**7. Discussion**

Overall, the direct effect of the forced air flow on the development of a refractive index field around a hot cylinder was shown using the BOS method. The qualitative influence was similar in the simulation and measurement (compare Figures 5 and 7).

However, the effect was less distinguishable in the FPS measurements (see Figure 8 and Table 3). Here, the activation of the air flow yielded higher reconstruction errors for the immediate use (*td* = 0.3 s) of the forced air flow compared to the uninfluenced measurement (compare the dashed and solid red and cyan lines in Figure 8d). This indicated a contradiction: the size of the light-deflecting refractive index field was reduced, but the reconstruction error was increased. In the presented setup, the change of the radiation color of the specimen might have an increasing influence on the reconstruction quality. A cooling of the object resulting in a lowering of the radiation through self-emission was observed during the experiment and could be concluded from the thermographic measurement (see Figure 6). To investigate the stated matter, the necessary time delay between the activation of the air flow and the start of the projection sequence was estimated to be between the start of the flow activation (dotted-dashed line) and the start of the linear temperature development at *tir* ≈ 8.0 s. Adding the activation time *td*,<sup>0</sup> resulted in an improved time delay of *td* = 3.3 s. The result of the implementation of this improved delay is shown in Figure 8 (blue/green lines). It is obvious that the improved time delay resulted in an improvement in the estimated reconstruction quality compared to the initial time delay *td* = *td*,<sup>0</sup> = 0.3 s, as well as the measurement without the forced air flow.

Any concerns about an induced vibration through the forced air flow and a resulting decrease of reconstruction quality could be dispelled through the comparison of the measurements at *Tc* ≈ 300 K. If any vibration would have been induced, a significant increase in the reconstruction error metric would have been noticed.
