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Peer-Review Record

Robustness of Laser Speckles as Unique Traceable Markers of Metal Components

Digital 2021, 1(1), 54-63;
Reviewer 1: Anonymous
Reviewer 2: Anonymous
Digital 2021, 1(1), 54-63;
Submission received: 28 December 2020 / Revised: 27 January 2021 / Accepted: 10 February 2021 / Published: 18 February 2021

Round 1

Reviewer 1 Report

The paper proposes an interesting approach to register unique features of surfaces via the speckle pattern generated by them once illuminated with coherent/laser light.

While the idea is elegant, I do not think this can be extended to any metal component. For example, I can think of processes affecting the metallic surface in time which would vary the detected speckle patterns if taken at different times over untreated surfaces. I am thinking of corrosion over the surface. In such a case, how would the authors use the same patterns if the surface no longer has the same roughness associated with it?  In addition, I do not think this can be either extended to liquid metals. Surfaces subject to thermal variations may also rearrange the scattering centers within the surface that lead to the required constant speckles, so the approach is not applicable. So, I think the authors must restrict their approach to real cases where the surface is not affected at all and do not generalize this application to every possible metallic fingerprint.

Speckles have been used recently in crumpling phenomena including thin sheets, check Appl. Opt. 58[24], 6549 (2019). I think that, given the nature of this approach, the authors should elaborate more in the effects of the surface in the fingerprint degradation. This is loosely addressed in their manuscript and it merits more in-depth discussion. Please add such discussion in a revised version of your paper.

Effectively, as stated by the authors in lines 43-48, the speckles arise mostly with coherent laser light. This is indeed the most typical used setup. However, in a production line, mentioned multiple times in the manuscript, one could think of implementing this using distant incoherent light. As stated by the van Cittert-Zernike theorem, incoherent light could gain in coherence with distance. Can the authors elaborate on this idea in their manuscript? This is probably a groundbreaking approach of the idea disclosed in this manuscript.

The methodology is well presented, and the results are in accordance with the objectives. Very simple experiments are conducted. The results comply with the original idea. They just need to be better bonded with the requested extension of the content commented above.


Some English corrections to be implemented.

Line 34. It must be ‘generates’ instead of generate.

Line 97. Check the phrase “The choice of these sheets are somewhat random…”; it must be is and not are for ‘the choice’.

Line 160. It must be every 20 ‘minutes’ and not minute.

Line 203. It must be ‘indicate’ instead of indicates.


In its current form, this paper is not suitable for publication but can be made publishable after a new round of review. This reviewer will be available to assess a new revised version if the editor decides to continue evaluating this contribution.

Author Response

The manuscript has been updated in response to the comments given. Below follows a summary of the updates.

Comment 1 includes several issues that are commented below separately.

It is now clarified that the scope of the paper is limited to solid surface scattering metal surfaces. A comment about that is included towards the end of the introduction. By this any volume scattering materials are excluded as is particulate and colloidal material.

The effect of corrosion is now considered and is included as a discussion point in the discussion section. With this a reference is added.

The effect of thermal variations, i.e. strain, is considered by an extension of the discussion about motion-induced decorrelation.

As a final comment I may add that the purpose of the paper is to investigate how resiliant a speckle pattern is for surfaces changes typically met in practice. So the technique is not restricted to application in which the pattern doesn't change at all but to applications in which the pattern doesn't change significantly, in this case 50%.

Comment 2

The reference included in the comment concerns decorrelation from a volume scatterer and is not directly related to the scope of this paper. There are hundreds of papers where decorrelation from a volume scatterer is utilized, mainly from the medical field where this effect frequently is used for the detection of microscopic activity or flow. I therefore chose not to include that reference. Instead three new references are included that are more related to the scope of this paper.

Comment 3

A new discussion point is added at the end of the discussion section addressing this issue. I think this is an interesting idea that needs further investigation.

Comment 4

Thanks for finding the typographical errors. These are now corrected.

Reviewer 2 Report

Reviewer remarks to the article:




This paper is well constructed. The results of investigations are valuable and very interesting from the point of view of the traceability of manufactured components. This is the correct planned and done scientific work. The authors applied a methodical apparatus adequate to the assumed goals. But the following comments should be addressed before considering of publication:


1)     Abstract could be slightly changed. This abstract partially looks like introduction and it is not enough specific. The abstract is expected to include a brief digest of the research, that is - new methods, results, concepts and conclusions only. The abstract needs to be more focused and achievements should be clearly mentioned. Should be brief but should contain the key parts of the paper background and research gaps identification research approach/method, results and conclusions. Must underline the scientific novelty/claim of the paper. Should not be a re-statement of the conclusions.

2)     What is the source of equation (1)?

3)     Acronyms should be given in full when first used in the text.

4)     What about the scientific gap - please explain clearly what is the novelty of the research presented in this paper compared to the previous works, especially those cited in references. Insert it at the end of "Introduction". Besides this, it would be good to give some summary sentences at the end of the chapter "Introduction".

5)     Literature analysis is somewhat poor. Although it could be enriched with the additional latest items in the considered area.

6)     There are some typographical errors.

7)     I propose to separate the final conclusions from chapter 4 as a separate chapter 5. And I would like to add something. In my opinion, there are no proper details in the Conclusions. Conclusions are somehow simplistic as it seems to be observational without revealing findings of generic academic value. What I mean that based on the results some generic and fundamental academic conclusions need to be drawn. In Conclusions, please try to emphasise the novelty, put some quantifications and comment on the limitations. The conclusions should also highlight the progress in understanding of knowledge presented in the work.

8)     Please emphasize more, what are the specific conclusions useful for workshop practice.

Author Response

The paper has been updated in line with the comments. Below follows a summary of the updates and responses to the comments.

Comment 1,

The abstract is updated and replaced to about 50% indicating more quantitative results.

Comment 2,

A reference is given as background to Eq. (1). In response to this also a new discussion point is added to the discussion section commenting issues with the algorithm chosen and possible alternatives. This point is added from line 260 to 278.

Comment 3,

OCR is written out.

Comment 4,

The introduction is updated and broadened to include more background information and to indicate the gap. A section is included between line 48 and 66 that comment and refer to similar investigations albeit with a different scope. In addition, the scope of this paper has been specified more clearly towards the end of the section between line 74 and 85.

The end of Introduction has been slightly updated with a peak look at some of the conclusions from this investigation.

Comment 5,

I agree the literature is somewhat poor. It is now updated with eight new references. The scope has been slightly broadened to fit in these references, which has resulted in a deepened discussion. As a result, line 279 to 327 is added to the manuscript.

Further, as a result from a comment from reviewer 1 a new discussion point is added that appear between line 328 and 344.

Comment 6,

Some typographical errors are corrected. There may still be some, but these are the ones I found.

Comment 7,

As a response to this comment, the previous last part of the discussion section has been removed and the new section Conclusions is added. The text in this section is new and should meet the standard for a conclusions section.

Comment 8,

I haven't really been able to add so much about workshop practice as that may involve special arrangements not considered. However, I do feel that some rule of thumbs do exist in the paper and that anyone attempting this approach in a broader context will be able to find useful information in the paper to avoid the worst pitfalls.

Round 2

Reviewer 1 Report


Review Report for Revised Version

  • The phrases within lines 76-80 “With this choice volume scatters are excluded as well as colloidal and particulate material. The reason for this choice is that volume scattering materials such as tissue and paper are known to decorrelate easily in response to very small changes and as such they are not expected to generate a speckle pattern stable enough for fingerprinting” need a reference.
  • No mention to my comment regarding whether the proposed method could be also implemented with incoherent light in the far field. I regret this, given that this would have established the foundations for further development on this front. Without this, I’m afraid this will be a so what? paper (as defined by former Editor of Applied Optics Prof. Driggers)
  • I leave the Editor the final decision, but the revision is in my view too poor to warrant acceptance. In its current form, I doubt the paper will be cited by recognition among the speckle/optics community or the digital image processing community.

Author Response

I have tried to consider the last comments from the reviewer and have slightly updated the manuscript, I hope, in response to the comments.

  1. A reference is added to the comments about the susceptibility of volume scatterers to small changes in the material structure. As mentioned in my previous response, there are several branches of optical metrology that in essence utilize this sensitivity for metrological purposes including Dynamic Light Scattering (or photon correlation spectroscopy) and speckle flowmetry. I have however chosen not to include an excessive number of references in those fields as that would take away the focus of the current paper. The scope is after all significantly different from those applications.
  2. I am a bit surprised by this second comment as I don't feel the reviewer comment was ignored at all. In fact the last section of the discussion was added to address considerations regarding the coherence of the light source in direct response to the comment. A guess is that it was unclear that only image plane speckles are considered in the current investigation. Diffraction field speckles are left out. However, to clarify this the phrase image-plane laser speckles are added where appropriate. I have two further remarks that I chose not to include in the paper. Firstly, diffraction field speckles would have the practical problem of having to locate exactly the same spot on the surface during registration and detection, a problem that is circumvented by an imaging geometry. Because of this, diffraction field speckles were excluded from the investigation. Secondly, I think the reviewer have mixed up the properties of speckles and spatial coherence. For sure, spatial coherence increase with propagation distance (rather with the solid angle to the source) but that doesn't mean the speckle contrast will increase. Instead, the speckle generation depends on the coherence properties at the generating surface. What will change with propagation distance is the speckle size but not the speckle contrast. In fact, a common way to emulate a spatially incoherent source is to illuminate a rotating diffuser with a laser beam and integrate the result. The investigation suggested by the reviewer would therefore be meaningless. What is of interest, however, is what level of speckle contrast that is required for a successful detection, a fact that also is included in the manuscript.
  3. I feel it is a bit unfair to label the contribution a so-what paper if not the ideas of the reviewer is included in the paper, particularly if this idea appears to be physically unsound as I see it. If this is a so-what paper or not is rather for future readers to judge.

Reviewer 2 Report

I accept in present form

Round 3

Reviewer 1 Report

The authors did not include major revisions to their third version for this manuscript. 

Kindly note that by no means I have any reason to be angry by reviewing your manuscript and I have no conflic of interest with what is attepted to be published in it. I feel it's my responsability to provide a critical report on what I see with respect to the rest of the published literature (at least the part of the literature that I'm aware of). That's why I provided you with a tool to stress the significance that your paper may have. You have chosen not to proceed that way and that's a choice to respect.

I, therefore, leave the Editor the decision to accept or not this manuscript in its present form.

Best of luck.


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