## 2 Materials and Methods
### Data Collection Procedure
#### 2.3.2 Experimental Procedure  
Following the initial evaluation, the experiment itself began. A within-subject experimental design was used in order to account for inter-individual variability of psychophysiological reactions. The experiment consisted of repeatedly glaring the participants (\"drivers\", who had their own headlamps on) by different oncoming vehicle's light sources while monitoring their ECG, SCR, breathing, and blinking frequency, followed by an assessment of their discomfort glare rating (using the modified [PERSON] scale [23], which was explained to the participants) and changes in visual acuity and contrast sensitivity. In total, each participant went through eight different light source combinations (two different driver's headlamps \(\times\) four different light samples of the oncoming vehicle) and were glared four \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Photometry Test Point** & **Halogen Bulb** & **Series LED** & **Bluish LED** & **Yellowish LED** \\
**and Gradient** & **Reflector** & **Module** & **Module** & **Module** \\ \hline HV & 475 cd & 436 cd & 248 cd & 558 cd \\ BR & 261 cd & 275 cd & 199 cd & 298 cd \\ B50L 1 & 267 cd & 238 cd & 205 cd & 271 cd \\
50L & 5934 cd & 13,280 cd & 7732 cd & 7536 cd \\ \hline
75R & 15,256 cd & 25,730 cd & 12,260 cd & 22,150 cd \\ Gradient & 0.233 & 0.291 & 0.377 & 0.198 \\ \hline \hline \end{tabular} 1  
\end{table} 
Table 3: Right light samples and their main photometric values as per ECE standards. times by each of these combinations, totaling 32 glare events per participant (Due to technical errors while manually operating the carousel, some participants were glared more than four times by some combinations, in which case only four glare events for this particular combination were selected.) The order of the light source combination presentation was counterbalanced across participants so that the light samples were presented to each participant in a different order. This procedure is summarized in Figure 3.  
More precisely, the participants were instructed to keep their head on the chin rest and look straight forward \"as if they were driving\", and not to talk except when answering a question (in order to minimize bias in the psychophysiological data). They were also handed a button with the instruction to press it \"as soon as the oncoming vehicle's light hits their eyes\" during each trial to validate their responses. Their psychophysiological responses were recorded using the BIOPAC MP 36 (BIOPAC Systems, Inc., Aero Camino, Goleta, CA, USA), a bio-signal acquisition device often used for psychophysiological studies [35; 36]. First, the driver's headlamps were turned on (either white LEDs or halogen bulb reflectors). Next, the researcher also turned on the headlamps of the \"oncoming vehicle\" (one of the four samples, varied randomly in order across the participants) mounted on a carousel and set in a position in which they do not glare the driver. At a variable interval from 1 to 10 s (so that the participants could not predict the situation and prepare for it), the researcher smoothly tilted the carousel to glare the driver for approximately 0.5 s. Due to the carousel not being motorized, this had to be done manually, so the length of each glare might have slightly varied. Upon inspection, no glare was shorter than 300 ms and longer than 1 s; within this range, we did not assume variations in reactions caused by this variable glare duration [37]. Following a 30 s interval when the lights were not shining into the driver's eyes, the researcher tilted the carousel again and glared the driver, subsequently  
Figure 3: Experimental procedure. waiting for another 30 s. This procedure was repeated once more, so there were a total of four glares from one source of the oncoming vehicle, with 30 s intervals in between the glares. The length of the interval was chosen with respect to the typical onset and duration of the psychophysiological responses to the stimulus and stabilization back to baseline.