A wearable device , comprising: a first light emitter configured to illuminate a target via an external body surface; a second light emitter configured to illuminate a target via the external body surface a photodetector configured to detect data corresponding to a first property of the target in response to illumination by the first light emitter and data corresponding to a second property of the target in response to illumination by the second light emitter, the first and second property being indicative of first and second relative locations of the target with respect to the first and second light emitters; a mount configured to mount the first and second light emitters and the photodetector to an external body surface proximate to the target; and a controller configured to: operate the first and second light emitters to emit respective amounts of light alternating between the first and second light emitters over different periods of time; and receive data from the photodetector; determine whether the target is aligned based on the first and second relative locations of the target, wherein the target is aligned when the target is within a specified region between the first light emitter and the second light emitter; wherein the determination that the target is aligned is based on the first and second property detected by the photodetector during respective periods of operation of the first and second light emitter being substantially equal. a first light emitter configured to illuminate a target via an external body surface; a second light emitter configured to illuminate a target via the external body surface a photodetector configured to detect data corresponding to a first property of the target in response to illumination by the first light emitter and data corresponding to a second property of the target in response to illumination by the second light emitter, the first and second property being indicative of first and second relative locations of the target with respect to the first and second light emitters; a mount configured to mount the first and second light emitters and the photodetector to an external body surface proximate to the target; and a controller configured to: operate the first and second light emitters to emit respective amounts of light alternating between the first and second light emitters over different periods of time; and receive data from the photodetector; determine whether the target is aligned based on the first and second relative locations of the target, wherein the target is aligned when the target is within a specified region between the first light emitter and the second light emitter; operate the first and second light emitters to emit respective amounts of light alternating between the first and second light emitters over different periods of time; and receive data from the photodetector; determine whether the target is aligned based on the first and second relative locations of the target, wherein the target is aligned when the target is within a specified region between the first light emitter and the second light emitter; wherein the determination that the target is aligned is based on the first and second property detected by the photodetector during respective periods of operation of the first and second light emitter being substantially equal. <EOS>