Apparatus and methods for stable and reproducible optical diffuse
reflection measurement are described. Light sources and photodetectors
are used such that at least two points of light entry and at least two
points of light exit are established on the surface of the sample. The
light sources are modulated and the detector outputs are demodulated such
that the signals associated with the at least four optical measurement
beams are individually detected. The signals are processed such that all
instabilities generated at the surface of the sample are canceled out. In
effect, the accuracy of the measurement is increased by having only the
deeper layers of the sample contribute to the result. The geometry of the
probe can be optimized for specific sampling depths and for specific
measurement applications. Spectroscopic referencing can be eliminated
since instabilities of light emission and detection are also canceled
out.