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 .