This Phase I SBIR will establish the feasibility of developing a collinear three wavelength source for an in-situ, 1800, back scatter nephelometer. The three wavelength source is being developed to facilitate on-board validation measurements of airborne remote backscatter lidar. Currently, commercial nephelometers utilized for in-situ validation measurements employ wavelengths that are both not matched to the operational wavelengths and not at the 1800 angle used in near nadir remote sensing backscatter lidar systems. Uncertainties in the wavelength and angle dependence of the return signal make in-situ validation of the remote measurement difficult. The performance of remotely operated lidar systems will be greatly improved by the successful demonstration of an in-situ backscatter measurement with the proposed collinear three wavelength source.