# opacity

Opacity (lat. Opacitas "cloudiness", "shading") generally denotes the opposite of transparency , that is, lack of transparency or lack of permeability. The associated adjective is opaque (Latin opacus "cloudy, dark, blurred").

The term is used in particular in optics and with reference to materials, but also in philosophy.

## optics

In physics, the opacity is a measure for the light opacity ( haze ) of translucent ( scattering translucent) materials and layers.

In contrast, clear materials (with little scatter) are called transparent . Their light (in) permeability is given as optical density (logarithmic) or transmission (linear).

Bacharach calculator

Bacharach is an outdated measure of the gray value of smoke plumes (see figure).

### Opacity and transmission

Opacity is the reciprocal of the transmission T :

${\ displaystyle O = {\ frac {1} {T}} = {\ frac {I_ {0}} {I}}}$

with the incident luminous flux and the transmitted luminous flux . ${\ displaystyle I_ {0}}$${\ displaystyle I}$

It should be noted that on the exit side, it must in principle be integrated over the entire half-space , for example with an integrating sphere . Whether this is actually done depends on the application; opacimeters for sooty exhaust gases only measure in a straight line because soot mainly absorbs light instead of scattering it.

Examples:

• Flue gas with high transmission  T = 0.97 (over the given measuring section ) has an opacity of  O = 1.03.
• Clothing fabric with transmission  T = 0.2 has an opacity of  O = 5.
• The paper of a greeting card with transmission  T = 0.01 therefore has an opacity of  O = 100. (However, it is common in the paper sector to specify the opacity for a sheet or a sheet in accordance with ISO 2471. This is approximately defined there as O = 100 % - light transmission. A sheet of paper with a light transmission of one percent has an opacity of 99%.)

The decadic logarithm of the opacity is called the extinction E :

${\ displaystyle E = \ lg O}$

### Opacity and opalescence

In finely dispersed media , depending on the size of the scattering particles, there is a transition from opacity to opalescence :

• if the particles are larger than the wavelength of the light, the wavelength-independent Mie scattering occurs; the scattered light is white, as can be seen from the clouds , for example . This is called opacity.
• If the scatterers are smaller than the wavelength, then instead the wavelength-dependent Rayleigh scattering occurs ; the scattered light becomes bluish, while the transmitted light becomes reddish. Because of this color, one speaks of opalescence. It can be observed , for example, in the blue of the sky and the red of a sunset .

### Opaque materials

Among other things, opacity is a physical property of:

• Minerals
• Paper , whereby a higher opacity is created by adding fillers or by a higher proportion of wood (lignin)
• Printing inks , a distinction being made between a translucent (translucent) color and an opaque color (covering color)
• Painting , a distinction being made between wet and dry opacity
• Gases, gas mixtures or loose accumulations of matter in astronomy , such as B. the atmosphere of stars (see main sequence ), planets and moons, of interstellar matter such. B. dark clouds or the tail of a comet
• Smart glass
• Sun protection clothing, in general, wetting reduces the opacity of bath textiles, where a higher level of opacity is required, they are therefore backed with a second layer of white active substance
• Latex balloons are available in opaque or transparent coloring, all of which become significantly more translucent when inflated by stretching.

### Milky substances

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

However, such names do not always indicate a milky appearance, for example in the case of Liebfrauenmilch .

## Opacity in philosophy

In philosophy , the concept of opacity appears around the middle of the 20th century in such different currents as critical theory and analytical philosophy and describes the impenetrability of a fact or linguistic content.

In critical theory, opacity denotes the inaccessibility of what is immediately given. That which appears or is issued as opaque immediacy is thereby withdrawn from access by reason and criticism. Critical theory criticizes the fact that social as well as philosophical issues are stamped as opaque and thereby put aside as inaccessible to discourse.

In analytical philosophy, the concept of opacity appears above all in a linguistic-philosophical context. There it describes the referential opacity in the narrower sense .