CHECK-TILE FOR COLOUR MEASUREMENT INSTRUMENTS

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional 61 /424,790, filed

December 20, 2010, which is hereby incorporated by reference in its entirety.

FIELD OF INVENTION

The invention relates to a check-tile for validating instrument scales of colour measurement instruments, specifically of goniospectrophotometers.

BACKGROUND OF INVENTION

Colour measurement instruments used in colour development, pigment testing, or production control must provide the quality and quantity of data required with high reliability, serviceability, and speed of operation, independent of operating

environment, and at reasonable cost. In a multi-plant operation, uniformity of testing equipment also becomes an important consideration. Whenever reflectance spectra are stored for use as numerical standards, a program of careful monitoring of the colour measurement instrumentation and of providing preventive maintenance at regular time intervals should be instituted. Without the associated support, a program of maintaining numerical standards will probably produce less than satisfactory results. Hence, a basic requirement of any colour measurement instrument is stability, i. e., it must give the same readings for an identical sample over a period of time.

All instruments consist of various optical and electrical components which all are subject to ageing processes. These ageing processes give rise to a gradual drift of the instrument readings over time. Eventually, the drift of an instrument can be traced back to a deteriorating detector element, to a distorted spectral power distribution of the lamp spectrum, or a shift in colour position of the calibration white tile. As a consequence of instrument drift it might be necessary to send it in to the

manufacturer for service. However, when it is returned from recertification service the measured spectra of standards might differ from the previous average values and can not be made to coincide exactly with the gathered historical data. The change in reflectance measurements before and after repair must be considered as a design defect. This defect can be kept negligibly small by means of an efficient instrument profiling technology.

Methods have been devised, for example, in A. R. Robertson, Diagnostic

performance evaluation of spectrophotometers, in Advances in Standards and Metrology in Spectrophotometry, ed. by C. Burgess and K. D. Mielenz, p. 277, Elsevier Science Publishers B. V. (1987) for diagnosing the performance of a spectrophotometer with respect to a reference instrument or reference data sets by measuring the spectral reflectance (or transmittance) factors of a set of special samples exhibiting a variety of spectral shapes and features. Different types of errors are separated by matching a composite error model to the experimentally observed spectral differences between sample and reference data sets. Any error whose effect on a particular sample can be expressed as a mathematical function of its spectral reflectance (or transmittance) factor can be included in the analysis.

For a daily quick-check of the performance of a colour measurement system the diagnostic tools described above are too time-consuming and complex for the operators of the equipment. Generally a system specialist will be responsible to thoroughly diagnose and document the performance of measuring instruments in regular intervals as, e. g., every six months. In addition to that process flows in industry require an efficient quick-test allowing the operator to validate the overall performance of instrumentation in possibly a single measurement step with easily interpretable results.

The major factors affecting the performance of colour measuring instruments are stability, photometric scale, wavelength scale or filter-fit, angle scale, and stray light. To define and validate these instrument scales and performance criteria, material standards have to be used. These material standards represent physical systems, whose properties depend on intrinsic variables as, e. g., the ambient temperature. Hence, the above list has to be extended to include as a further performance factor the material standards used.

It is recognised that the capability to make accurate measurements requires more than just the provision of a calibrated white standard. Additional quality assurance methods as, e. g., the provision of performance test samples, instructions for their use, the analysis of the results, diagnosis of any errors and instructions on how to remedy them, and retesting until satisfactory performance is achieved.

The performance of an instrument should be tested directly after it has been calibrated. It is convenient to test several aspects of performance at once. Obviously different material standards must be used for calibration and testing, since an error in standardisation or a defect in the standard could not be detected otherwise. Repeatability data for each instrument are routinely obtained using a set of dedicated stable coloured material standards as, e. g., the set of BCRA ceramic tiles (BRCA = British Ceramic Research Association). This set comprises 4 achromatic and 8 chromatic tiles. The achromatic (grey) tiles are useful to test photometric linearity, but are not useful to test errors associated with the wavelength scale. The eight chromatic tiles comprising brilliant red, yellow, and orange colour shades as well as saturated green, blue, and violet colour shades are useful in testing for wavelength errors. These chromatic colours exhibit steep slopes at various spectral ranges and serve to detect spectral shifts in the instrument's wavelength scale. Readings of these tiles provide the basis for determining the extent of instrumental problems and serve as reference point to which the instrument can be adjusted at each scheduled preventive-maintenance service call (at least every six months). As an instrument approaches the time for a preventive action, the colour difference calculated from the check-tile measurement begins to increase. On occasion, an increase in the check- tile's readings may result in the preventive maintenance to be performed ahead of schedule.

In case of modern goniospectrophotometers three different instrument scales have to be maintained and validated: the photometric scale, the wavelength scale, and the angle scale. Only the former two photometric and wavelength scales can be tested and adjusted by means of an appropriate set of solid colour standards, while a performance test of the latter angle scale requires gonioapparent reflectance standards. So far only solid colour standards as the set of BCRA ceramic tiles are commercially available and are widely used in industrial applications for validating the performance of colour measurement instruments. Since there is no accepted means in industry for checking the performance of the angle scale of

goniospectrophotometers, any user of such colour measurement equipment has to design and produce his own proprietary standards for performance testing. Typically masstones of brilliant aluminium pigments exhibiting an extreme lightness travel are a good choice for the described purpose, since in addition to the high lightness flop the colour position of such paint formulations is almost free of thermochromism. A lightness drop of ΔΙ_*«180 units can easily be achieved for bright metallic colours between both viewing angles ε=15° and ε=45°, thus giving rise to a sensitivity of the order of AL7A&¾6/deg. Special accessories are offered by instrument manufacturers or designed and build by users for standardised measurement of product samples. Such instrument holders allow the user to always position the goniospectrophotometer at the same spot at the sample surface and to fix and hold the instrument at the measurement position by exerting a certain amount of pressure on the top face of the instrument ensuring a close contact between sample surface and measurement head. Compared to a purely manual operation the variance of the measurement results can be significantly reduced when utilising such equipment. However, if instruments are integrated into robotic systems for contactless measurement of objects, slight mechanical instabilities or malfunctions may affect the orientation of the measurement plane and have an unintentional impact on the angle scale. Early recognition of geometrical malfunctioning by means of a suitable performance test would help to avoid downtime, time-consuming and costly complaints, and waste material.

Besides black and white calibration tiles, instrument manufacturers frequently also supply a special check tile with the instrument which can be used routinely for validating the performance of colour measurement instruments and correctness of colour measurement results. In general such tiles are formulated solid green or cyan colour shades and as such can only be utilised for testing the photometric and wavelength scales.

Stability of standards is a highly desired property, since they have to keep their measurement values over a long period of time (perhaps many years). Stability to environmental parameters other than time is likewise important. Standards should be stable to temperature, humidity, and light. This can not always be ensured. For instance brilliant red and yellow pigments are known to be extremely thermochromic. Such standards must be used at the temperature at which they are calibrated, and care must be taken that they do not change temperature due to absorption of energy while being measured. Other materials can be photochromic or hygrochromic.

In order to reduce the dependence of measurement values on position, uniformity of colour (reflected or transmitted) over the entire surface of the standard is important. This requirement can be extended to the lack of azimuth-dependence (orientation of the standard) and to independence of the state of polarisation of the incident light field. Considerations of uniformity also apply to the surface topography of standards. Generally they should be flat and highly polished, although in practice both ceramic tiles and porcelain enamels exhibit a slight surface pattern known as orange peel.

When analysing the reflectance properties of opaque gonioapparent coatings, for example, of colour- and/or effect-imparting base-coats as used in automotive coatings the reflectance level of high-angle readings is very low and the

corresponding colour appears desaturated (less chromatic) and dark. Opaque gonioapparent colour shades can therefore only serve to validate the angle scale of a goniospectrophotometer. For a test of the other two photometric and wavelength scales further independent reflectance standards are required.

Therefore, there is still a need for a check-tile allowing a simultaneous quick check of all three instrument scales of colour measurement instruments such as

goniospectrophotometers.

SUMMARY OF THE INVENTION

The objective of the present invention is to provide a check-tile allowing a

simultaneous quick check of all three instrument scales (photometric scale, wavelength scale, angle scale) of colour measurement instruments such as goniospectrophotometers. Such a check-tile shall quickly validate the performance of a colour measurement instrument, i.e. a single reflectance standard of sufficient sensitivity for changes in all three instrument scales is needed. Furthermore, the following properties are also desirable for the check-tile: stability, uniformity, durability, non-fluorescence, opacity, easy to clean and easy to handle.

The present invention relates to a check-tile for validating instrument scales of colour measurement instruments, in particular of goniospectrophotometers, said check-tile comprising a non-transparent (opaque) substrate coated with a multi-layer coating, said multi-layer coating comprising:

A) a pigmented ground coat, wherein said pigmented ground coat is opaque and made of a base coat coating composition I comprising at least one opaque solid coloured pigment, and

B) a pigmented mid-coat, wherein said pigmented mid-coat is translucent and made of a base coat coating composition II comprising at least one

transparent interference pigment. The pigmented ground coat and the pigmented mid-coat are adjacent to each other in the multilayer coating, i.e. the pigmented mid-coat is applied over the pigmented ground coat.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 shows an example of a reflectance surface of a check-tile according to the present invention (with a multilayer coating with ground coat; mid-coat and clear coat) exhibiting an extreme two-tone colour travel from green to red. The figure displays the reflectance as a function of wavelength (λ) and viewing angle (Θ).

Figure 2 shows the temperature variation of the colour position of the multilayer coating displayed in Figure 1 . The graph displays the residual colour difference ΔΕ94 of the respective sample colour at temperature T with respect to the starting colour position at room temperature.

DETAILED DESCRIPTION

These and other features and advantages of the present invention will be more readily understood, by those of ordinary skill in the art, from a reading of the following detailed description. It is to be appreciated those certain feature of the invention, which are, for clarity, described above and below in the context of separate embodiments, may also be provided in combination in a single embodiment.

Conversely, various features of the invention that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any subcombination. In addition, references in the singular may also include the plural (for example, "a" and "an" may refer to one, or one or more) unless the context specifically states otherwise.

As used herein:

The term "colour measurement instrument" shall mean here and in the following a device to detect light reflected by a target and to disperse the spectrum in its components within the desired range of wavelengths, preferably within the visible spectral range.

The term "goniospectrophotometer" shall mean here and in the following a colour measurement instrument, where the illumination/viewing geometry is selectable, allowing the user to create multiple combinations of angles of light incidence and viewing of reflected light. Three different concepts have been realised in commercially available instruments: (i) fixed angle of illumination and variable angle of viewing, (ii) reciprocal convention of variable angle of illumination and fixed angle of viewing, and (iii) variable angle of illumination and variable angle of viewing. A typical set of measurement geometries consists of a fixed illumination angle of o=45° with respect to the sample surface normal and five viewing angles at e= 5°, 25°, 45°, 75°, and 1 10° with respect to the specular angle (see Fig.1 ).

Goniospectrophotometers are ideal for measurement and analysis of gonioapparent material such as metallic or pearlescent paints and interference pigments.

The term "check-tile" shall mean here and in the following a device qualified to validate the performance of a colour measurement instrument and the correctness of all instrument scales. To fulfil the requirements for a quick test of all instrument scales (photometric, wavelength, and angle scales) such a check-tile has to possess suitable spectral features.

The terms "transparent", "translucent" and "opaque" shall mean the following:

When light passes through a medium it may become continuously attenuated. If attenuation is complete and no light is transmitted, the medium is called opaque (or non-transparent). If light passes through a medium without any attenuation, it is termed transparent, while a body with partial attenuation is called semi-transparent. A medium that allows a fraction of light to pass through, while scattering the transmitted light into many different directions is called translucent. Whether a medium is transparent, semi-transparent, translucent, or opaque depends on the material as well as on its geometrical thickness.

Transparent solid pigments: Usually the transparency of a pigmented system denotes its ability to scatter light as little as possible. Examples of transparent pigments are, e. g., organic carbon black pigments (P.BI.7). There is a plethora of transparent organic blue, yellow, and red pigments available on the market as, e. g., P.B.15:1 , P.Y.1 10, and P.R.202.

Opaque solid pigments: The majority of inorganic solid pigments belongs to the class of opaque or well-hiding pigments. A typical example is the standard Ti0 ₂ white pigment (P.W.6), whose size is adjusted to attain the maximum scattering power, or iron-oxide yellow (P.Y.42) or red pigments (P.R.101 ) or cobalt blue pigments (P.B.28). Semi-transparent solid pigments: A limited number of pigments just falls between the above two extreme categories and are termed semi-transparent. Typical

representatives are diketo-pyrrolo-pyrrole red pigments (P.R.254), yellow azo- pigments (P.Y.151 ), or the micronised variant of Ti0 ₂ called μ-Τί0 ₂ (P.W.6).

Achromatic solid pigments: Achromatic pigments or colours are located in the close proximity of the lightness axis (L*) of the colour space. A realistic and acceptable limit can be defined by chroma values C*<10 units or C*<18 and L*>27 units (DIN6175 part 2). Titanuim dioxide pigment is an example for an achromatic solid pigment.

The term "solid pigment" shall mean here and in the following an inorganic or organic substance consisting of small particles which are practically insoluble in the applied medium and used owing to their colouring, protective, or electromagnetic properties. Solid pigments can be characterised by their chemical composition and their optical and technical properties. Their optical properties are determined by their light scattering and absorbing properties which can be selective (solid colour pigments) or aselective (solid achromatic pigments, e. g. black and white pigments). Solid pigments can be achromatic solid pigments and coloured solid pigments.

The term "effect pigment" shall mean here and in the following generally platelet-like pigment particles which besides colour imparting a substrate additional optical properties as angle-dependent colour, lightness travel and visual texture. The pigment particles themselves are practically insoluble substances utilised as colorant or corrosion hindering, or electromagnetic properties. The palette of special effect pigments is diverse and can be divided into interference and mirror-like reflective pigments. In order to fully exploit their colouristic potential a parallel alignment of the pigment particles to the surface of the embedding medium should be achieved.

The term "interference pigment" shall mean here and in the following a class of pigments whose colour is generated completely or predominantly by the

phenomenon of interference of light. Such pigments contain a transparent lamellar core (as, e. g., natural mica or synthetic Si0 ₂ ) coated with a high refractive index metal oxide (as, e. g., Ti0 ₂ or Fe ₂ 0 ₃ ).

The term "gonioapparent reflectance standard" or "gonioapparent coating" or

"gonioapparent colour shade" shall mean here and in the following a reflectance standard, colour shade and coating with the optical property of anisotropic reflecting a beam of collimated incident light. If, e. g., such a colour shade is illuminated by a collimated beam of light at a constant angle, the level of the reflected light and with it the colour will be strongly dependent on the viewing angle. Such colour shades can be formulated by means of special effect pigments as, e. g., interference pigments.

The check-tile of the present invention fulfils the optical profile for testing all three instrument scales of colour measurement instruments. The check-tile comprises a non-transparent substrate coated with a multi-layer coating of specific pigmentation. The multi-layer coating comprises a pigmented ground coat made of the base coat coating composition I and a pigmented mid-coat made of the base coat coating composition II. The pigmented ground coat and the mid-coat are adjacent to each other in the multilayer coating. Preferably the multi-layer coating comprises a clear top coat on top of the pigmented mid-coat, said clear top coat is transparent and made of a clear coat coating composition. The clear top coat is applied to the pigmented mid-coat. More preferred the multi-layer coating consists of the pigmented ground coat applied to the substrate and made of the base coat coating composition I, the pigmented mid-coat applied to the pigmented ground coat and made of the base coat coating composition II and the transparent clear top coat applied to the mid-coat and made of a clear coat coating composition.

Useful substrates that can be used in this first embodiment include a variety of metallic and non-metallic substrates such as plastic substrates. Useful metallic substrates that can be used include un-coated substrates or previously painted substrates, cold rolled steel, phosphatised steel, and steel coated with conventional primers by electro-deposition. Useful plastic material include polyester reinforced fibreglass, reaction-injection molded urethanes, partially crystalline polyamides, and the like or mixtures thereof, primed or un-primed. The substrate can generally be pre- coated with one or more coating layers. Preferably the substrate is a metallic panel of sufficient thickness and stability, for example, of a thickness of 0.5 to 1 .2 mm.

The ground coat A) is opaque and made of base coat coating composition I.

Base coat coating composition I comprises at least one opaque solid coloured pigment. Preferably base coat coating composition I is free of effect pigments.

Opaque solid colour pigments are, for example, iron-oxide yellow (P.Y.42) or red pigments (P.R.101 ) or cobalt blue pigments (P.B.28). Suitable opaque solid colour pigments are commercially available. The mid-coat B) is translucent and made of base coat coating composition II.

Base coat coating composition II comprises at least one transparent interference pigment. Base coat coating composition II is free of other pigments contributing to the colour of the mid-coat B), i.e. is, for example, free of transparent solid pigments and opaque solid coloured pigments. Examples of transparent interference pigments are those as defined above. Preferred transparent interference pigments are those having a transparent core coated with titanium dioxide.

The base coat coating compositions I and II further comprise the conventional constituents of a pigmented base coat coating composition as used, for example, in automotive coatings. They can comprise one or more binders, and optionally at least one of the following constituents: cross-linking agents, conventional coating additives, water and organic solvents.

Examples of binders are conventional film-forming binders for preparing water- based or organic solvent based base coat coating compositions and familiar to the person skilled in the art. Examples are polyester resins, (meth)acrylic copolymer resins or polyester/(meth)acrylic copolymer hybrids and polyurethane resins or polyurethane/(meth)acrylic copolymer hybrids. These may be functional or nonfunctional resins. In order to ensure sufficient water dilutability of the binders in case of water-based coating compositions, these binders are modified in a suitable manner to render them hydrophilic. The binders may be ionically (anionically and/or cationically) and/or non-ionically modified. An anionic modification and an anionic modification in combination with a non-ionic modification is preferred. Preferably water-dilutable binders may contain carboxylic acid groups, sulfonic and/or

phosphonic acid groups. Carboxylic acid groups are most preferred. Also, emulsifiers can be used, alone or in addition to a hydrophilic modification of the binders A).

Preferably the base coat coating compositions comprise polyurethane resins, optionally in combination with other resins, e.g. (meth)acrylic copolymers, and with dispersants. Examples of polyurethane resins are those, for example, with a number average molecular weight Mn of 500 to 500 000 g/mol, preferably, of 1 100 to 300 000 g/mol, most preferably, of 5000 to 300 000 g/mol, an acid value of 10 to 100 mg KOH/g, preferably of 20 to 80 mg KOH/g. Appropriate polyurethane resins which may be used are, for example, prepared by reacting compounds which are reactive with respect to isocyanate groups and polyisocyanates having at least 2 free isocyanate groups per molecule. The thus obtained polyurethane resins can still be subjected to chain extension to increase the molecular weight. For example, NCO-functional polyurethane prepolymers can be reacted with compounds, which are reactive with respect to isocyanate groups. Compounds, which are reactive with respect to isocyanate groups, are in particular compounds with hydroxyl and/or secondary and/or primary amino groups. OH-functional polyurethane prepolymers can be chain extended for example with polyisocyanates. Other preferred binders are acrylic or methacrylic copolymers.

The base coat coating compositions may be physically drying or chemically cross-linking. Accordingly, the base coat coating compositions can contain cross- linking agents, such as, for example, polyisocyanates with free isocyanate groups. Selection of the optionally used cross-linking agents depends on the type of cross- linkable groups in the binders and is familiar to the person skilled in the art.

The base coat coating compositions I and II may contain conventional coating additives in conventional quantities, for example, of 0.1 to 5 wt.%, relative to the solids content of the base coat coating compositions. Examples of additives are neutralizing agents, antifoaming agents, wetting agents, adhesion promoters, catalysts, levelling agents, anticratering agents, thickeners, rheology control agents, e.g. layered silicates and light stabilizers.

The base coat coating compositions are liquid coating compositions comprising the pigments, binders, optionally cross-linking agents and a liquid carrier. The liquid carrier can be an organic solvent, a mixture of organic solvents and/or water. Therefore, the base coat coating compositions can be organic solvent-based or water-based.

Organic solvent-based base coat coating compositions are coating

compositions, wherein organic solvents are used as solvent or thinner when preparing and/or applying the coating composition. The organic solvent-based coating compositions can contain about 20 to 80 % by weight of organic solvents, based on the total amount of the coating composition.

Water-based base coat coating compositions are coating compositions, wherein water is used as solvent or thinner when preparing and/or applying the coating composition. The water-based coating compositions can contain about 20 to 80 % by weight of water and optionally, up to 15% by weight, preferably, below 10% by weight of organic solvents, based on the total amount of the coating composition.

Preferably the multi-layer coating applied to the opaque substrate further comprises a clear top coat applied to the pigmented mid-coat, wherein said clear top coat is transparent and made of a clear coat coating composition. The clear coat coating composition is also a liquid coating composition comprising binders, optionally cross-linking agents and a liquid carrier. The liquid carrier can be an organic solvent, a mixture of organic solvents and/or water. Therefore, the clear coat coating compositions can be organic solvent-based or water-based. The clear coat coating composition can contain conventional coating additives in conventional quantities, for example, of 0.1 to 5 wt.%, relative to the solids content thereof.

Examples of additives are those as mentioned above for the base coat coating compositions. Preferably the clear coat coating composition comprises at least one hydroxyl-functional binder and a polyisocyanate cross-linker.

Due to the translucent mid-coat layer the optical properties of the background

(ground coat layer) contribute to the total colour appearance and colour shade with an extreme colour travel with viewing angle and a comparably highly saturated colour at the flat and high angles can be designed. The ground-coat and the mid-coat both contribute to the colour appearance of the coated substrate. The outer top-coat layer is formed by an un-pigmented or colourless transparent clear-coat conveying a glossy appearance and serving as a protective film.

The ground-coat and the mid-coat contribute to the colour appearance of the coated substrate. To design the desired colour appearance and colour travel a translucent mid-coat layer containing only the transparent interference pigment or mixtures of transparent interference pigments is used in combination with the coloured

background (ground-coat layer) containing the opaque solid colour pigment or a complex mixture of opaque solid colour pigments and which is opaque and have good hiding properties. The background colour (colour of the ground coat layer) should have a sufficiently high chroma, which can preferably be realised with masstones of red, orange, or yellow solid colour pigments or complex mixtures of less brilliant pigments (masstones as, e. g., blue solid pigments) with a white pigment. Therefore the ground-coat can be formulated as a solid colour coat containing only a single opaque solid colour pigment of high chroma and brightness, or a solid colour coat with a complex pigmentation of high chroma and brightness. The mid-coat is formulated as a special effect coat containing only a single transparent interference pigment or a mixture of transparent interference pigments. The transparent interference pigment or the mixture of transparent interference pigments should be present in sufficient dilution. The dilution must not be too high as to preserve the high reflectivity of the mid-coat close to the specular angle. The necessary dilution can be achieved by varying the film thickness of the mid-coat or by varying the pigment- binder ratio in the base coat coating composition II. For example, the base coat coating composition II can be diluted with binder. Preferably this binder should be the same as the main binder of the base coat coating composition II. The mid-coat may have a resulting dry film thickness of, for example, 15 to 30 μπΊ. The combined ground coat and mid-coat layers exhibit a colour travel between the interference colour of the interference pigment close to the specular angle and the highly chromatic and straight colour of the single or complex pigmented ground-coat at the flat and high angles.

Transparent interference pigments shall be selected which do have sufficient transparency in general and high reflectivity and high chroma close to the specular angle. The design of typical useful interference pigments is based on a transparent platelet-like core (as, e. g., natural mica or artificially produced Si0 ₂ flakes ) covered by a thin layer of a metal-oxide, preferably of titanium dioxide (Ti0 ₂ ). Hence such interference pigments can consist of three layers with different refractive indices and four interfaces. In particular the types having a Ti0 ₂ layer are sufficiently transparent, since they do not absorb light in the visible spectral range and only may produce high-chroma colour by interference. Therefore interference pigments having a platelet-like core coated with a layer of titanium dioxide (Ti0 ₂ ) are preferred.

The thickness of the Ti0 ₂ layer determines the interference colour. With increasing film thickness of the Ti0 ₂ coating interference colours develop varying from gold, red, and blue to green. The overall solids content of the mid-coat composition can range, for example from 10 to 30 % by weight, in particular from 15 to 25 % by weight. The specific pigment to binder ratio can vary widely as long as it provides the requisite colour and colour travel. The pigment to binder ratio can be in a range of, for example, 5 to 20, in particular 5 to 15. Pigment to binder ratio means the amount of pigments in the composition divided trough the amount of binder in the composition, multiplied with 100. A person skilled in the art is able to select the appropriate amounts of pigments and a suitable pigment to binder ratio. The application of the multilayer coating can be performed as follows:

Base coat coating composition I is applied to the opaque substrate thereby forming the ground coat on the substrate. The base coat coating composition I is applied in a resulting dry film thickness of, for example, 15 to 30 μπΊ . The base coat coating composition II is applied to the ground coat layer in a resulting dry film thickness of, for example, 15 to 30 μπΊ , thereby forming the mid coat of the multilayer coating on the substrate. The base coat coating composition II can be applied by using the wet- on-wet process. Following a flash-off period, if allowed for, both layers can be hardened or dried together. Hardening or drying can take place, for example, at 30 to 80°C for 15 to 45 minutes. However, it is also possible to harden or dry both layers separately, optionally after a flash-off period.

Preferably a transparent top coat layer is applied on the mid coat. Again, the transparent top coat layer (clear coat composition) can be applied by using the wet- on-wet process. Following a flash-off period, if allowed for, ground coat, mid coat and clear coat layers can be hardened or dried together. Hardening or drying can take place, for example, at 30 to 80°C for 15 to 45 minutes. However, it is also possible to harden or dry one or more of the three layers separately, optionally after a flash-off period. Even if not preferred the clear coat coating composition forming the transparent top coat layer may comprise at least one transparent or semi- transparent solid colour pigments

The coating compositions described above can be applied by conventional techniques, preferably by spraying.

The present invention also relates to the use of the afore described check-tiles for validating instrument scales, in particular the photometric scale, the wavelength scale, and the angle scale, of colour measurement instruments, or with other words relates to the use of said check-tiles for testing the performance of colour measurement instruments.

Before utilising the check-tile for testing the performance of a colour measurement instrument, the instrument has to be calibrated thoroughly and thereby forced into a well-defined state. Calibration is considered to be the process by which an instrument is adjusted to read, as closely as possible, the same values as those assigned by a recognized standardisation laboratory. After the calibration process, the instrument performance should be tested by means of the check-tile.

EXAMPLES Figure 1 shows the reflectance surface of a check tile according to the present invention. The measurements were made with typical portable

goniospectrophotometers equipped with five different measurement geometries. In this particular case the angle of illumination was fixed at an angle 45° with respect to the sample surface normal. The reflected light field was detected at five different viewing angles at 15°, 25°, 45°, 75°, and 1 10° with respect to the gloss angle.

Figure 1 displays the reflectance surface a multi-layer coating according to the invention formulated with water-based base coat coating compositions I and II. The spectra at the high angle are those of a saturated red colour shade. Close to the specular angle the colour of the utilised transparent interference pigment (rutile pearl green) dominates the reflectance picture.

Details of the base coat coating compositions I and II are listed below. Ti0 ₂ -coated interference pigments have been utilised, since they have a higher degree of translucency than Fe ₂ C>3- or Cr ₂ 03-coated interference pigments or combination pigments. The ground coat layer is formed by base coat coating composition I, a masstone of a red diketo-pyrrolo-pyrrole pigment (P.R. 254) covered by the mid-coat formed by base coat coating composition II containing the rutile pearl green interference pigment (Iriodin 9235)). The top-coat layer is formed by a colourless clear-coat composition (Standocryl® VOC Easy Clear, from Standox GmbH)

For all formulations tints and special effect tints of a typical water-based Refinish mixing system have been utilised. In order to adjust the required transparency for the mid-coat, the tints for preparing base coat coating composition II have been diluted with binder by a factor of 4.

A metallic panel (30x30 cm, 0.8 mm thick) has been coated with the ground coat, the mid-coat and the clear coat according to the invention. The ground coat has been applied in three spray passes in a resulting dry film thickness of about 20 μπΊ . The mid-coat has been applied wet-on-wet in three spray passes in a resulting dry film thickness of about 20 μπΊ. The clear coat has been applied wet-in wet in four spray passes in a resulting dry film thickness of about 40 μπΊ. The multi-layer coating has been cured at 80°C for 15 minutes.

The reflectance surface depicted in Figure 1 exhibits steep slopes in the entire angle range. The temperature variation of colour position of the check-tile has been measured by means of an experimental setup. The temperature range covered is typical for processes in an industrial environment. Relevant data for the

thermochromism of the check-tile are shown in Figure 2 in form of the temperature variation of the total residual colour difference ΔΕ94 at all viewing angles. The colour position at room temperature has been taken as the reference point. The respective temperature coefficient of the check-tile is about a factor of 6 higher and of an order of magnitude that in a practical application a temperature correction has to be applied in case of non-reference measurement conditions to take advantage of its high spectral sensitivity.

Colour travel Green-Red

Base coat coating composition I:

Standohyd® Mix 361 Rubin Red (Standox GmbH, containing 7.2 % by weight of Irgazin Red BO (P.R. 254), based on the entire formulation.

Base coat coating composition II:

The following ingredients have been mixed together:

20 g of Standohyd® Mix 324 Light Green (Standox GmbH, containing 9.4 % by weight of a rutile green pearl pigment (Iriodin ^® 9235)), based on the entire formulation

80 g of an aqueous polyurethane binder dispersion (21 % by weight solids)

10 g of VE water