the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 May 2026
Development and Characterization of an Advanced Holographic Instrument for Atmospheric Research
Abstract. In this study, we develop and characterize an advanced holographic instrument — HOLODROPS (HOLOgraphic DROPlet Sensor) — along with a complementary event-based Particle Image Velocimetry (ePIV) system. HOLODROPS operates in two configurations: (1) HOLODROPS, designed for long-term, high-precision measurements of cloud droplet size distributions, and (2) HOLODROPS-TRACK, designed for short-interval particle tracking to study turbulence and collision-coalescence processes. Unlike traditional optical scattering or imaging probes, both configurations can simultaneously capture the size, shape, and three-dimensional position of multiple droplets and ice crystals larger than 8.8 µm within a sample volume, eliminating the need for multiple instruments and yielding more accurate and cohesive data. However, the detailed three-dimensional particle tracking provided by HOLODROPS-TRACK requires significant computational resources for hologram reconstruction and data processing, limiting its use over extended measurement periods. To address this challenge, we developed the ePIV system, which leverages an event-based camera to assess key cloud motion features in real time with substantially lower data volume and processing. By rapidly characterizing the flow conditions, ePIV identifies optimal measurement windows and informs targeted deployment of HOLODROPS-TRACK, paving the road for efficient, high-resolution particle tracking when conditions are most scientifically valuable. All three systems have been successfully characterized and validated in the laboratory. Additionally, we tested HOLODROPS successfully in an outdoor setting with a drizzle event.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-2141', Jan Henneberger, 08 Jun 2026
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RC2: 'Comment on egusphere-2026-2141', Anonymous Referee #2, 23 Jul 2026
This manuscript presents an framework integrating HOLODROPS for long-term observation, HOLODROPS-TRACK for short-term particle tracking, and ePIV (an event-camera-based particle image velocimetry system) for atmospheric research (cloud microphysics and turbulence). The attempt to overcome the limitations of traditional single-particle sampling probes using digital holography is interesting and valuable. However, due to the prototype stage of the system, significant shortcomings remain regarding data uncertainty treatment, specification consistency across instruments, and control over experimental environments. I hope the following comments will be helpful before the manuscript can meet the standards for publication.
Major comments
1. In Section 3.2, known-size glass beads were used to calibrate instrument accuracy, but sizing errors (about 10-25%) were observed in the resulting data: The authors attribute this to "turbulence and background intensity variations (noise) caused by the carrier airflow." However, real cloud conditions and outdoor atmospheric conditions exhibit far more severe turbulence and water vapor inhomogeneities than controlled laboratory setups. If the instrument exhibits a 25% sizing error under laboratory carrier airflow, it cannot be deemed "high-precision." Algorithmic noise reduction (e.g., improved background subtraction) must be implemented, or quantitative error margins (error bars) must be explicitly provided.
2. One of the core scientific contributions claimed in this manuscript is the scenario where the lightweight, high-speed ePIV system runs continuously to guide targeted deployment for the computationally intensive, data-heavy HOLODROPS-TRACK system. While the Abstract implies a seamless integrated workflow where ePIV enables "targeted deployment" of HOLODROPS-TRACK, the Conclusion retreats to stating that "ePIV can inform users and can inform when the conditions would be ideal." This indicates a passive monitoring interface for manual user intervention rather than automated, data-driven closed-loop system triggering. The authors must clarify whether this connection is manual or automated. To prove interoperability, time-series data demonstrating ePIV detecting a flow event and subsequently triggering HOLODROPS-TRACK must be presented for a single flow phenomenon. Instead, Sections 3.4 and 3.5 show that the two instruments imaged separate, independent nebulizer flows at different times. This proves that both devices can measure similar velocities, not that ePIV dynamically informs the deployment timing of TRACK. The authors should include a software flowchart or signal triggering timeline for real-time control. If automatic triggering was not implemented, the claim must be toned down to describe the setup as "two complementary, independent measurement instruments."
3. HOLODROPS-TRACK uses a 532 nm pulsed laser (Line 188), whereas ePIV utilizes a 532 nm continuous-wave (CW) laser shaped into a laser sheet (Lines 205–207). If operated simultaneously in a field deployment, the high-intensity CW laser sheet from ePIV could spill directly into the sensitive image sensor of HOLODROPS, introducing severe speckle artifacts and noise into the reconstructed holograms.
Minor comments
1. Throughout the text, micrometers are frequently written as um instead of using the standard micro symbol. Meanwhile, μm is correctly used sometimes. Units should be standardized to μm across the entire manuscript.
2. Line 303: The word “lactation” appears to be a typo for locations. Additionally, in in is duplicated.
3. Line 306: “general articles tracking software's” should be corrected to general particle tracking software.
4. Figure10: The individual subplots only bear duplicate headers (vx distributions and vy distributions), making visual interpretation difficult. I suggest that the author can add explicit row labels (e.g., "Front-View" / "Side-View") to the left or top of each subplot group.
Citation: https://doi.org/10.5194/egusphere-2026-2141-RC2
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Please find my review in the attached file.