Abstract: A non-invasive device and method for monitoring concentration levels of blood and tissue constituents within a living subject such as a human or animal utilizes a polychromatic light source that emits light over a broad spectrum of wavelengths in the near infrared range. The light is passed through, or reflected from, a part of the subject such as a finger, ear lobe or other part of the body. That light is then separated into its various components by means of a grating or prism, and the near infrared band is focussed onto a linear array detector. A microprocessor uses the output of the array detector to measure the light transmitted (T), calculate the absorbance (log 1/T) and calculate the second derivative of the absorbance. A calibration equation is used for each constituent to be monitored to convert the second derivative measurements to a concentration level for that constituent. The device is programmed to take measurements between heart beats and to adjust for the temperature of the sample being taken.