An apparatus for effecting a control action in respect of a function within a virtual or mixed reality system, the control action corresponding to a predefined bodily movement of an authorized user of said function, wherein an authorized user is defined by a predetermined criterion in respect of a selected body part and/or a passive device carried thereon. The apparatus comprises a detection module for detecting a predefined bodily movement; a multi-spectral imaging system for capturing spectral reflectance and/or emission data at a plurality of wavelengths in respect of said selected body part and/or the passive device carried thereon; and an analysis module for comparing at least a portion of the spectral data with data corresponding to an authorized user of said function to determine if said criterion is met, and outputting a signal to effect said control action only if said criterion is met.