Current definitions of immersion describe its relationship to presence and allow for relative comparisons between the immersive qualities of Virtual Reality (VR) systems, but lack the ability to describe the immersion supported by a system as an absolute quantity. In this article, we present an abstract model of perception, defining sensory units as the smallest biological registers of sensation within the body. Two metrics of immersion are introduced: the immersed sensory range, and the immersed sensory field, which can be defined for both individual sensory units and entire sensory categories. We define an isolated sensory unit as one that is shielded from non-VR stimuli, and derive the terms isolated field and isolated range from this definition. These metrics are further described as ratios, resulting in a set of theoretical and practical attributes which can be used to quantify the immersive potential of a VR experience.