In 1966 the British Calibration Service was set up to operate a nation-wide measurement scheme which includes the calibration of optical equipment. Under this scheme, industrial and other laboratories may apply for approval to issue official calibration certificates covering the measurement of optical transfer functions, and the OTF laboratory at the Royal Aircraft Establishment is being organized, in collaboration with the National Physical Laboratory, as the national standards centre to which such measurements can be referred. This work has led to a critical study of the sources of error in OTF measurement, and the main features of this study are described here.