A G-source, an abstract sensor component representing either a light source or a TV camera, is defined. G-source illumination conditions are investigated to describe the condition under which each G-source illuminates (or observes) surface regions with respect to its illumination/observation directions. Sensor detectability is defined as a function that takes object boundaries and produces a set of points on the object from which the sensor can obtain useful data. It is shown that sensor detectability can be represented by G-source illumination conditions and/or operations between them. The way in which G-sources are combined is described by a sensor-composition tree in which leaf nodes represent G-source illumination conditions and branch nodes represent set operations. Applications of the sensor composition tree to object boundaries are investigated and a set of detectable points is obtained using the VANTAGE geometric modeler. Several 2D structures are proposed to represent these detected points given by VANTAGE. A brief overview of how to use these capabilities in building model-based vision systems is given.>