Machine Vision Basics: Optics Part Two

Transcription

1 Machine Vision Basics: Optics Part Two Webinar Gregory Hollows Director, Machine Vision Solutions Edmund Optics, Inc. Celia Hoyer Product Marketing Vision Systems Cognex Corporation

2 Agenda Quick Review of Part One Optics Tutorial: Part Two Depth of Field Distortion Perspective Q&A 2

3 Review of Part One Celia Hoyer, Cognex Corporation

4 Three Requirements for Machine Vision Solution 1. Vision System 2. Part 3. Illumination 4. Optics 4

5 Adding a Lens to Enhance Your Image 1. Locate features within the Field of View (FOV) 2. Ensure features are in focus 3. Avoid perspective distortion 4. Maximize contrast 5

6 Review of Terms Field Of View (FOV): The viewable area of the object Working Distance (WD): The distance from the front of the lens to the object Resolution: The minimum feature size of the object 6

7 Optics Tutorial: Part Two Gregory Hollows, Edmund Optics

8 Image Quality Resolution Depth of Field Contrast Image Quality Perspective Distortion 8

9 Depth of Field Depth Of Field (DOF): The maximum object depth that can be maintained in desired focus The difference between the closest and farthest distances an object may be shifted before an unacceptable blur is observed Sometimes called Depth of Focus DOF should not be confused with Working Distance 9

10 What Is F/#? Infinite conjugate F/# = Focal length/ Diameter Image Space close conjugate F/# = Image distance/ Diameter Object Space close conjugate F/# = Object distance/ Diameter 10 Illustration adapted from Smith, Modern Optical Engineering: The Design Of Optical Systems, New York, McGraw-Hill, 1990

11 How Does Diffraction Affect Performance? The smallest achievable spot from a lens = 1.22 x wavelength of light x (F/#) F/# describes the light gathering ability of an imaging lens (lower F/# lenses collect more light) As lens aperture decreases, F/# increases 11

12 How Do We Test Depth Of Field? The frequency of the lines on target = resolution DOF is to be tested at

13 Case Study 1: 8.5mm fixed focal length lens Effects on resolution and depth of field with changing aperture setting Iris completely open Iris half open Iris mostly closed 13

14 Case Study 1: 8.5mm fixed focal length lens Effects on resolution and depth of field with changing aperture setting Iris completely open Iris half open Iris mostly closed 14

15 Case Study 2: 50mm Double Gauss lens Effects on resolution and depth of field with changing aperture setting Iris completely open Iris half Iris open half open Iris mostly closed 15

16 Case Study 2: 50mm Double Gauss lens Effects on resolution and depth of field with changing aperture setting Iris completely open Iris half Iris open half open Iris mostly closed 16

17 Image Quality Resolution Depth of Field Perspective Image Quality Contrast Distortion Perspective Distortion 17

18 Distortion An optical error (aberration) caused by the lens that results in a difference in magnification at different points within the image Expressed in terms of % 18

19 Distortion Negative distortion % Distortion = (AD-PD) x 100 PD AD = Actual Distance of the image points from the center of the field PD = Predicted Distance that the real world points would be from the center of the field if distortion was not present 19

20 Avoiding Perspective Distortion (Perspective Error) The further an object is from the camera, the smaller it appears through a normal lens Specially important for gauging or high precision applications To avoid perspective distortion, keep the camera perpendicular to the FOV 20

21 Minimizing Perspective Error Conventional Lens Telecentric Lens 21

22 Perspective Error Caused by variations in the working distance (even within the same part) Telecentric lenses optically correct for this occurrence Must be especially avoided when doing precise measurements Conventional Lens Telecentric Lens 22

23 Maintaining Consistent Feature Size 23

24 Maintaining Consistent Feature Size Real world example 24

25 Maintaining Consistent Feature Size Standard 8.5mm Fixed Focal Length Lens Telecentric Lens 25

26 Review Depth of Field Of View (DOF): The maximum object depth that can be maintained in focus at a desired resolution Distortion: The change in magnification laterally across the field of view of the image plane in relation to the real world position of details in the object plane Perspective Error: The change in magnification associated with different working distances Telecentric Lens: A lens that maintains the same magnification over a given working distance range essentially eliminating perspective error 26

27 Summary Lighting and Optics are important Keys to the overall solution Much more involved than most think Not dependent upon each other but complement each other Specify what you need as a system not just components Expect a lot from optical suppliers Find trusted partners 27

28 Future Webinar Topics Machine Vision Basics Selecting a lens Filters and how they can enhance your images Advanced Machine Vision Topics Edge inspection Geometric pattern matching Color inspection And more! 28

29 Thank You