by the access through reason and criticism is withdrawn. The critical theory criticizes the fact that both social and philosophical issues are labeled as opaque and thus set aside as the discourse inaccessible.

In analytic philosophy, the concept of opacity emerges above all in on sprachphilosophischem context. He called there in the narrower sense, the referential opacity.