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Chromatic aberration

Chromatic aberration is caused by the variation of the refractive index with the wave length. Therefore, there is no. chromatic aberration when monochromatic light is used. The world is poly chromatic; Hence, the human eye suffers for chromatic aberration.

We can divide chromatic aberration into two sub catergories: longitudinal and transverse.

Longitudinal chromatic aberration (ACL).

Longitudinal chromatic aberration: for a simple depiction, (not real) symetrical and rotationally symetrical eye is sufficient (the pupil is also centered).

chromatic aberration and longitudinal

Longitudinal chromatic aberration in a simple model eye: for better visualization, the respective paths of the green and red components of a polycrhomatic source are shown separately. Red light is focused behind the green light. This causes the spread of the red light to be larger at the retinal flat if the green is in focus.

The point source is poly chromatic, and we the optical axis (which is to the visual axis coincide, owing for the symmetry of the eye). For clarity, the figure has been spitted in two, to show the respective paths of green (shorter wavelengths) and red (longer wavelengths).  For this centered eye and year "on axis" point of attachment, longitudinal chromatic aberration causes a "spread" of the focal spot. In this example, when green wavelengths are in focus at the retinal flat (top), red wavelengths are focused behind the retina; the eye have 'less vergence' (less optical power) for the red wavelengths.  This is due to the fact the refractive index decreases when the wavelength increases (red lightwaves are longer than green - less energetic - light waves). In thoery, optical calculation show that between the two ends of the visible spectrum, there are 2 diopters of longitudinal chromatic aberration (ACL). In practice, the effect of ACL gradients the optical quality of the eye in the same proportion as 0.2 D of monochromatic defocus, the human eye being most sensitive to yellow light (yellow light is in the middle of the spectrum).

 

Transverse chromatic aberration (CAW)

Transverse chromatic aberration is caused by the same physical mechanisms than longitudinal chromatic aberration, owing to the fact that the eye has not centered refractive surfaces, and year aperture stop (iris pupil), which is not centered on the visual axis.

chromatic aberration transverse

Transverse chromatic aberration is due to the absence of rotationnal symmetry of the eye. It is reduced by aligning the poly chromatic source on the visual axis (bottom).

Thesis asymmetries explain that the spot diagram of multiple wavelengths emitted by a polychromatic source are not in focus, and that the respective contours of these spots are not concentric. However, it is worth noticing that for rays traveling close to the visual axis, the transverse chromatic aberration if low (zero being the visual axis, which can be defined as the axis of null transverse chromatic aberrations). In fact, the visual axis can be defined as the axis where the transverse chromatic aberration is null. The visual axis is sometimes called the "achromatic axis.

Consequently, it is important to center refractive surgical procedures on the visual axis, especially when they incur a reduction of the functional optical area; If, not, the contrast of the retinal image will be severely degraded by large amounts of transverse chromatic aberration.

The difficulty of achieving proper focus on the visual axis explains why it is difficult to correct the chromatic aberration for the human eye; any apparatus which would aim at compensating for LCA (ex: achromatizing doublet) must be well centered on the visual axis, to avoid the induction of large amounts of transverse aberration.

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