The pupil diameter (PD) has been found to respond to cognitive and emotional processes. However, the pupillary light reflex (PLR), is known to be the dominant factor in determining pupil size. In this paper, we attempt to minimize the PLR-driven component in the measured PD signal, through an Adaptive Interference Canceller (AIC), with the H^∞ time-varying (HITV) adaptive algorithm, so that the output of the AIC, the Modified Pupil Diameter (MPD), can be used as an indication of the pupillary affective response (PAR) after some post-processing. The results of this study confirm that the AIC with the HITV adaptive algorithm is able to minimize the PD changes caused by PLR to an acceptable level, to facilitate the affective assessment of a computer user through the resulting MPD signal.