Extension and Method-to-Method Agreement Assessment of a Visible-Image-Assisted Thermal Imaging Method for Directional Longwave Radiation Characterization of Building Heating Equipment

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.