The two types of chromatic aberration are Longitudinal Chromatic Aberration (LCA) and Axial Chromatic Aberration (ACA). LCA is described further here. ACA describes the colour aberration caused by the variation in depth of the focal point with wavelength. In the case of a simple lens with a single lens element, ACA appears as symmetric colour fringing that is not dependent on image location (colour plane misfocus). Refraction in an optical system is governed by Snell's Law, which relates the incidence and transmitted angles of a light ray passing from one material directly into another material through the sine of the incident ray angle and the difference in refractive indices of the two materials. Due to dispersion the refractive index of a material is wavelength dependent, and thus the refractive index of a lens element will affect the locations at which light of varying wavelength but from a single scene point will impinge on the image sensor. The result is misfocus of one or more of the colour planes in the image. The fringing effect of ACA is particularly noticeable at the boundary of saturated regions. Note that LCA and ACA typically coexist in an image.

 

Removal Solution

The chromatic focus variation of ACA results in image blur of one or more colour channels, and so the ACA removal task can be seen as a deblurring problem. Blur reduction in a digital image can be achieved by using deconvolution methods, but this approach has several drawbacks. Firstly the blur kernal must be known accurately in order to achieve results that do not exhibit 'ringing' effects, and estimating the blur kernal from image data is not straightforward. Secondly, image deconvolution is very sensitive to image noise, thus when it is used to reduce ACA the deconvolution artifacts may actually be worse than the ACA artifacts in the uncorrected image. Thus deconvolution for ACA correction is not ideal.

We have developed a method for ACA removal that does not use deconvolution. Instead, a proprietary technique is applied to the chromatic channels in order to selectively reduce defocus and thus remove ACA. Since the technique does not involve deconvolution, ringing is not introduced into the corrected image, and there is no noise amplification. Importantly, a calibration stage is not required, so ACA can be corrected on a per-image basis using our method. Additionally, as part of the proposed technique the color planes are optionally realigned so as to simultaneously remove LCA. Thus complete chromatic aberration correction is enabled without any calibration step.

 

Key Advantages

  • Fully automatic process that works on a per-image basis, no pre-calibration required
  • All chromatic aberrations, both axial and lateral, are removed

 

Below is shown a crop of an image that exibits significant ACA in addition to some LCA, and (on mouse over) the image after automatic CA correction using our correction technique.

Crop of original image showing significant colour fringing due to CA, and crop after CA correction using our correction technique.
Mouse over the image to see correction.

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