1. A method of measuring a material comprising: irradiating a region of interest in the material with spatially coherent light having a first beam with a first wavelength and a second beam with a second wavelength; directing reference light having the first wavelength and the second wavelength along an optical path having a variable path length; detecting scattered light from the material in response to the irradiating light and detecting the reference light while varying the path length; and generating a heterodyne signal from the detected scattered light and the detected reference light.