Physics
Chromatic Aberration
Chromatic aberration refers to the phenomenon in which different colors of light are refracted by different amounts when passing through a lens, resulting in a blurred or distorted image. This occurs because the refractive index of a lens material varies with the wavelength of light, causing colors to focus at different points.
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- Douglas B. Murphy, Michael W. Davidson(Authors)
- 2012(Publication Date)
- Wiley-Blackwell(Publisher)
Figure 4.10 Aberrations of a simple lens. (a) Chromatic Aberration: Parallel incident rays of different wavelength are focused at different locations. (b) Spherical aberration: Incident rays parallel to the optical axis and reaching the center and the periphery of the lens are focused at different locations. (c) Coma: Off-axis rays passing through the center and periphery of the lens are focused at different locations. (d) Astigmatism: An off-axis aberration causes waves passing through the vertical and horizontal diameters to focus an object point as a streak. (e) Field curvature: The image plane is curved and not planar. (f) Distortion: So-called barrel and pincushion distortions produce images that are not high in fidelity compared with the object.Chromatic Aberration occurs because a lens refracts light differently, depending on the wavelength. Blue light is bent inward toward the optical axis more than red light. The result is disastrous: Blue wavelengths are focused in an image plane closer to the lens than the image plane for red wavelengths. Even at the best focus, point sources are surrounded by color halos, the color changing depending on the focus of the objective, the image never becoming sharp. Since each wavelength is focused at a different distance from the lens, there is also a difference in magnification for different colors (chromatic magnification difference). The solution is to make compound lenses made of glasses having different color-dispersing properties. For example, glass types known as crown and flint are paired together to make an achromatic doublet lens that focuses blue and red wavelengths in the same image plane.Spherical aberration
eBook - ePub- Randy O. Wayne(Author)
- 2019(Publication Date)
- Academic Press(Publisher)
Lessing, 1970 ).Fig. 2.35 Spherical aberration occurs because the rays from any given object point that hit a lens with spherical surfaces far from the principal axis are refracted too strongly. This results in a circle of confusion. Spherical aberration can be reduced by grinding the lens so that it has aspherical surfaces.Rays of every color emanating from a point on the object are focused to the same image point by mirrors. However, since the refractive index of a transparent medium depends on wavelength, the lenses show Chromatic Aberration. That is, rays of different colors coming from the same point on the object disperse and do not focus at the same place in the image plane. Consequently, instead of a single image, multiple monochromatic images with varying degrees of magnification are produced by a lens with Chromatic Aberration (Fig. 2.36 ). Newton believed that all transparent materials had an equal ability to disperse white light into colored light and therefore Chromatic Aberrations could not be corrected. This is why Newton constructed a reflecting telescope rather than a refracting telescope. However, Newton did not have sufficient observational data. John Dollond said (Martin, 1962 ), “As I saw Reason to doubt of Sir Isaac, I thought it high time to begin to think for myself, and endeavor to find out the truth by Experiment.” Dollond (1758) then showed that by combining two materials with different dispersive powers, for example, crown glass and flint glass, color-corrected lenses in fact could be made (Fig. 2.37 ; Kelly, 1804 ). Since flint glass has a greater refractive index and dispersion than crown glass, it is possible to make a concave lens out of flint glass that will not completely cancel the refraction due to a convex lens made out of crown glass, yet at the same time bringing two wavelengths to a common focus (Bergmann and Schaefer, 1999
eBook - ePub- Elizabeth Allen, Sophie Triantaphillidou(Authors)
- 2012(Publication Date)
- Routledge(Publisher)
superachromat. The wavelengths chosen are in the blue, green, red and infrared regions, so that no focus correction is needed between 400 and 1000 nm. Using materials such as silica and fluorite, an achromatic lens may be made for UV recording, needing no focus correction after visual focusing.Other optical materials require chromatic correction. Plastics (polymers) are used in photographic lenses either as individual elements or as hybrid glasseplastic aspheric combinations. It is possible to design an achromatic combination using plastics alone.The use of reflecting surfaces that do not disperse light, in the form of ‘mirror lenses’, offers another solution, but most mirror designs are for long-focus lenses only. Most of these, too, are catadioptric lenses with some refracting elements and these still require some colour correction.Lateral Chromatic Aberration
Lateral or transverse Chromatic Aberration, also called either lateral colour or chromatic difference of magnification, appears in the form of dispersed colour fringes at the edges of the image (see Figure 6.4 ). It is an off-axis aberration, i.e. it is zero at the centre of the focal plane but increases as the angle of field increases. Whereas axial Chromatic Aberration concerns the focused distance from the lens at which the image is formed, lateral Chromatic Aberration concerns the size of the image. It is not easy to correct: its effects worsen with an increase in focal length, and are not reduced by closing down the lens aperture. It can be minimized by a symmetrical lens configuration and at least three types of optical glass. Almost full correction is achieved by use of special optical materials (Figure 10.3 ). These include optical glass of anomalous or extra-low dispersion (ED), which may be used in long lenses, at increased cost. Another material with very low dispersion characteristics is fluorite
eBook - ePub- Galen C. Duree(Author)
- 2011(Publication Date)
- For Dummies(Publisher)
(see Chapter 7) is the blurring of an image you’re seeing through a lens. Light passing through the center of a lens focuses at the intended point, but light passing through the outer edges of the lens doesn’t focus at the intended point because the edges focus at different distances than the center does (a function of lens thickness and shape). Spherical aberration results in a less-clear, less-sharp image.Chromatic Aberration with a Magnifying GlassTo easily witness (and fix) spherical aberration, grab your magnifying glass and do a little experimenting. Look through the magnifier at an object and notice the blurriness. Using dark, thick paper, cut out a donut slightly larger than the outside diameter of the magnifying glass; the donut’s inside hole should leave at least three-quarters of the glass visible. Place this donut over the lens and look back at the same object to see how this addition affects the clarity and sharpness of the image. You can make a second donut that covers the outside half of the lens and recheck the image. As you eliminate the light passing through the outer edges of the glass, you’re eliminating the unfocused light, and the object should become clearer.If you look through a cheap simple magnifying glass, you see a slight rainbow fringe around objects that is the result of Chromatic Aberration. Chromatic Aberration (head to Chapter 7) occurs becausethe index of refraction of all optical materials changes for the different wavelengths (or colors) of light. As such, the lens is really focusing a number of images at varying distances from the lens, one for each wavelength present in the incident light, which creates an unfocused overall image.To combat Chromatic Aberration, you can place light filter elements (if they’re available to you) over the magnifying lens. These filters block out certain wavelengths, so you may find that you can affect the rainbow colors. You can also stack these filters to block out multiple wavelengths simultaneously.
