###### Abstract  
The paper focuses on the use of holographic interferometry in the research of thermal modification and its effect on the heat transfer from the wood surface to the surrounding air. In the experiment, spruce wood samples modified at 160 \({}^{\circ}\)C, 180 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, 220 \({}^{\circ}\)C and an unmodified control sample were used. A radiant heat source was placed under the sample. The top of the sample represented the boundary where the observed heat transfer occurred. The temperature fields above the sample were visualized by real-time holographic interferometry and the heat transfer coefficient \(\alpha\) was calculated from the obtained interferograms. During the heating of the samples, a decrease of the heat transfer coefficient was observed. The heat transfer coefficient of the control unmodified sample decreased from a maximum of \(\alpha\) = 22.66 Wm\({}^{2}\)K\({}^{-1}\) to a minimum of \(\alpha\) = 8.6 Wm\({}^{2}\)K\({}^{-1}\). In comparison with these values, the heat transfer coefficients of the modified samples treated at 160, 180, 200 and 220 \({}^{\circ}\)C, respectively, decreased to 99%, 93%, 68% and 51% of the maximal control value at the beginning of experiment and to 95%, 86%, 80% and 64% of the minimal control value by the end of the experiment. Moreover, an analysis of variance was used to determine the significance of the heat treatment effect on the heat transfer coefficient. A high significance (\(p<5\%\)) was observed between the control sample and the modified samples treated at 200 \({}^{\circ}\)C and 220 \({}^{\circ}\)C. Experiments with the use of holographic interferometry produced results consistent with previous studies conducted by different methods. t 27 January 2021  
8 March 2021  
11 March 2021  
**Problems:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. staypack wood, shaped under increased temperature and humidity conditions. [PERSON] and [PERSON] [5] focused on the absorption properties of thermally modified wood. [PERSON] [6] investigated thermal modification as a method to improve the dimensional stability of wood. [PERSON] also addressed this issue [7]. [PERSON] and [PERSON] [8] investigated the mechanical properties and mold resistance of thermally modified wood. [PERSON] [9] described the changes in wood that occurs during a two-phase thermal modification of wood. [PERSON], [PERSON] and [PERSON] [10] studied structural changes in thermally modified wood. [PERSON] and [PERSON] [11] compared the thermal conductivity of wooden samples with different degrees of thermal modification. [PERSON] and [PERSON] [12] researched thermal conductivity of fir and beech wood modified at various temperatures. [PERSON] [13] studied the effects of heat treatment on the surface characteristics of wild cherry wood. [PERSON] [14] researched the thermal conductivity of solid wood panels made from heat treated spruce and lime wood strips. [PERSON] [15] led research on the effect of heat treatment duration on the thermal conductivity of spruce and poplar wood. [PERSON] [16] studied the effect of heat treatment on the thermal properties of European beech wood. [PERSON] [17] dealt with the influence of the tool and the parameters of abrasive machining on the chip size of thermally modified wood. [PERSON] [18] observed a change in the properties of wood that had been modified at different temperatures. [PERSON] [19] investigated the effect of the thermal modification of pine wood on the energy consumption of a milling machine. [PERSON] [20] solved a similar topic with samples from spruce wood and [PERSON] [21] compared the quality of milled surfaces on samples modified at different temperatures. [PERSON] and [PERSON] [22] observed changes in the color of the surface of thermally modified wood during the weathering process. [PERSON] [23] dealt with the change of lignin in wood due to thermal modification.