Abstract: A method and apparatus for the measurement of short distances by means of an interferometer using incoherent light is described. An incoherent beam of light is transmitted via a beam-splitter (2) both to the reference point (3) and to the object (4) to be measured, and the position of the reference device is adjusted until interference is detected, at which time the distances passed by the beams are equal. The position of the reference device (3) is adjusted periodically by means of an automatic actuating device, which may be, e.g., a mirror attached to a loudspeaker mechanism, and the interference is detected by means of an electronic detector (5), as well as, at the moment of interference, the position of the actuating device is read, e.g., on the basis of the momentary value of the supply signal controlling the actuating device.

Abstract: In the present publication, a method is described for the measurement of the properties, particularly of the thickness, of a plastic film (2) by transmitting infra-red radiation (1) through the film (2) to be measured, whereby part of this radiation is absorbed in the film (2) concerned. The face, form, and position of the plastic object to be measured as well as irregularities in the structure of the material cause variations of intensity in the absorption measurement that do not illustrate the quantity to be measured, e.g. thickness. The object of the present invention is to eliminate said disturbance effects. The invention is based on that the disturbances caused by refraction and scattering of the IR beam as well as by unhomogeneity of the material are compensated by separating from the radiation that has passed through the plastic film two different wave-length bands and by measuring the ratio of the intensities of the different wave-length ranges.