Lighting and Optics for Machine Vision

Presented by Dave Coleman – Regional Director for Advanced Illumination

SE07

Come to Solution Area #5 and

Cognex Booth #704 to see:

Advanced Illumination and

Edmund Optics products

See our other 5 Vision and Bar Code Tech

Sessions

Also at the Werner Solution Expo

Tips For better Imaging Systems

Slides Courtesy Edmund Optics

Edmund Optics® is a global OPTICS and IMAGING

company that manufactures and supplies the worldwide

technical community

with precision optical components and subassemblies.

Who we are

World’s Largest Inventory of Optical Components.

WHY ARE WE HERE

#1. Allow room for your imaging system

• In order to achieve certain levels of performance many systems need to grow larger

• High resolution optics generally require larger diameters and cannot compromise size easily

• For measurement applications, very large optics may be required. For a 100mm FOV a lens might be 170mm in diameter

• The lighting footprint can increase especially for large objects and objects that are very reflective

• Some illumination types will require optics that have very long working distances

#1. Allow room for your imaging system

#2. Create the right Working Distance to Field of View 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

#3. Choose the right illumination

Standard Backlight Illumination

#3. Choose the right illumination

Telecentric Illumination

#3. CHOOSE THE RIGHT ILLUMINATION

660nm Light 470nm Light

# 4. Color (wavelength) Matters

# 4. Color (wavelength) Matters

# 4. Color (wavelength) Matters

# 5. YOU HAVE TO COMPROMISE BETWEEN RESOLUTION AND DOF

# 5. RESOLUTION AND DOF TRADEOFF

# 6. NO LENS CAN SOLVE EVERY APPLICATION

• To obtain the highest levels of performance many lenses will have to be employed to solve different applications.

• This is due to the way optical aberrations behave making it impossible to have one “catch-all” lens.

# 7. UNDERSTAND WHAT YOU ARE LOOKING AT AND NEED TO ACHIEVE

• The properties of an object and the desired results will drive the final system composition.

• Bring all variations and marginal parts into the discussion early in the process.

# 8. ASK LOTS OF QUESTIONS

• Understanding why a system or imaging component will or will not work is critical for success.

• Suppliers should welcome your questions and be able to give you a good explanation about what your expectations should be.

# 9. UNDERSTAND THE FUNDAMENTALS OF AN IMAGING SYSTEM

Defined at a

Resolution

and Contrast

Tips For better Imaging Systems

Slides Courtesy Advanced Illumination

Ai is a Machine Vision lighting solutions company,

based in Rochester, Vermont

50+ Employees; in business since 1993

Largest independent US company focusing on Machine

Vision Illumination

Fully US owned and operated

Who is Advanced Illumination

We manufacture Standard and Custom LED

vision lighting and drive electronics

Long history of Technology Innovation –

Evenlite®

, Signatech®

Full custom lighting design facility for OEs and

partners

We Can Design and Build a Prototype Light in as Little as three Days

Who is Advanced Illumination

Why use lighting for machine

vision?

Create a proper and consistent lighting

environment

What we really require is control of the lighting

environment for producing: - Sample inspection & system with appropriate lighting

- To the extent possible, standardization of components,

techniques, implementation and operation

- Reproducibility of inspection results

- Robustness to handle variations of “all sorts”

Objectives of Vision Lighting

Sample-Appropriate Lighting

It’s All About (creating) Contrast Contrast!!

1) Maximum contrast • features of interest (Signal)

2) Minimum contrast • features of no interest (Noise)

3) Minimum sensitivity to normal variations (ROBUST)

• minor part differences • presence of, or change in ambient lighting • sample handling / presentation differences

Types of light sources

Four most common light sources:

• LED - Light Emitting Diode

• Quartz Halogen – W/ Fiber Optics

• Fluorescent

• Xenon

Primary Vision Light Sources

Primary Vision Light Sources

Brief Review of Light and

Optical Properties for

Illumination

Graphics courtesy of Midwest Optical, Palatine, IL

Vision Lighting Spectrum

Typical CCD

Sensor

Human Visual

System

Sample / Light Interaction

Illumination

Reflect

Emit

Absorb

Transmit

Total Light In =

Reflected + Absorbed + Transmitted + Emitted Light

Basic Lighting Techniques

Change Light / Sample / Camera Geometry

3-D spatial relationship

Change Light Pattern (Structure)

Light Head Type: Spot, Line, Dome, Array

Illumination Type: B.F. – D.F. – Diffuse – B.L.

Change Spectrum (Color / Wavelength)

Monochrome / White vs. Sample and Camera Response

Warm vs. Cool color families – Object vs. Background

Change Light Character (Filtering)

Affecting the wavelength / direction of light to the camera

Need to understand the impact of incident light on both the part of interest and its

immediate background!

4 Lighting Cornerstones

How do we change/maximize contrast?

Common Lighting Techniques

Full/Partial Bright Field Dark Field Back Lighting

Diffuse Dome Axial Diffuse Flat Diffuse

Typical Co-axial Ring Light – Sample Geometry

Bright Field

Bright Field vs. Dark Field

Dark Field

Dark Field vs. Bright Field

Dark Field Lights in

Grey Areas

Mirrored Surface

Partial Bright Field

Lights in White Area

Scratch

45

• Angled light – 45 degrees or less

• Used on highly reflective surfaces

• OCR or surface defect applications

Dark Field Example

Reading under Cellophane

UPC Bar Code

Broad Area Linear Array Dark Field Ring Light Bright Field Ring Light Axial Diffuse Illuminator

Diffuse Dome

• Similar to the light on an overcast day.

• Creates minimal glare.

Ink Jet OCR

• Purple Ink

• Concave, reflective

surface

Diffuse Dome Axial Diffuse Illuminator Dark Field Ring Light Bright Field Ring Light

Axial Diffuse

• Light directed at beam splitter

• Used on reflective objects

Flat Diffuse

• Diffuse sheet directed downward

• Long WD and larger FOV

• Hybrid diffuse (dome and coaxial)

Coaxial BF

Ring Coaxial DF

Ring

Diffuse

Coaxial

Diffuse

Dome Flat Diffuse

• Locates edges – Gauging

• Internal defects in translucent parts

• Hole-finding

• Presence / Absence

• Vision-Guided Robotics / Pick & Place

• Useful for translucent materials

Backlight Illumination

• Small Bottle – Determine Fill Level

• Consider colors and materials

properties also.

• Longer wavelength isn’t always

best for penetration!

660 nm Red Backlight

880 nm IR Backlight 470 nm Blue Backlight

Back Lighting Example

Using Color to your Advantage

Use Colored Light to Create Contrast

Using Color

• Use Like Colors or Families to

Lighten (green light makes green

features brighter)

• Use Opposite Colors or

Families to Darken (red light

makes green features darker)

Warm Cool

R V

O B

Y G

Increasing Contrast with Color

Red Green

Blue White

Consider how color affects both your object and its background!

White light will contrast all colors, but may be a contrast compromise.

Warm Cool

R V

O B

Y G

Using Near IR and UV Light

400 nm 500 nm 600 nm 700 nm

390 455 470 505 520 595 625 660 695 735

The Visible Light Spectrum

All light is seen differently by film, humans and CCD/CMOS Imagers

UV IR

Human Visible Range

Infra-red (IR) light interacts with sample material

properties, often negating color differences.

Imaging Beyond “Visible” – Near IR

White light – B&W Camera IR light – B&W Camera

Near UV light when

used w/ a matched

UV excitation dye,

illuminates codes

and structural fibers.

Imaging Beyond “Visible” – UV

Imaging Beyond “Visible” – UV

Near UV light

fluoresces many

polymers, including

nylon.

Filters and Other Techniques

Controlling and Negating Ambient Light

Turn off the ambient contribution Most effective . . . Least Likely!

Build a shroud Very effective, but time-consuming, bulky and expensive

Overwhelm the ambient contribution w/ strobing Effective, but requires more cost and complexity

Control it with pass filters Very effective, but requires a narrow-band source light

Ambient Light

Any light, other than the vision-specific lighting that the camera collects.

Pass Filters in Machine Vision

• Pass filters exclude light

based on wavelength.

• Reduce sunlight and mercury

vapor light 4X

• Reduce fluorescent light 35X

715 nm Long Pass

660 nm Band Pass 510 nm Short Pass

UV light w/ strong Red 660nm

“ambient” light.

Pass Filters

Same UV and Red 660 nm “ambient”

light - with 510 nm Short Pass filter

applied.

Polarizing Filters in Vision

On-axis Light

w/o Polarizers

On-axis

Light w/

Polarizers

Off-axis Light

w/o Polarizers

w/o Polarizers

w/ Polarizers

2 ½ f/stops

more open!

Summary of a Standard Lighting Method

• Determine the Exact Features of Interest

• Analyze Part Access / Presentation • Clear or obstructed, Moving / Stationary

• Min / Max WD range, Sweet Spot FOV, etc.

• Consider Robot(s) Motion

• Analyze Surface Characteristics • Texture

• Reflectivity / Specularity

• Effective Contrast – Object vs. background

• Surface flat, curved, combination

• Light Types and Applications Techniques Awareness • Rings, Domes, Bars, ADIs, Spots, Controllers

• Bright Field, Diffuse, Dark Field, Back Lighting

• Determine Cornerstone Issues (Slide 11)

• 3-D Geometry, Structure, Color & Filters

• Ambient Light Effects / Environmental Issues

Come to Solution Area #5 and

Cognex Booth #704 to see:

Advanced Illumination and

Edmund Optics products

See our other 5 Vision and Bar Code Tech

Sessions

Also at the Werner Solution Expo