A method of assembling an opto physiological (OP) sensor comprises: (i) modelling the opto physiological properties of at least one body tissue type to be monitored (ii) determining through application of the model an optimal optical design for an opto physiological (OP) sensor operable to monitor the opto physiological properties of the at least one body tissue type and (iii) making the OP sensor to the determined optical design. The optimal optical design for the OP sensor comprises: (i) determining the optimum separation of each of a plurality of light sources from a photodetector based on modelled optical path lengths for light travelling from each light source through the body tissue type to be monitored to the photodetector and (ii) locating light sources of different wavelengths at different distances from the photodetector.
