There are many mechanisms to sense arousal. Most of them are either intrusive, prone to bias, costly, require skills to set-up or do not provide additional context to the user's measure of arousal. We present arousal detection through the analysis of pupillary response from eye trackers. Using eye-trackers, the user's focal attention can be detected with high fidelity during user interaction in an unobtrusive manner. To evaluate this, we displayed twelve images of varying arousal levels rated by the International Affective Picture System (IAPS) to 41 participants while they reported their arousal levels. We found a moderate correlation between the self-reported arousal and the algorithm's arousal rating, r(47)=0.46, p<.01. The results show that eye trackers can serve as a multi-sensory device for measuring arousal, and relate the level of arousal to the user's focal attention. We anticipate that in the future, high fidelity web cameras can be used to detect arousal in relation to user attention, to improve usability, UX and understand visual behaviour.