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Abstract
The condition assessment of large-scale overhead timber components in ancient Chinese architecture — such as caisson ceilings (zaojing), bracket sets (dougong) and roof framing — is challenging because contact-based techniques (stress-wave timing, ultrasonic velocity, resistance drilling) are impractical for elements 5–8 m above floor level, while visible-light inspection is masked by centuries of grime, soot and oxidation. This work presents a lightweight, handheld dynamic thermal scanning protocol for rapid in-situ survey of inaccessible timber heritage. A 1 kW halogen spotlight is manually swept across the ceiling from ground level while an uncooled long-wave infrared (LWIR, 7.5–14 µm, 640×480, NETD 20 mK, 50 Hz) camera records the full thermal transient. Physically grounded thermal feature maps are extracted, including the temporal standard deviation of each pixel encoding surface thermal effusivity, a high-pass filtered version isolating material-intrinsic contrast, cooling-rate maps as a proxy for thermal diffusivity, and peak-arrival-time maps reconstructing the scan trajectory. The method is validated on the octagonal central caisson of the Main Hall of Baoguo Temple (1013 CE), revealing faded Song-dynasty floral scrollwork, wood knot and grain-orientation distributions, board-to-board joints, a grafted historical repair section, and localised anomalies indicative of biological degradation, all entirely invisible in the conventional visible-light photograph. The induced surface temperature rise is 1–2 °C, well below any damage threshold for aged timber.
Keywords: infrared thermography; dynamic thermal scanning; timber heritage; caisson ceiling; bracket set (dougong); thermal effusivity mapping; non-destructive evaluation; Song dynasty architecture; Baoguo Temple.
1. Introduction
Established non-destructive techniques for timber heritage — including stress-wave timing, ultrasonic velocity measurement and resistance drilling — require physical contact with or close proximity to the inspected surface, which is impractical for overhead components 5–8 m above floor level without scaffolding [1]. Visible-light inspection, although the most widely used, is severely constrained on millennium-old surfaces where accumulated grime, soot and oxidation products mask the underlying material characteristics and fade painted decorations to near-invisibility [2]. A rapid, non-contact and ground-based screening tool is therefore needed for periodic condition monitoring of large-scale, inaccessible timber heritage.
2. Experimental method
A 1 kW halogen spotlight is manually swept across the ceiling surface from ground level while an uncooled long-wave infrared (LWIR, 7.5–14 µm, 640×480, NETD 20 mK, 50 Hz) camera records the full thermal transient. The total equipment mass (camera, tripod, spotlight, cabling) is below 5 kg, and a full scan of one bay requires less than 60 s of acquisition. From the resulting spatiotemporal dataset, a suite of physically grounded thermal feature maps is extracted: (i) the temporal standard deviation of each pixel's temperature time series, encoding the spatial distribution of surface thermal effusivity; (ii) a spatial high-pass filtering of this standard-deviation field, which removes the slowly varying illumination envelope arising from handheld scanning and isolates material-intrinsic contrast from excitation artefacts; (iii) a cooling-rate map providing a qualitative proxy for thermal diffusivity; and (iv) a peak-arrival-time map that simultaneously reconstructs the scanning trajectory and encodes local thermal-inertia variations.
3. Results and discussion
The method is validated on the Main Hall of Baoguo Temple, Ningbo — a National Key Cultural Heritage Protection Unit housing one of the oldest surviving timber-framed structures in southern China (1013 CE) — focusing on the octagonal central caisson and the surrounding bracket-and-beam framework. Thermal scanning reveals a wealth of material-level information that is entirely invisible in the conventional visible-light photograph (Fig. 1): faded Song-dynasty floral scrollwork (juancaowen) and geometric motifs along the tie beams and in the corner bracket regions; spatial distributions of wood knots and grain orientation; board-to-board material differences at panel joints; a grafted historical repair section on an exposed load-bearing beam; and localised anomalies potentially indicative of biological degradation or moisture ingress (Fig. 2). The induced surface temperature rise of 1–2 °C above ambient is well below any damage threshold for aged timber, ensuring complete non-invasiveness [3, 4].
Fig. 1. Visible-light photograph of the octagonal caisson ceiling and surrounding bracket-and-beam framework in the Main Hall of Baoguo Temple (1013 CE), acquired from ground level at a working distance of approximately 6 m.
Fig. 2. High-pass filtered temporal standard deviation map of the same area obtained from a 60 s handheld thermal scan, revealing decorations, wood knots, grain orientation and localised anomalies.
4. Conclusion
A lightweight (<5 kg), handheld dynamic thermal scanning protocol is demonstrated for rapid in-situ inspection of inaccessible timber heritage. Combined with physically grounded thermal feature mapping, the protocol resolves faded painted decorations, hidden material heterogeneity and localised defects in a millennium-old caisson ceiling, all from ground level and within a 60 s acquisition. The proposed protocol therefore establishes a viable rapid-screening tool for periodic condition monitoring of large-scale, inaccessible timber heritage.
References
[1] Lin Y., Chun Q., Zhang C., et al., Research on seismic performance of traditional Chinese hall-style timber buildings in the Song and Yuan dynasties: a case study of the main hall of Baoguo Temple, J. Wood Sci., vol. 68, p. 1, 2022.
[2] Dritsa V., Orazi N., Yao Y., et al., Thermographic imaging in cultural heritage: a short review, Sensors, vol. 22, no. 23, p. 9076, 2022.
[3] Ding Y., Hu J., Sfarra S., et al., Fusion of infrared and terahertz imaging for non-invasive inspection of marqueteries coupled with finite element analyses, Infrared Phys. Technol., vol. 141, p. 105470, 2024.
[4] Ding Y., Russo G., Tshiangomba R.K., et al., Stabilization system for solar loading thermography applied on cultural heritage objects exposed outdoors, J. Therm. Anal. Calorim., vol. 150, pp. 1687–1707, 2025.
