## 3 Results
### Concept of Sensors and Illumination  
A laser sensor with safety class I was chosen to enable a precise distance measurement at one point and to achieve an unobstructed view over the treated area. Despite the recommended perpendicular treatment of the skin, in reality, the attending specialist mostly applies the kINPen(r) MED-\(\beta\) with an incidence angle of 25\({}^{\circ}\) to 40\({}^{\circ}\). This results in an elliptical (comet-like) effective zone and in an elliptical reflection area of the sensor signal (Figure 3).  
Two laser sensors (VL6180X) are distributed around the kINPen(r) MED-\(\beta\). The laser proximity sensor works based on absolute time of flight measurement. The time required for the signal to be detected after transmission and reflection is proportional to the distance the signal travels. The chosen sensor possesses a cross talk elimination option and ambient light detection. In this way, it can detect the signal precisely even at short distances and independent of the reflectance and color of the reflecting  
Figure 3: Concept of sensors: (**a**) front view, (**b**) side view; blue: visible plasma effluent, red: laser beams.  
surface. The sensors are aligned directly with the effluent at a distance of 7 mm to 10 mm (on average 8.5 mm) to ensure the maximum efficacy when the kINPen(r) MED-\(\beta\) is tilted, but also in case of a vertical application. In order to further minimize the signal area, an aperture is placed in front of the sensor. This ensures that the sensor area lies directly on the effluent and scarcely more area is detected than the visual effluent contact area of the kINPen(r) MED-\(\beta\). This eliminates errors caused by unevenness and mistakes through tilting. It has to be mentioned that the conceivable laser sensor VL6180X works with a wavelength of 850 nm. The plasma generated by kINPen MED-\(\beta\) and argon as a feed gas also emits light of this wavelength, which may result in an incorrect distance measurement. In case this could be a problem under application-oriented conditions, the focus of the sensors can be shifted, albeit the possibility of errors due to unevenness may increase again. In addition, it has to be ensured that the device is not tilted in such a way that the sensors lie on the bottom side. Thus, the upper side should therefore be clearly marked or four sensors should be used with a automatic recognition of the sensor position. It is known that the effective area of the kINPen(r) MED-\(\beta\) is larger than the visible effluent [2; 30]. Thus, this area has to be visualized for the attending specialist. This illumination should be visible under application-oriented light conditions and should not influence the functionality of the kINPen(r) MED-\(\beta\). Thus, several LEDs can be arranged symmetrically around the kINPen MED-\(\beta\), illuminating a large part of the surface (Figure 4). The LEDs have to be positioned in such a way that the beam geometries of the illumination cones and of the efficacy cones are adjacent to each other. In addition, a ring aperture should be placed in front of the LEDs, which would only allow a narrow gap of light to pass through. The gap forms a circle that encloses the effective area.  
Figure 4: Concept of visualization; Illumination of treatment area using circular aperture for fixed incidence angle (aperture replaceable); (**a**) circular aperture for perpendicular (0\({}^{\circ}\)) and (**b**) tilted application, (**c**,**d**) including different perspectives using tilted device; green: estimated effective plasma zone, yellow: LED-illuminated zone.  
To confirm the efficacy of the introduced concept, some exemplary test methods were used to determine the influence of the incidence angle on the ROS distribution, (V)UV irradiation and antimicrobial property of the plasma source. This approach may be a possibility for the determination of the effective area dependent on a novel developed or re-adjusted concept for the tip of the handset of the kINPen(r) MED.