8 tips for optimal machine vision lighting

7/29/2019 8 Tips for Optimal Machine Vision Lighting

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Eight Tips for Optimal

Machine Vision Lighting

Tips for Choosing the Right Lighting

for Machine Vision Applications

Technology White Paper




Eight Tips for Optimal Lighting

Machine vision systems rely on quality images or quality

output. High quality images enable the system to accu-

rately interpret the inormation extracted rom an object

under inspection, resulting in reliable, repeatable system

perormance. The quality o the image acquired in any

vision application is highly dependent on the lighting con-

fguration: the color, angle, and amount o lighting used

to illuminate an object can mean the dierence between

a good image, resulting in good perormance, and a poor

image, yielding poor results.

Machine vision lighting should maximize eature contrast

while minimizing contrast o the rest, thereby allowing the

camera to clearly “see” the part or mark. High contrast

eatures simpliy integration and improve reliability; im-

ages with poor contrast and uneven illumination require

more eort rom the system and increase processing

time. The optimal lighting confguration is dependent on

the size o the part to be inspected, its surace eatures

and part geometry, and the system needs. With a broad

range o wavelength (color), feld o view (size), and ge-

ometry (shape) options available, machine vision lightingcan be tailored to specifc application requirements.

This white paper provides tips for choosing the optimal lighting solution for a machine vision application. Proper lighting is critical to the

success of a machine vision application, and should be the first consideration when setting up a system. A well planned lighting solu-

tion will result in better system performance and save time, effort, and money in the long run. This paper presents the following tips:

1. Use Bright Light to Detect Missing Material

2. Use Appropriate Wavelength for Accurate Component Placement

3. Use Non-Diffused Light to Detect Cracks in Glass

4. Use Diffused Light to Inspect Transparent Packaging

5. Use Color to Create Contrast

6. Use Strobed Light for Rapidly Moving Parts

7. Use Infrared Light to Eliminate Reflections

8. Use Infrared Light to Diminish Color Variation

Microscan Systems, Inc.

Effects of lighting techniques on a ball bearing

The ball bearing Fiber optic

ring light

Fluorescent

ring light

Diused

dome light

NERLITE

DOAL

NERLITE

SCDI

NERLITE

CDI

1 www.microscan.com

Five Considerations When Choosing Lighting:

• Is the surface flat, slightly bumpy or very bumpy?

• Is the surface matte or shiny?

• Is the object curved or flat?

• What is the color of the barcode or mark?

• Are you inspecting moving parts or stationary objects?

BEFORE

CORRECT LIGHTING

AFTER

CORRECT LIGHTING

How Does Lighting Affect a Vision Application?




Identiying missing material rom plastic molding applications

is important to ensure good sealing suraces. When material

is missing, you have a “short shot” condition (i.e. insufcient

quantity o material injected into the mold). To identiy short

shots, a NERLITE®DOAL®(Diuse On-Axis Light) illuminator

is used to highlight missing material on the sealing surace.

The bright feld lighting technique relies on surace texture

and at topography. Light rays hitting a at specular suracereect light strongly back to the camera, creating a bright

area. Roughly textured or missing suraces scatter the light

away rom the camera, creating dark areas.

Tip #1: Use Bright Light to Detect Missing Material

Good part presents a bright, well-defnedannular area to the camera

Bad (short shot) par t reects light away rom camera,presenting dark areas that indicate rejects

Problem:

Solution:

Material is absent during the molding opera-

tion (short shot), presenting a ailure in the

bottle sealing surace.

NERLITE® DOAL® light source reects

brightly o the at sealing surace o a good

bottle. This presents the camera with a

well-defned bright annular area. A short shoteliminates the at specular surace, scatter-

ing the light away rom the camera and allow-

ing easy identifcation.

Identiying “Short Shots” in Plastic Molding

Are your shots falling short?

Illumination Technique: Bright Field

Illumination Example:

2 www.microscan.com




Tip #2: Use Appropriate Wavelength for Accurate Component Placement

Proper placement easily identifed by bright appearance o components

“Flipped” components appear dark, enabling reject detection

3 www.microscan.com

Identiying proper component orientation is a common ma-

chine vision application in PCB assembly. In this example,

chips were incorrectly “ipped” in an automated assembly

step. Instead o being placed onto the substrate with copper

side down or proper electrical connection, the chip is ipped

over, silver side down, causing component and assembly

ailure.

To identiy problem assemblies, a NERLITE®DOAL®

illuminator with a blue wavelength is used to highlight chip

orientation. This lighting technique relies on wavelength and

coaxial lighting geometry. The blue wavelength (460 nm)

sharply dierentiates the silver and copper suraces: copper

absorbs the blue light and appears dark, silver reects the

blue light and appears bright. The unique NERLITE®coaxial

lighting geometry eliminates the root cause o alse rejects:

unwanted glare, reections and dark spots.

Problem:

Solution:

During circuit assembly, chips can be acci-

dentally “ipped” upside down on the circuit

boards, resulting in both component and

assembly ailure.

A NERLITE®DOAL®light source in the

proper color will reect brightly o o prop-

erly installed components, while improperly installed components absorb the light and

appear dark. The sharp dierence in contrast

is quickly recognized by the machine vision

system, enabling real-time process correc-

tions.

Are your chips flipped?

Illumination Technique: Bright Field


Inspecting “Flipped Chips”




Tip #3: Use Non-Diffused Light to Detect Cracks in Glass

Good part presentsa eatureless darkbackground

Field o View (FOV)

Bad (cracked) partpresents a bright, eas-ily detected eature o

interest

Field o View (FOV)

4 www.microscan.com

Detecting cracks during sidewall inspection o glass containers

prior to packaged-goods shipment is an important way to mini-

mize waste, decrease returns and increase consumer confdence.

To detect cracks in transparent material, a NERLITE®Area Array is

used to highlight the imperection.

In this application, dark feld lighting is used to create a bright,

easily detectable eature o interest within a dark background.

Light rays are directed at the transparent bottle rom a dark feld

region (i.e. rom outside the camera’s feld o view). Most o the

rays pass through the transparent object undetected by the cam-

era. I a material irregularity such as a crack exists, some light

rays strike that irregularity. Cracks, in particular, create an internal

void, where light rays reract and reect, scattering in many direc-

tions including back to the camera. These are the rays that turn

an otherwise difcult to detect crack into a bright eature on the

dark background.

Problem:

Solution:

Cracks in glass containers occurring dur-

ing manuacturing and handling must be

identifed prior to fll operations.

Use o a non-diused NERLITE®Area Ar-

ray in the dark feld region provides light

which will reract and reect o cracksin the glass. The resulting image depicts

a bright eature (the deect) on a dark

background.

You’ve had your fill of glass defects

Illumination Technique: Dark Field


Detecting Cracks in Glass Containers




Empty wells appear dark and are easily detected by the vision system

High contrast enables detection o misshapen (damaged) contents

Tip #4: Use Diffuse Light to Inspect Transparent Packaging

Detecting missing, damaged and/or incorrect contents in the cells

o blister packages is an important inspection step or ensuring the

quality o fnished pharmaceutical goods. Illuminating the packages to

avoid reections rom cell coverings can oten be a tough challenge.

To present an accurate image o the cells and their contents to the

machine vision camera, a NERLITE®CDI®(Cloudy Day Illuminator)

employing white LEDs is used. The CDI eliminates reections rom the

clear cell coverings, making them virtually disappear, while at the same

time providing a high contrast, neutral color view o the cell contents.

The continuous diuse lighting technique de-emphasizes surace tex-

tures and changes in elevation. It provides a very large solid angle o

illumination to guide light rays to the object rom a multitude o angles,

thereby eliminating the reections and shadows typical o unidirection-

al or single-plane light sources.

Problem:

Solution:

Manuacturer needs to detect miss-

ing, broken and/or incorrect objects

(by their shape or color) in the cells

o blister packages.

Use NERLITE®CDI®light source to

bathe the object in uniorm white

light rom every angle. This presents

the camera with a glare-ree image

o blister pack cell contents and the

clear blister coverings virtually disap-

pear rom view.

What’s inside that blister?

Illumination Technique: Continuous Diffuse


Veriying Contents o Blister Packs

5 www.microscan.com




Tip #5: Use Color to Create Contrast

A useul method or creating a high contrast image in

a machine vision application is to illuminate the object

with light o a particular wavelength (color). A light’s

wavelength can make eatures with color appear eitherbright or dark to the monochrome camera. Using the

color wheel as a reerence, choose light o an opposing

color to make eatures dark; choose light o the same

color to make eatures light.

Blue IlluminationRed Illumination

A light should be strobed when a part is moving quickly and

the image captured appears blurred. Use the ormula on

the right to calculate Strobe Pulse Width (maximum accept-

able blur image = 1 pixel).

In the ormula, feld o view (FOV) and pixels are rom the

axis o travel. For example, assuming a typical 1 inch FOV

and a 640 pixel rame, and a part speed o 500 inches per

second, a strobe pulse width o 3.1 μs is needed.

NERLITE® strobe models are available or high speed applications

Tip #6: Use Strobed Light for Rapidly Moving Parts

Tip #7: Use Infrared Light to Eliminate Reflections

Reection rom AmbientLight Results in Inaccu-rate Cap Measurement

Inrared Light RemovesUnwanted Reection

Original Part

Machine vision systems rely on transitions o gray

levels in a digital image. In many vision applications,

ambient light (i.e. overhead room lighting) sources

contribute unwanted bright reections that make it

difcult or impossible or the vision system to detect

the eatures o interest. An inrared (IR) light source

can be used to eliminate this problem.

Examples:

• I the eature you want to make darker is red,

use a green light.• Use a green light to make a green eature

appear lighter.

• Note the dierences in red and blue lighting on

printed aluminum below:

Strobe Pulse Width =

Field o View (in) ÷ Pixels

Part Speed (in/s)

6 www.microscan.com




Inrared light can be used to diminish the grayscale dier-

ence between colored objects. Dark objects absorb inrared

light waves, thereby creating uniormity in objects o other-

wise varying shades. This lighting solution acilitates detec-

tion o inconsistencies where color or shade variation is

expected and should not aect inspection.

Tip #8: Use Infrared Light to Diminish Color Variation

Infrared LightGrayscale Camera

White LightGrayscale Camera

www.microscan.com

North America (Corporate Headquarters)Email: [email protected]

EuropeEmail: [email protected]

Asia PacificEmail: [email protected]

7

Machine Vision L ighting solutions from Microscan

Microscan’s innovative NERLITE® line o products is the longest established brand o machine vision lighting, enabling machine

vision and auto ID systems to perorm reliably in thousands o applications worldwide.

A global technology leader with extensive solutions or ID tracking, traceability and inspection, Microscan’s line o 1D and 2D bar-

code readers, machine vision systems, and machine vision lighting is backed by one o the world’s most robust patent portolios

and years o proven perormance in the feld. Supported by a highly skilled global integrator network, Microscan products are known

and trusted by customers worldwide or their high precision and reliability in industrial automation and control applications, with

solutions ranging rom basic barcode reading to complex machine vision inspection, gauging, and measurement.

More inormation on Microscan is available at www.microscan.com.

©2011 Microscan Systems, Inc. 06/1

8 tips for optimal machine vision lighting

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