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ABERRATIONS. The Perfect Image. There is no such thing as a perfect image. All light rays passing through optical systems are subject to distortions. Aberrations.
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The Perfect Image • There is no such thing as a perfect image All light rays passing through optical systems are subject to distortions
Aberrations “When light from a point source goes through a correctly powered spectacle lens yet fails to create a perfect image, the cause is lens aberration.”
Classifications of Aberrations • Chromatic vs. Monochromatic • Depends on the material of the lens • Requires the beam of light to contain more than one wavelength
Chromatic vs. Monochromatic • First there are changes in the image with the color. These are referred to as chromatic aberrations. • Then there are a subset of aberrations that are called monochromatic aberrations. One type of monochromatic aberration is a failure to get a point image of a point object. Another type of monochromatic aberration is called distortion, which is a failure to get the same shaped image as the object.
Classifications of Aberrations • In Focus vs. Out of Focus • Out of focus aberrations cause fuzzy images where clear sharp images should be • In focus aberrations cause images to be the wrong shape (distorted).
Classifications of Aberrations • Wide Beam vs. Narrow Beam • Wide beam aberrations are not as important when the light goes through a narrow opening or aperture, such as the pupil of the eye. • Wide beam aberrations are important for optical instruments such as telescopes. • Narrow beam aberrations are the important aberrations when making glasses.
Classifications of Aberrations • On Axis vs. Off Axis • On axis aberrations effect vision when looking straight ahead through the lens. • Off axis aberrations effect peripheral vision.
Lens Aberrations • Chromatic • Spherical • Marginal Astigmatism • Coma • Distortion
Chromatic Aberration • The lens material breaks white light into its component colors
Chromatic Aberration • A lens will not focus different colors in exactly the same place. • the focal length depends on refraction and the index of refraction • Short wavelength has higher n and is refracted more than long wavelength • The amount of chromatic aberration depends on the dispersion of the glass. Lens Eye http://micro.magnet.fsu.edu/primer/java/aberrations/spherical/index.html
Chromatic Aberration • Dispersive power (abbe value) is based on change in index for different wavelengths • If the index is the same for all wavelengths, there is NO DISPERSION • The n increases as wavelength decreases
Longitudinal (axial) • The placement of the various focal points on the axis.
Longitudinal (axial) • Longitudinal chromatic aberration (LCA) occurs when different wavelengths focus at different points along the horizontal optical axis as a result of dispersion properties of the glass. The refractive index of a glass is wavelength dependent, so it has a slightly different effect on where each wavelength of light focuses, resulting in separate focal points for F, d, and C light along a horizontal plane
Lateral (magnification)/ Transverse • Different image sizes • Result in colored ‘ghost’ images
Lateral (magnification)/ Transverse • Transverse chromatic aberration (TCA) occurs when the size of the image changes with wavelength. In other words, when white light is used, red, yellow, and blue wavelengths focus at separate points in a vertical plane.
Chromatic Aberration • Material dependent. • Results in out of focus image. • The higher the power of the lens, the more the chromatic aberration.
Chromatic Aberration • Correction: • Doublet lens (for instruments: cameras, telescopes, microscopes). • Change lens materials.
Correction of Chromatic Aberration • An achromatic doublet does not completely eliminate chromatic aberration, but can eliminate it for two colors, say red and blue. • The idea is to use a lens pair – a strong lens of low dispersion coupled with a weaker one of high dispersion calculated to match the focal lengths for two chosen wavelengths. • Cemented doublets of this type are a mainstay of lens design. • Achromatic Doublets
Correction of Chromatic Aberration APOCHROMATIC LENS The addition of a third lens corrects for three colors (red, blue and green), greatly reducing the fuzziness caused by the colors uncorrected in the achromatic doublet.
Lens Aberrations • Chromatic • Spherical • Marginal Astigmatism • Coma • Distortion
Spherical Aberration • When light is refracted by spherical surfaces the rays do not all converge to a point, even if they are of one wavelength. Monochromatic aberrations can arise from surfaces with irregularities (The human eye is a good example) but they also naturally arise from spherical refracting surfaces, or ‘perfect lenses’. • If you apply Snell’s law rigorously at every surface for a bunch of rays hitting a lens, you will discover that rays do not all meet at a single point.
Spherical Aberration Spherical lens: Peripheral rays have shorter focal length than paraxial rays. Light distribution of a blurred image
Spherical aberration results in uneven focus from the middle to the outside of an image... Stopping down can cause barrel distortion at the image corners: In focus Out of focus
Spherical Aberration • Peripheral rays refract more than paraxial rays. • Correct with parabolic curves, aplanatic lensdesign. • Results in out-of-focus image.
Lens Aberrations • Chromatic • Spherical • Astigmatism • Coma • Distortion
Astigmatism is when incoming light when passed through a lens is unable to focus at any point. Astigmatism can’t be reduced by stopping down the lens because the hurt of the problem is the actual shape of the lens. Astigmatism reduces image definition, detail and contrast the further away from the optical axis. • Astigmatism aberrations are found at the outer portions of the field of view in uncorrected lenses, and cause the ideal circular point image (Airy pattern) to blur into a diffuse circle, elliptical patch, or line, depending upon the location of the focal plane
Spherical lens, narrow beam entering off-axis.
Hope College, PHYS 352, Spring 2013 Circle of least confusion
Complete removal of astigmatism is difficult, but can occur in optical systems when the two curves, S and T, become flatter and coincide. • Correct Curve lens design for glasses corrects for this aberration. • How?
Lens Aberrations • Chromatic • Spherical • Marginal Astigmatism • Coma • Distortion
COMA Coma is an aberration which causes rays from an off-axis point of light in the object plane to create a trailing "comet-like" blur directed away from the optic axis. Image – cone or comet shaped. Object, way off to the left)
Coma • A lens with considerable coma may produce a sharp image in the center of the field, but become increasingly blurred toward the edges. Negative coma: Marginal rays focus closer to the optic axis. Positive coma: …farther from..
Coma The resulting image is called a comatic circle. The coma flare, which owes its name to its cometlike tail, is often considered the worst of all aberrations, primarily because of its asymmetric configuration.
COMA • Wide beam aberration, so not important in glasses design • Corrected with parabolic curves, aplanatic lens design. • Results in out-of-focus image. • For very high plus lenses, aspheric designs will improve coma.
Curvature of Field Curvature of field is an aberration that results in an image of a flat object positioned perpendicular to the lens’ optical axis lies upon a surface that is either concave or convex relative to the lens. This aberration causes uneven image field sharpness. When the central part of the image is sharply focused, its edges will lie out of focus and will appear to lack sharpness. If the sharpness is set along the edges of the image, then its center will end up lacking sharpness.
Curvature of Field A lens aberration that causes a flat object surface to be imaged onto a curved surface rather than a plane.
