*6.1. Experimental Plan*

To evaluate the influence of the forced air flow on the light deflection effect, different experiments were conducted:


The cooling effect (1) was measured using the thermographic infrared (IR) camera (VarioCAM from InfraTec, Dresden, Germany, with a maximum frame rate of 10 Hz) mounted underneath the projector (see Figure 4). To this end, a red-hot cylinder was measured for a duration of 18 s at the maximum frame rate. During the temperature measurement, the magnetic valve was opened for 8 s. The emission coefficient was set to one (*ε* = <sup>1</sup>), and the mean temperature of an arbitrarily selected rectangle is plotted in relation to the passed time in Figure 6.

**Figure 6.** Thermographic images from the IR camera and the development of the mean temperature. (**b**–**d**) are taken at the specified points in time indicated by the green spots in (**a**). The window from which the mean temperature data in (**a**) is taken is marked by a crosshatched rectangle in (**b**).

To estimate the size of a refractive gradient field qualitatively (2), the BOS method was used. To this end, a video camera (not shown) was placed in front of the random pattern background without the measurement object in between both. The video capture was started before the hot cylinder was placed. After an undisclosed time delay, the magnetic valve was opened. The first image of the video was used as the reference image to calculate the results. Here, the pixel displacement was used as a means to quantify the light deflection relatively, since a reconstruction of valid refractive index values required extensive a priori knowledge about the setup and the measured field. This procedure was valid in the presented case, since the experiments were conducted without moving any parts of the setup during the video sequence.

To analyze the effect of the interaction of the forced air flow actuator and refractive index field on triangulation measurements (3), the multi-camera FPS was used. Here, the cylinder was placed, and a reference measurement without active air flow was conducted. This was directly succeeded by activating the air flow actuator, waiting a defined delay time *td*, and then, triggering the start of the measurement. This procedure reduced the uncertainties induced by moving or exchanging the objects, as discussed in [15]. There was a difference between the direct and the indirect method of estimation for the light deflection effect. This was mainly due to the different paths of light that were examined. When evaluating Figure 4, it is clear that the light for the FPS was traversing different paths compared to the view rays for the BOS method. Due to the 2D nature of the BOS measurement setup, the pixel-wise magnitude of the light deflection effect could not be correlated directly to the reconstructed 3D points. Additionally, the random pattern background used for BOS measurements was blocking the field of view of the lower cameras and of the IR camera. Therefore, the experiments were not carried out simultaneously.
