Abstract: In a method for detecting impurities in a transparent material, the material (2) is scanned line by line and the light transmission through the material is measured and compared with a reference value. If the transmission value at one point of the material is in a predetermined relation to said reference value, indicating the occurrence of impurities, the light transmission is studied in an area around the measured point for this transmission value in order to determine the extent and shape of the impurity, the light transmission values in the central area of the impurity being compared with transmission values measured in surrounding parts of the impurity to determine whether the impurity is of gel type or an impurity of some other type.

Abstract: Equipment for detecting impurities in transparent material comprising a light source to illuminate the material, a camera to detect light transmitted through the material and signal-processing apparatus for processing and analysing signals from the camera representing the light transmission through the material. This type of equipment is calibrated by dark areas being displayed on a film of transparent material (22, 24, 26) and the actual sizes (30) of the areas being determined. The sizes of the dark areas are then determined using the detecting equipment (34) and in this way determined sizes are compared with actual sizes (36) for calibration of the detecting equipment (38).

Abstract: Equipment for determining the surface uniformity of film or sheet material, in which the material is arranged to pass over a rotating measuring shaft, a light source being arranged to illuminate the horizon formed by the outer surface of the material exactly where it is in contact with the periphery of the measuring shaft, and also a light detector for detecting light transmitted from the light source and emitting corresponding electric signals to subsequent signal-processing apparatus in order to determine the dimensions, number and locations of the irregularities from the shadows produced by the surface irregularities. In a method for calibrating equipment of this type, at least one calibration shaft (10, 12) is produced with a diameter corresponding to the diameter of the measuring shaft and the thickness of the material being calibrated, after which a number of elevations (No.1, No.2, No.3) are produced on the peripheral surface of the shaft and the actual dimensions of the elevations are determined.