NEW: Goniochromism

Dent Mater. 2009 Jun;25(6):802-9. doi: 10.1016/j.dental.2009.01.003. Epub 2009 Feb 8.

Mechanisms of goniochromism relevant to restorative dentistry.

Chirdon WM1, O’Brien WJ, Robertson RE.

Author information

Abstract

OBJECTIVES:

This study aims to determine the effects of multiple translucent layers, the alignment of composite structures, and specular reflecting backings as goniochromatic mechanisms relevant to dentistry.

METHODS:

Rectangular composite specimens were filled with very short E-glass fibers (120 microm length, 16 microm diameter). The fibers were oriented random, perpendicular, and parallel to the surface normal using an electric field. A Minolta CS-100 colorimeter was used to measure the color at various angles of samples with various filler alignments, translucent sublayers, and specular reflecting backings.

RESULTS:

All three investigated mechanisms were proven to have a goniochromatic effect. Filler alignment perpendicular to the surface made the composites more transmissive and reduced the dependence of lightness on observation angle. Backing composites with a pigmented sublayer caused the color of the sublayer to be more apparent when the observation angle is perpendicular to the surface. The specular reflection of a gold backing was only partially diffused by the specimen.

SIGNIFICANCE:

The determination of the effects of these goniochromatic mechanisms is relevant to dentistry, because teeth are naturally aligned composites composed of multiple translucent layers. Therefore, understanding these goniochromatic effects is important to recreating them in restorative materials. Also, gold backings were previously found to give restoratives a more vital appearance, and this perceived vitality may be related to goniochromism.

Iridescence (also known as goniochromism) is the property of certain surfaces that appear to change colour as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, butterfly wings and sea shells, as well as certain minerals. It is often created by structural coloration (microstructures that interfere with light).

The word iridescence is derived in part from the Greek word ἶρις îris (gen. ἴριδος íridos), meaning rainbow, and is combined with the Latin suffix -escent, meaning “having a tendency toward.”[2] Iris in turn derives from the goddess Iris of Greek mythology, who is the personification of the rainbow and acted as a messenger of the gods. Goniochromism is derived from the Greek words gonia, meaning “angle”, and chroma, meaning “colour”.

Iridescence is an optical phenomenon of surfaces in which hue changes with the angle of observation and the angle of illumination.[3][4] It is often caused by multiple reflections from two or more semi-transparent surfaces in which phase shift and interference of the reflectionsmodulates the incidental light (by amplifying or attenuating some frequencies more than others).[3][5] The thickness of the layers of the material determines the interference pattern. Iridescence can for example be due to thin-film interference, the functional analogue of selective wavelength attenuation as seen with the Fabry–Pérot interferometer, and can be seen in oil films on water and soap bubbles. Iridescence is also found in plants, animals and many other items. The range of colours of natural iridescent objects can be narrow, for example shifting between two or three colours as the viewing angle changes,[6][7] or a wide range of colours can be observed.[8]

Iridescence can also be created by diffraction. This is found in items like CDs, DVDs, or cloud iridescence.[9] In the case of diffraction, the entire rainbow of colours will typically be observed as the viewing angle changes. In biology, this type of iridescence results from the formation of diffraction gratings on the surface, such as the long rows of cells in striated muscle. Some types of flower petals can also generate a diffraction grating, but the iridescence is not visible to humans and flower visiting insects as the diffraction signal is overruled by the coloration due to plant pigments.[10][11][12]

In biological (and biomimetic) uses, colours produced other than with pigments or dyes are called structural coloration. Microstructures, often multilayered, are used to produce bright but sometimes non-iridescent colours: quite elaborate arrangements are needed to avoid reflecting different colours in different directions. Structural coloration has been understood in general terms since Robert Hooke‘s 1665 book Micrographia, where Hooke correctly noted that since the iridescence of a peacock‘s feather was lost when it was plunged into water, but reappeared when it was returned to the air, pigments could not be responsible.[13][14] It was later found that iridescence in the peacock is due to a complex photonic crystal.[15]

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