Implementations disclosed describe a collimator assembly having a collimator housing that includes an interface configured to optically couple to a process chamber that has a target surface, and a port to receive an optical fiber to deliver, to an enclosure formed by the collimator housing, a first (second) plurality of spectral components of light belonging to a first (second) range of wavelengths, and an achromatic lens located, at least partially, within the enclosure formed by the collimator housing, the achromatic lens to direct the first (second) plurality of spectral components of light onto the target surface to illuminate a first (second) region on the target surface, wherein the second region is substantially the same as the first region.