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aberrations

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lens aberrations are the deviations of performance of an optical system from pridicted paraxial optics...

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aberrations

  1. 1. Microscopy 2/10/2016 1bhargava
  2. 2. • Microscopy: – Light Microscopy-Introduction, Geometrical optics, Image formation, Magnification and Resolution, Lens aberrations, Distortion of image and curvature of field, Types of microscopes- Compound, Comparison, Fluorescence, Polarized, Stereo, – Their basic principles, working and Forensic Applications. • Electron Microscopy- – Introduction, Historical review, Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), – Theory and basic principles , Instrumentation, Forensic applications. Curriculum 2/10/2016 2bhargava
  3. 3. Need..? • The applications of microscopy in forensic science are limitless. – The ability of microscopes to locate/detect, to recover, to resolve, to compare and to image the smallest items of evidence, often without alteration or destruction. – Magnification – Resolution 2/10/2016 3bhargava
  4. 4. Laws governing the image formation – law of reflection determines the imaging properties of mirrors, – Snell’s law of refraction determines the imaging properties of lenses. 2/10/2016 4bhargava
  5. 5. Light rays undergoing reflection (a) and refraction (b) at plane surfaces 2/10/2016 5bhargava
  6. 6. Lens • Lenses are at the heart of many optical devices (cameras, microscopes, binoculars, and telescopes). • Lenses are essentially light-controlling elements, used primarily for image formation with visible light. • A lens is made up of a transparent refracting medium, generally of some type of glass, with spherically shaped surfaces on the front and back. • A ray incident on the lens refracts at the front surface (according to Snell’s law) propagates through the lens, and refracts again at the rear surface. 2/10/2016 6bhargava
  7. 7. Types of Lens • Positive lens converges parallel incident rays and forms a real image; such a lens is thicker in the middle than at the periphery and has at least one convex surface. – Positive lenses magnify when held in front of the eye. • Negative lens causes parallel incident rays to diverge; negative lenses are thinner in the middle than at the periphery, and have at least one concave surface. – Negative lenses do not form a real image, and when held in front of the eye, they reduce or demagnify. 2/10/2016 7bhargava
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  9. 9. Terminology • The focal length is shown as the distance ‘f’ from the principal plane of the lens to its focal point F, the front and rear focal lengths having the same value. • The optic axis is shown by a horizontal line passing through the centre of the lens and perpendicular to its principal plane. • The object distance ‘a’; distance from the object to the principal plane of the lens. • The Image distance ‘b’; distance from the image to the principal plane of the lens. • Front face & Rear face 2/10/2016 9bhargava
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  11. 11. •Geometrical optics of a simple lens. The focal length f, focal point F, object-lens distance a, and lens-image distance b are indicated. 2/10/2016 11bhargava
  12. 12. OBJECT-IMAGE MATH The well-known lens equation describes the relationship between focal length ‘f’ and object and image distances, ‘a’ and ‘b’: The magnification factor ‘M’ of an image is described as: 2/10/2016 12bhargava
  13. 13. 2/10/2016 bhargava 13 Image formation by convex lens
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  16. 16. Optical Aberration • An optical aberration is a departure of the performance of an optical system from the predictions of paraxial optics. • Aberration leads to blurring of the image produced by an image-forming optical system. • Makers of optical instruments need to correct optical systems to compensate for aberration. 2/10/2016 16bhargava
  17. 17. Types of Aberrations • Monochromatic aberrations are caused by the geometry of the lens or mirror and occur both when light is reflected and when it is refracted. – They appear even when using monochromatic light. • Chromatic aberrations are caused by dispersion, the variation of a lens's refractive index with wavelength. – They do not appear when monochromatic light is used. 2/10/2016 17bhargava
  18. 18. Chromatic Aberrations • Chromatic Aberrations - This type of optical defect is a result of the fact that white light is composed of numerous wavelengths. • When white light passes through a convex lens, the component wavelengths are refracted according to their frequency. 2/10/2016 bhargava 18
  19. 19. 2/10/2016 bhargava 19 • Blue light is refracted to the greatest extent followed by green and red light, a phenomenon commonly referred to as dispersion. • The inability of the lens to bring all of the colours into a common focus results in a slightly different image size and focal point for each predominant wavelength group.
  20. 20. 2/10/2016 bhargava 20 Chromatic Aberration Chromatic Aberration in an image of a bird.
  21. 21. 2/10/2016 bhargava 21 1. Longitudinal Chromatic Aberration 2. Lateral Chromatic Aberration
  22. 22. Corrections • Dollond, Lister reduce longitudinal chromatic aberration: – By combining crown glass and flint glass, where each lens has a different refractive index and dispersive properties. – They succeeded in bringing the blue rays and the red rays to a common focus, near the green rays. – This combination is termed a lens doublet or achromatic doublet (a – without & chroma - colour) which means the lens that makes image without colours. • Different lens materials (crystal fluorite) may also be used to minimise chromatic aberration. 2/10/2016 bhargava 22
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  24. 24. 2/10/2016 bhargava 24 Spherical Aberration • Spherical aberration causes beams parallel to, but distant from, the lens axis to be focused in a slightly different place than beams close to the axis. • Variation of focus • This manifests itself as a blurring of the image. • Rays striking the surface at a greater distance (marginal rays) are focused closer to the vertex ‘V’ than are the paraxial rays and creates spherical aberration.
  25. 25. 2/10/2016 bhargava 25 •Marginal rays are bent too much and focused in front of paraxial rays. •Distance between the intersection of marginal rays and the paraxial focus is known as the LSA (longitudinal spherical aberration). •TSA (transverse SA) is the transverse deviation between the marginal and paraxial rays on a screen placed at F. •Rays close to optical axis come to focus near the paraxial focus position. •As height increases, the focus moves farther.
  26. 26. 2/10/2016 bhargava 26 Spherical Aberration
  27. 27. COMA • ‘Coma’ aberration is a result of refraction differences by light rays passing through the various lens zones as the incident angle increases and derives its name from comet shaped aberrated images. • This type of aberrations are only encountered with off-axis objects. • Variation of magnification. • Oblique rays incident on a lens with coma, the rays passing through the edge may be imaged at a different height than those passing through the center. • It is also one of the easiest aberrations to demonstrate. – On a bright sunny day, use a magnifying glass to focus an image of the sun on the sidewalk and slightly tilt the glass with respect to the principal rays from the sun. – The sun's image, when projected onto the concrete, will then elongate into a comet- like shape that is characteristic of comatic aberration. 2/10/2016 bhargava 27
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  29. 29. 2/10/2016 bhargava 29 •The imaging of a point at ‘S’ can result in a “comet-like” tail, known as a coma flare and forms a “comatic” circle on the screen (positive coma in this case). •This is often considered the worst out of all the aberrations, primarily because of its asymmetric configuration.
  30. 30. 2/10/2016 bhargava 30 Marginal rays give smaller image  negative coma Marginal rays give larger image  positive coma
  31. 31. Astigmatism • Astigmatism occurs when the tangential and sagittal images do not coincide. • The image of a point turns into two separate lines. • An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different focus. • If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances. 2/10/2016 bhargava 31 Rays along x-axis: sagittal; along y-axis: tangential Astigmatism increases when moving further from the axis.
  32. 32. 2/10/2016 bhargava 32 Astigmatism
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  34. 34. 2/10/2016 bhargava 34 Astigmatism • Astigmatism causes difficulties in seeing fine detail.
  35. 35. Corrections • The amount of aberration due to astigmatism is proportional to the square of the angle between the rays from the object and the optical axis of the system. • With care, an optical system can be designed to reduce or eliminate astigmatism. Such systems are called anastigmats. • Astigmatism can be often corrected by- 1. Glasses with a lens that has different radii of curvature in different planes (a cylindrical lens), 2. Contact lenses, or 3. Refractive surgery of cornea. 2/10/2016 bhargava 35
  36. 36. Petzval Field Curvature • Petzval field curvature, named for Joseph Petzval, describes the optical aberration in which a flat object normal to the optical axis cannot be brought into focus on a flat image plane. • This makes a planar object looks curved in its image. • Image points near the optical axis will be in perfect focus, but rays off axis will come into focus before the image sensor. • This is less of a problem when the imaging surface is spherical, as in the human eye. 2/10/2016 bhargava 36
  37. 37. 2/10/2016 bhargava 37 Field curvature: the image "plane" (the arc) deviates from a flat surface (the vertical line).
  38. 38. Distortion • Distortion is a deviation from rectilinear projection. 2/10/2016 bhargava 38
  39. 39. Distortion • Distortion can be irregular or follow many patterns, • As we are mainly concerned with the distortion caused by symmetrical photographic lens, so we will be discussing the radially symmetric distortions which falls in two categories : – Barrel distortions or – Pincushion distortions. 2/10/2016 bhargava 39
  40. 40. Barrel distortion & Pincushion distortion 1. In barrel distortion, the image magnification decreases with distance from the optical axis. •The apparent effect is that of an image which has been mapped around a sphere (or barrel). 2. In pincushion distortion, image magnification increases with the distance from the optical axis. •The visible effect is that lines that do not go through the centre of the image are bowed inwards, towards the centre of the image, like a pincushion. 2/10/2016 bhargava 40
  41. 41. Distortion 2/10/2016 bhargava 41
  42. 42. 2/10/2016 bhargava 42 Images of a square grid for “pincushion distortion” (left) and “barrel distortion” (right).
  43. 43. 2/10/2016 bhargava 43 Aberration Character Correction Chromatic Aberration White light, on & off axis, image blur Contact doublet, spaced doublet, crystal fluorite Spherical Aberration Monochromatic light, on & off axis, image blur Diaphragm, stoppers COMA Monochromatic light, off axis only, comet shaped image Spaced doublet, stoppers Field Curvature Monochromatic light, off axis only, Flat field objectives, spaced doublet Distortion Monochromatic light, off axis only, distorted image Spaced doublet Astigmatism Monochromatic light, off axis, image blur Cylindrical lens Corrections
  44. 44. Corrections • Spherical Aberration: limiting the outer edges of the lens from exposure to light using diaphragms • Chromatic Aberrations: combining crown glass and flint glass, fluorspar • Field Curvature Aberrations: flat-field objectives. • Comatic aberrations: spherical aberrations or by designing lens elements of various shapes • Astigmatism aberrations: design of the objectives to provide precise spacing of individual lens 2/10/2016 bhargava 44

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