Enhance Your Vision Applications with
Optical Filtering
Jason Dougherty
Managing Director
Midwest Optical Systems
Agenda
Why should you use optical filters?
Optimize your lighting with optical filters
Additional benefits and features of optical filters
Things to consider when choosing filters
Application examples
Q&A
3 | © 2013 Cognex Corporation
How to think about filters
Oftentimes
optical filters
are viewed as
an afterthought
Lens
Camera
Filter
Optical filters are a necessity
not an accessory
Filter
Light
Lens
Light
Camera
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Optical filtering in action
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Why should you use optical filters?
Why optical filters?
Control quality and quantity
of light
Block all unwanted
ambient lighting
Pass only the output of
lighting used for inspection
Increase contrast
and resolution
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Increase contrast and resolution
A lens with
Chromatic
Aberration
doesn't focus all
wavelengths of
light to a single
point resulting
in less than
optimal images.
White
Light
Lens
No Bandpass Filter
Before Bandpass Filter
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Multiple
Focal
Points
Single
Focal
Point
White
Light
Bandpass Filter
After Bandpass Filter
How filters control light
Digital Camera Spectral Response
Interfering Light Reaching the Application
Bandpass Filters Block Interfering LIght
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How filters can increase contrast
Pass Light = Highlight Color
Selectively pass or block light
wavelengths which can highlight
or darken areas of an image
Block Light = Darken Color
Pass Light With Green Bandpass Filter And
Highlight Green Colored Marker
Blocked Wavelengths With Green Bandpass
Filter Darken Blocked Colors
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Does filter quality matter?
Filters designed for
photographic film are
not suitable for digital
imaging systems!
Color Image, No Filter
Green Photographic Filter
Red Photographic Filter
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And here’s why…
Relative Spectral
Response
CCD/CMOS
Photographic
Film
Relative Spectral Response: Film vs Digital Camera
Sharpcut / Longpass Filters Are Suitable for Photography
But Not Machine Vision
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Photographic Filters Limit Transmission When
Used with a Digital Camera
Machine Vision Filters Isolate the Desired Wavelengths
MV filters optimize image quality
Camera, No Filter
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Photographic Filter
Machine Vision Green Bandpass Filter
Optimize your lighting with optical filters
Monochromatic LED lighting
Monochromatic (single color) LED lighting is
most commonly used for illumination in applications
to isolate the desired wavelength
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Designed for modern day illumination
Designed for High Transmission
 All machine vision bandpass filters
should have an antireflective
coating to increase transmission
More light = faster speeds and
added efficiency
Allows LED to operate with
less intensity
 Maximize output and increase
lifespan of the LED which results
in less light intensity on the
factory floor
Mimic the output of the LED
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100%
75%
50%
25%
0%
Designed for modern day illumination
Typical monochromatic LEDs have a total light output
of 60-70+/-10nm – filters are designed purposefully
broad to accommodate the entire output of the LED
Solid Blue 470 LED
Spans 60-70 nm
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Acutely Narrow
Filter Spans 20 nm
Bandpass Filter
Spans 80-90 nm
Narrow Bandpass
Filter Spans 40-50 nm
White light
Made up of the entire visible spectrum
=
White LED
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=
Visible Light
Spectral curve of a white
LED shown spanning the
entire spectrum
Test white light and bandpass filters
=
=
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Blue LED highlights
push pin
=
White LED no filter,
no contrast
Blue Bandpass Filter
highlights blue push pin
Additional benefits and features of
optical filters
Protect your lens investment
Filters protect the
surface of the lens
and internal threads
from dust, scratching,
cleaning and inhospitable
environments
Safe Lens
Broken
Filter
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Save money and space
Optical filters can
eliminate light shields
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Camera resolution flexibility
A filter improves contrast
significantly making it
possible to get acceptable
detection results using a
lower resolution camera
High Resolution Camera
Orange Fluorescent Spot Detected
In The Application
Low Resolution Camera With
Orange Bandpass Filter
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Extreme durability
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“Insures” repeatability and stability
Filters create a low-cost “Insurance Policy” for the lifetime of the application
Achieve consistent results
with Optical Filters
Image Lab
Consistent Lighting Environment
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Factory Floor
Inconsistent Lighting Environment
Things to consider when choosing a filter
Define
Imaging
Objective
Light &
Wavelength
Monochrome
or Color
Camera
Filter
Mount
Size
Define imaging objective
Define
Imaging
Objective
Low Contrast
Interfering Ambient Light
Color Sorting
Edge Detection
Reduce Glare
Remove Barcode Interference
Camera Bloom
Overpowering Light Source
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Reduce Light Intensity
What’s your optimal LED wavelength?
Light &
Wavelength
Utilize white light with bandpass filters
to test & determine the optimal LED
wavelength for your application
Original Color Image
Blue Bandpass / UV Block
Light Green Bandpass
Orange Bandpass
Light Red Bandpass
IR Bandpass
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How do you find the right filter?
Light &
Wavelength
Find the most
suitable
wavelength to
optimize image
quality
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Filter Test Kits
Test before
investing In
hardware
Find the
most suitable
match for the
light source
Find the
best filter to
maximize
contrast
Find the
best filter to
control
interfering
light
Monochrome or color?
Monochrome
or Color
Camera
A Bayer filter array
runs over a
monochrome sensor
Analyzes each pixel’s color
information and, combined with
that of adjacent pixels, recreates
the full color image
Resolution loss is most
pronounced when detection of a
single or a few colors is all that is
required
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RGB sensor
25%
50%
25%
Red Pixel
Green Pixel
Blue Pixel
Monochromatic imaging
Monochrome
or Color
Camera
Color
Significantly increase camera efficiency
Monochrome
Monochrome with Filter
Blue Bandpass Filter
90% Efficient
25% Efficient
Entire sensor is used to detect
intensity.
Original Color Image,
UV 395nm LED
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No Filter
Blue Bandpass Filter
Combined with a blue bandpass
filter that transmits 90+%, a
monochrome sensor vs a color
sensor is at least 3 times more
efficient
Mount to any system
Filter
Mount
Size
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Application examples
Date/lot code reading
Challenge: Low contrast
between yellow background
and white letters
White Text Yellow Background
Solution: Blue bandpass filter
Why does it work?: Blue
bandpass filter blocks yellow
reflection, darkening background
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White LED, Blue Bandpass
2-D code reading
Challenge: Reading 2-D codes
with ambient light changes on
a curved metal surface
Red LED, No Filter
Solution: Red bandpass filter
Why does it work?: Only pass
illumination from system,
block overhead lighting
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Red LED, Red Bandpass Filter
Color sorting/counting
Challenge: Low contrast
between two colors
Solution: Blue or red
bandpass filter
Original Color Image
Why does it work?: Red
bandpass filter highlights red,
blue highlights blue
Monochrome, No Filter
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Color sorting results
No Filter
Blue Bandpass Filter
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No Filter
Red Bandpass Filter
Reading exterior barcodes
Challenge: Overcome
interference from
barcode’s background
No Filter
Solution: Infrared (IR)
bandpass filter
IR Bandpass Filter
Why does it work? The ink
on the beverage label
reflects IR
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IR
PASS
Finding the fabric edge
Challenge: Very low contrast
between two colors
No Filter
Solution: IR bandpass filter
creates the effect of using an
IR camera
IR Bandpass Filter
Why does it work? Material
on left reflects IR, material
on right absorbs IR
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IR
PASS
Reducing saturation
Challenge: Reduce excess IR
emitted through heat in the
production process
No Filter
Solution: Shortpass filter
Why does it work? Shortpass
filters block IR light generated
from heat
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IR/Red Block – Visible Pass Filter
IR
Block
Counting LEDs
Challenge: Over saturation of
light on the part being inspected
Solution: Neutral density filter
Why does it work? Filter
evenly decreases overall
light transmission
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Iris Set To Smallest Aperture
To F/16, No Filter
Iris Set To Smallest Aperture To F/16, With
1% Transmission) Neutral Density Filter
IR
Block
Verifying presence of sockets
Challenge: Overcome specular
glare on exterior packaging
Solution: Polarizer on filter and lens
Why does it work? Reduces
specular reflections by cross
aligning polarizers for
maximum extinction
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No Filter Specular Glare
Linear Polarizer For Lens &
Linear Polarizer Film For
Light Source
Fluorescence and optical filters
Excitation
light source
(395nm LED)
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Emission from object
(Light blue fluorescence)
Fluorescence in action
No Filter, 395nm UV Light
Excitation
light source
(395nmLED)
Emission from object
(Light blue fluorescence)
395nm UV Light With Blue Bandpass Filter
Excitation
light source
(395nmLED)
Emission from object
(Light blue fluorescence)
Blue Bandpass Filter
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The bottom line
Of all the available tools to “control” what a camera
sees, optical filters are:
 The simplest
 The quickest
 The most cost effective way
To improve repeatability and stability in any machine
vision system!
45 | © 2013 Cognex Corporation
Thank you!