Dispersive Raman spectrometers measure the wavelength and intensity of inelastically-scattered light. They use visible lasers, a grating, and charged-coupled detectors (CCD) to collect data. In most dispersive Raman spectrometers, the CCD is a silicon array. The laser acts as a light source and irradiates the sample. In a process known as Rayleigh scattering, most of the radiation elastically scatters or "reflects off" the sample at the same energy as the incoming laser radiation. Nevertheless, a small amount (approximately one photon out of a million, or 0.00001%) scatters from the sample at a wavelength that is slightly shifted from the original wavelength.