When an impurity appears in material such as polyethylene tape or film, it migrates for hydrodynamic reasons towards the surface of the tape or film and is deposited in the surface so that approximately half its diameter protrudes from the surface. Surface uniformity determination is therefore included when checking the quality of such tape or film.

US A, 4 656 663 describes a system for inspecting transparent film, pri- marily intended for use as videotape, to detect irregularities in the surface of the film. A sample of the film is coated with metal by means of steaming on, the metal being applied at a certain angle. The film is then illuminated and the distribution of irregularities is determined with a scanning video camera from the number and length of the"shadows"obtained in the metallization, caused by irregularities in the surface of the film. The signal obtained at the scanning is digitalized, stored in a computer memory and processed pixel by pixel.

The present invention relates to equipment for determining the surface uniformity of film or sheet material which is caused to pass over a measuring shaft and the horizon formed by the outer surface of the material where it is in contact with the periphery of the measuring shaft is illuminated and the light transmitted is detected. The dimensions, number and locations of the irregularities are detected from the transmitted light detected, which is related to shadows produced by the surface irregularities. The object of the invention is more specifically to achieve a new, reliable method and provide a new, reliable device for calibrating this type of equipment for determining surface uniformity.

This object is achieved with a method and a device of the type described in the introduction, having the characteristics defined in claim 1 and claim 8, re- spectively.

A very substantial advantage with the calibration method and device ac- cording to the invention is that the calibration can be traced backwards, which is a requirement according to the ISO 9000 standard.

The equipment for determining surface uniformity is intended for use for film or sheet material, e. g. tape of various thicknesses, that is to say material ex- truded at various speeds to various thicknesses. According to an advantageous embodiment of the method according to the invention calibration shafts are pro- duced having different diameters corresponding to different material thicknesses.

The equipment is thus calibrated for different material thicknesses, using different calibration shafts.

According to another advantageous embodiment of the method according to the invention said elevations are produced on the calibration shafts by applying drops of glue on the peripheral surfaces of the shafts and causing them to harden.

Before the glue, suitably epoxy plastic, hardens the drops spread out so that their shape substantially resembles a normal distribution curve. Hardening is suitably performed in heat.

According to yet another embodiment of the method according to the in- vention the actual dimensions of the elevations are determined with the aid of a reading microscope. The relative uncertainty of this determination is estimated to 1% from the spread in measured data and also estimated uncertainties in the equipment used.

According to another embodiment of the method according to the inven- tion the calibration shafts are rotated several turns to enable determination of hit rate and standard deviations, as well as determining the height and width of the elevations.

According to yet another advantageous embodiment of the method ac- cording to the invention a spectrography camera is used as light detector, said camera emitting signals representing light transmission data scanned pixel by pixel.

According to a further advantageous embodiment of the method accord- ing to the invention, before measurement is commenced on the calibration shaft a scale is applied in the measuring position of the detecting equipment, with the aid of which the desired pixel resolution is adjusted and the camera focus fixed on the measuring position. The scale used is suitably a commercially available Heiden- hain scale designed for such purposes. The camera is fixed mechanically follow- ing correct setting and thereafter maintains this correct setting.

To explain the invention further a number of examples of the device ac- cording to the invention will be described in more detail by way of example, with reference to the accompanying drawings in which

Figure 1 illustrates schematically the type of equipment for determining sur- face irregularities, to which the invention relates, Figure 2 shows the calibration unit in the arrangement according to the inven- tion, Figure 3 shows an example of a calibration shaft, and Figures 4-6 shows examples of elevations applied on the peripheral surface of the calibration shaft.

Figure 1 shows schematically an equipment for determining the surface uniformity of film or sheet material, such as tape extruded from black polyethyl- ene, for instance. The material 2 runs through the equipment over a number of pulleys and drive rolls and at 4 a measuring shaft is shown in the actual measur- ing unit 6. Upon passage over the measuring shaft 4 the outer surface of the ma- terial 2 forms a horizon which is illuminated by a light sou. rce placed immediately below the shaft 4, for instance, (not shown in detail in the figure) in the measuring unit 6, and a light detector placed immediately above the shaft 4, for instance, is arranged to detect transmitted light. Irregularities in the outer surface of the mate- rial 2 will thus block the light to a varying extent, thereby influencing the amount of light transmitted. The light detector thus emits an electric signal representing transmitted light to subsequent signal-processing apparatus in order to determine the dimensions, number and locations of the surface irregularities.

Other parts of the equipment illustrated in Figure 1 are not a part of the invention and will therefore be described more briefly.

The mid-section of the equipment thus comprises a label printer 14 to mark the tape 2 with a label where a surface irregularity is encountered.

In the section furthest to the right in Figure 1 the tape 2 is cut into pieces of predetermined length immediately after it has passed between two rolls 16, by a rotating knife 18. Flawless lengths fall down to the left of the partition 20 into the container 22 for flawless material. When a label has been applied to the tape 2 to indicate a surface irregularity, as mentioned above, the knife 18 is temporarily stopped so that a longer piece of tape with the detected irregularities will be cut off. Since the cutting process is temporarily stopped the tape will protrude over the upper edge of the partition 20 and when the longer piece of tape has been cut it will fall down to the right of the partition 20 into the container 24 for rejected mate- rial.

The calibration unit shown in Figure 2 is used for calibrating the equip- ment shown in Figure 1. A calibration shaft 8 of predetermined dimension and with elevations of predetermined dimensions and number applied on its peripheral surface can be mounted in the calibration unit for rotation. With this calibration unit the calibration shaft 8 is applied in the measuring position in which the measuring

shaft 4 is normally located. Dimensions, sizes and locations of the elevations on the calibration shaft 8 are now determined in the manner described above, from transmitted light detected by the light detector.

The actual dimensions, number and locations of the elevations on the calibration shaft 8 are determined in a manner described in more detail in the fol- lowing with reference to Figures 3-6 and the equipment is calibrated by compari- son of the dimensions determined using the equipment, with the actual dimen- sions of the elevations.

Rotating the calibration shaft 8 several turns ensures that the elevations are repeated at each turn. This also enables determination of hit rate, that is to say the number of elevations encountered in relation to the actual number of ele- vations passed, and the standard deviation, as well as the heights and widths of the elevations. This hit rate is normally very close to 100%.

Before running the calibration shaft in the equipment, the desired pixel resolution is set with the aid of a"Heidenhain scale"and the focus of the light de- tector, preferably a spectrography camera, is set. The position of the detector is then fixed mechanically and this position need not subsequently be adjusted.

As mentioned, a spectrography camera is preferably used as light detec- tor, this emitting signals representing scanned light transmission values pixel by pixel. A spectrography camera is particularly suitable since it has rectangular pix- els of 13 x 500um. The pixels'height of 500pu is then used to measure the height of the irregularities. An objective is arranged in front of the detector. This objective may be 26 mm long, in which case a width of 20 mm of the material or the calibration shaft is viewed, i. e. the objective projects 20 mm of the material or calibration shaft onto 26 mm of detector surface, thus reducing the height resolu- tion from 500pu to 400, um. Irregularities thus appear in a poorer transmission.

The irregularities will shadow more or less, which is thus detected in pixels that are projected on the material or calibration shaft with a width resolution of ap- proximately 1 0pm since the size is reduced from 26 mm to 20 mm.

Figure 3 shows an example of a calibration shaft intended for application in the calibration unit shown in Figure 2. The calibration shaft comprises a meas- urement part 10 and a conical shaft part 12 for mounting in the calibration unit.

The calibration shaft is suitably made of steel and the measurement part 10 may typically be 70 mm long and have a diameter of 8 mm. The diameter of the cali- bration shaft is equal to the diameter of the measuring shaft plus the thickness of the material to which the calibration relates. Since the detecting equipment is in- tended for use with materials of different thicknesses, calibration shafts with dif- ferent diameters are produced to enable calibration of the equipment for each thickness of material.

Three elevations No. 1, No. 2 and No. 3 are applied on the peripheral sur- face of the measurement part 10 of the calibration shaft. These elevations suitably consist of drops of epoxy plastic that have been caused to harden. Data as to the dimensions of the drops or elevations can be found in Figures 4-6 illustrating the drops in section on a larger scale, and stating height, half-width and base exten- sion.

The actual dimensions of the drops or elevations are determined by reading microscope and the measuring equipment can thus be calibrated by com- parison of the dimensions determined using the equipment with these actual di- mensions.