An endpoint detection system for detecting an endpoint of a process comprises a polychromatic light source which emits polychromatic light. The light is reflected from a substrate. A light wavelength selector receives the reflected polychromatic light and determines a wavelength of light at which a local intensity of the reflected light is maximized during the process. In one version, the wavelength selector comprises a diffraction grating to generate a plurality of light beams having different wavelengths from the reflected polychromatic light and a light detector to receive the light beams having different wavelengths and generate an intensity signal trace of the intensity of each wavelength of the polychromatic reflected light.