Airborne Constant Ground Resolution Imaging Optical System Design

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Strengths (Pros)

1. Innovative Approach to Ground Resolution
• The study introduces the concept of field-of-view focal length (FFL) to maintain constant ground resolution (GR), which is a crucial improvement for UAV-based imaging systems.
• The method effectively reduces variations in ground resolution that typically arise due to object distance changes in oblique aerial imaging.
2. Advanced Optical System Design
• The use of free-form optical surfaces allows high flexibility in correcting aberrations, leading to better imaging quality.
• The design of an off-axis three-mirror system reduces optical distortions and enhances clarity, making it suitable for high-resolution air-to-ground target imaging.
3. Robust Mathematical Framework
• The paper presents a well-detailed mathematical derivation, including FFL calculations and aberration correction formulas, which demonstrate the theoretical soundness of the approach.
4. High-Quality Imaging Performance
• The system's Modulation Transfer Function (MTF) is close to the diffraction limit across the entire field of view, indicating excellent imaging capabilities.
• The point spread function (PSF) analysis confirms that diffraction-limited performance is achieved.
• The ground resolution variation is significantly reduced by 80.4%, ensuring a more uniform imaging result.
5. Real-World Applications
• This system is highly suitable for reconnaissance, surveillance, remote sensing, and mapping applications where constant ground resolution is necessary for accurate data interpretation.

Weaknesses (Cons)

1. Limited Experimental Validation
• The study primarily relies on simulations and theoretical models. While these results are promising, experimental validation through real-world testing is lacking.
• No real UAV flight test data or physical prototype results are included.
2. Complexity of Optical Manufacturing
• The use of free-form optical surfaces increases the difficulty and cost of fabrication and alignment. The paper does not provide insights into manufacturing feasibility.
• The integration of mirrors in a compact airborne system may require high-precision assembly that could introduce additional practical challenges.
3. Field-of-View Limitations
• The field of view (10°×8°) is relatively narrow, which might limit the system's application in wide-area mapping scenarios.
• The system does not discuss how it could be adapted for larger field-of-view imaging.
4. Lack of Comparative Analysis
• The paper references previous works but does not provide a direct performance comparison against other existing UAV optical systems.
• A comparative study showing improvements over traditional systems would strengthen the study’s significance.
5. Energy Efficiency and Weight Considerations
• The optical system’s power consumption and weight impact on UAV flight endurance are not addressed.
• A lighter or more power-efficient version might be required for smaller UAVs.

Future Prospects

1. Real-World Flight Testing
• Future work should focus on building and testing a prototype mounted on a UAV to validate the theoretical findings.
• Conducting field experiments in different terrains and lighting conditions would help refine the model.
2. Integration with AI for Image Enhancement
• AI-based image processing techniques could be used to further enhance image resolution and clarity.
• Real-time image stabilization algorithms could improve usability in dynamic UAV environments.
3. Adaptation for Wider Field of View
• Extending the design to wider-angle imaging (e.g., 30°×30°) would improve its applications in large-scale mapping and environmental monitoring.
4. Miniaturization for Lightweight UAVs
• Reducing system weight and integrating compact optical components could enhance UAV endurance and portability.
5. Multi-Spectral and Hyperspectral Imaging
• The system could be adapted for multi-spectral or hyperspectral imaging, increasing its usability in applications such as agriculture, geology, and disaster management.

Suggestions for Improvement

1. Include Experimental Validation
• Adding real-world test results with images captured from a UAV would strengthen the study’s reliability.
• Flight test comparisons with traditional systems would highlight the improvements in ground resolution uniformity.
2. Discuss Manufacturing and Alignment Challenges
• A section discussing fabrication feasibility, alignment tolerance, and calibration procedures would be beneficial.
• Cost analysis of free-form surface production should be considered.
3. Provide Comparative Performance Metrics
• A performance comparison between the proposed system and existing UAV imaging solutions should be included.
• Key metrics such as resolution, distortion, weight, and power consumption should be analyzed.
4. Consider Broader Applications
• The system's potential for earth observation, security monitoring, and industrial inspections should be discussed.
• Future modifications for autonomous UAV integration could be explored.

Final Verdict

Overall, this is a well-researched and technically strong paper that presents an innovative approach to improving UAV imaging systems. However, experimental validation, broader field-of-view capabilities, and real-world applications need further exploration to make the work more impactful.

 

 

 

Comments on the Quality of English Language

Weaknesses in English and Areas for Improvement

1. Grammatical Issues

 

Some sentences are too long and complex, making them difficult to read.

There are instances of incorrect verb usage, missing articles (a, an, the), and awkward sentence structures.

Example Issues & Suggested Improvements:

replace the following: 

 "The system employs different field-of-view focal lengths across various view angles to maintain constant ground resolution over the entire field of view."

with this: 

 "The system uses different focal lengths for various view angles to ensure a uniform ground resolution across the entire field of view." (More concise and natural)

replace the following:

 "After obtaining the above formula, the desired distribution of focal lengths for each field of view, i.e. ( )FFL ω in the formula, can be obtained given equal values of ground resolution, individual pixel cell size and camera elevation, such that GR under each field of view is equal to a uniform value of 0.53 m."

with this:

 "Using the formula above, the required focal length distribution for each field of view can be determined. By maintaining equal values for ground resolution, pixel cell size, and camera elevation, the ground resolution (GR) remains constant at 0.53 m across all view angles." (Simplified and clearer)

2. Awkward Sentence Structures

 

Some phrases are unnatural, possibly due to direct translation from another language.

Example Issues & Suggested Improvements:

replace the folllowing : 

 "This paper adopts the concept of field-of-view focal length, a local focal length, to extend the focal length from zero to the full field of view."

with this: 

 "This paper introduces the concept of field-of-view focal length (FFL) to extend the focal length dynamically across the full field of view." (More natural and precise)

replace the following:

 "The system's modulation transfer function (MTF) approaches the diffraction limit across the full field of view, with the dispersion spots remaining within Airy's disk at each view angle."

with this:

 "The system's modulation transfer function (MTF) is close to the diffraction limit across the entire field of view, ensuring that dispersion spots remain within the Airy disk at all view angles." (Better readability and technical clarity)

3. Inconsistent Use of Tenses

 

Some sentences shift between past and present tense unnecessarily, making the writing inconsistent.

Example Issues & Suggested Improvements:

replace the following:

 "The optical system is designed and optimized using free-form surfaces. The field-of-view focal length was calculated to control the ground resolution."

with this:

 "The optical system was designed and optimized using free-form surfaces. The field-of-view focal length was calculated to control the ground resolution." (Keep past tense for completed research work)

replace the following:

 "This method improves the image quality and was verified through simulation results."

with this:

 "This method improves image quality and has been verified through simulations." (Consistent verb tense)

4. Repetitive Use of Words & Phrasing

 

Some terms are repeated too frequently, making the text sound redundant.

Example Issues & Suggested Improvements:

replace the following:

 "The field-of-view focal length is an important parameter in the design. The field-of-view focal length helps control the imaging properties. The field-of-view focal length is calculated using the following equation."

with this:

 "The field-of-view focal length (FFL) is a crucial parameter in the design, as it helps control imaging properties. The FFL is calculated using the following equation." (More concise, avoids excessive repetition)

5. Use of Informal or Overly Complex Phrasing

 

Some sentences are either too informal or overly technical, making them hard to understand.

Example Issues & Suggested Improvements:

replace the following:

 "This paper talks about how an optical system can keep ground resolution steady."

with this:

"This paper explores how an optical system can maintain uniform ground resolution." (More formal and academic)

replace the following:

 "The calculations for the focal length adjustments were made, and the results were analyzed carefully."

with this:

 "The focal length adjustments were calculated and thoroughly analyzed." (More direct and professional)

 

Clarity: Moderate (Some sections are clear, while others are overly complex or vague).

Grammar & Syntax: Needs improvement (frequent minor grammar mistakes).

Technical Writing Style: Acceptable, but could be more precise and concise.

Readability: Moderate (some sentences are too long and complex).

Suggestions for Improvement

✔ Use a Professional Proofreader or Native English Speaker

 

A native English-speaking editor with a technical background could help refine sentence structures.

1. Shorten & Simplify Sentences

 

Some sentences are unnecessarily long and could be broken into shorter, clearer statements.

1. 1. Ensure Consistency in Tenses

 

Maintain past tense for completed work (e.g., "The system was designed") and present tense for ongoing principles (e.g., "The system maintains constant resolution").

1. Avoid Repetitive Phrasing

 

Vary sentence structures and word choices to improve readability.

1. Use More Formal & Concise Language

 

The paper should use professional, academic writing style while avoiding excessive complexity.

Final Recommendation

If this paper is submitted to a high-impact journal, professional English editing is recommended.

If it is for conference proceedings or internal use, some self-editing and proofreading would be sufficient.

With improved grammar, sentence structure, and clarity, the paper would become much more readable and impactful.

Author Response

Dear reviewer,

We are very grateful to you for the critical comments and thoughtful suggestions. Based on these comments and suggestions, we have made careful modification on the original manuscript. Please check draft.Thank you.

Best regards,

Lei

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

In the present work the authors present a novel approach to designing an off-axis three-mirror optical system using free-form surfaces to achieve constant ground resolution (GR) across a 10°×8° field of view (FOV) for UAV-based imaging. The application of field-of-view focal length (FFL) to control GR uniformity is innovative, and the integration of XY polynomial free-form surfaces demonstrates effective aberration correction. The design meets key specifications, including MTF values approaching the diffraction limit and dispersion spots within the Airy disk. While the work is technically sound and addresses a significant challenge in aerial imaging, several issues require clarification and improvement. Below are my detailed comments, which are split into major and minor comments.

Major comments:

1. The terms "three-reversal" and "three-mirror" are used interchangeably, which may confuse readers. Please standardize terminology (e.g., "off-axis three-mirror system").
2. The distinction between FFL and traditional focal length needs clearer elaboration in the Introduction. A brief mathematical definition of FFL early in the text would help readers unfamiliar with the concept.
3. The choice of a 4th-order XY polynomial for free-form surfaces is not justified. Higher-order polynomials might offer better aberration correction. Please discuss why 4th-order terms were sufficient and whether trade-offs between optimization complexity and performance were considered.
4. The manuscript mentions dividing the FOV into nine segments for GR analysis. Clarify how these segments were selected (uniformly or adaptively) and whether this sampling density sufficiently captures GR variations.
5. The description of structural constraints (e.g., preventing coaxial blockage) is unclear. A diagram or expanded explanation of the constraint implementation (e.g., PMVA/ABGT operands) would strengthen reproducibility.
6. The design lacks a tolerancing analysis, which is critical for assessing manufacturability and robustness, especially for free-form surfaces. Include an error budget or sensitivity analysis.
7. Mid-wave IR systems (3–5 μm) are sensitive to thermal changes. Discuss how the design accounts for thermal expansion or defocus, particularly for airborne applications.
8. The 80.4% improvement in GR uniformity (Fig. 13) is stated without clarifying the metric (e.g., standard deviation reduction). Provide a quantitative basis for this claim.
9. Free-form surfaces pose fabrication challenges. Briefly address how the proposed XY polynomial coefficients align with current machining capabilities (e.g., diamond turning tolerances).
10. While prior work is cited, a direct comparison of GR uniformity, MTF, or size/weight metrics against recent designs (e.g., Wu et al., 2021; Zhao et al., 2023) would better contextualize the advancement.

Minor comments:

1. Figure. 9: Label the primary, secondary, and tertiary mirrors in the system layout for easier interpretation.
2. Equation 7: The term "cos(-28°)" appears unexplained. Clarify its origin (e.g., camera tilt angle).
3. Please use consistent notation for FFL(ω) and GR(ω) throughout the text.

Comments on the Quality of English Language

1. Minor grammatical errors exist (e.g., "subscope" → "secondary mirror" in Table 2, "dispersive spots" → "spot sizes"). A thorough proofread is recommended.
2. Some errors like “Sur-face” “com-pact” should be corrected.

Author Response

Dear reviewer,

We are very grateful to you for the critical comments and thoughtful suggestions. Based on these comments and suggestions, we have made careful modification on the original manuscript. Please check draft.Thank you.

Best regards,

Lei

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors,

This study focus on studying calculating and optimizing the field-of-view focal lengths to control the ground resolution, and the design of a mid-wave infrared optical system with a 10°*8°field of view and an F-number of 3 has been put forward.

 

I get the feeling that the paper was rushed out while still in a draft phase. I suggest you to pay more attention and care to your articles in order to provide for publication a more refined and polished version. It would greatly improve readability and help the review process, among other considerations.

 

1. The English writing needs careful revision, to make sure the meaning is accurate. There are too many Chinglish descriptions, as Photonics is an international journal, for all readers all over the world, the manuscript needs major modifications.
2. About the formulas, the specific parameters should be annotated in the paper. Besides, about the figures, the detailed illustrations are required.
3. In Section 2, the common optical systems are introduced briefly, and according to the purpose of this study that the application requirements of the airborne optical system. About the technical specifications of the optical system proposed, how did you list the parameters? On what basis? And what are the application scenarios of this system? together with the design of UAV…
4. On the optical system design and optimization, the descriptions of your work and advantages are abbreviated, as well as that in Section 4.

On the whole, the contents and the layout of this paper look sloppy, I suggest the manuscript needs seriously reconstruction.

 

Comments on the Quality of English Language

The English writing needs careful revision, to make sure the meaning is accurate. There are too many Chinglish descriptions, as Photonics is an international journal, for all readers all over the world, the manuscript needs major modifications.

Author Response

Dear reviewer,

We are very grateful to you for the critical comments and thoughtful suggestions. Based on these comments and suggestions, we have made careful modification on the original manuscript. Please check draft.Thank you.

Best regards,

Lei

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript could be accepted without modification.

Author Response

Dear reviewer,

Thank you for your reply.We are very grateful to you for all the critical comments and thoughtful suggestions.

Best regards,

Lei

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors,

   This study just proposed a design of an imaging optical system of UAV, through introducing the field-dependent focal length concept for constant ground resolution. Mainly based on MATLAB, as well as introducing the system design indicators, the performance of the designed system was presented. However, the ideal system in practical application, the relevant experiments and so on are absent. In the Conclusions, the ideal analysis results were given, and in my opinion, the discussions of the realization of the ideal system in practical application should be given.

Author Response

Dear reviewer,

We are very grateful to you for the critical comments and thoughtful suggestions. Based on these comments and suggestions, we have made careful modification on the original manuscript. Please check the attachment.Thank you.

Author Response File: Author Response.pdf