Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B11/00—Measuring arrangements characterised by the use of optical techniques
  • G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
  • G01B11/06—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material
  • G01B11/0616—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material of coating
  • G01B11/0658—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material of coating with measurement of emissivity or reradiation

Definitions

• the present invention is directed to a method and apparatus for measuring the thickness of a coating on a substrate. More specifically, the invention is directed to a method for measuring the thickness of a coating by directing fluorescent light onto the coated surface and analyzing the intensity of light that is emitted from the coating due to fluorescence at a specific wavelength.
• Performance of products containing coating compositions often depends upon the accuracy with which the coating compositions are applied, and the thickness ofthe coating compositions can be critical to performance and price. Variations in thickness of a coating composition can occur for a number of reasons, including the inherent variability of mechanisms used to apply the coatings, as well as variability of the properties of the coating composition being applied, such as variation in the temperature and the pressure at which the coating compositions are applied.
• the present invention relates to a method of measuring the thickness of a coated surface and a system for performing such measurements.
• the coated surface has fluorescent properties.
• a first wavelength of light is directed from a light source onto the coated surface such that the coated surface absorbs the light and fluoresces to emit light of a second wavelength.
• the intensity of the light of the second wavelength is measured by use of a solid state array of light-sensitive elements.
• the measured intensity of the light of this second wavelength is used to determine the thickness of the coating, and is substantially proportional to the thickness of the coating on the coated surface.
• the coating on the surface is very thick in specific implementations, and may be less than 1 percent of the thickness of the substrate which it coats. In specific implementations, the coating is less than 50 nanometers thick, and in certain implementations is less than 25 nanometers thick.
• a surface having an unknown coating thickness is provided in order to measure coating thickness.
• a second reference surface is provided in order to measure coating thickness. This second surface has light emitting properties that are the same as a surface having a known coating thickness. This second surface functions as a reference "standard” or "control”. The intensity of light emitted from the surface having an unknown thickness is compared to the measured intensity of light from the second surface in order to determine coating thickness. The comparison is made according to the formula:
• Tft thickness of reference
• I ft intensity of reference
• the first and second wavelengths of light are in the ultraviolet spectrum.
• the fluorescent properties of the coated surface are developed by either adding a fluorescing material to the coating or by using a coating composition having inherent fluorescent properties.
• one or more optical band pass filters may be positioned intermediate the light source and the coated surface, and intermediate the coated surface and the solid state array of light sensitive elements. The optical band pass filters limit the passage to a narrow wavelength of light and allow more accurate measurements.
• FIG. 1 is a schematic side elevational view of a thin film measuring system constructed and arranged in accordance with the present invention.
• FIG. 2 is a schematic front elevational view of a linear solid state array of light- sensitive elements constructed and arranged in accordance with the present invention.
• FIG. 3 is a schematic side elevational view of a second thin film measuring system constructed in accordance with the present invention, showing a feedback control loop for adjusting application of the thin film.
• the present invention is directed to a method and apparatus for measuring the thickness of a coating composition.
• the method includes providing a radiant energy source and directing radiant energy onto a coating composition that has fluorescent properties.
• the radiant energy is, for example, visible or ultra-violet light.
• the coating composition absorbs the light and emits light of a second wavelength.
• the intensity of the light of the second wavelength is measured by use of a solid state array of light sensitive elements.
• the thickness of the coating composition is determined at multiple points along the coated surface based upon the intensity of the light of the second wavelength measured by the solid state array of light sensitive elements.
• FIG. 1 shows a schematic cross section of a system 10 for measuring the thickness of a coating.
• Coated film 12 includes a base film 14 and a coating composition 16 applied to the top of the base film 14. It will be appreciated that coated film 12 is a layered product: a base film 14 onto which is deposited the coating composition 16. In specific implementations of the present invention, the objective is to determine the thickness of coating composition 16. The present invention is also applicable to certain implementations wherein the combined thickness of the base layer 14 and the coating composition 16 is measured. In such implementations, the base layer 14 and coating composition 16 are both fluorescent.
• the coated surface 12 is either a single layer or is multiple layers.
• System 10 includes light source 18 and solid state array of light sensitive elements 20.
• Light waves 22 having a wavelength of ⁇ i are emitted from light source 18 and
• Coating composition 16 absorbs light of wavelength ⁇ i, and
• Light waves 24 having wavelength ⁇ .
• Light waves 24 are received by solid state array 20, which measures their intensity.
• the intensity of the light waves 24 provides an indication of the thickness of coating composition 16. This indication of coating thickness is provided because coating
• composition 16 is fluorescent and selectively absorbs light of the first wavelength ⁇ i and
• the intensity of the emitted light waves 24 is dependent on the fluorescent properties of the coating composition 16 and the intensity and wavelength of the incoming light waves 22. When the intensity and wavelength of the incoming light is constant, then the intensity of the emitted light waves 24 varies with the fluorescent properties of the coating composition 16. If the coating composition 16 is substantially homogeneous, then the intensity of the emitted light 24 varies with the thickness of the coating composition 16. A thicker coating composition 16 will contain a greater quantity of the fluorescing material than a thin coating and will produce light waves 24 of a greater intensity than a thin coating. This characteristic is observed on coatings sufficiently thin that they do not demonstrate significant absorbency
• wavelengths of light are referred to as ⁇ i and ⁇ 2 , each of
• wavelengths and the other wavelengths referred to herein, are not necessarily discrete single wavelengths but may be one or more wavelengths denoted generally, for
• the light waves 22 emitted from light source 18 may be more
• light source 18 emits a spectrum of light containing a number of different wavelengths.
• ⁇ ⁇ denotes this spectrum of wavelengths of light rather than a single wavelength.
• light waves ⁇ 2 may be a spectrum of light. However, while ⁇ i and ⁇ 2 may each be discrete spectra of light and may have some overlap, the two spectra are not identical.
• Light source 18 is a radiant energy source.
• the radiant energy is any type of radiant energy that will cause fluorescence of the fluorescer within coating composition 16.
• Examples of useful radiant energy sources include those that emit thermal energy (heat or infra-red radiation), e-beam radiation, microwave radiation, UV radiation, gamma radiation, visible radiation, and the like.
• Solid state array 20 includes a multitude of light sensitive elements. These elements are characterized by the ability to absorb light and provide an electronic output of the simultaneous light intensity at a multitude of measured points along the coated surface 12. At each measured point, the thickness of the coating composition is determined by the formula:
• T ⁇ thickness of coated surface
• Tft thickness of reference
• I ft intensity of reference
• the output of the solid state array tends to be a non-linear curve. Therefore, the intensity of the light of the second wavelength cannot always be determined by an arithmetic average of the outputs of the array elements because, for example, the optics may return a greater percentage of the light from the central array elements than from the peripheral array elements.
• a smooth curve, P(x) is fitted to the array output, where x is the crossweb position.
• the maximum of this fitting curve is defined as the measured intensity, and the ratio of the maximum value to the fitting curve provides a normalization coefficient, G(x), for each array element.
• G(x) is time invariant.
• the thickness vs. intensity curve is predetermined by a calibration.
• the curve is generated by coating uniform samples of the backing with different known coating thickness' T R j and then measuring their intensity I Ri and the intensities of backing I B j. Experimentation has shown that the relationship between coating thickness and fluorescence intensity is linear in the range of thin coating thickness.
• a line is fitted to the calibration data (T RJ , IR J -I BJ )- This line is the Thickness vs. Intensity curve in the range of thin coating thickness'.
• the coating thickness' of these samples should be preferably distributed uniformly in the range of thin coating thickness.
• the intensity of a uniform sample I Rc that is measured for the calibration of the Thickness vs. Intensity curve may be different from the real-time measured intensity of the same sample, say I R ⁇ .
• the intensity of a uniform backing I t , c measured during the calibration may be different from the real time measured intensity of the same backing, say I r -
• experimental results indicate that the ratio:
• p is a coefficient that compensates for any change in light intensity, etc.
• I ⁇ r is also stable in a relatively long time interval and is independent of the crossweb position when the backing is uniform.
• the real-time output of an array element say Is(x,t), where x and t represent crossweb position and time respectively, can be determined by:
• T s (x,t) F(I(x,t))
• I(x,t) (I s (x,t) * (G(x) - I Br ) * p
• FIG. 2 showing a schematic front elevational view of a solid state array of light-sensitive elements 20.
• the light-sensitive elements are indicated as individual elements are listed as A, B, C, D, etc.
• the array of light-sensitive elements 20 is oriented above the coating composition as shown in FIG. 1.
• Each element A, B, C, D, etc. corresponds to a specific portion of the coating composition. In general, the larger the array, the higher the resolution and detailed measurements that are made.
• the fluorescer within the coating composition is selected from compounds that absorb radiant (excitation) energy of a first wavelength, and fluoresce (emit) radiant
• the excitation wavelength ⁇ i is typically among a range
• excitation wavelengths wherein one or more wavelengths within the range of the excitation wavelength is useful to excite the fluorescer to fluoresce radiant energy.
• emitted wavelength ⁇ 2 is included within a range of wavelengths emitted or fluoresced by the fluorescer upon excitation.
• the excitation wavelengths ⁇ i fall somewhere within
• wavelength ⁇ 2 falls somewhere within the range of 300 to 600, or from 300 to 450
• the excitation wavelength ⁇ i is shorter than the emitted
• the intensity of emitted light from the coated film 12 will vary based upon the quantity of fluorescer within the coating composition 16.
• Preferred fluorescers include, but are not limited to: biphenyl, fluorine, and fluorine derivatives such as NN-Decyl fluorine, 9-9 dibutyl fluorine, and 9,9-decyl, 9-methyl fluorine.
• the intensity of the radiant energy emitted by a fluorescer can be measured at
• any wavelength ⁇ i of the excitation spectrum can be used to excite a fluorescer and cause fluorescence through the range of the wavelengths of the emission
• the excitation wavelength ⁇ i is chosen to prevent interference with the
• the excitation energy used to measure fluorescence emission at a wavelength of about 290 manometers is preferably less than about 270 manometers.
• the wavelength of the excitation energy is shorter than the wavelength of the emission energy.
• FIG. 3 depicting a schematic cross-section of a system 110 for measuring the thickness of a coated surface.
• the system 110 gives high accuracy, high definition, real time measurements of the thickness of coated film 112.
• Coated film 112 includes a base film 114 and a coating composition 116.
• Light sources 118 and solid state array 120 respectively emit and receive radiant energy.
• a are emitted from the
• the light waves 122a are filtered through bandpass filters 126.
• Bandpass filters 126 narrows the spectrum of light waves 122a and only light waves 122b
• ⁇ u is a subset of ⁇
• waves 122b are a subset of light waves 122a.
• the intensity of light waves 122b is necessarily reduced from the intensity of light waves 122a. Passing of light waves 122a through the band pass filter 126 allows for selection of a specific spectrum or wavelength
• the coating composition 1 16 removes light of the emission wavelength o the coating composition 1 16. Removal of the emission wavelength is important, for example, to prevent reflection of light from the coating composition from intei fering with the measurements of emitted light. If the reflected lig it and emitted light have the same wavelength, the measured thickness of the coating composition can be improperly increased by measuring reflected light.
• Coating composition 1 16 absorbs excitation light 122b of wavelength ⁇
• Emitted light 124a passes through a second band pass
• FIG. 3 also shows a processor unit 1 0 and feedback loop 132 joining to the coating composition 1 4.
• Processor unit 1 0 receives the measured intensity o the light 124b and determines whether or not the coating is evenly applied and at a desired thickness. If the coating is not being properly applied, processor unit 130 communicates by feedback loop 132 to the coating applicator 134, and adjustment is made to the thickness of coating composition as it is applied.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Length Measuring Devices By Optical Means (AREA)
• Investigating, Analyzing Materials By Fluorescence Or Luminescence (AREA)
• Length-Measuring Devices Using Wave Or Particle Radiation (AREA)

Priority Applications (4)

Applications Claiming Priority (2)

Publications (1)

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Cited By (7)

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Citations (5)

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• 1998
  • 1998-07-15 US US09/116,009 patent/US6252237B1/en not_active Expired - Lifetime
• 1999
  • 1999-01-14 WO PCT/US1999/000888 patent/WO2000004340A1/en not_active Ceased
  • 1999-01-14 DE DE69924378T patent/DE69924378T2/de not_active Expired - Lifetime
  • 1999-01-14 EP EP99904103A patent/EP1097352B1/en not_active Expired - Lifetime
  • 1999-01-14 AU AU24575/99A patent/AU2457599A/en not_active Abandoned
  • 1999-01-14 JP JP2000560410A patent/JP2002520606A/ja not_active Withdrawn

Patent Citations (5)

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