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.
