Transparency and translucency
Other categories of visual appearance, related to the perception of regular or diffuse reflection and transmission of light, have been organized under the concept of cesia in an order system with three variables, including transparency, translucency and opacity among the involved aspects.
Scattering centers (or particles) as small as 1 μm have been observed directly in the light microscope (e.g., Brownian motion).
[5][6] Optical transparency in polycrystalline materials is limited by the amount of light scattered by their microstructural features.
Primary scattering centers in polycrystalline materials include microstructural defects such as pores and grain boundaries.
In addition to pores, most of the interfaces in a typical metal or ceramic object are in the form of grain boundaries, which separate tiny regions of crystalline order.
Computer modeling of light transmission through translucent ceramic alumina has shown that microscopic pores trapped near grain boundaries act as primary scattering centers.
The volume fraction of porosity had to be reduced below 1% for high-quality optical transmission (99.99 percent of theoretical density).
[7] Transparent ceramics have created interest in their applications for high energy lasers, transparent armor windows, nose cones for heat seeking missiles, radiation detectors for non-destructive testing, high energy physics, space exploration, security and medical imaging applications.
The development of transparent panel products will have other potential advanced applications including high strength, impact-resistant materials that can be used for domestic windows and skylights.
Perhaps more important is that walls and other applications will have improved overall strength, especially for high-shear conditions found in high seismic and wind exposures.
Currently available infrared transparent materials typically exhibit a trade-off between optical performance, mechanical strength and price.
For example, sapphire (crystalline alumina) is very strong, but it is expensive and lacks full transparency throughout the 3–5 μm mid-infrared range.
Yttria is fully transparent from 3–5 μm, but lacks sufficient strength, hardness, and thermal shock resistance for high-performance aerospace applications.
A combination of these two materials in the form of the yttrium aluminium garnet (YAG) is one of the top performers in the field.
Chemically pure (undoped) window glass and clean river or spring water are prime examples of this.
If a dielectric material does not include light-absorbent additive molecules (pigments, dyes, colorants), it is usually transparent to the spectrum of visible light.
Color centers (or dye molecules, or "dopants") in a dielectric absorb a portion of the incoming light.
For example, water, cooking oil, rubbing alcohol, air, and natural gas are all clear.
The ability of liquids to "heal" internal defects via viscous flow is one of the reasons why some fibrous materials (e.g., paper or fabric) increase their apparent transparency when wetted.
Light transmission will be highly directional due to the typical anisotropy of crystalline substances, which includes their symmetry group and Bravais lattice.
For example, the seven different crystalline forms of quartz silica (silicon dioxide, SiO2) are all clear, transparent materials.
This resonant mode of energy and data transmission via electromagnetic (light) wave propagation is relatively lossless.
[citation needed] An optical fiber is a cylindrical dielectric waveguide that transmits light along its axis by the process of total internal reflection.
This effect, called total internal reflection, is used in optical fibers to confine light in the core.
The size of this acceptance cone is a function of the refractive index difference between the fiber's core and cladding.
[16] Further attenuation is caused by light absorbed by residual materials, such as metals or water ions, within the fiber core and inner cladding.
Light leakage due to bending, splices, connectors, or other outside forces are other factors resulting in attenuation.
Some marine animals such as jellyfish have gelatinous bodies, composed mainly of water; their thick mesogloea is acellular and highly transparent.
[18] For the same reason, transparency in air is even harder to achieve, but a partial example is found in the glass frogs of the South American rain forest, which have translucent skin and pale greenish limbs.
[19] Several Central American species of clearwing (ithomiine) butterflies and many dragonflies and allied insects also have wings which are mostly transparent, a form of crypsis that provides some protection from predators.
