An opto-physiological (OP) sensor 12, e.g. a photoplethysmography sensor, comprises: a plurality of light sources 28； and at least one photodetector 30 configured to capture radiation from the light sources, after the light from said sources has travelled through a body tissue type to be monitored, wherein the light sources are located relative to the photodetector at positions determined by a model 14 of opto-physiological properties of the body tissue type to be monitored. The optimum separation of each light source from the photodetector is based on modelled optical path lengths for light travelling from the source, through the body tissue type, to the photodetector, and wherein light sources of different wavelengths are located at different distances from the photodetector. The body tissue type may comprise one or more of: a tissue site, a tissue colour, a tissue thickness, a tissue age and a tissue condition. The opto-physiological properties of the body tissue type may comprise one or more of: a coefficient of absorption, a coefficient of scattering, a refractive index and optical density.
