Abstract

Directional longwave radiation from building heating equipment depends on surface temperature, geometry, projected area, viewing direction, and emissivity. Previous visible–thermal fusion and three-dimensional thermography studies mainly produce fused maps or visual models; this study systematizes a workflow for projected-area-weighted directional radiant intensity. Geometric contours from synchronized visible images are transferred to thermal images to reconstruct a three-dimensional thermal model. For a small radiant electric heater, the visible-image-assisted method was compared with an indirect method using the same archived thermal input, target object, calculation convention, and unit-emissivity setting (ε = 1). Across 23 directions, the mean absolute error (MAE), root mean square error (RMSE), mean absolute percentage error (MAPE), and coefficient of determination (R2) were 0.47 W/sr, 0.56 W/sr, 3.55%, and 0.996, indicating close method-to-method agreement rather than absolute accuracy. Application to a building-integrated thermal-storage heater produced 28.7–52.76 W/sr. A reusable three-dimensional thermal model can support multi-directional post-processing from one synchronized imaging campaign and may reduce repeated instrument repositioning; however, no time–cost study was available to quantify labor or operational savings. Independent traceable reference measurements, repeated trials, surface-specific emissivity verification, and registration/geometry uncertainty analysis are still required.

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