An improved sensor (102) for hydration monitoring in mobile devices, wearables, security, illumination, photography, and other devices and systems uses an optional 
phosphor-coated 
broadband white LED (103) to produce 
broadband light (114), which is then transmitted along with any ambient light to target (125) such as the ear, face, or 
wrist of a living subject. Some of the 
scattered light returning from the target to 
detector (141) is passed through a 
narrowband spectral filter set (155) to produce multiple 
detector regions, each sensitive to a different waveband 
wavelength range, and the detected light is spectrally analyzed to determine a measure of hydration, such as fluid losses, fluid ingested, 
fluid balance, or rate of fluid loss, in part based on a noninvasive measure of components of the bloodstream. In one example, variations in components of the bloodstream over time such as 
hemoglobin and water are determined based on the detected light, and the measure of hydration is then determined based on the in components of the bloodstream over time. In the absence of the LED light, ambient light may be sufficient illumination for analysis. The same sensor can provide identifying features of type or status of a tissue target, such as 
heart rate or 
heart rate variability, 
respiratory status, or even confirmation that the tissue is alive. Hydration monitoring systems incorporating the sensor, as well as methods, are also disclosed.