Surface indication
measurement
Using the remote visualinspection non-destructive
evaluation method forsurface indication
measurement
precision andaccuracy

A video borescope can meet your advanced measurement needs using the Remote Visual Inspection (RVI) non-destructive
evaluation (NDE) method. This paper will answer many of your questions about measurement precision and accuracy, including:• What is the smallest error that can be expected under optimal conditions with Waygate Technologies Real3D™ Stereo& Real3D™ Phase measurement?
• Which is better—Real3D Stereo or Real3D Phase structured light?
• Can Real3D measure on your specific surfaces?
• How small of an indication can be measured and with what precision and accuracy?
• What is MTD and does it affect accuracy?
• Does “measurement performance” differ between the various Optical Tip Adapter options?
• How do the various measurement types affect measurement uncertainty?
For further details on this topic, the Waygate Technologies’ Real3D™ Measurement Handbook is an excellent resource.
The intent of this paper is to better help one understand advanced measurement technology with a video borescope
and achieve the answers to the questions above on the items, assets, areas of interest one is considering
measuring various surface indications upon using the Remote Visual Inspection (RVI) NDE method.
For detailed reference on measurement types and best practices,
please consult the Real3D Measurement Handbook.

Making the right decision

sequentially repeated during an inspection event, enabling
the Auto Repeat feature decreases the measurement setup
time each time a measurement image is captured.

When making safety-of-flight or Fit for Service/continued
operation decisions, you likely will need to accurately
determine the size, area, or depth of an indication, theamount of missing material, or a blade tip clearance.
Waygate Technologies’ Real3D Measurement technology
provides the quality data you need to help you make thebest possible decision.

A VideoProbe from Waygate Technologies may support
various measurement technologies including Shadow,
traditional Stereo, Real3D Stereo and Real3D Phase. Whether
you choose one of these technologies, or one offered byanother supplier, there is only one correct answer to questions
regarding achievable precision and accuracy: It depends!

Unmatched and unavailable in any other video borescope,Waygate’s Mentor Visual iQ VideoProbe™ has 1.2 mega-pixel
imaging and patented 3D structured light measurement witha fully surfaced point cloud.

What you need to know
Many application-dependent factors impact the accuracyof Stereo, Real3D Stereo, and Real3D Phase measurements.
These factors include:

The reliable, precise, and accurate Mentor Visual iQ cansave significant time and millions of dollars by uncovering
component flaws before they become a serious problem. Inaddition, it can generate a detailed catalog of component
degradation, thereby affecting subsequent component
design considerations.

• Measurement data viability– Real3D Phase Measurement (3DPM) generates a plurality
of XYZ points using structured light fringe patterns
projected on a surface. These are then processed in analgorithm that generates a fully surfaced point cloud
representing points on a surface to be measured. The
white light image can be out of focus and a viable XYZdata set still may be generated.

For reference, eight measurement types are available on theflame-colored Mentor Visual iQ VideoProbe:• Length			• Multi-segment• Point to line		• Depth profile• Depth			• Area depth profile• Area			• Blade tip clearance– Stereo and Real3D Stereo measurement work on theprinciple of matching pixels in stereoscopic images using
a 3D coordinate system. Cursor placement in both images
is critical; they must both be on the same pixel to obtainmeaningful results. Thus, the image to be analyzed witheither measurement type must be in sharp focus.

Several of the above-mentioned types can be assisted byMeasurement Plane which allows for measurements using amathematical plane. If any of these measurement types is2 of 6• Tip-to-target distance
All current 3D measuring borescope systems rely on
triangulation. As the tip-to-target distance increases, theaccuracy of the computed 3D surface points decreases.
Thus, measurements made with the borescope tip close
to the surface generally will be more accurate than
measurements made from farther away.

systems to determine accurate 3D surface points.
• Aspect ratio
Systems may be unable to both view and illuminate thebottom of narrow, deep features with either structured lightpatterns or normal illumination, making them unable todetermine accurate 3D data at the bottom of such features.
Through proper training, RVI inspectors who must performmeasurements can learn how to:This is especially true of all depth-type measurements.
Real3D measurement technologies provide a Maximum
Target Distance (MTD) value, indicating the distancefrom the measurement tip adapter to the viewed surface
at the location of the farthest cursor in a measurement,and advise the user when the MTD is excessive for thatmeasurement type and size.

• Choose the most appropriate measurement
technology, optical tip adapter, and measurement typefor a given application.
• Position the probe tip to maximize the fidelity of the3D surface data generated by the system and used
to compute measurement results.

• Incident angle
The angular relationship between the probe tip and thetarget feature can have a profound effect on the resultant
measurement calculation. Try to avoid situations where
this angle exceeds approximately 60 degrees. Re-adjust
the probe or perhaps the target component if possible. Ifcircumstances prevent both possibilities, then consider
using an alternative measurement tip such as the forward
or side-facing option.

• Place cursors at the correct locations on an indication in
the image to obtain the desired dimensions.
• Leverage the 3D Surface Mask and fully surfaced 3D
point cloud view to evaluate 3D data quality andcorrectness of cursor placement when using Real3D
Stereo or Real3D Phase.
Given the above factors and variability in inspector skill
levels, the best way for an organization to establish expectedprecision and accuracy on any given surface type and/orexpected indication type is to do a Gage Repeatability andReproducibility (R&R) study on a part that is representative of
the actual inspection requirement. The inspectors doing theR&R study should have similar skill levels as those who will be
performing the actual measurements.

• Surface conditions
Shiny surfaces often result in glare and/or dark images
that make it difficult for systems to accurately determine
3D surface coordinates. Smooth surfaces with little texture
may be problematic for stereo systems, which rely on
differentiating surface detail to determine 3D surface
data. Oily, transparent, or translucent surfaces also canbe challenging.

The American Quality Society (AQS) has a description of thatprocess on its website.

• Reflections
Polished surfaces such as shiny turbine compressor blades
and shrouds or high-purity piping may reflect mirror
images of other parts/areas or cause projected patterns
to bounce between different surfaces, making it difficult for

On precision and accuracy
The Real3D™ Measurement Handbook provides detailed
information on Real3D measurement technologies
including accuracy curves, which were developed througha study performed by a third party. These represent thetechnologies’ capability when used by a well-trained
inspector under ideal conditions.

3 of 6

The best way to establish expected precision and accuracy
on any given surface type and/or expected indication type isto do a Gage R&R study on a part that is representative of theactual inspection requirement.

The Mentor Visual iQ displays measurement results witha precision of 0.01 mm (0.0004”) in mm mode or 0.001”(0.0254 mm) in inch mode. While all displayed digits may besignificant on small measurements made from very small
tip-to-target distances, the right-most digits may not bemeaningful on measurements made from larger distances.

Under ideal conditions, and for several indication types andsizes, the relationship between expected measurement
accuracy at a given MTD is published in the Measurement Data
Accuracy section of the Real3D™ Measurement Handbook.

Keep in mind that measuring in millimeters allows for a more
precise value:

While a lower MTD is desirable, neither a high nor low MTD
ensures the captured measurement data is viable. MTD is adata point providing more information for the analysis process.

• 0.0100 mm = 0.0004”• 0.0254 mm = 0.0010”
The Mentor Visual iQ shows two decimal points in mm(x.xxmm) and three decimals in inches (x.xxx”).

For example, see Figures 1 and 2 from a third party’s Gage
R&R study, as seen in the Real3D™ Measurement Handbook.

It is recommended that 0.125mm (0.005”) be considered the minimum feature size for measurement.
Under ideal conditions, a general guideline to consider when measuring these smaller features is that Real3D Phase andReal3D Stereo measurement usually can achieve errors in the +/- 0.05mm (+/- 0.002”) range or better on both length anddepth type measurements.
Caution: This does not express or imply that you will be able to achieve these results for all measurements attempted. A Gage
R&R study will determine expected results for any given measurement requirements using a VideoProbe.
Ideal conditions vary by measurement technology as described below.
Real3D Phase Measurement ideal conditions• Measurements by a trained, qualified RVI technician• Clean and calibrated optical tip adapters (OTAs) and camera lens
• Camera positioned very close to the indication, even if slightly out of focus• Camera positioned in a non-perpendicular incident angle to the surface• Measurement upon a matte, non-reflective surface finish• Minimal or eliminated reflections and shadows of structured light• Absence of ambient light• Systems may be unable to both view and illuminate the bottom of narrow, deep features with either structured light patterns
or normal illumination, making them unable to determine accurate 3D data at the bottom of such features.
Real3D Stereo and Stereo Measurement ideal conditions:• Measurements by a trained, qualified RVI technician• Clean and calibrated OTAs and camera lens
• Camera positioned very close to the indication, and in sharp focus• Measurement on surfaces that are feature-rich and not bland• No glare on the indication• Pixel pattern differentiation around the indication

While a lower MTD is desirable, neither a high nor low MTD ensures the captured measurement data is viable. MTD is a data
point providing more information for the analysis process.

4 of 6

For example, see Figures 1 and 2 from a third party’s Gage R&R study, as seen in the Real3D™ Measurement Handbook.
Pt-Line error vs. MTD

Length error vs. MTD

5.00%

0.003

4.00%
3.00%

0.002

2.00%
0.001
01.00%

0.2

0.4

0.6

0.8

1

1.2

0.00%

0.004
1.50%

0.003

1.00%

0.002

0.50%

0.001
00.2

0.4

MTD (in)
3DPM

0.6

0.8

11.2

0.00%

Average absolute error (%)

0.004Average absolute error (in)Averaged over 0º to 50º viewing angle
0.21” (5.33 mm) long feature
Average absolute error (%)

Average absolute error (in)Averaged over 0º to 50º viewing angle
0.071” (1.803 mm) edge ding feature

MTD (in)
3DST

3DPM

Figure 1

3DST

Figure 2

Note that in Figure 1, a lower MTD yielded a lower Average Absolute Error for 3DPM. However, in Figure 2, a lower MTD did not yielda lower Average Absolute Error for 3DPM.
In addition, increasing MTD has a much greater effect on depth measurement types than it does on length or area types. Thisis because the Z value of a given pixel becomes more difficult to calculate as the tip-to-target distance increases (greaterMTDs). The main reason: A pixel on an imager has differing X and Y values based on a given Z (MTD) value and the actual sizeof the imager’s pixels.
Additionally, precision and accuracy of an indication’s measurement values are affected by several variables, one of which isMTD. Another is interpreting the surface and not having measurements tilted to the desired plane.
This is where the value of a fully surfaced 3D point cloud is a phenomenal analysis tool in Real3D measurement. See Images 1and 2 below as an example.
Image 1 is a typical view of a traditional stereo measurement. In this case, the blade tip clearance—a jet engine’s turbine blade
clearance to the shroud—is being measured. Using a Depth measurement, three selected points establish a mathematicalreference plane on the shroud. The distance (clearance) from the tip of the blade to the Depth plane on the shroud is shown as
1.24mm (0.049”).
The goal is to place a cursor on the left image, and the processor must precisely match the same pixel on the right image.
Many qualified technicians have looked at this image and have said it exhibits good technique.

Image 1

Image 25 of 6

Image 2 depicts this same image using the Real3D Stereo
Measurements’ fully surfaced point cloud in the right halfof the image. Obviously, based on cursor placement, thismeasurement is both very accurate and very wrong! Oneof the Depth reference plane’s cursors is on the blade andnot on the shroud with the other two cursors. The results arethat the measurement plane is tilted to the shroud yielding
a gross error. In this case, it could lead to a $1 million-plus
engine removal and replacement—or worse.
In Image 3, when the errant cursor is moved, in either thewhite light image or the point cloud and from the blade to
the shroud, all three cursors of the Depth reference plane
are on the shroud. In addition, the shroud is now aligned withthe reference plane (the dark blue square frame around thepoint cloud data), and the depth cursor (fuscia colored) is onthe blade tip and now perpendicular to the Depth plane andshroud. The result is now 1.56mm, a significant difference of0.32mm (0.013”). Also, note that all pixels on the shroud thatare very close to the Depth plane are now masked in green.

Image 3

Summary
Many factors can affect the precision and accuracy ofmeasurements in each application. Training is critical tomaking reliable measurements. While we can provide
guidelines and accuracy curves that reflect systemcapability under ideal conditions, those guidelines/curves
may not represent achievable accuracy under different
conditions at your site. We therefore recommend that trainedand qualified inspectors/technicians perform a Gage R&R
study for critical applications.
The Mentor Visual iQ Ø6.1mm system with Real3D Stereo
measurement and Real3D Phase measurement using
structured light has been evaluated by a third party using aGage R&R process. The results are published in the Real3D™
Measurement Handbook.
Real3D measurement, both Stereo and Phase structured
light, has proved to have excellent precision and accuracy

Copyright 2021 Baker Hughes Company. All rights reserved.
BHCS38986		

(05/2021)

in hundreds of applications globally in aerospace, power
generation, petrochemical and general manufacturing.
Enhanced precision of measurement results is enhanced bychoosing a Ø6.1mm or Ø8.4mm Mentor Visual iQ VideoProbewith 1.2 mega-pixel high definition (HD) cameras. In some
applications, this allows inspectors to place cursors more
precisely on an indication in a measurement image.
Since the measurement technologies and types are thesame for standard definition (SD) and high definition (HD)cameras, accuracy is expected to be about the same.
Finally, do not overlook the importance of trained and qualified
inspectors performing any Non-Destructive Inspection ortesting method, including Real3D measurement with aVideoProbe, or any measuring video borescope. A trained andqualified RVI inspector should achieve the best possible results
on a given surface for a given indication type.

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