Illumination unit is a term used to describe a device that produces light. The three crossword solutions for this term are PHOT (4 letters), LUX (3 letters), and NEONLAMP (8 letters). PHOT stands for 'photoelectric cell', which is a device that converts light into electrical energy. LUX is a unit of measurement for the intensity of light, and NEONLAMP is a type of lamp that uses neon gas to produce light.