Wearable heart rate monitor

A wearable fitness monitoring device comprising:

• a motion sensor configured to provide output corresponding to motion by a user wearing the wearable fitness monitoring device;
• a photoplethysmographic heart rate monitor comprising (1) a periodic light source, (2) a light detector positioned to receive periodic light emitted by the periodic light source after interacting with a user's skin, and(3) circuitry determining a user's heart rate from an output of the light detector;
• and
• one or more processors configured to:
• (a) operate the heart rate monitor in a first mode while also operating in a skin characterization mode for determining a characteristic of the user's skin, wherein the first mode is configured to determine one or more characteristics of the user's heartbeat waveform, and wherein operating in the skin characterization mode comprises:
• (i) emitting a plurality of light pulses using the periodic light source in the heart rate monitor,
• (ii) detecting the plurality of light pulses using the light detector in the heart rate monitor after the plurality of light pulses interact with the user's skin,
• (iii) generating a plurality of data points, each data point comprising an intensity value of a respective light pulse of the plurality of light pulses as emitted in (i) and an intensity value of the respective light pulse of the plurality of light pulses as detected in (ii) that corresponds to the emitted light pulse of the plurality of light pulses as emitted in (i), and
• (iv) fitting a mathematical relationship to the plurality of data points;
• (b) using the mathematical relationship to determine a light source emission intensity setting that provides a pre-determined light detection level;
• and
• (c) adjusting a light source emission intensity to said light source emission intensity setting determined in (b) for operating in the first mode.