A light guide system for a physiological sensor includes a light channel (38) in optical communication with an emitter and a light channel (40) in optical communication with a detector. The light channels are formed of a bio-compatible resilient plastic, such as a silicone, that is blended with a colorant. Each light channel has an optical transmission spectrum that passes the emitter spectral band and rejects most or all ambient light. The colorant can be a pigment provided as a suspension in a liquid or a dye provided in liquid form. The optical transmission spectrum defines a longpass optical filter where the shortest wavelength of the optical spectrum of the emitter is greater than the transition wavelength of the longpass optical filter. Each of the light channels is configured to be in contact with the skin, thereby reducing or eliminating optical loss due to transmission through an air gap.