High Accuracy Solar Diffuser BRDF Measurement for On-Board Calibration in the Solar Reflective Band
Round 1
Reviewer 1 Report (Previous Reviewer 2)
Figure 14 should be improved (the legend is too small) and the red and black curves should be explained.
I understand the value of using the reciprocity assumption to reduce uncertainties in the angle of incidence. Nevertheless, I think it is essential to discuss the validity of this assumption. The slightest anisotropy of the surface state can make the hypothesis invalid.
Author Response
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Reviewer 2 Report (New Reviewer)
This manuscript is more like a device instruction than a researh paper, the structure needs to be reorganized
Author Response
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Reviewer 3 Report (New Reviewer)
Line 401, the analysis of uncertainty is very confusing. In the formula (19), what does each term refer to? Line 435, according to Figure 3, the uncertainty should be 0.054% (≥0.015%)
I suggest the section 4 Uncertainty budget and section 5 will be rewritten and reorganized.
The English of this paper should be improved.
Author Response
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Reviewer 4 Report (New Reviewer)
This paper presents a methodology for the absolute measurement of the BRDF of solar diffusers used for on-board calibration of satellite measurement calibration. The proposed methodology combines a high-brightness integrating sphere as a radiation source, a six-axis robot arm, and a large rotation stage. The paper's topic is crucial and relevant for remote sensing. However, I have a concern regarding the writing of the paper. Many parts are unclear and hard to follow, and some definitions are missing. Besides, some of the text is written in a red and underlined font for an unclear reason. I recommend the authors consider rewriting the paper and sending it to professional language editing to make it straightforward. Several comments are provided as follows:
Abstract:
· The authors state, "At an incident zenith angle of 75°, the improved method reduces the measurement uncertainty of the SD BRDF by 52% at 410 nm to 480 nm and by 70% at 480 nm to 1000 nm compared to the absolute measurement method." What about the other bands?
1. Introduction:
· Line 43: "It is defined as the ratio of the radiance reflected by an object surface to the irradiance incident on the object surface." This definition lacks the directional reflection aspect.
· Line 45: use "an SD" instead of "a SD", since the spilling of the "s" in an abbreviation starts with the vowel "e," i.e., "es-de"
2. Description and calibration of the facility
· Line 135-142, The stability of the sphere radiator: provide statistics about all the bands, not only the one presented in Figure 3.
· Lines 153-162, this paragraph is unclear, and sentences are long and hard to follow. I recommend rephrasing it.
· Equations 1-4: Which information regarding the measurement location are provided to the system? If it is not the BRDF angles, then present the inverse of these equations, i.e., converting from "Alpha", "Beta" and "Gamma" to "Theta" and "Phi".
· Line 172, "For some measurement wavelengths, the difference will even exceed 4 orders of magnitude." Which ones, and why?
· In Figure 5, make the text clear and readable. It is tough to understand what the numbers are in many places. Where the reflected light from plane mirror 2 enters the optical platform?
· Line 197: The monochromator has a spectral resolution of approximately 3 nm in the 350 nm to 600 nm and 6 nm in the 600 nm to 1700 nm. What about the rest of the range you are using? i.e., 1700-2400 nm.
· In Equation 5: explain what each parameter stands for.
· The results in Figure 8: The motivation behind using a maximal zenith angle of 75 degrees is clear, but why did you not measure for angles lower than 35?
· In Equation 9: explain what "d-omega" stands for.
· In line 389, to which angle 0°/45° is related. Moreover, explain more why these angles are used as references.
5. Measuring the SD BRDF for on-board calibration
· Demonstrate the Sun-Earth-satellite (SES) angle in the figure.
· Provide more results regarding the BRDF in the different reflection angles, not only the one angle, as presented in Figure 22.
The paper's language quality must be improved.
Author Response
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Round 2
Reviewer 4 Report (New Reviewer)
I'm pleased with the improvements.
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
Comments for author File: 
Comments.pdf
Author Response
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Reviewer 2 Report
The paper presents a BRDF measurement device and establishes a study of the uncertainties. The whole study seems correct but I am not expert enough to detect incorrect things.
I am more circumspect about the "improved algorithm" which seems to be one of the main contributions of the article. It is based on the assumption of reciprocity. This may be acceptable if we assume the material isotropic. But it seems unwise for metrology to base the level of uncertainty on an assumption about the material. Or else, one would have to estimate the uncertainty brought by this assumption. And in practice, it is not so obvious to prove the reciprocity (it presupposes a symmetry of the solid angles of incidence and collection).
On the other hand, we would like to know what other teams do in the same situations. Do they have alternatives to this "improved algorithm"?
In the introduction, there are references presenting the uncertainties on the BRDF obtained by the national metrology institutes. But these references are more than 10 years old. Are you sure there are no more recent results? In other words, the state of the art should be improved.
I think it is important to give reasoned answers to the above points so that the article can be accepted in my opinion.
Miscellaneous
p5: "there will be an approximately 4 orders of magnitude difference between the incident signal and the reflected signal." I don't understand the estimate of 4 orders of magnitude difference. "Signal" is too imprecise. Is it flux, irradiance or radiance?
p6: Can't you give the range and spectral resolution in nm instead of cm-1 for FTIR?
Eq5 p7: can you explicit the acronym DN?
Eq19 p12: can be removed (the same eq in the previous line)
Author Response
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Reviewer 3 Report
A robot-based gonioreflectometer for spectral BRDF measurement was introduced in this manuscript. To measure the SD BRDF at large observation geometries, an improved algorithm based on the reciprocity theorem is proposed. However, the key problems to be solved in the manuscript are not clearly explained. In other words, what is the difference and innovation from the existing research work mentioned in the introduction?
More detailed comments are shown below.
(1) In the introduction, it is necessary to supplement the new work compared with the previous work.
(2) In line 192, what’s the “V80”. The abbreviation was not explained clearly when it first appeared.
(3) In line 232, how to get the origin angle before correction.
(4) There are two configurations in the detection system. What are the working wavelengths of the two detectors, and how do they work together?
(5) In section 4.3, the uncertainty of the BRDF measured using the two methods in Sections 3.1 and 3.2 was compared. However, the uncertainty analysis for the absolute BRDF measurement method wasn’t introduced in the manuscript. In addition, what can we get from this uncertainty comparison? Can you add a comparison between BRDF measurements using different methods.
(6) The format of references in the article is different. Please unify the format of references according to the requirements of the magazine.
Author Response
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Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
Comments for author File: Comments.pdf
Reviewer 2 Report
The authors have made many improvements to their text. However, they have only partially answered my main questions.
The bibliographic references including the new ones are more than 10 years old. Are there no more recent references? The contribution of the article should be situated in relation to what other research teams are currently doing.
The assumption of reciprocity that underlies the method must also be discussed. The authors must discuss its limits and try to show how it is not "too" problematic.
Reviewer 3 Report
I think some previous problems and suggestions have also been well addressed after the last round of revision. Therefore, I suggest that the paper be published.
