Visual Inspection

1
Visual Inspection
2
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

3
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.

4
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.

5
Introduction

• 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.

6
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.

7
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.

8
Basic Principles The Human Eye
The retina is a mosaic of two basic types of
photoreceptors, rods and cones.

• 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.

9
Basic Principles Visual Acuity

• 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.

10
Basic Principles Visual Acuity
There is a limit to what the unaided eye can see.

• 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

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.)
11
Basic Principles 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 lighteror darker than the
  background

The graph to the right plots thevisibility of a
spot as a function of theabove variables
12
Basic Principles Contrast Sensitivity
It should be noted, however, that larger objects
are not always easier to see than smaller objects
as contrast is reduced.

• 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.

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
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.
13
Basic Principles Light Levels

• 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.

14
Basic Principles Light Intensity Measurement

• 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.

15
Basic Principles 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.

16
Basic Principles Perspective
The eye/brain need visual clues to determine
perspective.
Is the book facing towards or away from you?
17
Basic Principles Optical Illusions
Sometime the eye/mind has trouble correctly
processing visual information.
Are the horizontal lines parallel or do they
slope?
How many black dots do you see?
18
Basic Principles Vision

• 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.

19
Basic Principles

• 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.

20
Inspection Applications
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

• 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.

21
Inspection Applications Flaw Detection

• 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.

22
Inspection Applications Flaw Detection
In-service inspections of existing components and
structures is commonly accomplished visually.

• 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.

23
Inspection Applications Flaw Detection

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

24
Inspection Applications Flaw Detection
Over 80 percent of all aircraft inspections are
performed visually.
25
Inspection Applications Flaw Detection

• Weld quality requirements are commonly determined
  through visual inspection.
• Many standards have established acceptance
  criteria for welds.

Transverse weld crack
Slag rolled into toe of weld
26
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.

27
Dimensional Conformance

• 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.

Palmgren gage
Fillet gage set
VWAC gage
Cambridge gage
28
Dimensional Conformance

• 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.
29
Dimensional Conformance

• 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.
30
Dimensional Conformance

• 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.
31
Dimensional Conformance 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.
32
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.

33
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.

34
Equipment Precision Measurements

• 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.

35
Equipment Precision Measurements

• 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.

36
Equipment Precision Measurements

• 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.

37
Equipment Precision Measurements

• 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.

38
Equipment Precision Measurements

• 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.

0.100 (2 X 0.025) 0.050 (15 X .001)
0.015 0.165
39
Equipment Precision Measurements

• 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.

• 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
40
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.

41
Equipment Transferring Gauges

• 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.

42
Equipment Transferring Gauges

• 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.

43
Equipment Transferring Gauges

• 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.

44
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.

45
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.

46
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.

47
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.

48
Visual Inspection With A Borescope
Clean Surface
Corrosion Damage
49
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.

50
Machine Vision Inspection
51
Machine Vision Basic Principles

• 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.

52
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?

53
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
54
Machine Vision - Equipment

• 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.

55
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.

56
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.

57
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.

58
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.

59
Glossary of Terms, Cont.

• 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.

60
Glossary of Terms, Cont.

• 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.

61
Glossary of Terms, Cont.

• 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.

62
Glossary of Terms, Cont.

• 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.

63
Glossary of Terms, Cont.

• 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.