## 2 Materials and Methods  
A total of 44 third-year dental students who attended the preclinical phantom course in Conservative Dentistry at the University of Zurich participated in the study. Informed consent was obtained from the participants, and they were anonymized by individual codes. Under these terms, the study was exempted from the need to get ethical approval by the local ethics committee, and a declaration of non-jurisdiction was obtained from the Swiss Ethics Committees on Research (BASEC-Nr 2020-01309). Before they attended the practical phantom course, all students had received a theoretical lecture on light-curing in the regular curriculum, which provided them with basic knowledge about light polymerization. The students were then randomly divided into a control group (\(n=22\)) and a training group (\(n=22\)). At the start of the phantom course, the participants from both groups were asked to light-cure a simulated posterior restoration (tooth 27) in the MARC Patient Simulator (BlueLight Analytics, Halifax, Canada) in order to assess their baseline light-curing performance. All students used the same light-curing unit (SmartLight Pro, Dentsply Sirona, Bensheim, Germany), and the irradiation time was standardized to 10 s. The output irradiance of the light-curing unit was monitored for consistency throughout the whole study period using a calibrated FieldMaxII-TO power meter and PM2 thermopile sensor (Coherent, Santa Clara, CA, USA). The irradiance delivered to the simulated restoration in the MARC Patient Simulator was determined using a 3.9-mm diameter cosine-corrected light detector (CC3-UV, Ocean Optics, Dunedin, FL, USA) attached by a fiber-optic cable to a laboratory-grade spectrophotometer (USB4000, Ocean Optics, Dunedin, FL, USA) inside a mannequin head. The detector was located at the bottom of a Class I cavity preparation in the upper left second molar, 2 mm below the cavosurface margin and 4 mm from the cusp tip. In order to better imitate clinical conditions, the MARC Patient Simulator was fixed in a dental chair, and its mouth opening was limited to 40 mm. Furthermore, dental rubber dam, sectional matrices, and a separation ring (Palodent V3 Sectional Matrix System, Dentsply Sirona, Bensheim, Germany) were placed as in routine filling therapies. Prior to the light-curing procedure, the students were instructed to position the head of the patient simulator as they would do with a real patient. At their free disposal, various auxiliary tools lay in front of them on the try of the dental chair, such as protective orange eyeglasses, standard protective glasses, an orange light shield, a mirror, and a probe. After testing the baseline performance of both groups, the training group received a three-part training with the patient simulator: In the first part, the students were given a practical demonstration with the simulator on how to optimize the light-curing technique. For this purpose, they were taught how to correctly adjust the dental chair and position the head of the patient according to ergonomic aspects and in order to provide the light guide with a straight-line access to the restoration. The students were then shown how to position and stabilize the light-guide tip correctly, i.e., at right angles and as close as possible to the restoration surface. Furthermore, the relevance of wearing blue-light-blocking orange protective glasses was demonstrated to the students, which allow them to look directly at the restoration during the light-curing process and to stabilize the light guide with their free hand. During the practical demonstrations, the integrated visual feedback mechanism of the patient simulator allowed the students to follow in real time how small changes in the light-curing technique, such as changes in the position, distance, and tilt angle of the light guide, affect the amount of light delivered to a restoration. In the second part of the training, the students were asked to light-cure the simulated restoration in tooth 27 of the MARC Patient Simulator.