However, if we restrict ourselves to locations outside the convex hull (think shrink-wrap) of the object, i.e. in free space, then we can measure the plenoptic function by taking many photos using a digital camera. Moreover, in this case the function contains redundant information, because the radiance along a ray remains constant from point to point along its length, as shown at left. In fact, the redundant information is exactly one dimension, leaving us with a four-dimensional function (that is, a function of points in a particular four-dimensional manifold). Parry Moon dubbed this function the "photic field" (1981), while researchers in computer graphics call it the "4D light field" (Levoy 1996) or "Lumigraph" (Gortler 1996). Formally, the 4D light field is defined as radiance along rays in empty space.