Spektriläikkää

Spektriläikkää, often translated as spectral spot or spectral stain, is a visual phenomenon observed in certain materials under specific lighting conditions. It refers to a localized area within a material that displays a distinct color or series of colors, differing from the surrounding material. This coloration is not due to a pigment or dye but arises from the interaction of light with the material's microstructure.

The appearance of spektriläikkää is a result of thin-film interference. When light strikes a thin layer of material, such as a coating or a surface with microscopic irregularities, it reflects off both the top and bottom surfaces of that layer. These two reflected waves can then interfere with each other. Depending on the thickness of the layer and the wavelength of the light, certain wavelengths will be constructively interfered with (appearing brighter) and others will be destructively interfered with (appearing dimmer or absent). Since white light contains all visible wavelengths, this selective interference causes the thin film to appear colored. The specific colors observed can change as the viewing angle or the thickness of the film varies, creating iridescent or shimmering effects.

Spektriläikkää can be observed in a variety of natural and artificial contexts. Examples include the iridescent colors on oil slicks on water, the shimmer on a soap bubble, or the colored patterns seen on some bird feathers. In industrial applications, it can be an indicator of coating uniformity or surface damage. The study and understanding of spektriläikkää are rooted in principles of optics and wave phenomena.