Flexible manipulators have promising applications in minimally invasive surgery as it allows the surgical tools reach targets which are prohibited by conventional rigid surgical instrument. However one of the technical difficulties of implementing the flexible manipulator is to measure the bending curvature. This paper proposes the design of a novel optical sensor for measuring the bending curvature of a flexible manipulator based on light intensity modulation. The sensor is low cost and is temperature independent. A theoretical model of using the sensor design to deduce the curvature of a flexible robot has been created. Implementing the proposed theoretical model, the developed sensor has been used to measure the bend of a section of a flexible segment. Validation tests have been carried out; the results demonstrate that the developed sensor has good accuracy in measuring the bending angles, the orientation of the bending and the bending radius.