A photometry method for estimating the full-field opening for fluid-filled fractures in transparent materials is presented. Measurements are based on analysis of the intensity drop, according to an exponential extinction law, of a backlight source as it passes through the dye-laden fluid that fills the fracture. The ability to analyse full-field fracture opening in three-dimensional experimental configurations permits many new insights into the physics of fluid-driven fractures. An example is presented that illustrates analysis of the near-tip asymptotic behaviour of the fracture opening which, under certain conditions, may be used to compute the mode I stress intensity factor. The example both provides a means by which the fracture toughness may be estimated without considerations of the details of the internal loading and illuminates some pitfalls associated with estimation of fracture toughness from analysis of fluid pressure records.