Spectroscopic Light Scattering (SLS) is an informative in-situ monitoring method that irradiates the surface of thin films by white light, and monitors the scattered light by a spectrometer (Figure). This newly introduced technique provides information on surface roughness, deposition speed, composition and optical properties, while it can be easily attached to existing systems, and also low in cost. We have developed this SLS technique under collaboration with HMI (Germany), and have applied to the three-stage deposition process of Cu(In,Ga)Se₂ (CIGS) thin films. We have found SLS useful for controlling the optical and physical properties of the film during the deposition, and consequently improve the performance of the CIGS solar cells. For futher information, see: http://unit.aist.go.jp/energyelec/cispvc/Research/SLS/