GE
Inspection Technologies
Remote Visual Inspection Products
REMOTE VISUAL INSPECTION (RVI)
MEASUREMENT TECHNOLOGY BEST PRACTICES
GENERAL COMMENT
Use The Latest Remote Visual Inspection Technology. Video borescopes which are used to perform
defect measurements have continued to improve over the last 20 years. Earlier generation video
borescopes do not have many of the advanced measurement features and core image resolution
capability as the current generation of video borescopes. The latest generation of video borescopes
provide more accurate measurements than previous generation products, and are generally easier
to use and train users on their operation. Latest generation RVI products also provide indications to
the operator of attributes of the image and measurement which contribute to greater measurement
accuracy.
USE OF COMPARISON OR SCALAR MEASUREMENT CAUTIONS
Most video borescopes support a comparative or scalar measurement technique whereby
measurements are performed by using a detail in the inspection view of a known dimension ( such as
the thickness of a safety wire introduced into the field of view, or the distances between cooling
holes) as the reference for performing subsequent measurements. This measurement technique
requires that all measurements be made on the same plane as the source of the reference
dimension – and that this plane must be perpendicular to the video probe’s tip. If measurements
are made on surfaces not in the same plane as the reference and perpendicular to the video probe,
the optical magnification at the measurement locations could be greater or lesser than the reference
point resulting in a significant measurement error which may grossly overstate or understate
dimensions of an indication.( see illustration on next page showing measurement inaccuracies
when probe not perpendicular to target)
Best Practice: Whenever possible, use the measuring function of the video borescope which uses
the specialized Stereo, 3D Phase, or Shadow probe measurement technologies which require the
installation of optical tip adapters designed to support these measurements – and do not use the
scalar reference measurement technique. Measurement images which result from the use of a
measurement technique which was performed using specialized optical tips can be identified by the
inclusion of the serial number of the optical measurement tip in the measurement image.
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EFFECT OF NOT HAVING COMPARISON MEASUREMENT IMAGE
PERPINDICULAR TO PROBE TIP
( All measurements should be ~ 0.250” – the same as the reference measurement “C”)
Stereo
Measurement Image
Shadow Probe
Measurement Image
3D Phase
Measurement Image
TYPICAL MEASUREMENT IMAGES SHOWING MEASUREMENT TIP SERIAL NUMBERS IN THE IMAGE
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STEREO MEASUREMENT BEST PRACTICES
STEREO MEASUREMENT FUNDAMENTALS
Stereo Measurement technology is based upon the use of specialized measurement optics on the
VideoProbe which splits the image into two images – with each image having a slightly different
optical viewing angle toward a target area of interest. These two images are shown on the video
borescope as two images, side-by-side on the video borescope’s LCD monitor. Stereo measurements
are performed by having an operator position a measurement cursor on the left half-screen image
and the video borescope software automatically positioning a cursor in the right half-screen image.
The video borescope uses a form of pattern recognition software to correctly match and locate its
cursor in the right half-screen image The video borescope is able to calculate measurement results
through embedded software that uses information concerning the optical characteristics of the
measurement tip, and the location of “matching cursors” on the LCD monitor screen. If the stereo
image is not crisp and sharp, the video borescope’s software will not be able to accurately perform
this matching process, resulting in it incorrectly placing its matching cursor – resulting in an
erroneous measurement.
CAPTURE A SHARP, CRISP IMAGE WITH SIGNIFICANT SCENE DETAIL


Use a clean probe - Clean the lens on the end of the VideoProbe ( use the special mini- cotton-tipped
cleaning sticks)
Use a clean stereo tip - Clean the stereo tip
COMPOSE YOUR MEASUREMENT IMAGE IN ORDER TO MAXIMIZE THE SYSTEM’S ABILITY TO MEASURE –
GET AS CLOSE AS POSSIBLE TO THE DEFECT WITH THE PROBE

Get the Tip as Close as Possible to the Target
o Navigate the video borescope as close as possible to the measurement target, try to fill the
screen with the indication
o CAUTION - do not position the probe tip so close that the image becomes out of focus
o In general, try to get the tip to within one half-inch or closer to the target, or achieve a
magnification of 5.0 or greater (measurement software on the video borescope will provide
either actual tip to target distance, or a magnification index value).
o If defect is so large an INDEX of 5.0 is not achievable ( because probe must be positioned
further away in order to capture full defect) - position the probe as close as possible to the
indication
Probe Tip Too Far Away From Indication
Probe Tip Closer to Indication
Probe Rotated to Fill Screen
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Reduce Reflections and Glare
o
o
o
Stereo Measurement does not require the probe to be straight-on to the indication – Stereo
can measure from any angle.
Since stereo software relies on pattern-matching, reflections and glare may show up in
different locations on the left and right half-screen images, potentially causing the matching
software to operate incorrectly and position its cursor erroneously
Stereo measurement does not require the target indication to be measured to be
perpendicular to the probe tip – in fact being slightly off-axis from perpendicular may help to
reduce glare and reflections.
Angle of Probe to Indication
Creates glare in image
Probe Rotated to Reduce Glare
on upper Leading Edge
USE A VIDEO BORESCOPE WITH MAXIMUM LIGHT OUTPUT


The sharpness of an image is dependent on having adequate light shined on the area of interest. A
video borescope with greater light output will generally be able to provide a sharper image, thus
allowing better Stereo cursor matching by the system.
If the video borescope uses a replaceable lamp, insure the light output of the lamp is within
specification (replace the lamp if necessary)
PLACE CURSORS CAREFULLY AND ACCURATELY

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Use the “ Zoom Window” feature of the video borescope to zoom in on the area where the cursors are
being placed in order to accurately place the cursor
Errors from incorrect cursor placement become much more problematic ( the measurement error gets
greater if the cursor is off by even one pixel position) the further a tip is from the target area of interest
Depth Measurements are extremely susceptible to cursor placement errors as the reference plane
must be created accurately ( and not be tipped) in order for the depth or height above the reference
plane to be calculated
VISUALLY VERIFY THE CURSOR MATCHING BY THE VIDEO BORESCOPE


After the operator has placed a cursor in the left half-screen, visually verify that it appears that the
video borescope has correctly placed its cursor in the correct corresponding location in the right halfscreen
If the video borescope supports a feature to indicate a confidence level of the matching cursor
placement ( on a scale of 0 to 5) , always try to obtain a match strength figure of three <3> or greater
PERFORM CRITICAL MEASUREMENTS AT LEAST TWICE

For critical measurements, always perform the measurement twice – capturing a second image and
performing the measurement a second time, verifying the two measurements are consistent
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VERIFY MEASUREMENT ACURACY CAPABILITY OF VIDEO BORESCOPE
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
Before performing a measurement on an asset, perform a “Measurement Verification” process by
measuring the calibrated NIST-traceable test target block included with the measuring video
borescope.
Insure that the video borescope can accurately measure the test target dimensions before and after
every critical measurement.
USE ONLY MEASUREMENT TIPS WHICH ARE CALIBRATED TO THE VIDEO BORESCOPE

Measurement tips must be calibrated to a given video probe. If you perform a measurement using a
measuring tip which has not been calibrated to the probe – you will get errors in measurements - or
the system will not allow measurements (depending on video borescope model)
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3D PHASE MEASUREMENT BEST PRACTICES
3D Phase Measurement technology captures a full surface scan of a target area and created a 3D
point cloud representation of the area of interest. Al measurements are performed by the video
borescope’s software in this 3D Point Cloud Space. Most of the best practices for Stereo
Measurement apply for 3D phase measurement as well. The following items are unique
characteristics of the 3D Phase Measurement process
COMPOSE YOUR MEASUREMENT IMAGE IN ORDER TO MAXIMIZE THE SYSTEM’S ABILITY TO MEASURE
As with Stereo Measurements:




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Get as close as possible to the target area of interest
Measurements from a tip to target distance of 0.5 inches or less are preferred ( however
measurements from 2.0 inches away are possible
Slightly out of focus images are acceptable if the tip is very close to the target – as long as the cursors
can be placed accurately
Minimize reflections of the phase pattern back into the probe by positioning the probe at an angle to
the indication being measured ( straight-on viewing is not recommended)
3D Phase measurement is also capable of performing measurements from any incident angle.
USE THE POINT CLOUD TO VERIFY CRSOR PLACEMENT


Always view the point-cloud image to verify you have placed your cursors accurately on the area of
interest
Verify that the area of interest shows up clearly defined in the point-cloud view
Wrong !
Measurement View
Point Cloud View
Rotated Point Cloud View
(notice measurement not vertical)
Correct !
Measurement View
Point Cloud View
Rotated Point Cloud View
(notice measurement is vertical)
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MAXIMIZE THE SIGNAL STRENGTH

Move the probe close as possible to the indication - trying to maximize the signal-strength bar-graph
icon on the measurement screen
CHOOSE THE CORRECT MEASUREMENT TECHNIQUE


Understand when a depth profile measurement is appropriate to use – and when a traditional 4-point
Depth measurement should be used (as in previous illustration)
Do not use Depth Profile measurement on curved surfaces – reference cursors for Depth Profile
measurement must be on the same plane. Position Depth Profile cursors along length of a surface –
not across curved surface.
0.003”
corrosion pit
depth
3D Phase Measurement Showing Measurement View, Profile View, and Point Cloud View

measurements Use of traditional 4-point traditional Depth measurement should be used, with the
reference plane surrounding the indication
-----------------------------------------------------------------------------------------------------Thomas D. Britton
Aviation Applications Specialist
GE – Inspection Technologies
Remote Visual Inspection
721 Visions Drive
Skaneateles, NY 13152
T 315 554-2056
M 315 317-6490
E tom.britton@ge.com
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