intro_to_visual inspection

Visual Inspection

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.

Basic Principles – 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 /2)2

b

2 ’

d d’

n

b’

c’

~425 by 425 cones

180,000 Cones

1 mm

1 mmX

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.

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

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

Basic Principles – 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.

Basic Principles –Light Intensity Measurement

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.

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.

Basic Principles – 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.


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.


Over 80 percent of all aircraft inspections are performed visually.


• 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


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.

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.

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


• 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.


• 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.


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”

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”

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.

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.

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.

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.

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 Inspection

• 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

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