Axial Chromatism

Definition - What does Axial Chromatism mean?

Axial chromatism is an optical distortion where the location of the focal point of light on the optical axis through an optical system varies depending on the wavelength of the light (i.e., the color). It is one of the two types of chromatic aberrations, the other being transverse chromatism.

The effect of axial chromatism is relevant for any optical application, such as photography, telescopy and microscopy. The presence of axial chromatism in a lens system will distort the images, making them look out of focus, blurry and with different colors dispersed. Axial chromatism is generally reduced by a technique called stopping down, which increases the depth of field and increases the amount of focus.

While most applications seek to reduce the effects of axial chromatism, some use cases, such as in non-contact profilometers, make use of this effect to provide useful information about a material's surface.

Corrosionpedia explains Axial Chromatism

Axial chromatism arises from the fundamental property of the refraction of light. As light passes through a transparent material with a refractive index, the components of the light with different wavelengths pass through the material at different angles. When the light leaves the material, the light is now split apart according to it's wavelength, which correspond to different colors. White light passing through a lense or raindrops create the appearance of rainbows. Axial chromatism occurs along the optical axis through a lens. Because the angle of refraction for each wavelength differs, the focal point of each wavelength is either closer or farther away from other wavelengths.

Combinations of different lens types, such as using a negative lens after a positive lens, can reduce the effects of axial chromatism by reversing the effect of refraction by a complimentary effect in the second lens. These systems are called achromats and are used widely in optical microscopy and telescopes to reduce chromatic image distortions.

The properties of axial chromatism help evaluate the roughness or profile of a material’s surface. This application is a non-contact profiling technique called chromatic confocal profilometry. Instead of minimizing the effect of axial chromatism, the optical profilometer uses axial chromatism to create a topographical map of the material's surface. The variations in the wavelength of light coming from different heights on the surface correspond to various z-coordinate values on the surface. As non-contact profilometers do not damage the surface of the material at hand, chromatic confocal profilometers are a useful in evaluating the quality of a surface or coating, and non-destructive testing.

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