A method and apparatus for a downhole refractometer and attenuated reflectance spectrometer

The present invention provides a method for distinguishing between gas and liquid based on the much lower refractive index of gas. It can also be used to monitor fluid sample cleaning over time. The refractive index of a wellbore fluid is determined from the proportion, R, of light reflected from the interface between a transparent window having a known refractive index and this fluid. The refractive index is preferably measured at a light wavelength for which the fluid is not very attenuating. However, the nearby transmission spectrometer can be used to connect the refractive index measurement to attenuation at the wavelengths it monitors. This reflection-based refractometer design can also be used as a damped reflection spectrometer at very attenuating wavelengths.