## Materials and Methods
### Experimental Set-Up
#### 2.1.2 Technical Realization of Laser Projection Unit  
The in vivo recording situation and the subsequent image processing impose several fundamental requirements on the laser projection unit. On the one hand, the visibility of the projected laser pattern on the vocal folds in the camera recordings is essential for robust detection. Green laser light is advantageous for this purpose since most cameras (color and grayscale) display an increased sensitivity for green and red light whereas green can be easily distinguished from the vocal fold tissue beneath. Typical high-speed recording frame rates for human phonation are 4000-8000 fps causing very short exposure times for each frame. In order to guarantee sufficient brightness and contrast of the laser pattern to the tissue surface, we have to ensure enough illumination power. On the other hand, it is mandatory to realize the design within the safety regulations for the exposure of skin tissue to laser radiation and therefore limit the laser power respectively. To the best of our knowledge, there are no official limits specifically for laser exposure of mucosal tissue. According to the guidelines of the International Commission on Non-Ionizing Radiation Protection (ICNIRP) [23], the limits for skin exposure in the wavelength range of 400-1400 nm depend on the duration of the exposure. Typically, an examination of the vocal fold dynamics takes less than 1 min in total (inserting, positioning, recording and removing). However, the actual exposure duration during phonation is even shorter due to the natural relative movement between the test subject and the clinical operator. According to our measurements, the exposure of the exact same tissue point never exceeds 0.5 s. In the duration range between 100 ns and 10 s, the exposure limit \(EL\) given by the ICNIRP is determined by means of the following formula  
\[EL=11\cdot(t)^{0.25}\frac{\mathrm{kJ}}{\mathrm{m}^{2}}. \tag{1}\]  
For safety reasons, we calculated the exposure limit based on 1 s exposure duration, resulting in a maximum exposure of 11 kW/m\({}^{2}\). The local ethics commission has approved this approach (reference number: N\({}^{\circ}\) 123_15B). For our present set-up, as shown in Figures 2 and 3, we chose a continuous wave, frequency doubled Nd:YAG laser at a wavelength of 532 nm. By the use of a single diffractive optical element (DOE) in combination with a lens system, the initial laser beam is split into a regular laser grid of  
Figure 2: Overview of the experimental measuring device. The imaging unit comprises a rigid 70\({}^{\circ}\)-Jaryngoscope, a zoom coupler and a high-speed camera. A frequency-doubled Nd:YAG laser source is connected to the laser projection unit via a glass fiber. Different perspectives on the tip of the miniaturized endoscopic laser projection unit are provided on the left. \(31\times 31\) points, resulting in \(n_{\mathrm{grid}}=961\). The total output power at the tip of the LPU can be continuously adjusted up to \(P_{\mathrm{max,total}}=450\) mW. At a working distance of 50-80 mm below the tip of the LPU, the \(1/\mathrm{e}^{2}\) radius of the Gaussian beam profile ranges in \(r_{\mathrm{spot}}=175\)-\(225\,\mathrm{\SIUnitSymbolMicro m}\). Assuming \(I_{\mathrm{max}}\approx 2\cdot I_{\mathrm{mean}}\) for a Gaussian profile, as in Figure 4, the maximum available intensity \(I_{\mathrm{max}}\) is approximated by  
\[I_{\mathrm{max}}=2\cdot\frac{P_{\mathrm{max,total}}}{n_{\mathrm{grid}}\cdot \pi\cdot r_{\mathrm{spot}}^{2}}. \tag{2}\]  
This yields \(I_{\mathrm{max}}=5.9\)-\(9.7\,\mathrm{kW}/\mathrm{m}^{2}\) for the present configuration which is below the maximum exposure of \(11\,\mathrm{kW}/\mathrm{m}^{2}\). We believe this to be the optimal balance between acceptable tissue exposure and sufficient conditions for an automated image processing. In the calculation of the maximum exposure, the characteristics of the grid as well as the beam propagation have to be considered.