An imaging based interferometric pressure sensor apparatus compromise a fluid pressure sensor unit (1) and an optical monitor (2). The disposable pressure sensor part comprises a rigid transparent cover plate 3 and a flexible diaphragm (4), in between forming an air-gap cavity (8). By illuminating the air-gap cavity (8) with a light source (6), the air-gap cavity generates an interference pattern which is captured by the optical imaging device (7). The pressure of the fluid (5) to be sensing is applied to flexible diaphragm, causing the deformation of the diaphragm and the variation of the air-gap thickness. Hence the interference pattern varies with pressure of fluid. The optical monitor (2) includes light source 6 and optical image device (7) which records the interference pattern from the fluid pressure sensor unit (1). The pressure of the fluid is measured by processing the captured image.