The disclosed watch may include a watch band, a watch body that includes a front surface and a rear surface, at least one light emitting diode, and an image sensor. The watch body may be configured to contact a user's wrist when the watch body is attached to the watch band and donned by the user. The at least one light emitting diode and the image sensor may be on the rear surface of the watch body. The at least one light emitting diode may be configured to provide a light source for a heart rate monitoring function of the watch when the watch body is attached to the watch band and to provide a light source for the image sensor when the watch body is detached from the watch band. Various other related methods and systems are also disclosed.