methods and apparatus utilizing back scattered laser light measurement techniques for the measurement of particle size distribution in a fluid are disclosed . such methods and apparatus are particularly suited for but not limited to the measurement of sub - micron particle size distribution in fluids such as petroleum fluids which are colored or opaque and strongly absorptive of incident light . utilizing an optical transceiver partly immersed in the fluid , the methods and apparatus include provision for the measurement of particle size distribution while the fluid is subjected to conditions of high pressure and / or high temperature .