A method of assembling an opto-physiological (OP) sensor comprising: modelling the opto-physiological properties of at least one body tissue type to be monitored; determining, through application of the model , an optical design for an opto-physiological (OP) sensor operable to monitor the opto-physiological properties of the at least one body tissue type; and making the OP sensor to the determined optical design ; wherein the optical design for the OP sensor comprises: determining a 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 locating a plurality of light sources of a first wavelength in an array on the circumference of a first circle, the first circle being centred on the photodetector and having a radius which is a function of the first wavelength; and locating a plurality of light sources of a second wavelength in an array on the circumference of a second circle, the second circle being centred on the photodetector and having a radius which is a function of the second wavelength; and providing a controller configured to estimate a degree of tilt between the sensor and tissue based on relative changes in intensity from each light source of a single wavelength with respect to the overall intensity of all light sources of that wavelength. modelling the opto-physiological properties of at least one body tissue type to be monitored; determining, through application of the model , an optical design for an opto-physiological (OP) sensor operable to monitor the opto-physiological properties of the at least one body tissue type; and making the OP sensor to the determined optical design ; wherein the optical design for the OP sensor comprises: determining a 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 locating a plurality of light sources of a first wavelength in an array on the circumference of a first circle, the first circle being centred on the photodetector and having a radius which is a function of the first wavelength; and locating a plurality of light sources of a second wavelength in an array on the circumference of a second circle, the second circle being centred on the photodetector and having a radius which is a function of the second wavelength; and determining a 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 locating a plurality of light sources of a first wavelength in an array on the circumference of a first circle, the first circle being centred on the photodetector and having a radius which is a function of the first wavelength; and locating a plurality of light sources of a second wavelength in an array on the circumference of a second circle, the second circle being centred on the photodetector and having a radius which is a function of the second wavelength; and providing a controller configured to estimate a degree of tilt between the sensor and tissue based on relative changes in intensity from each light source of a single wavelength with respect to the overall intensity of all light sources of that wavelength. <EOS>