The disclosure relates to optical measuring methods and apparatus for determining the transmission and/or reflection properties of translucent objects with utility for process monitoring and quality inspection in the manufacture of surface-coated substrates. According to the disclosure the transmission and reflection properties are determined in such a way that sequentially: a first large surface of the object is illuminated by a first illuminating device, with a photodetector measuring the total transmittance (Ttotal), a second, large surface of the object, lying opposite and parallel to the first one, is illuminated by a second illuminating device, with a photodetector measuring the diffuse transmittance (Tdiffuse), and optionally the first large surface of the object is illuminated by the first illuminating device, with the photodetector measuring the reflectance, and/or the second large surface of the object is illuminated by the second illuminating device, with the photodetector measuring the reflectance.