Hello Everyone,
It is found that most of the researchers in the field of thermography considered the reflection mode during the experimental procedure while some of few also focussed on the transmission mode.
From the literature review, I came to know that reflection mode enables a high resolution for the determination of defect depth. However, only inclusions close to the surface could be detected reliably. Thus, the lateral size of defects along the whole cross section of the samples could only be determined by transmission measurements. In several cases due to the accessibility of only one side of the structure, only reflection measurements are possible.
Beside these, is there any other significance of using reflection and transmission mode in infrared thermography during the NDT&E of materials?
It is appreciated if anyone can provide information about this issue.
Thanks,
its a choice of individual for cases where both sides are accessible and for thin samples. But as you have already mentioned for practical applications very seldom you have access to both sides. Hence reflection mode is preferred.
besides for pure R&D you need to consider the thickness. Thermal waves are highly damped inside materials, hence allowing them to pass through the whole material for surface or sub-surface defects may not be ideal. You will loose spatial as well as temperature resolutions. Apart from that it will also allow lateral flow of heat which will make life very difficult for doing analysis of pulsed or lock-in data.
On the other hand, for deeper burried defects transmission mode is ok. But it depends on your sample properties, dimension and source strength. I have seen researchers using transmission mode for thin rubber sheets and measuring thermal diffusivity (Perker and similar methods). Hope this will be useful. Thanks
Dear Lahiri,
Thank you very much for your response. One more query is about the extraction of phase image with transmission mode and reflection? What is your opinion regarding this issue? Which method will be more appropriate? As I have no experience working with the transmission mode, I have not gone through this. Of course, I am interested in transmission mode and will consider i near future but time matters. It will be good to know in advance.
Thanks,
for extraction of phase image i think reflection mode will be better. in transmission mode you will encounter additional phase changes during wave propagation through the material. In my opinion, unless you have an experimental constraint its better to deal with reflection mode which is conventional and equations are straight forward.
You will find an answer in textbooks like The American Society for Non Destructive Testing (ASNT). Handbook: “Infrared and Thermal Testing”,
It depends on the context and on the relative depth of the defects. Defects close to the back face are difficult to detect in the reflection configuration from the front face; in that case, the transmission configuration will allow you not to miss the defect. Then, however, it will be difficult to evaluate the defect depth. If you have access to both faces, the best is to perform 3 experiments: one in transmission and two in reflection from each face.
Pulse thermography is generally easier to interpret than modulated thermography or chirped thermography. Provided the same amount of energy is introduced into the sample, after applying common image processing tools (logarithmic averaging/filtering for pulse thermography, IFT/DFT in the pulse-phase approach, lock-in or signal cross-correlation) the final signal to noise (i.e. defect detectability) is about the same. Applying the transient, modulated or chirped approach is essentially a matter of taste and tradition. Trying new pulse-functions is an easy way to endlessly publish on this subject...
Fundamentally, there is no real difference between reflection and transmission modes. They are described by the same differential equation and solution, with differences only in source and detector positions. However, in many real applications, transmission is not an available option as the part under test may be closed or attached to core. For multilayer structures, there are some advantages to reflection mode, where techniques such as TSR can identify and characterize each layer, while transmission flattens the result to appear as a single layer, regardless of the actual complexity of the part.
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