SI Traceable Solar Spectral Irradiance Measurement Based on a Quantum Benchmark: A Prototype Design
Round 1
Reviewer 1 Report
General Comments
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This is a useful study presenting a new approach to establish SI traceability by means of quantum optical radiometry onboard satellites, using the Spontaneous Parametric Down-Conversion as referred to by the authors. The authors state that this has considerable advantages over current satellite calibrations and is essentially insensitive to degradation of optical components.
The concept would benefit a wider audience as the proposed calibration chain could be useful in other areas of the Earth Sciences and radiometry where long term calibration stability is required. For that though, the manuscript needs to be expanded throughout to assist the reader (both in terms of its English and the descriptions of the key concepts).
Whilst I understand that English is not the authors' first language, there are a few areas throughout that could do with some refining which would help the clarity of the text. This is often at the level of the making sure the correct prepositions are included.
More than that the description of the underlying physical process needs more explanation, as do the key steps towards a complete system. How do the two channel system enable an absolute source / detector to be created? More information is needed about the BBO crystals and their behaviour to produce broad spectral sources. More detail is needed to justify the error budget, as is a better distinction between sources of error that are avoided through the proposed system (degradation) and those that still persist.
Some specific examples of previous traditional calibration chains would be useful to include, and then it would be clearer to the reader where the benefits of this new approach arise.
Overall a promising manuscript and piece of work, but more detail is needed throughout for it to realise its full potential
Specific Comments:
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L13: "outer standard" it is not clear what this phrase means. It is used several times so some clarification is needed
L23: "on-orbit". Please clarify?
L31: "uncontrollable atmospheric propagation properties in vicarious calibrations". Please clarify
L36: "Comparing with a concrete standard device, physical constants can be more competitive for a space benchmark." Please clarify this sentence.
L40-52: This is the section of the text where SPDC is introduced and described. It would help the reader who is not familiar with this effect to have a more extensive description of the effect and why it is absolute and why it is insensitive to the degradation of downstream optical components. There are some comments throughout the text, but as this is the bedrock of the proposed calibration an expanded description is warranted. How is it absolute? Does it not depend on the stability of the laser power source? Why is it "magic"? A more appropriate word is needed here.
L60: Please number equations here and throughout.
L64-66: Some more detail is required on this process to help the reader understand how the physical process allows pumping with a monochromatic beam to create a spectrally broad source
L75: Define BBO
L83: Define "Geiger mode"
L93: It is not obvious. This requires more explanation as to how the system is insensitive to degradation and other difficulties that would be problematic for traditional calibration chains. It is also not clear how the proposed system is insensitive to simultaneous degradation effects in both detection channels
Fig 3: Is the only source the sun, or is the intention to also measure the Earth? How is the total (integrated) solar irradiance measured? Is this shown in the figure? If the first laser suppression modules suppresses wavelengths other than 266nm, how does the spectrally broad emission from the BBO crystals pass through?
L134: Why is this specific stability required? What is the time required for a spectral scan, or are these array detectors? How does the scan time effect the stability threshold?
L146: It is not clear how these are "absolute light sources"? Some more explanation is needed.
L180: It is not clear what the 400 calibration channels are here. Are the different wavelengths? They are presumably different to the two channels mentioned earlier.
L181: this seems a long time for a calibration. What is the reason?
L185: How could the channels be reduced? Are they not set by the spectral range?
L200: "Stray light could be the main source..." This seems very speculative. Is it the main source or not. Generally more support is needed for the error budget and assessment
L207 and Table 1: if experimental uncertainties are unavailable at present, then how have the estimates in table 1 been calculated?
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The paper aims at presenting a quantum benchmark for solar spectral irradiance with SI traceability. While the work is appreciated and interesting, I recommend a major revision.
Point 1: Major problem #1.
How far this work is beyond the state of the art, and demonstrates innovation potential?
Point 2: Major problem #2.
For scientific purpose, the measurement of the UV solar spectral irradiance is important. Could you be more specific in the introduction to provide accurate information on this subject? So, how your quantum benchmark could be revolutionary if it does not allow a measurement of the solar spectral irradiance below 380 nm?
Point 3: Major problem #3.
There is a significant lack of references to previous and recent work. Add new references in your manuscript (lines 27-28). Indeed, new space-based instruments require new expected requirements for total solar irradiance, solar spectral irradiance and "Earth reflective radiance". Swartz et al. (Remote Sens. 2019, 11, 796) and Meftah et al. (Remote Sens. 2020, 12, 92) bring new light for scientific requirements. It must be clarified whether these uncertainties are absolute uncertainties and to highlight the notion of stability over time (the instrument objective is to provide accurate measurement over a long-time period). Today, the proposed uncertainties in this manuscript are no longer relevant.
Point 4: Major problem #4.
Introduce in details the importance of intensive pre-flight calibrations, which are crucial for such instruments. Pre-flight calibration can be achieved with detectors and transfer radiation-source standards, both traceable to a primary standard source found in synchrotron-radiation facilities, while the instruments themselves can be calibrated at the synchrotron facility or locally, at the instrument test facility, by transporting a transfer source standard to that facility. Introduce also in details the importance of in-flight calibrations with different sources (lamps with SOLSPEC, stars with SOLSTICE/SIM, electrical substitution with TIM & PREMOS & SOVAP & VIRGO, …). All these in-flight calibrations suffered degradations (BenMoussa et al., 2014). Why your system would escape it? It is necessary to introduce this problem in the introduction and to argue a little later in the discussion of the manuscript.
Point 5: Major problem #5.
Provide a better and rigorous description of the design instrument (section 3). Provide also a definition of all acronyms such as BBO.
Generally, for solar spectral irradiance measurements, we use a Czerny-Turner double monochromators with concave gratings (limitation of optical aberrations, …). Why you did not use a similar system? You must support the discussion.
Point 6: Major problem #6.
You have to be more specific about the elements that will age in space. Contamination of the optics, oxidation of the filters, impact of the radiation on optics, detectors aging and temperature effects in space? Impact of Beta Barium Borate crystal aging in space? More critical, how do you validate the stability of pump laser in space?
Point 7: Major problem #7.
Estimated uncertainty budget.
You need to provide an uncertainty budget for absolute measurement (reference), the stability per year, the stability per decade for long-term measurements (essential for solar observations and Earth-Sun connection).
How do you manage the non-equivalence between the Sun (with limb darkening function) and what simulates the Sun (pump laser & optics with aging in space)?
For both systems (non-equivalence between Sun & pump laser), what will be the impact of the slit functions (FWHM, bandpass, …) and their evolution with time on the measurements?
How you manage the effect of pointing uncertainties during observations? Generally, diffusors are placed in front of the entrance slit to ensure full illumination of the first grating and also to reduce the effect of pointing uncertainties during observations.
What is the influence of slits dimensions and locations (entrance slit, intermediate, slit, exit slit close to the detectors?) on the uncertainty budget?
Finally, an external calibration system (for ground calibration) possesses a black body simulator for which we can know the spectral radiance from Planck's function and the temperature. From the spectral radiance, we can retrieve the spectral irradiance. However, this ground calibration is done at an ambient temperature (comparison between a direct source and the pump laser system). However, how you will manage the effect of the temperature (temperature distribution in space vs. temperature distribution in laboratory, ...) and vacuum (water vapor correction, ...)?
Minor Remark:
You obtain an uncertainty of about 0.35% for the full 380-2500 nm band? Do you have a strategy for the margin philosophy of this result?
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
The authors propose a set-up for spectral solar irradiance measurement based on spontaneous parametric down-conversion because this technique is intrinsically absolute, insensitive to degradation, independent fom other standards and hence of large interest for satellite on board calibration.
The paper describes the parametric down conversion technique for absolute calibration of detector responsivity, introduces design of solar radiometer for VIS to SWIR wavelength and analyze a preliminar uncertainty budget based on figures derived from references.
The paper is well written, has proper references and describes a smart application of parametric down conversion.
The uncertainty table shows many contributions, most of them described along the paper; at least crystal transmittance is not introduced. How crystal transmittance is measured with the set-up of figure 3? Could be done at all wavelength in the measurement range ?
The paper should be considered for publication after revision which explain how crystal loss is measured.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Dear Authors,
The manuscript has improved greatly. Thanks for the update. However, I have still comments and you need to improve the article manuscript.
Point 1: Major problem #1.
How far this work is beyond the state of the art, and demonstrates innovation potential?
You still need to develop this question in the text. You need to explain the limits of your design compared to a cryogenic radiometer. How you justify a potential of innovation in this manuscript?
Point 3: Major problem #3.
There is still a significant lack of references to previous and recent work. You must to discuss on-orbit degradation of solar instruments (Ben Moussa et al. (2014)) and lessons learned.
How this feedback is used to consolidate your design?
Space benchmark sensors are expected to measure total solar total (TSI), solar spectral irradiance (SSI) and Earth reflective radiance with uncertainties of about 0.02%, 0.2% and 1%. You need to add new references and to explain if it is absolute or relative uncertainties. For TSI, absolute uncertainties are close to 0.1% (PREMOS, SOVAP, TIM, …). For SSI, as example, SOLAR/SOLSPEC has an absolute uncertainty of 1.26% at 1σ from 165 to 3000 nm (SSI). Same for solar instruments onboard SORCE mission.
You need also to add references about other calibration system used such as Total solar irradiance Radiometer Facility (TRF) calibration.
Finally, you need to add new references of recent SSI observations (during solar cycle 24, absolute measurements of TSI, solar spectrum, …).
Point 4: Major problem #4.
Introduce in details the importance of intensive pre-flight calibrations, which are crucial for such instruments.
Point 7: Major problem #7.
You partially answered to the questions. Then, could you answer point by point to these questions?
How do you manage the non-equivalence between the Sun (with limb darkening function) and what simulates the Sun (pump laser & optics with aging in space)?
For both systems (non-equivalence between Sun & pump laser), what will be the impact of the slit functions (FWHM, bandpass, …) and their evolution with time on the measurements?
What is the influence of slits dimensions and locations (entrance slit, intermediate, slit, exit slit close to the detectors?) on the uncertainty budget?
Finally, an external calibration system (for ground calibration) possesses a black body simulator for which we can know the spectral radiance from Planck's function and the temperature. From the spectral radiance, we can retrieve the spectral irradiance. However, this ground calibration is done at an ambient temperature (comparison between a direct source and the pump laser system). However, how you will manage the effect of the temperature (temperature distribution in space vs. temperature distribution in laboratory, ...) and vacuum (water vapor correction, ...)?
Point 8: Major problem #8.
About instrumental equation (line 261). All effects are not taken into account. How you estimate impact of systematic bias? Temperature correction factor of the aperture area? Opto-electrical non-equivalence of the system (direct & indirect measurements)? R and tau dependency with temperature?
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
