A method of monitoring an OLED production process for making an OLED device is disclosed. According to the method, at least one reference OLED device similar to said OLED device is fabricated. Said at least one reference OLED device has a layered structure corresponding to said OLED device and a range of hole injection and/or transport layer thicknesses. A spectral variation of a light output of said at least one reference OLED device with respect to variation in said hole injection and/or transport layer thickness is characterized. A said OLED device is partially fabricated by depositing one or more layers comprising at least said hole injection and/or transport layer and a thickness of said one or more layers is measured such that a light output for said partially fabricated OLED device can be predicted, in a target color space, from said measuring, using said characterized spectral variation.