The invention relates to a method in the control of the quality or the condition of a fibre web on the basis of optical imaging diagnostics, wherein the fibre web under examination andor means relating to the processing of the fibre web are monitored with at least one optical imaging measuring device. The dielectric body has a laminated structure including a folded flexible dielectric layer. The intermediate component can be constructed of a softer material than either the ball or stem, the intermediate component can further include a sleeve exhibiting a plurality of interior threads and facilitating mounting of an exteriorly threaded end location of the stem. The output signal of the coil is pulsed with a constant amplitude, the pulse frequency depends on the rotation speed of the toothed wheel. Immediately upon being dried, the bond faces of the first and second wafers are abutted together and the wafers are subjected to a preliminary anneal at a temperature of at least 400\xb0 C. The group of observers then analyzes the second lighting condition signal.