GB1566398A

GB1566398A - Thickness measurements

Info

Publication number: GB1566398A
Application number: GB4621075A
Authority: GB
Original assignee: GV Planer Ltd
Current assignee: GV Planer Ltd
Priority date: 1975-11-07
Filing date: 1975-11-07
Publication date: 1980-04-30
Also published as: NL7612285A; DE2651003A1

Description

(54) IMPROVEMENTS IN OR RELATING TO THICKNESS MEASUREMENT (71) We G.V. PLANER LIMITED, af Windmill Road, Sunbury-on-Thames, Middlesex, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the measurement of thickness, and is particularly concerned with measurement of thickness of materials that are transparent to electromagnetic radiation.

One application of the present invention is in the measurement of the thickness of a wet ink film on a metal roller used in printing by the Intaglio process. However the measurement of the thickness of other and thicker material is also envisaged, for example, measurement of the thickness of a coating of paint.

In accordance with one aspect of the invention there is provided a method of measuring the thickness of transparent material on a base member, wherein electromagnetic radiation is directed onto one surface of the material at an angle of incidence substantially equal to the Brewster angle, a detector is arranged to receive the radiation emergent from the material after transmission therethrough and reflection from another surface thereof, and wherein a polariser is arranged to prevent radiation reflected substantially at the Brewster angle from said one surface of the material reaching the detector. Thus, the radiation incident on the material and received by the detector is substantially only that which has been transmitted through the material.

The detector may also be arranged to receive as a reference beam radiation from the same source as said electromagnetic radiation, for effecting a comparison between the intensities of the reference and emergent beams. The intensity of the detected emergent radiation with respect to the reference radiation is thus substantially modified only by the thickness of the material, which thickness is thereby measurable.

The method requires no mechanical contact with the transparent material, and is therefore, of particular, though not exclusive, application in measuring thickness of wet material. The thickness of a dry mass of material, provided it is transparent to the radiation, may also be measured in accordance with the present invention.

According to a further aspect of the invention there is provided apparatus for measuring the thickness of transparent material, comprising a source of electromagnetic radiation and a lead sulphide electromagnetic radiation detector. the source and the detector being disposed so that radiation from the source incident on one surface of the material at an angle effectively equal to the Brewster Angle is subsequently directed towards the detector; and polarising means disposed in the radiation path between said one surface and the detector so that that portion of the incident radiation reflected from said one surface is prevented from passing to the lead sulphide detector, and that portion of the incident radiation transmitted through the material and reflected from another surface thereof is passed to the lead sulphide detector; and wherein the lead sulphide detector comprises an electrical circuit that provides an output signal in accordance with its radiation input signal that is indicative of the thickness of the material between said two surfaces.

If the electromagnetic radiation is chosen to be in the infra-red region of the spectrum, then conveniently the source of the radiation may be a tungsten filament light source, and in this case the lead sulphide detector may be coupled with a silicon filter to provide a band width between approximately one and three microns.

For convenience only, the transparent material will hereinafter be referred to as a film, but this should not be considered as restricting the material thickness to which the invention is applicable.

When a beam of unpolarised light meets an air/film interface at an angle of incidence substantially equal to the Brewster angle for that film, then some of the light is refracted, passes through the film, is reflected at the surface of the base member back through the film, and is refracted at the film/air interface. The loss in intensity of the transmitted emergent light, with reference to the intensity of the incident light, is dependent on absorption by the film, absorption on reflection of the ink/base member interface, and loss by reflection on a particular base member. For a particular angle of incidence, the last two losses are constant. Therefore, the intensity loss of the emergent light is substantially dependent only on absorption by the film, and thus on the film thickness. Light is also reflected from the initial beam at the air/film interface, and this light is plane polarised. It is usually found that a light detector arranged to receive the transmitted emergent light also receives the directly reflected light, which is usually much more intense than the transmitted emergent light. However, by placing a polarising filter in advance of the detector and arranging the filter to be cross-polarised with respect to the plane of polarisation of the reflected light, the effect of the directly reflected light can be removed from the detector, which can then be calibrated to provide an output signal substantially representative of the thickness of the film.

Method and apparatus in accordance with the present invention, and for use in measuring the thickness of a film of wet ink on a printing roller, will now be described, by way of example, with reference to the accompanying drawings, in which: Figure I is a diagrammatic representation for explaining the theory employed in the present invention; Figure 2 is a schematic diagram of apparatus used in the invention; and Figure 3 is an electrical circuit diagram of a detector of the apparatus for converting an optical input signal to an electrical output signal.

Referring to Figure 1, the surface 1 of an engraved printing roller used in the Intaglio printing process is a highly reflective metal surface. A film 2 of printing ink is deposited on the roller for use during the printing process, and is usually of thickness t up to 100 microns.

A ray of light 3 is arranged to be incident on the upper surface 4 of the ink film 2 at an angle of incidence equal to Brewster's angle for the material of the film. In such case, the directly reflected ray 5 is plane polarised, with its electric vector perpendicular to the plane of incidence. The refracted ray 6 is at right angles to the reflected ray 5. and is only partially polarised. The ray 6 is reflected at an angle at the surface 1 of the roller, and is sent back through the ink film 2 and refracted at the upper ink film surface 4 to provide a partially polarised emergent ray 7. The two rays 5 and 7 leaving the ink film 2 are parallel to one another, ray 5 being plane polarised and ray 7 containing rays polarised perpendicular to the plane of polarisation of the ray 5. The ray 7 has travelled through the ink film 2 a distance 2t/cos 8 = 2t/sin 6B and, due to optical losses in this path length. has been reduced in intensity. The intensity of the ray 7 is dependent on the thickness t of the film 2. so that measurement of its intensity provides the measurement of the thickness of the ink film. However, the intensity of the reflected ray 5 is far in excess of that of the emergent ray 7, so that the effect of the ray 5, which has not passed through the ink film 2, has to be removed. A polariser 8 is inserted in the path of the rays 5 and 7, and is arranged to be cross-polarised with the plane of polarisation of the reflected ray 5. By this means, the only light passed through the polariser 8 is plane polarised with its electric vector parallel to the plane of incidence, and is thus that light from the emergent ray 7. A lead sulphide detector 9 that includes a photocell is arranged to receive the optical output of the polariser 8 and to convert this into a measurement of the thickness t of the ink film 2.

Figure 2 shows an apparatus employing the theory described with reference to Figure 1, in which a light source 10 and a detector 11 are mounted respectively to direct radiation to and to receive emergent reflected radiation from the ink film on the surface 12 of a printing roller 13. Two apertured plates 14,15 are placed adjacent the roller 13 so as to restrict the angle of view of the light source 10 and detector 11 respectively.

A polariser 16 is disposed between the plate 15 and the detector 11, and a mechanical chopper 17 is mounted behind the polariser 16. The chopper 17 is effective to chop the light output from the polariser 16 twice per revolution, the effective chopping frequency being about 50 Hz., so as to enable the detector 11 better to distinguish source radiation from ambient radiation. A stabilised DC power supply is used for the source 10 to reduce variations in the intensity of the incident light. The output of the detector may be amplified and rectified by means of the circuit shown in Figure 3.

Referring to Figure 3, a very high input impedance amplifier 18, such as an RCA (Registered Trade Mark) Type 3130, is employed to amplify the output signal of the photocell 19 of the detector, since this imposes very little loading on the photocell. The a.c.

coupling between the photocell and the amplifier and between the amplifier and the rectifier 20, as well as the feedback components are chosen so that the circuit provides adequate response at the chopping frequency (which may be other than 50 Hz) whilst being insensitive to much higher frequencies and to d.c. signals.

The final output voltage, which is proportional to the peak intensity of the light entering the detector, is measured with a digital voltmeter.

When printing by the Intaglio process, it is desirable that the thickness of the film of ink applied to the printing roller be accurately controlled in order to assist the printer in maintaining a consistent quality of the final print and to avoid excessive wastage of ink. To achieve this control, it is of advantage to measure the thickness of the ink film on the rotating printed roller, and it is important that the measurement should be insensitive to eccentricity of the roller.

As an alternative to the determination of the thickness of the ink film from only the emergent light, the detector may also be arranged to receive light directly from the source 10 i.e. light that has not been passed through or reflected from the ink film. Preferably this light, serving as a reference beam, is incident on the detector after reflection from an un-inked surface of the printing roller 13, so as to effect simple compensation for the reflectivity of the roller surface. The emergent and reference beams are then supplied to the detector in time-spaced sequence, which then effects a comparison of the intensity therebetween so as to determine the thickness of the ink film. It is also envisaged that the emergent and reference beams would be supplied to respective detectors, whose outputs are subsequently compared for derivation of the ink film thickness.

In descnbing the method of thickness measurement with reference to Figure 1, a single incident ray of light has been considered. However, in practice a multitude of rays of light is incident on the ink film, and this provides a range of angles of incidence. Moreover, the mean angle of incidence will vary if the ink surface is rippled. These two factors, in combination with the fact that polarisers are not perfect in that they do not provide a sharp cut-off, mean that in a practical apparatus the light reflected directly off the upper surface of the ink film is not completely prevented from reaching the detector. The accuracy of the measurement is thus adversely affected. However. it has been found in practice that provided the transmittance of the ink film exceeds a few percent, then the method is still sufficiently accurate for practical applications, when the fact that the transmittance is dependent on the wavelength of the incident radiation is taken into account. That is to say, the relationship between the wavelength of radiation and the particular ink used should preferably be chosen so as to provide a transmittance of the film as a whole of at least 10% (say). On the other hand, at high values of transmittance of the film as a whole, the sensitivity of the apparatus is not sufficient to provide a practical measurement of film thickness.

To take account of the different properties of inks, or other liquid films, which give rise to different transmittances, the apparatus has to be calibrated and can be provided with a control knob graduated for units of transmittance of the particular liquid film under examination.

Furthermore, a band of wavelengths, rather than monochromatic light may be used, and this provides the advantages of a less complex optical system, higher usable light intensities, and use with films having narrow transmission bands. By employing a filter, for example a filter having a transmission range of 1.l microns to 3.3. microns, it has been found that transmittances similar to those for monochromatic light can be obtained.

The determination of the Brewster angle for the particular transparent material employed can be made by any known suitable means, and so will not be described here.

However, it has been found that, at least for the typical inks used in Intaglio printing, variation of the angle of incidence between about 45" and about 62" does not seriously affect the accuracy of the readings. Accordingly, it is convenient to consider the angle of incidence to be 55 , which is substantially equal to the Brewster angle, and to set up the optical system accordingly, for all such inks.

It has been found that with certain of the typical inks used in the Intaglio printing process, a particularly simple method of calibrating the apparatus for use will all of these inks is possible. By restricting the range of film thickness to, for example, between 15 and 35 microns, the optical aperture defining the incident beam, or the electronic gain setting of the amplifier, can be chosen so that the voltage output signal of the apparatus is directly indicative of the film thickness. A control knob on the apparatus then varies the optical aperture or electronic gain setting in accordance with the particular ink involved. The thickness range is then adjusted by a second, separate gain control.

In the practical application of this invention to measurement of the thickness of ink in the Intaglio printing process, the apparatus may well have to work with ink at a temperature of 60-70"C, and it is important that the temperature change be taken into account since the transmittance of films may well change with temperature. Either a correction may be applied to the output of the apparatus, or calibration may be made at the operating temperature.

In the arrangement whereby a reference beam is produced, it is also envisaged that all the source radiation be derived from the same part of the lamp filament, when the source is, for example, a tungsten filament lamp, and two beams are derived therefrom by means of a sPlit-mirror optical system. That beam which is to be used as a reference beam is then directed on to an unwetted surface of the base member of the film i.e. on to the inking roller of the example given, and the reflected radiation is detected, and its intensity measured, by a first photocell. That beam which passes into the film and which is then reflected from the base member back through the film is received by a second photocell. The intensity of the source of radiation can conveniently be monitored by the first photocell, and kept at a substantially constant value by connecting a servo-amplifier in a feed-back loop between the first photocell and the radiation source.

Preferably, the radiation used is broad spectrum infra-red radiation, since this avoids transmission band effects.

Furthermore, as an alternative to the arrangement of Figure 3, the photocells and the amplifiers may be d.c. coupled so that variations of the thickness of the film due to the shape of the liquid/air interface, for example due to rippling of the ink surface, can be measured without the necessity of providing a very fast optical chopping system.

WHAT WE CLAIM IS: 1. A method of measuring the thickness of transparent material on a base member, wherein electromagnetic radiation is directed on to one surface of the material at an angle of incidence substantially equal to the Brewster angle, a detector is arranged to receive the radiation emergent from the material after transmission therethrough and reflection from another surface thereof, and wherein a polariser is arranged to prevent radiation reflected substantially at the Brewster angle from said one surface of the material reaching the detector.

2. A method according to claim 1. wherein a portion of the electromagnetic radiation directed towards the material is deflected therefrom and is arranged to be incident on the detector for use as a reference beam, and wherein the detector is arranged to effect a comparison between the intensities of said portion of the radiation and of the emergent radiation to determine the thickness of the material.

3. A method according to claim 2, wherein the reference beam is reflected off a part of the base member that is not covered by the transparent material, before reaching the detector.

4. A method according to claim 1, wherein a portion of the electromagnetic radiation directed towards the material is deflected therefrom and is arranged to be incident on a further detector after reflection from the base member, and wherein the output of the further detector is supplied to the source to maintain substantially constant the intensity of the radiation reaching the further detector.

5. A method according to any preceding claim, wherein the electromagnetic radiation is obtained from a tungsten filamen light source.

6. A method according to any preceding claim, wherein the electromagnetic radiation is infra-red radiation.

7. A method according to any preceding claim, wherein the or each detector is a lead sulphide detector.

8. A method according to claim 7. wherein the or each lead sulphide detector is operatively coupled with a silicon filter so as to limit the bandwidth of radiation detected.

9. A method according to any preceding claim wherein the emergent radiation is periodically interrupted before reaching the detector to reduce the effect on the measurement of any variations in intensity of ambient electromagnetic radiation incident on the material.

10. A method according to any preceding claim wherein the thickness of the material is less than one hundred microns.

11. A method according to claim 10. wherein the thickness of the material is between fifteen and thirty-five microns.

12. A method according to any preceding claim wherein the material is a printing ink and the base member is the surface of a printing roller.

13. A method according to claim 12, wherein the ink is Intaglio printing ink, and the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. aperture or electronic gain setting in accordance with the particular ink involved. The thickness range is then adjusted by a second, separate gain control. In the practical application of this invention to measurement of the thickness of ink in the Intaglio printing process, the apparatus may well have to work with ink at a temperature of 60-70"C, and it is important that the temperature change be taken into account since the transmittance of films may well change with temperature. Either a correction may be applied to the output of the apparatus, or calibration may be made at the operating temperature. In the arrangement whereby a reference beam is produced, it is also envisaged that all the source radiation be derived from the same part of the lamp filament, when the source is, for example, a tungsten filament lamp, and two beams are derived therefrom by means of a sPlit-mirror optical system. That beam which is to be used as a reference beam is then directed on to an unwetted surface of the base member of the film i.e. on to the inking roller of the example given, and the reflected radiation is detected, and its intensity measured, by a first photocell. That beam which passes into the film and which is then reflected from the base member back through the film is received by a second photocell. The intensity of the source of radiation can conveniently be monitored by the first photocell, and kept at a substantially constant value by connecting a servo-amplifier in a feed-back loop between the first photocell and the radiation source. Preferably, the radiation used is broad spectrum infra-red radiation, since this avoids transmission band effects. Furthermore, as an alternative to the arrangement of Figure 3, the photocells and the amplifiers may be d.c. coupled so that variations of the thickness of the film due to the shape of the liquid/air interface, for example due to rippling of the ink surface, can be measured without the necessity of providing a very fast optical chopping system. WHAT WE CLAIM IS:

1. A method of measuring the thickness of transparent material on a base member, wherein electromagnetic radiation is directed on to one surface of the material at an angle of incidence substantially equal to the Brewster angle, a detector is arranged to receive the radiation emergent from the material after transmission therethrough and reflection from another surface thereof, and wherein a polariser is arranged to prevent radiation reflected substantially at the Brewster angle from said one surface of the material reaching the detector.

angle of incidence is fifty-five degrees.

14. A method of measuring the thickness of transparent material on the base member, substantially as hereinbefore described with reference to the accompanying drawings.

15. Apparatus for measuring the thickness of transparent material, comprising a source of electromagnetic radiation and a lead sulphide electromagnetic radiation detector, the source and the detector being disposed so that radiation from the source incident on one surface of the material at an angle effectively equal to the Brewster Angle is subsequently directed towards the detector; and polarising means disposed in the radiation path between said one surface and the detector so that that portion of the incident radiation reflected from said one surface is prevented from passing to the lead sulphide detector, and that portion of the incident radiation transmitted through the material and reflected from another surface thereof is passed to the lead sulphide detector; and wherein the lead sulphide detector comprises an electrical circuit that provides an output signal in accordance with its radiation input signal that is indicative of the thickness of the material between said two surfaces.

16. Apparatus according to claim 15, wherein a portion of the electromagnetic radiation directed towards said one surface of the material is deflected therefrom and is arranged to be incident on the detector for use as a reference beam, and wherein the detector comprises means for comparing the intensities of the emergent and reference beams to provide said output signal.

17. Apparatus according to claim 16, including means for interrupting one of said beams such that the detector receives the beams at time-spaced intervals.

18. Apparatus according to claim 15, comprising a further lead sulphide detector, said further detector being arranged to receive radiation from said source after reflection from a base member supporting said material, wherein the output of the further detector is supplied to the source, thereby to maintain substantially constant the intensity of the radiation output thereof reaching the further detector, thus stabilising the intensity of radiation corresponding to zero thickness of the material being measured.

19. Apparatus according to any of claims 15 to 18. wherein the source is a source of infra-red radiation.

20. Apparatus according to any of claims 15 to 19, wherein the or each lead sulphide detector is operatively associated with a silicon filter so as to limit the bandwidth of the radiation detected.

21. Apparatus according to any of claims 15 to 20, wherein the source of the electromagnetic radiation is a tungsten filament lamp.

22. Apparatus according to any of claims 15 to 21, comprising a metal printing roller, on which the transparent material, in the form of printing ink. is to be deposited.

23. Apparatus for measuring the thickness of transparent material, substantially as hereinbefore described with reference to the accompanying drawings.