Abstract: A method and apparatus for normalizing spectral data for an optical sensor. The sensor is comprised of a probe, a color sensor, a series of reflective targets, and a data processing means. Before the sensor is put into operation, a model is created, the use of which can normalize spectral data output by the color sensor for a variety of defined parameters. During operation of the sensor, the model maybe updated by rotating the targets individually in front of the probe. The targets have known reflectivities and the difference between the actual reflectivity and the spectral data is used to update the model.