A process for re-refining used lubricating oil having thermally unstable additives. An additional source of light that can be controllably switched on and off between a first light intensity and a second light intensity which is greater than the first one uniformly illuminates the infrared detector for deriving correction values for correcting the signals from each of the photoconductors during a measurement. The first wall includes at least a portion of a longitudinal cavity. \u201d The signal light provides a particularly valuable tool for pizza delivery companies and the like because it serves as an advertising tool reminding the consumer of the company, as well as providing the company with a way to more quickly and efficiently render its delivery services.