16421: Vision Sensors

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16421 Vision Sensors

• Lecture 3 Optical Elements of Imaging
• Instructor S. Narasimhan
• Wean 5310, T-R 130pm 250pm

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Lenses
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Types of Lenses
Wikipedia
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Bi-convex Lens
Wikipedia
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Bi-concave Lens
Wikipedia
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Lensmakers Equation
f is the focal length of the lens, n is
the refractive index of the lens material, R1
is the radius of curvature of the lens surface
closest to the light source, R2 is the radius
of curvature of the lens surface farthest from
the light source, d is the thickness of the
lens (the distance along the lens axis between
the two surface vertices).
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Optics of a Two Lens System
object
final image
image plane
intermediate virtual image
lens 2
lens 1

• Rule Image formed by first lens is the object
  for the second lens.
• Main Rays Ray passing through focus emerges
  parallel to optical axis.
• Ray through optical center passes
  un-deviated.

• Magnification

Exercises What is the combined focal length of
the system? What is the combined focal
length if d 0?
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Changing Diameter of Collimated Beam
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Focusing a Laser beam to a point
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Fanning a Laser beam to a sheet
Used in Range Scanners
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Cylindrical Lenses
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Telecentric Lenses
Object-side and Image-side telecentricity
Aperture at the focal point of both lens systems

• Sizes of object and image do not change as they
  are translated.
• However, focus does change as in any lens.

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Eliminating Perspective Distortion
Telecentric Lens
Regular Lens
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Illusions with Telecentric Lenses
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Relay Lenses (11 imaging)
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Wide angle Lenses
Circular Fisheye
Full Frame Rectangular Fisheye
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Common lens related Issues
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Vignetting
B
L3
L1
L2
A
More light passes through lens L3 for scene point
A than scene point B
Results in spatially non-uniform brightness (in
the periphery of the image)
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Lens Vignetting

• Usually brighter at the center and darker at
  the periphery.

Reading http//www.dpreview.com
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Vignetting
photo by Robert Johnes
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Chromatic Aberration
longitudinal chromatic aberration
transverse chromatic aberration
(axial)
(lateral)
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Chromatic Aberrations
longitudinal chromatic aberration
transverse chromatic aberration
(axial)
(lateral)
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Chromatic Abberations
Reading http//www.dpreview.com
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Lens Glare

• Stray interreflections of light within the
  optical lens system.
• Happens when very bright sources are present in
  the scene.

Reading http//www.dpreview.com
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Geometric Lens Distortions
Radial distortion
Tangential distortion
Photo by Helmut Dersch
Both due to lens imperfection Rectify with
geometric camera calibration
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Radial Lens Distortions
No Distortion
Barrel Distortion
Pincushion Distortion

• Radial distance from Image Center

ru rd k1 rd3
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Correcting Radial Lens Distortions
Before
After
http//www.grasshopperonline.com/barrel_distortion
_correction_software.html
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Our Eyes

• Index of refraction cornea 1.376, aqueous
  1.336, lens 1.406-1.386
• Iris is the diaphragm that changes the aperture
  (pupil)
• Retina is the sensor where the fovea has the
  highest resolution

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Accommodation
shorter focal length
Changes the focal length of the lens
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Myopia and Hyperopia
(myopia)
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Astigmatism
The cornea is distorted causing images to be
un-focused on the retina.
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Blind Spot in Eye
Close your right eye and look directly at the
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Eyes in Nature
Mosquito
http//ebiomedia.com/gall/eyes/octopus-insect.html
Mosquitos have microscopic vision, but to focus
at large distances the eye would need to be 1 m!
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Curved Mirrors in Scallop Eyes
Telescopic Eye
More in the second part of the course
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Filters
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Neutral Density Filters

• Spectrally flat from 400-700nm Homogeneous
  Glass Blocks by absorption or by reflection
• Light/Exposure Control for Imaging
• Transmittance 10(-optical density) 100

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Variable Neutral Density Filters
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Variable Neutral Density Filters
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Color Filters
Filter Book
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Linear Variable Interference Filters
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Mirrors
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Cold Mirrors
Reflect VIS Transmit IR
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Hot Mirrors
Reflect IR Transmit VIS
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Prisms
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Image Reflection
Right Angle Prism
Penta Prism
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Image Rotation
Dove Prism
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Single Lens Reflex Cameras

• What you see is what you get
• Single Light Pathway for viewfinder
• and image

1 - Front-mount Lens (4 element Tessar design)2
- Reflex Mirror at 45 degree angle3 - Focal
Plane Shutter4 - 35 mm Film or Sensor
5 - Focusing Screen6 - Condenser Lens7 -
Optical Glass Pentaprism (or Pentamirror)8 -
Eyepiece (can have diopter correction ability)
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Canon.com
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Common Lens Related Issues - Summary
Vignetting
Compound (Thick) Lens
B
principal planes
A
nodal points
thickness
more light from A than B !
Chromatic Abberation
Radial and Tangential Distortion
ideal
actual
ideal
actual
image plane
Lens has different refractive indices for
different wavelengths.
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Biological Cameras
Mosquito Eye
Human Eye