A correlation between the diameter of the pupil and the mental workload of the brain has been found as early as in the 70s. With the simple detection of the pupil size in modern eye tracking systems it is now possible and practical to study this measurement systematically.
Significance for the emotional intensity: high
Informative value on the kind of emotion: low
Informative value on the mental workload: high
Effort: measurements through standard eye tracking systems