Aberrations of the eye

Optical imperfections in the eye

Aberrations of the eye are imperfections in the optical system of the human eye that cause light rays to deviate from their intended path, leading to a blurred or distorted image on the retina. These aberrations can be classified into lower-order and higher-order aberrations, with the former including common refractive errors such as myopia, hyperopia, and astigmatism, and the latter including more complex distortions that cannot be corrected with standard eyeglasses or contact lenses.

Types of Aberrations

Lower-order Aberrations

Lower-order aberrations are the most common and include:

• Myopia (nearsightedness): A condition where distant objects appear blurry because the eye focuses images in front of the retina.
• Hyperopia (farsightedness): A condition where close objects appear blurry because the eye focuses images behind the retina.
• Astigmatism: A condition where the eye does not focus light evenly onto the retina, often due to an irregularly shaped cornea.

Higher-order Aberrations

Higher-order aberrations are more complex and include:

• Spherical Aberration: Occurs when light rays that pass through the periphery of the lens focus at a different point than those that pass through the center.
• Coma: An aberration that causes points of light to appear as comet-shaped blurs, often due to off-axis light rays.
• Trefoil: A type of aberration that results in a three-lobed pattern of distortion.

Diffraction through a pinhole

Measurement of Aberrations

Aberrations of the eye can be measured using various techniques, one of which is the Shack-Hartmann wavefront sensor. This device uses a grid of lenslets to measure the wavefront of light as it exits the eye, allowing for precise mapping of aberrations.

Shack-Hartmann wavefront sensor

Correction of Aberrations

While lower-order aberrations can be corrected with conventional lenses, higher-order aberrations require more advanced solutions. These include:

• Custom LASIK: A laser eye surgery that reshapes the cornea based on a detailed map of the eye's aberrations.
• Wavefront-guided contact lenses: Specially designed lenses that correct specific aberrations by altering the wavefront of light entering the eye.

Zernike Polynomials

Higher-order aberrations are often described using Zernike polynomials, a mathematical representation that allows for the decomposition of complex wavefront shapes into simpler components. This method is crucial for understanding and correcting aberrations in advanced optical systems.

Zernike polynomials

Related pages

• Refractive error
• Optometry
• Ophthalmology
• Vision correction