Abstract: The calibration of an instrument, such as a spectrophotometer, for measuring baking contrast units (BCUs) is described. In one example, a spectrophotometer includes a filter wheel including, among other filters, a bandpass filter to transmit a range of broadband light through the spectrophotometer. The spectrophotometer also includes a detector to detect a reference sample value based on a reflection of the range of light off a BCU reference standard. Processing circuitry in the spectrophotometer is configured to calculate a reflectance ratio value based on a ratio of the reference sample value and a reference reflectance value, and calibrate the spectrophotometer for measuring BCUs based on the reflectance ratio value and a known BCU value for the BCU reference standard. The reference sample value and the reference reflectance value can also be corrected for measurement drift attributed to a source of the broadband light and/or the detector over time.