Intro_to_Visual Inspection.ppt

Outline
• Introduction
• Basic principles
• Manual Vision Inspection
– Human Vision
– Common Inspection applications
– Equipment
• Automated or Machine Vision Inspection
– Machine Vision
– Common Inspection Applications
– Equipment
• Advantages and Limitations
• Glossary of terms

Introduction
• Visual inspection is commonly defined as “the examination of a
material, component, or product for conditions of
nonconformance using light and the eyes, alone or in
conjunction with various aids.
• Visual inspection often also involves, shaking, listening, feeling,
and sometimes even smelling the component being inspected.
• Visual inspection consists of at least two major processes.
– The first is a search process.
– The second is a process of combining relevant knowledge, sensory
input, and pertinent logical processes to provide an identification
that some anomaly or pattern represents a flaw that poses a risk to
the performance of the part.
• Visual inspection is commonly employed to support other NDT
methods.
• Digital detectors and computer technology have made it
possible to automate some visual inspections. This is known
as “machine vision inspection.”

Introduction
• Visual inspection is the most basic and most
commonly employed NDT method.
• It is applicable to a wide variety of material types and
product forms.
• Several characteristics about the part being examined
may be determined, which include dimensional
conformance, the presence of discontinuities, general
fit and wear, and simple cosmetic compliance.
• It can be performed by direct or indirect methods
during various stages of manufacturing or after the
component has been placed in-service.

• The quality of an inspection are affected primarily by
four factors.
– The quality of the detector (eye or camera).
– The lighting conditions.
– The capability to process the visual data.
– The level of training and attention to detail.
Introduction

Introduction –
Manual Versus Automated Inspection
• The majority of visual
inspections are completed
by an inspector, but
machine vision is becoming
more common.
• The primary advantage of an
inspector is their ability to quickly adapt to a variety of
lighting and other non-typical conditions, and their
ability to use other senses.
• The primary advantage of a machine vision inspection
system is their ability to make very consistent and rapid
inspections of specific details of a component.
• Machine vision is primarily used in production
applications where a large number of components
require inspection and the inspection conditions can be
closely controlled.

Basic Principles –
The Human Eye
• Light enters the eye through
the pupil and an image is
projected on the retina.
• Muscles move the eyeball in
the orbits and allow you to
focus the image on the
central retina or fovea.

• Rods are sensitive to blue-green light and are used for vision under dark
or dim conditions.
• Cones operate only in relatively bright light, but they provide us with our
sharpest images and enable us to see colors. There are three types of
cones
– L-cones are red absorbing cones or those that absorb best at the relatively
long wavelengths peaking at 565 nm
– M-cones are green absorbing cones with a peak absorption at 535 nm
– S-cones are blue absorbing cones with a peak absorption at 440 nm.
The Human Eye
The retina is a mosaic of two basic types of photoreceptors, rods and cones.

• Cones provide us with our sharpest
images because most of the 3 million
cones in each retina are confined to a
small region just opposite the lens
called the fovea. The maximum
concentration is about 180,000 cones
per square mm.
• Our sharpest and most colorful images
are produced in the fovea.
• Outside of this region our vision is
relatively poor but, since we can quickly
redirect our eyes we tend not to be
aware of our poor peripheral vision.
Basic Principles – Visual Acuity

•Normal visual acuity or 20/20 vision is defined as the ability to resolve a
spatial pattern separated by a visual angle of one minute or 1/60 of a
degree of arc.
•One degree of a scene is projected on about 290 micrometers of the retina.
•In 290 micrometers there are 123 cones and in 1/60 of a degree there 2
cones which is the number required to resolve an object.
•The size of an object that can be seen at a given distance can be calculated
using the following formula: X = (d tan q/2)2
b
q
/
2
q q
’
d d’
n
b’
c
’
~425 by 425 cones
180,000
Cones
1 mm
1 mm
X
X/2
Basic Principles – Visual Acuity
When visually inspecting an object for a defect, a comfortable viewing distance “d” might
be around 12 inches. At 12 inches, the normal visual acuity of the human eye is 0.0035
inch. (It must be noted that this value is for the situation where there is good lighting and high contrast
between the objects being viewed.)
There is a
limit to what
the unaided
eye can see.

Contrast Sensitivity
• Contrast sensitivity is a measure of how faded or
washed out an object can be before it becomes
indistinguishable from a uniform field
• It has been experimentally determined that the
minimum discernible difference in gray scale level
that the eye can detect is about 2% of full brightness
• Contrast sensitivity varies with
– the size or spatial frequency
of a feature
– The lighting conditions
– Whether the object is lighter
or darker than the background
The graph to the right plots the
visibility of a spot as a function of the
above variables

In this image:
– The luminance of pixels is varied
sinusoidally in the horizontal
direction. The spatial frequency
increases exponentially from
left to right.
– The contrast also varies
logarithmically from 100% at the
bottom to about 0.5% at the top.
– The luminance of peaks and
troughs remains constant along
a given horizontal path through the image.
It should be noted, however, that
larger objects are not always
easier to see than smaller objects
as contrast is reduced.
Campbell, F. W. and Robson, J. G. (1968) Application of Fourier analysis to
the visibility of gratings. Journal of Physiology (London) Image Courtesy
of Izumi Ohzawa, Ph.D. University of California School of Optometry
Contrast Sensitivity
If object visibility was dictated solely by image contrast, the alternating
bright and dark bars should appear to have equal height everywhere in the
image. However, the bars seem to be taller in the middle of the image.

• Under normal lighting conditions the
cones are operating and the eye has
good visual acuity and is most
sensitive to greenish yellow color,
which has a wavelength around 555
nanometers (photopic curve).
• When the light levels drop to near total
darkness, the response of the eye
changes significantly as shown by the
scotopic response curve on the left.
• At this level of light, the rods are most
active and the human eye is more
sensitive to any amount of light that is
present, but is less sensitive to the
range of color.
• At this very low light level, sensitivity
to blue, violet, and ultraviolet is
increased, but sensitivity to yellow and
red is reduced.
Basic Principles – Light Levels

• Effective visual inspection requires adequate
lighting.
• The type of inspection will dictate the lighting
requirements. Inspection of components with
fine detail and low contrast will require greater
illumination than components with large details
and high contrast.
• Light intensity may be measured with a suitable
light meter. The unit of measure for white light is
foot-candles (fc).
– A foot-candle is equal to the amount of direct
light thrown by one standard candle at a distance
of 1 foot.
• Inspection of components with fine detail and
low contrast may require 100 foot-candles or
more.
• Specification requirements for lighting should be
reviewed prior to performing an inspection.
Light Intensity Measurement

Light Directionality
• The directionality of the
light is a very important
consideration.
• For some applications,
flat, even lighting
works well.
• For other applications,
directional lighting is
better because it
produces shadows that
are larger than the
actual flaw and easier
to detect.

Is the book facing towards
or away from you?
Basic Principles – Perspective
The eye/brain need visual clues
to determine perspective.

Are the horizontal lines parallel or
do they slope?
How many black dots do you see?
Sometime the eye/mind has trouble correctly processing
visual information.
Optical Illusions

• When evaluations are made by an
inspector, eye examinations must
be done at regular intervals to
assure accuracy and sensitivity.
These examinations may consist
of the following:
• Near Vision (Jaeger)
• Far Vision (Snellen)
• Color Differentiation
• When using machine vision,
different but similar performance
checks must be performed.
Basic Principles – Vision

• For best results the inspector or
machine vision operator must have:
– A basic knowledge of material processing,
forming, machining and joining processes.
– A general understanding of design features,
application and service requirements.
– Specific instructions on what to look for and
specific accept/reject criteria.
Basic Principles

Inspection Applications
• Detection of surface anomalies such as scratches,
excess surface roughness, and areas void of paint
or plating.
• Crack, porosity, corrosion or other flaw detection.
• Dimensional conformance.
• Precision measurements.
• Foreign object detection.
• Component location.
Applications for visual inspection and many and range
from looking a product over for obvious defect to
performing detailed inspections. Some of the common
applications include:

• Visual inspection of
manufactured materials
and components is a
cost effective means of
identifying flaws.
• Visual inspection of a
casting reveals a crack
between a threaded
opening and a pressed
fit.
• The aluminum sand
casting has hot tears
and shrinkage at the
transition zones.
Inspection Applications –
Flaw Detection

• In this example, visual
inspection of a fire escape
reveals a failure in a
handrail tube.
• The failure is in the tube
seam and is likely the
result of ice expansion.
Flaw Detection
In-service inspections of
existing components and
structures is commonly
accomplished visually.

Normal inspection practices
for highway bridges rely
almost entirely on visual
inspection to evaluate the
condition of the bridges.
Flaw Detection

Over 80 percent of all aircraft
inspections are performed
visually.
Flaw Detection

• Weld quality requirements
are commonly determined
through visual inspection.
• Many standards have
established acceptance
criteria for welds.
Slag rolled into toe of weld
Transverse weld crack
Flaw Detection

Dimensional Conformance
• Visual inspection is commonly employed for
general dimensional conformance, assembly
fit, and alignment between components.
• Common applications include determining:
– Weld size and tolerance.
– Component dimensions.
– Material alignment and allowable distortion.

Welds are commonly inspected for dimensional tolerance.
• There are several types of gages used to inspect welding fit up
and finished weldments.
• These gages are intended for general inspection where close
tolerances are not required.
• The gage used is determined by the application.
Fillet gage set
Palmgren gage
VWAC gage
Cambridge gage

Visual inspection is commonly used to determine weld size and
tolerances according to standards and engineering specifications.
Throat measurement
using a Palmgren gage.
Leg size determination
with fillet gage.
Convexity measurement
with VWAC gage.

Undercut in a weld is readily seen visually. In many cases its
depth must be measured to determine if it exceeds code
requirements.
Measurement of undercut
depth with VWAC gage.

Component finish dimensions are checked with the
use of measurement devices, such as transferring
gages and precision measurement gages.
The finished depth of a
machined mold is determined
with a depth micrometer.
Small hole gage used in
determining hole diameter.

Alignment/Distortion
• Visual inspection frequently involves
checking materials and components for fit
and alignment.
• Many standards establish allowable
tolerances for fit and distortion.
• Structural fabrication requires dimensional
inspection of finished components prior to
shipment to the field site.
• Basic tools are used for the inspection. An
inspector will set up string lines at known
distances and plum them using a tape
measure. Measurements are then taken at
various locations and compared to code
requirements.
In this image a
fabricated girder is
being inspected for
distortion, sweep
and web flatness.

Equipment
• Visual inspection equipment includes a variety of
different tools. These may range from basic rulers,
tape measures and spring type calipers to rigid or
flexible borescopes and remote crawlers with
cameras.
• Many tools have been designed for specific
applications such as the various weld gauges.
• Some of the specialized tools such as crawlers have
been designed to satisfy the inspection needs in
applications where conventional techniques are not
feasible.

Equipment – Basic Measurements
• One of the most common tools
used in visual inspection is the
rule or scale.
• Used to measure linear
dimensions, when properly used
will measure within 0.015” or
1/64” and smaller.
• Rules are made in a variety
lengths, widths, and
thicknesses.
• They are graduated in common
fractions, decimal units, and
metric units, or combinations of
both.
• The specific type of rule is
typically chosen relative to the
application.

• Sliding calipers are a
precision refinement of the
common rule, which results
in greater accuracy of
measurements.
• They may incorporate either
a dial indicator or digital
readout.
• Sliding-type calipers are
commonly used to check
dimensional tolerances of
machined components, wear
on components, and fit
between components.
Equipment –
Precision Measurements

Equipment –
• Micrometers are precise
measurement instruments
used to make accurate direct
readings in contact
measurements.
• Micrometers are designed for
inside, outside, and depth
measurements, and are
available in a wide variety of
shapes and sizes.
• Micrometers may be either
thousandth inch (.001”) or ten
thousandth inch (.0001”)
measurement capable.

• Micrometers operate on the
principle that a precision
made screw with a pitch of
forty threads per inch will
advance one fortieth of an
inch (.025”) with each
complete turn.
• On a one inch micrometer,
the sleeve is marked
longitudinally with forty lines
to the inch which
corresponds to the number
of threads on the spindle.
Equipment –

• The reading line on the
sleeve is divided into forty
equal parts by vertical lines,
each designates 1/40th” or
.025” and every fourth line
denotes hundreds of
thousandths and is
numbered 1 – 0.
• The beveled edge of the
thimble is divided into
twenty five equal parts with
each representing .001”,
with every line numbered
from 0 -24.
Equipment –

Example:
– One major division on the sleeve
is visible, representing one tenth
of an inch.
– Two minor divisions are visible,
which each represent an
additional 25 thousandths.
– Line 15 on the thimble coincides
with the reading line on the
sleeve indicating that fifteen one
thousandths of an inch should
be added to the measurement.
– By adding all three values, the
micrometer reading is obtained.
Equipment –
0.100”
(2 X 0.025) 0.050”
(15 X .001) 0.015”
0.165”

Reading the ten thousandths micrometer.
• The ten thousandths micrometer incorporates a vernier scale.
• The vernier consists of ten divisions the sleeve, which occupies the
same space as nine divisions on the beveled edge of the thimble.
• The difference between one of the ten spaces on the sleeve and one
of the nine spaces on the thimble is 1/10th of a division or 0.0001”.
Equipment –
Example:
− The second major divisions (line 2) on the
sleeve is visible (2 x 0.100” = 0.200”)
− Three minor divisions after line 2 are visible
(3 X 0.025” = 0.075”)
− The beveled edge of the thimble is between
0 and 1 (0 x 0.001 = 0)
− The vernier scale has the sixth line on the
sleeve perfectly lined up with one of the
marks on the thimble ( 6 x 0.0001 = 0.0006”)
− The micrometer reading is: 0.2756”
0.200”
0.075”
0.000”
0.0006”
0.2756”

Equipment – Transferring Gauges
• Transfer instruments are
used to take measurements
which are transferred to
direct measurement devices.
• They consist of calipers,
dividers, telescoping gages
and small hole gages.

• Spring type calipers are available
for contact measurements of inside
and outside dimensions.
• They are useful for measuring
distances between and over
surfaces.
• They are commonly used to
transfer dimensions or sizes
between the work piece and
standard measuring devices, such
as graduated rules.
• The size of a linear or rounded
indication of a discontinuity may be
measured with dividers.

• Small hole gages are a type of transfer instrument used to
measure small holes or slots.
• They are generally supplied in sets with a range of 1/8” - 1/2”.
• The actual measurement is determined by transferring a
properly adjusted gage to a micrometer.

• Telescoping gages make
inside measurements such as
hole diameter and slot width.
• They are designed to be
measured by a micrometer
after being set to the hole or
slot size.
• To make accurate
measurements it is important
to make sure the telescoping
gage is aligned properly in
the measuring faces of the
micrometer.

Equipment – Screw Pitch Gage
• The screw pitch gage is a
basic visual aid for checking
the number of threads per
inch and rough inaccuracies
of threads.
• The gage consists of a steel
case with a number of folding
leaves at each end.
• Each leaf is number and
contains teeth corresponding
to a specific thread pitch.

Direct and Remote
Visual Inspection
• Many codes refer to direct visual examination as a
visual inspection which requires that access to the
area is sufficient to place the eye within 24 inches of
the surface to be examined and at an angle of not
less than 30º to that surface.
• If these requirements cannot be met, then remote
visual inspection may be used.
• Remote visual inspection may be accomplished with
the use of a number of optical aids such as, mirrors,
magnifiers, and rigid or flexible borescopes.

Optical Aids
• Mirrors are valuable aids in visual
inspection, they allow the
inspection of threaded and bored
holes, inside surfaces of pipes
and fittings, as well as many
others.
• Magnifiers assist the visual
inspector by enlarging the size of
the object being examined.
• Comparators are a magnifier with
a measuring capability. The
comparator has interchangeable
reticles which provide
measurements for threads,
angles, linear measurement,
diameters and radii.

Optical Aids
• Borescopes are visual aids used for the inspection of internal
surface areas.
• They are designed for remote viewing in difficult to reach
areas such as jet engines, cylinders, tanks, and various
enclosed chambers.
• Borescopes are available in many different diameters and
lengths, and are classified as rigid or flexible.

Visual Inspection With A Borescope
Clean Surface Corrosion Damage

Optical Aids
• Advances in technology has allowed video equipment
to be adapted to portable and robotic devices.
• Portable video probes allow inspectors to remotely
perform examinations in closed chambers which are
inaccessible by convention inspection means.
• Robotics have been developed whereby cameras can
be affixed to crawlers and submersibles.
– Retrieval tools can be affixed to robotics to remove
foreign objects.
• Conventional recording techniques may
be employed for image capture and
storage with many of the remote video
inspection methods.

• Machine vision technology uses an imaging system
and a computer to analyze an image and to make
decisions based on that analysis.
• In inspection applications, the machine vision optics
and imaging system enable the processor to "see"
objects precisely and thus make decisions about which
component meet a specific inspection criteria.
• Machine vision can eliminate human factor error that
might result from doing difficult, tedious, or boring
tasks. It also allows process equipment to be utilized
24 hours a day.
Machine Vision – Basic Principles

The following process steps are common to all
machine vision applications:
• Image acquisition: An optical system gathers an image, which is
then converted to a digital format and stored into computer memory.
• Image processing: A computer processor uses various algorithms
to enhance elements of the image that are of specific importance to
the process.
• Feature extraction: The processor identifies and quantifies critical
features in the image (e.g., the position of holes on a printed circuit
board, the number of pins in a connector, the orientation of a
component on a conveyor) and sends the data to a control program.
• Decision and control: The processor's control program makes
decisions based upon the data. Are the holes within specification?
Is a pin missing?
Machine Vision – Basic Principles

Machine Vision - Applications
As mentioned previously, machine vision is primarily used in
production applications where a large number of components
require inspection and the inspection conditions can be closely
controlled. Uses include:
• Assembly verification (caps, fasteners,
electronic board components, etc.)
• Surface inspection (dents, scratches,
porosity.
and other undesirable features)
• Verification of colors, gradients, patterns
in fabrics and labels.
• Confirmation of proper labeling for
medications, foods and other products.
• Inspection of coating coverage.
• Feature measurements.
Assembly Verification
Spark Gap Measurement

Key System Elements
• A variety of components are
included in a machine vision
system, which depend on the
environment, the application,
and the budget. However, the
following components are
common to all vision systems :
– Front-end optics: this includes the lighting, the lens, and the
camera.
– Frame grabber: this is a computer processor board that accepts
the video input from the camera, digitizes it, and stores it for
analysis.
– Processor: A computer processor is required to control the
vision application.
– Control Software: Computer software is used for controlling and
executing vision tasks.
Machine Vision - Equipment

Advantages of Visual Inspection
• Readily used on almost all materials.
• Simple to perform.
• Low in cost, (application dependent).
• Relatively quick.
• Results may be permanently recorded.
• Can be automated.

Limitations of Visual Inspection
• Direct inspections are limited to surfaces only.
• Indirect inspections require greater inspector
knowledge and training.
• Inspector dependent, knowledge of materials and
processing, eye sight.
• Standards (workmanship) may be difficult to obtain.

Glossary of Terms
• Borescope: A visual inspection aid used for the inspection of
internal surfaces. Borescopes are rigid and flexible.
• Corrosion: The deterioration of a metal by chemical or electro-
chemical reaction with its environment.
• Defect: A discontinuity which interferes with the usefulness of a
part. A fault in any material or part which is detrimental to its
serviceability.

Glossary of Terms, Cont.
• Direct Visual Inspection: Visual inspection conducted where the eye
can be place within 24” of the area to be inspected at an angle of not
less than 30˚.
• Discontinuity: An interruption in the normal physical structure or
configuration of a part.
• Foot Candle: The amount of direct light thrown by one standard
candle on a surface one foot away.

• Inherent Discontinuity: Discontinuities which are ordinarily normal to
the material at the time it originally solidifies from the molten state.
They consist of porosity, inclusions, and pipe.
• Magnifier: An optical glass which enlarges the real size of an object
being inspected
• Micrometer: A precise measuring instrument used to take accurate
readings in contact measurement.

• Pocket Comparator: A magnifier with measuring capability through a
system of interchangeable reticles.
• Power of Magnification: The amount that the real size of an object is
enlarged. The power if designated by “X”.
• Remote Visual Inspection: Inspection conducted where the eye
cannot be placed within 24” of the area to be inspected, or at an
angle less than 30˚.

• Reticle: A series of lines, dots, cross hairs, or wires in the focus of
the eyepiece of and optical instrument.
• Rule or Scale: A measuring device used to make linear
measurements.
• Screw Pitch Gages: A visual aid used to check the number of
threads per inch and rough inaccuracies in threads.

• Secondary Processing: Metal removal processes, heat treatment
and plating processes used to produce and article of the desired
shape and finish from the formed material.
• Service Discontinuity: Discontinuities induced in components after
they have been placed in service.
• Transfer Instruments: A group of instruments used to take
measurements which are transferred to precise direct measuring
devices. They consist of calipers, dividers, telescoping gages, and
small hole gages.

• Vernier Scale: A device for indicating a fraction of a whole division
of a scale reading
• Visual Scale: The ability to see. The keenness of perception.
• Wear: Wear is deterioration due to use. Wear will cause a
reduction in cross section and strength.
• Weld Inspection Gages: A group of measuring devices used to
inspect welding fit up and finished weldments.

Intro_to_Visual Inspection.ppt

Recommended

Recommended

More Related Content

Similar to Intro_to_Visual Inspection.ppt

Similar to Intro_to_Visual Inspection.ppt (20)

Recently uploaded

Recently uploaded (20)

Intro_to_Visual Inspection.ppt