## 3 Methodology
### The Instructional Film  
The institutional film prompts the replication of a series of predetermined facial movements as exemplified by an on-screen model. Hence, the expected signal was obtained by analysing the movements of this model. Specifically, this was done by manually coding the video based on [PERSON]'s FAC system through cross-referencing information found in the FAC Manual [46]. Therefore, the instructional film method compares the participant's movements, as measured by the camera-based FAC solution, to a transcription of the model's movements, made by the investigator, to gain insight into the performance of the camera-based FAC solution. This instructional film approach is considered desirable, as the common alternative of manually coding each video is highly labour-intensive. As over 4 h of footage was captured in this study, this would have taken approximately 220 h. However, inherently there is a trade-off associated with this approach. The discrepancy between the expected response signal and the camera-measured signal is influenced by two factors: system performance, reflecting the accuracy of the system in measuring the individual's physiological state; and task performance, involving the individual's ability to synchronously reproduce the prompted action in accordance with on-screen instructions. To produce a dataset [PERSON] et al. used a similar protocol in which a group of participants were asked to recreate facial actions to the best of their ability [57]. Moreover, [PERSON] et al. showed that over 70% of their participants tested were able to activate target action units--although they often co-activated other non-target action units when trying to do so [58]. Therefore, it was considered feasible that participants should be able to recreate facial action adequately enough to provide insight into differences in system performance for _OpenFace_ 2, between the illumination conditions as long as only the target expressions are analysed. This is, however, based on the assumption that participants perform the facial actions task consistently between trials. There are also some benefits of using an instructional film for assessing the performance of the camera-based heart rate measurement system. When assessing responses to screen-based media, facial expressions are often elicited. Hence, the task of reproducing facial movements can be considered representative of these spontaneous movements. Therefore, the influencing factor of motion is accounted for using this method when assessing the system performance. Moreover, as the instructional film is delivered via a screen, the changes in illumination on the skin are also representative of those which may be elicited from stimuli used for exploring psychophysiological responses to screen-based media. To create the instructional film, video content provided courtesy of IMotion was used to prompt the 18 facial actions _OpenFace_ 2 is capable of recognising [59]. These short clips, each showing a different facial action, were repeated three times in a row before moving on to the next one. The repetitions of each action were sequenced rhythmically and consistently, with the expression onsets starting every three seconds throughout the film so that the timing of the movement was predictable. To allow for the synchronisation of the video footage with the instructional film, a visual marker was placed in the instructional film in the form of a white flash at the start, which created a clear change in illumination visible on the viewer's skin. This was followed by a black screen which lasted 15 s, and then the instructions began. Figure 1 shows the overall brightness of the instructional film, showing the bright flash at the start followed by a dark screen which precedes the facial action stimuli. The instructional film was 177 s long and rendered at 30 FPS at 720p. It should be noted that further tasks were also included in the instructional film, which continued after the aforementioned 177 s, notably prompting head and eye movements.