A turbomachinery control system for controlling supercritical working fluid turbomachinery. The control system includes a light emitter to project light through working fluid of the turbomachinery toward a primary light detector provided within a line of sight to the emitter. The system further includes one or more secondary light detectors spaced from the line of sight, and a controller determining one or both of an intensity of light detected by the primary detector relative to the detected light intensity by the secondary detector, and wavelength of light detected by the primary detector relative to wavelength of light detected by the secondary detector. The controller determines the working fluid proximity of the critical point based on one or both of the determined relative intensity and determined relative wavelength, and controlling an actuator to control turbomachinery inlet or outlet conditions in accordance with the working fluid determined proximity of the critical point.