Progress in On-Aircraft Application of Thermography

Dr. Steven ShepardThermal Wave Imaging, Inc.

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Thermography System Evolution

1997

2006

1992 2011

2001

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Anatomy of a Thermography System

Detector

Excitation

Processing

Application Requirements

Physics

Inspection

NDT System

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Today…

• Frame rate: 30 – kHz• Array size: 64x64 – 1k x 1K• NETD 20-2000 mK• Cooled / uncooled• Cost: $2K - $250K

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Many Excitation Choices

500 W 1500 W 4800 J [watt-sec]

250 W600 W / m210 W / in2

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Best Solution?

FlashClose proximityNon-contactFOV: ~ 1 sq ft

ProjectionLong range (45’)Non-contactFOV: ~ 6 sq ft

Hot airClose proximityNon-contactFOV: ~ 2 sq ft

VibroClose proximityContactFOV: large

ScanningClose proximityNon-contactFOV: large stripe

All of these approaches detect the impact damage successfully, but they vary in sensitivity, cost, working distance, coverage area and inspection time.

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On-Aircraft Inspection Requirements

• Performance • Area coverage • Size / weight• Ease-of-use• Cost• Cost• Cost

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On-Aircraft Inspection Requirements

• Performance – Demonstrated for many applications– Some applications require laboratory scale systems

• Area coverage – Inherent feature of thermography

• Size / weight• Ease-of-use• Cost• Cost• Cost

Critical issues

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Optical Excitation

• Noncontact• Well-suited to area excitation• Pulse heating (xenon flash lamp)

– Precise high energy pulse facilitates high performance– Size, weight, cost issues

• Step Heating (halogen lamp)– Low cost– High power when applied over longer duration– Some applications may be inaccessible

Conventional Optical Heating IR NDT

reflector

IR camera

lamptarget

emitted IR

light

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Conventional Optical Heating IR NDT

reflector

IR camera

lamptarget

emitted IR

Visible + IR

reflected IR

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Spectral Filtering of Lamp Output

reflector

IR camera

halogen lamp (visible + IR)

IR filter target

emitted IR

visible

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Halogen Lamp Spectral Distribution

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Integtrate Planck Equation

Integrating Planck Equation over visible range (4-7 um):

temperature efficiency 3000 K 8.1% 3200 K 10.5% 4000 K 20.8% 5000 K 31.6% A typical 1500 W halogen lamp puts out ~ 150 W of visible light!

typical

Halogen lamps are inefficient generators of visible light!

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Spectral Distribution and Efficiency

visiblevisible + IR

filter

visible + NIR visible

INPUT OUTPUT

Solving Planck Equation for visible range (4-7 um):

temperature efficiency 3000 K 8.1% 3200 K 10.5% 4000 K 20.8% 5000 K 31.6% A typical 1500 W halogen lamp puts out ~ 150 W of visible light!

typical

Ref: Carl Zeiss

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Where Does Blocked Energy Go?

• Heat applied to one side of the filter passes through to the outer surface• This is the same heat conduction mechanism thermography is based on • Long wave IR (5-12 um) is emitted from the outer surface• Time constant for passage through filter is similar to inspection time scale• By blocking NIR, the filter creates a LWIR source.

visiblevisible + NIR visible + LWIRvisible +NIR

time

visiblevisible + NIR

filter

t0 t1 t2

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Uniformity, Efficiency and Lamp Geometry

Reflective optics can achieve excellent uniformity for a point source.

Point source

Parabolic reflector

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Uniformity, Efficiency and Lamp Geometry

• Actual lamps are not point sources, and may not be at exact focus of reflector. Collimation and uniformity suffer as source becomes larger.

• With a line source, a simple reflector illuminates a stripe. Diverging components may not hit the target.

• Can improve line source area uniformity with multiple lamps and reflectors in a reflective cavity, but system becomes larger.

Parabolic reflector

Line source

Diverging

Collimated

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Conventional Step Heating IR NDT System

• Poor efficiency– IR component of lamp output is blocked by filter.

Net efficiency is approx. 10%

• Non-uniform heating of target– Linear lamp array with reflecting enclosure required for

uniform heating of 2D area. Much of the output of a single linear tube does not hit the target

• Reflection artifacts– Light passing through raises filter temperature– Black paint may be required for reflective surfaces

• Transient reflection artifacts– Elevated temperature in reflector heats filter wall– Heat conduction through filter results in delayed

temperature increase on outer filter wall

• Slow onset and decay of heating– Processing methods are based on rectangular step

reflector

IR camera

Halogen lamp (visible + NIR)

IR filter target

emitted IR

visible

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Lamp and Focusing Reflector

Lamp

Focusing Reflector

• High intensity lamp, e.g. halogen

• Source element to allow focusing

• Reflector surface optimized for visible and IR wavelengths

Focal Point

Visible and IR beam

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Direct Visible and IR Onto Surface

Reflector

Lamp

Focusing Reflector

Focal Point

Visible and IR beam

Target

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Reflector Motion

Reflector rotates between open and closed positions

Beam directed away from target

Closed position

Target

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“Paint” Beam Onto Surface

Beam forward direction Beam off-axis

Target

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VoyageIR PROTM

Patents Pending

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VoyageIR PROTM

• Precise and efficient excitation• Compact, lightweight• 12” x 9” field of view• Uncooled microbolometer camera • Low cost (~ 40 K$)• TSR signal processing

Integrated touch screen control Large area inspection using MOSAIQ® software

Single case transport

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Applications: Moisture Ingress

+

++

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Drill Down Validation of Image Result

A320 Rudder

TSRRaw

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Drill Down Validation of Image Result

A320 Rudder

water

water

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Patch Identification: Raw IR Result

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TSR result

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Overlay Result Onto Aircraft

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Overlay Result Onto Aircraft

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Applications: Polymer FOD

Raw IR (video)

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Raw IR Result

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TSR Result

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TSR Result

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11.9”

0.5

” 1”

1” 0.75” 0.5”

0.25”

7.2”

poly insert

Hole 1 Hole 2

A B C D

1

2

0.12”

Lab flash system – cooled camera

VoyageIR Pro with uncooled camera

Boeing 7X7 Al Doubler Disbond Inspection

Raw IR result TSR result

Boeing disbond cal std

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Boeing 7X7 Al Doubler Disbond Inspection

TSR result

Boeing disbond cal std

UT

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ProjectIRTM

• Far-field thermography– Working distance 5 – 50 ft

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Summary

• VoyageIR Pro

– Unique approach to excitation removes

– Artifact reduction

– Advanced signal processing

– Apply to wide range of applications

– Drill-down confirmation of result

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