Optical metrology tools in a fleet of optical metrology tools can be matched using transforms. In particular, a first set of measured diffraction signals is obtained. The first set of measured diffraction signals was measured using a first optical metrology tool from the fleet of optical metrology tools. A second set of measured diffraction signals is obtained. The second set of diffraction signals was measured using a second optical metrology tool from the fleet of optical metrology tools. A reference diffraction signal is obtained. A first transform is generated based on the first set of measured diffraction signals and the reference diffraction signal. A second transform is generated based on the second set of measured diffraction signals and the reference diffraction signal.