A new and improved method and apparatus for determining cerebral oxygenation. In one basic aspect, a new processing methodology for oxygenation determination, in particular cerebral oxygenation, is provided based on the apprehension and discovery that time-based photon quantity determinations of the wavelength-specific light leaving the head of the patient may be significantly improved, and simplified, by determining the quantity of returning photons in at least one set of two different but closely spaced points in time following the instant of injection, and then taking the ratio of the determinations in each set, to thereby cancel and remove several important sources of error.