Measuring Apparatus

This invention relates to the measurement of the thickness of a film on a substrate to determine if the thickness is within prescribed maximum and minimum limits. One application of this invention is to measure the thickness of a protective plastic film on a sheet of metal or other self-sustaining material used in the can making industry.

It is old to measure film thickness upon a substrate by passing a beam of light therethrough and measuring the absorption of the beam in passing through the film, which absorption is a function of film thickness. If the substrate is moving at a high speed, measurements present a problem due to the bounce of the sheet, or due to the formation of creases, particularly if the sheet is quite thin and is subject to creasing. In the case of an opaque substrate that has reflecting powers, it is common to measure the film thickness by measuring the light absorption as a beam of light passes through the film and is reflected from the base or substrate to pass through the film again. Flexing of the substrate causes uncontrolled changes in the direction of the reflected light beam, and may drastically affect the measuring action. It is an object of this invention to provide a measuring apparatus of the above type wherein normal flexing or bouncing of the moving material under measurement, or the normal light dispersion within the material being measured, will not substantially affect the measurements. This is accomplished, in accordance with a preferred embodiment of this invention, by providing a light source which is a black-body radiator that is maintained at a regulated temperature so that the infra-red light rays emitted thereby are of substantially known wave lengths, and locating that radiator in a position such that it substantially encircles the area of the film that is being scanned at any particular instant in time whereby all observations of said area regardless of the angle of normal flexing of the material will still constitute an observation of some area of the radiator. The radiator is maintained at a uniform light intensity throughout its entire area. In one preferred embodiment, the black-body radiator is an isothermal cavity that is substantially hemispherical in shape, and the portion of the moving film that is being examined at any instant in time is a portion that is located approximately at the center of the hemisphere. A pyrometer is responsive to the infra-red or other rays radiated by the source. An infra-red pyrometer is useful for this measurement since it must operate on a wave length region of high absorption by the subject material of the film. Since the pyrometer responds not only to the radiation from the source passing through the film, but also the infra-red radiation from the surface of the film itself, it is important that the source radiation be of a high intensity compared to the radiation from the film surface itself. Therefore, the source radiation is maintained at a high temperature, say at the order of 500.degree.F (or more) when the film temperature is room temperature. The film whose thickness is being measured is at a temperature at the order of 125.degree.F or less. In the range of wave lengths used, the infra-red radiation is an exponential function of the temperature. That function is generally above the fourth power. Therefore, the infra-red radiation of the film being measured is a negligible part of the infra-red radiation of the black-body radiator.

The attainment of the above and further objects of this invention will be apparent from the following specification taken in conjunction with the accompanying drawing forming a part thereof.

FIG. 1 is a diagrammatic view of an apparatus embodying the present invention.

FIG. 2 is an enlarged cross-sectional view through a portion of a sheet or substrate having a protective film whose thickness is to be measured.

FIG. 3 is a fragmentary enlarged sectional view through the observation window of the black-body radiator of FIG. 1.

FIG. 4 is a diagrammatic view illustrating the use of the present invention for measuring thickness of a sheet that is transparent to the rays being sent therethrough.

In the drawings, like reference numerals designate like parts throughout.

At 1 there is indicated a coated sheet comprising a substrate 2 that has a reflecting metallic surface 3 on which is coated a protective film 4. The substrate 2 is opaque to infra-red rays. It may be a sheet of tinplate or aluminum, as used in the can making industry, or it may be a reflecting foil which is so thin as to require, and have, a paper backing to facilitate handling. At any rate, it is opaque to visible light. The film 4 on the sheet may be of a thickness of a few hundredths of an inch, or one or more thousandths of an inch. The film 4 is frequently a transparent plastic. It is transparent to infra-red rays and may be transparent also to other invisible rays as well as to visible rays. It is important to be able to measure the thickness of the film 4 and to provide an indication where the film thickness is outside of the prescribed maximum and minimum limits.

The sheet 1 is passed through a station 7 of a measuring apparatus 8 of FIG. 1. The sheet is maintained in position at said stations by pairs of rollers 10--10 and 11--11, the sheet moving between the rollers in the direction indicated by the arrow 12.

The measuring apparatus 8 utilizes a beam from a radiation source containing energy in a wave length region of strong absorption by the film 4. As the beam passes through the film, it will be strongly absorbed in its passage. The intensity of the energy of the beam will drop off exponentially as it passes through the plastic film 4. Since the substrate 2 is opaque to the wave length of the source of energy used, this invention utilizes the reflecting properties of the surface 3 to cause the radiation beam to traverse through twice the thickness of the film. The source of radiation should be such as to emit only the wave length region of strong absorption or the detector used must be such as to measure only in the wave length region of strong absorption. The instrument can be calibrated by passing through it a substrate 2 that lacks the film 4 (a film or zero thickness). Subsequently upon passing through the station a substrate having a film 4 thereon, the film absorbs some of the radiation. Since this absorption is a function of the thickness of the film, the instrument can be correspondingly calibrated.

The source of illumination, indicated at 15, is a black-body radiator electrically heated to a constant temperature under control of a thermostatic regulator 16 to produce radiation of a known wave length. The regulator controls the heating circuit for the radiator 15 from a source of electricity E. An adjusting knob 16' enables setting of the regulator 16 for any desired temperature of the radiator 15 within its range. The radiator 15 has a hemispherical cavity 17 having an observation opening or hole 18 in the form of a circular pin hole with a knife edge 19. Rollers 10--10 and 11--11 maintain a proper spacing 20 between the sheet 1 and the radiator 15, preferrably small as possible while avoiding contact between the sheet 1 and the radiator 15 as the sheet vibrates or bounces in its movement through the station 7. Infra-red light from the interior radiator surface 24 passes through the film 4, is reflected at 3 and then passes out through the film and through the opening 18 to the apparatus 8. The apparatus 8 is an electronic pyrometer of the type used to measure surface temperature by measuring the infra-red radiation from the surface. The output of the apparatus 8 is read on an instrument 25. The dial of the instrument 25 can be calibrated in terms of thickness of the film.

The reflective aluminum substrate serves to reflect the beam of energy back through the plastic and make the film appear to be twice its true thickness.

The substrate, although in this particular instance is aluminum, need not be of this material but can be of any material which reflects in the same wave length region as the film absorps.

The substrate may be visibly transparent and this is of no consequence as long as it exhibits a reflective characteristic at some wave length where the film absorps.

The reflecting surface of the substrate material may be of specular reflection characteristics or it may be rough and disperse the energy illuminating its surface. Under such circumstances, it is most desirable to use an extended area source of energy. The term "extended area" is used to mean that the source 15 should approach covering the entire hemisphere over the portion or area 30 being measured at any given instant. Thus, the reflection measured will be the hemispherical value and will eliminate alignment problems which might plague the system using a small source area and set up only for specular reflections where the reflecting substrate is rough and/or the coating is pigmented. For this reason, the diameter of the isothermal cavity is large in comparison with the open area of the opening 18 and its distance from sheet 1.

When the film temperature is substantially room temperature and the temperature of the black-body radiator is of the order of 600.degree.F, the temperature difference between the two is high and no material measurement error will result from the energy emitted by the film 4 due to its own temperature. In one construction the radiation pyrometer 8 operated at a wave length of 3.43 microns. The black-body cavity temperature was 600.degree.F.

While the cavity 17 herein shown is hemispherical, that particular shape is not crucial. It is desirable that a line drawn from any point on the edge 21 to the edge of the area 30 being scanned through the opening 18 shall make a minimum angle with the surface of the moving film.

If the substrate is highly transparent in the wave length region of film absorption, then the detector 8 and the black-body source of radiation would be placed on opposite sides of the object whose thickness is to be measured. This is illustrated in FIG. 4 wherein the substrate, indicated at 2', having a protective film 4', are both transparent to the wave length of infra-red rays emitted by the black-body radiator 15' (which is the same as 15 but may omit the pin hole opening 18 of FIG. 3). In this instance, the coated sheet is placed between the black-body radiator 15' and the radiation pyrometer 8. The instrument is calibrated to read film thickness as indicated by absorption of infra-red rays through the film 4' in a single passage therethrough (as distinguished from the double passage in the case of FIG. 1). The substrate 2' of FIG. 4 is of a precise thickness attained in normal manufacture thereof. The film 4' may be faulty as for instance by being interrupted or of zero thickness in some spaced small areas.