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"Introduction to Optics for Embedded Vision," a Presentation from Jessica Gehlhar

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For the full video of this presentation, please visit:
https://www.embedded-vision.com/platinum-members/embedded-vision-alliance/embedded-vision-training/videos/pages/may-2019-embedded-vision-summit-gehlhar

For more information about embedded vision, please visit:
http://www.embedded-vision.com

Jessica Gehlhar, formerly an imaging engineer at Edmund Optics, presents the "Introduction to Optics for Embedded Vision" tutorial at the May 2019 Embedded Vision Summit.

This talk provides an introduction to optics for embedded vision system and algorithm developers. Gehlhar begins by presenting fundamental imaging lens specifications and quality metrics such as MTF. She explains key parameters and concepts for specifying off-the-shelf or custom optics. Some optical design basics and trade-offs will be introduced.

Gehlhar also explores manufacturing considerations, including testing the optical components in your product, and the industrial optics used for a wide range of manufacturing tests. These tests and calibrations become important with designs that include multi-camera, 3D, color and NIR.

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"Introduction to Optics for Embedded Vision," a Presentation from Jessica Gehlhar

  1. 1. © 2019 Edmond Optics Introduction to Optics for Embedded Vision Jessica Gehlhar May 2019
  2. 2. © 2019 Edmond Optics Growing World of Embedded Vision Factory Automation Machine Vision + Robotics Medical Devices + Life Science Systems Defense, Space, Security Consumer! • Robots + Vision out of controlled environments • Benefit to every industry • Smaller, cheaper, accessible, more talent Autonomous Vehicles • Air, Land and Sea Crossover in all industries
  3. 3. © 2019 Edmond Optics Goal of Presentation Introduce challenges & effects of optics on your final image • Basic theory related to root cause Why care about optical effects on image? • Include all variations in set of test images • Better image = less development time • Processing power, ISP cleanup & estimation of what info was lost Why care about optical root cause/theory? • Better architecture up front • Less surprise/$ during manufacturing and test
  4. 4. © 2019 Edmond Optics Outline: Embedded Optics Considerations Smaller, Cheaper Rugged and often wide angle Creates challenges + common design deviations during manufacturing • Boresighting/pointing accuracy and tip/tilt • Focal length/Field of View and Working Distance tolerance • Defocus and Focus shift • Ruggedized • Color aberrations • Coatings • Small pixels, Diffraction Limit, Chief Ray Angle (CRA)
  5. 5. © 2019 Edmond Optics Imaging System Basic Terms FOV = Field Of View PMAG = Primary Magnification WD = Working Distance FL = Focal Length Res = Resolution f/# = f-number DOF = Depth of Field or Depth of Focus NA = Numerical Aperture MTF = Modulation Transfer Function
  6. 6. © 2019 Edmond Optics Imaging System Basic Terms Image Space Object Space
  7. 7. © 2019 Edmond Optics Basic Imaging Specs: Calculate FOV and WD       = 2 2/ AngularFOV TAN FOV WD             = 2 tan2 AngularFOV WDFOV FOV WD Angular FOV
  8. 8. © 2019 Edmond Optics Basic Image Specs: PMAG, Sensor Size Short FL Long FL
  9. 9. © 2019 Edmond Optics Sensor Format Names Format names do not match sensor dimensions Format names relate to vacuum tube sizes ¼” ½ ” 1” 35mm 16𝑚𝑚 8𝑚𝑚 = 2 8𝑚𝑚 4.5𝑚𝑚 = 1.78
  10. 10. © 2019 Edmond Optics AFOV (half):8° HFOV:74mm 1/3” Sensor AFOV (half):21° HFOV:204mm FOV and Sensor Size Dependency 1” Sensor
  11. 11. © 2019 Edmond Optics Basic Image Specs: Flange & Rear Protrusion C-mount CS-mount F-mount S-mount (M12) • Standard Flange Distance M8, M6 and smaller 1”-32TPI
  12. 12. © 2019 Edmond Optics Basic Imaging Specs: Resolution Basic FOV/Feature Size mapping to pixels • 2 pixels = 1 line pair • Divide FOV by # pixels/2 = limiting resolution • PMAG used to convert Object to Image Space
  13. 13. © 2019 Edmond Optics Resolution and Contrast 13
  14. 14. © 2019 Edmond Optics MTF: Resolution and Contrast
  15. 15. © 2019 Edmond Optics MTF (Modulation Transfer Function) Defined MTF shows modulation (relative contrast) at many frequencies MTF is absolute value of OTF (Optical Transfer Function) OTF is calculated by taking FFT (Fast Fourier Transform) of PSF (Point Spread Function) PSF can be measured by imaging a Point Source of light or a Slanted Edge target
  16. 16. © 2019 Edmond Optics OTF describes some interesting affects Phase Reversal when OTF goes negative
  17. 17. © 2019 Edmond Optics Distortion: Symmetric
  18. 18. © 2019 Edmond Optics Specifying and Calibrating Distortion
  19. 19. © 2019 Edmond Optics Distortion: Asymmetric Parallax/Keystone
  20. 20. © 2019 Edmond Optics Telecentric Lenses 2 afocal systems with stop in between, located at common focus • Stop located 1 focal length from front and rear assemblies • Chief rays all parallel • Maintains same magnification as working distance changes Creates symmetric blur and low distortion
  21. 21. © 2019 Edmond Optics Basic Imaging Spec: F/# and Numerical Aperture Measure of how much light can enter the lens F/# = Focal Length Entrance Pupil Diameter Numerical Aperture (NA) = 1 2∗𝐹/# Large F/# means less light, smaller apertures Large NA means more light larger apertures
  22. 22. © 2019 Edmond Optics Basic Imaging Spec: Depth Of Field Depth of Field is a measure of how much of an object is in focus measured along the working distance (Z-axis) Must be defined at a specific resolution
  23. 23. © 2019 Edmond Optics Maximizing DOF with Hyper Focal Distance Closest distance that appears sharp when lens is focused at  Closest distance that can be focused on while maintaining focus at  Circle of Confusion (CoC) is photographic term for limiting resolution in image space (equates to pixel size in digital imaging)
  24. 24. © 2019 Edmond Optics Focusing at Hyperfocal Distance Increases DOF If the lens is focused at infinity an object at the Hyper focal distance will have a smaller blur than the size of the pixel (blur is within CoC)
  25. 25. © 2019 Edmond Optics Pointing Accuracy and Tip/Tilt
  26. 26. © 2019 Edmond Optics Importance of Pointing Accuracy & Ruggedization
  27. 27. © 2019 Edmond Optics Causes of Pointing Variation Optical asymmetric tolerances Lens to Sensor alignment Opto-Mech tolerances Shock and Vibe
  28. 28. © 2019 Edmond Optics Focal Length/FOV
  29. 29. © 2019 Edmond Optics Focal Length Tolerance Causes of Focal Length variation = Axially Symmetric Tolerances • Power of Lens Elements • Spacer length
  30. 30. © 2019 Edmond Optics Working Distance
  31. 31. © 2019 Edmond Optics Working Distance Tolerances Causes of Working Distance variation = Axially Symmetric Tolerances • Power • Spacer length • Sensor location
  32. 32. © 2019 Edmond Optics Your Application: FOV and WD Ratio It is possible to get wide fields of view at short working distances. However, performance usually drops severely A working distance to field of view ratio of between 2:1 and 4:1 is recommend to gain higher performance at the most reasonable price 35mm lens 4.5mm lens
  33. 33. © 2019 Edmond Optics Effects of Working Distance 12mm lens 75mm lens
  34. 34. © 2019 Edmond Optics Relative Illumination (Vignetting/Lens Shading)
  35. 35. © 2019 Edmond Optics Consider Your Application
  36. 36. © 2019 Edmond Optics Consider Your Application Quantity and Cost Lead Time and Cost NRE
  37. 37. © 2019 Edmond Optics Focus Shift
  38. 38. © 2019 Edmond Optics Sources of Focus Shift and Reduced MTF Reduced MTF • spherical, field curvature, astigmatism, chroma • Lens spacing/index, flange distance, filter thickness • Veiling Glare • Manufacturing process Focus Shift • Thermal/Materials • Ruggedized
  39. 39. © 2019 Edmond Optics Color Aberrations
  40. 40. © 2019 Edmond Optics Color Aberrations
  41. 41. © 2019 Edmond Optics Purple Fringe
  42. 42. © 2019 Edmond Optics Coatings
  43. 43. © 2019 Edmond Optics Need for Coatings & Coating variations CFA (Color Filter Array) Curves UV or IRCF (Infrared Cut Filter) • Dichroic/Interference vs Blue Glass • Angle Dependence & batch variation/scratch
  44. 44. © 2019 Edmond Optics Pixel Size & Diffraction Limit
  45. 45. © 2019 Edmond Optics Challenges with Decreasing Pixel Sizes Standard Optics struggle to keep pace with sensor development Challenges are high resolution and light throughput The laws of physics create limitations • Manufacturability of high performance optics • Higher performing optics have limited range of usability • Higher performance will have come at the expense of depth of field and depth of focus
  46. 46. © 2019 Edmond Optics How Diffraction and f/# Affect Performance Smallest achievable spot size = 2.44 * λ * f/# f/2.8 f/8
  47. 47. © 2019 Edmond Optics Larger Chief Ray Angles (CRAs) to Handle Extreme Sensor Demands Small pixels, large formats • Pixels < 6µ fill factor issue • Ratio of CFA thickness & pixel pitch = color crosstalk Wide angle lenses, short total track lengths (TTL) Add micro lens arrays • Offset of micro lens from pixels • Large CRA allows TTL to reduce • Pushes molded aspheres/other shapes
  48. 48. © 2019 Edmond Optics Larger Chief Ray Angles (CRAs)
  49. 49. © 2019 Edmond Optics Color Shading due to CRA matching + Coatings
  50. 50. © 2019 Edmond Optics Lens Shading and Sensor Shading Lens Shading/Relative Illumination used to reduce aberrations during design Combined with sensor shading/CRA matching + protective windows Calibrate/correct for color and lens shading = color noise in corners RI ~ Cos4θ θ = 30° → RI = .56 θ = 45° → RI = .25 θ = 60° → RI = .06
  51. 51. © 2019 Edmond Optics Conclusions Choose right lens and sensor for your application Explore solutions from different industries • Match lens to sensor • Simple equations to get started • Remember required ‘resolution’ is more than just pixel calculation • Lens MTF, Aberrations, Diffraction Limit, Tolerances, DOF Many root causes for image to image variation • Isolate optical components, coatings, assembly, +mechanics, +sensor, + system integration & firmware • During design: think calibration requirements & manufacturing process
  52. 52. © 2019 Edmond Optics Resource Slide Image Credit and Active Alignment explanation Edmund Optics Tech Notes: • Fundamental Imaging System Parameters • Lens Design: Aberrations and MTF • Distortion • DOF • Athermalization • Aspheres Books for further reading: • Modern Optical Engineering, Warren J Smith • Principles of Color Technology, Roy S Berns

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