Apparatus for measuring and/or monitoring three-dimensional expansion features of applied glue on workpieces, preferably on sacks or sack semifinished products made out of cellulose material, - wherein provision is made of a radiation apparatus, by means of which it is possible to irradiate the workpieces with electromagnetic radiation in a first wavelength range and in a different wavelength range, - wherein the apparatus comprises a sensor device, by means of which it is possible to record properties of the electromagnetic radiation emanating from the workpieces, wherein it is possible, by way of the sensor apparatus, to record first light intensity values of the electromagnetic radiation reflected by the workpieces in the first wavelength range and second light intensity values of the electromagnetic radiation reflected by the workpieces in the other wavelength, - wherein it is possible, by way of the sensor apparatus, to record the first light intensity values from regions of the workpieces with glue and the second light intensity values from regions of the workpieces without glue, - and an evaluation and analysis apparatus is present, by means of which the first light intensity values and second light intensity values are related to one another and able to be taken into account when establishing or monitoring the thickness of the glue layer, characterized - in that provision is made of at least one sensor array for scanning the part of the surface of the workpieces to be examined, - in that the apparatus is configured in such a way that it is possible to carry out an examination of surface portions (An) by way of converting the surface of the workpiece into a grid, wherein a layer thickness (D(An)) is assignable to each surface portion, - in that a control apparatus is present, - and in that the measurement values are transmittable to the control apparatus, by means of which the measurement values are able to be taken into account when generating open-loop or closed-loop control signals for the glue application apparatus. - wherein provision is made of a radiation apparatus, by means of which it is possible to irradiate the workpieces with electromagnetic radiation in a first wavelength range and in a different wavelength range, - wherein the apparatus comprises a sensor device, by means of which it is possible to record properties of the electromagnetic radiation emanating from the workpieces, wherein it is possible, by way of the sensor apparatus, to record first light intensity values of the electromagnetic radiation reflected by the workpieces in the first wavelength range and second light intensity values of the electromagnetic radiation reflected by the workpieces in the other wavelength, - wherein it is possible, by way of the sensor apparatus, to record the first light intensity values from regions of the workpieces with glue and the second light intensity values from regions of the workpieces without glue, - and an evaluation and analysis apparatus is present, by means of which the first light intensity values and second light intensity values are related to one another and able to be taken into account when establishing or monitoring the thickness of the glue layer, - in that provision is made of at least one sensor array for scanning the part of the surface of the workpieces to be examined, - in that the apparatus is configured in such a way that it is possible to carry out an examination of surface portions (An) by way of converting the surface of the workpiece into a grid, wherein a layer thickness (D(An)) is assignable to each surface portion, - in that a control apparatus is present, - and in that the measurement values are transmittable to the control apparatus, by means of which the measurement values are able to be taken into account when generating open-loop or closed-loop control signals for the glue application apparatus.