Optical sensor for a wearable device

An apparatus comprising:

• a housing;
• a light source disposed at an edge of the housing in a first position, wherein the light source, when affixed to a surface of a body, is operable to emit light into the body from the first position;
• a first optical sensor disposed at the edge of the housing in a second position, the second position being a first fixed distance from the first position to measure a first backscatter of the light from an artery of the body at a first depth below the surface of the body;
• and
• a second optical sensor disposed at the edge of the housing in a third position, the third position being a second fixed distance from the first position to measure a second backscatter of the light from a vein of the body at a second depth below the surface of the body
• a processing device configured to determine a trend in a hydration condition of the body based on the first backscatter of light and the second backscatter of light.

A method comprising:

• emitting, by a light source affixed to a body at a first position, a first wavelength of light into the body;
• receiving, by an optical sensor affixed to the body at a second position, light from a depth below a of the body corresponding to a muscular-walled tube of the body, wherein:
• the first position is at a fixed distance from the second position to detect backscatter from the depth below the surface of the body;
• and
• the first position of the light source is at a first side of the muscular walled tube of the body and the second position of the optical sensor is at a second side of the muscular walled tube such that the light source and the optical sensor straddle the muscular walled tube;
• determining, by a sensor interface, a current sodium to potassium ratio of the body based on the detected backscatter of the first wavelength of light;
• comparing, by a processing device, the current sodium to potassium ratio to a previous sodium to potassium ratio of the body;
• and
• determining a hydration condition of the body when the current sodium to potassium ratio is different than the previous sodium to potassium ratio.