Method and apparatus for measuring color and/or composition

The invention relates to a method and apparatus for determining the color and/or composition of a material. A sample of the material is illuminated with at least three separate illumination bands singly or in combination, said illumination bands collectively substantially spanning at least the visible range. The light reflected or transmitted by the sample is measured with at least four light detector elements responsive to light in wavelength bands which substantially span the visible range when the sample is illuminated. The width of the illumination bands differs in such a manner that the illumination bands are narrowest near the expected fluorescence absorption bands.

BACKGROUND OF THE INVENTION
 The invention relates to a method for determining the color and/or
 composition of a material.
 The invention further relates to an apparatus for determining color and/or
 composition of a material.
 Color is a property of an object which depends on the object, conditions of
 illumination, and the observer. In general, the light reflected or
 transmitted by a non-self-luminous object depends on the nature of the
 light simultaneously incident on the object, and the geometrical relation
 of the light source and object. The perceived color of the reflected or
 transmitted light depends additionally on the visual receptivity of the
 observer, and the geometrical relation of the observer to said light.
 The apparent reflectance of a non-self-luminous object in a particular
 geometrical relation to the light source and observer is defined to be the
 ratio of the spectral power in each wavelength band of the reflected light
 to the spectral power of the same wavelength band of the incident light:
 ##EQU1##
 Similarly, the apparent transmittance of a non-self-luminous object in a
 particular relation to the light source and observer is defined to be the
 ratio of the spectral power in each wavelength band of the transmitted
 light to the spectral power of the same wavelength band of the incident
 light:
 ##EQU2##
 Absorbance is often used instead of transmittance, being the ratio of
 spectral power in each wavelength band of the absorbed light to the
 spectral power of the same wavelength band of the incident light. Thus, it
 is the complement of transmittance:
 ##EQU3##
 An alternative definition of absorbance is the logarithm of the absorbance
 as defined in (3). Absorbance and transmittance are interchangeable by
 trivial modification of any expression in which one or the other appears.
 In this specification, where either absorbance or transmittance is used,
 it is to be understood in each case that the equivalent formulation using
 the other is tacitly implied and within the scope of the specification.
 Similarly, while this specification expresses reflectance, transmittance,
 and other quantities as functions of wavelengths, equivalent expressions
 as functions of frequency or wave number are also in common use. These
 quantities can be easily converted between the different formulations.
 Thus, wherever a quantity is expressed as a function of wavelength, it is
 to be understood in each case that the equivalent formulations using
 functions of frequency or wave number are tacitly implied and within the
 scope of the specification.
 Clearly, reflectance and transmittance as defined in (1) and (2) have
 meaning only for wavelength bands in which the incident light has
 sufficient power to be detectable. Accordingly, rich light sources, having
 significant amounts of energy at all humanly visible wavelengths, are
 normally used for measuring them.
 Since the perceived color of an object depends on so many factors,
 standardization of definitions is most important for each of the
 variables. Standards authorities, such as the CIE (Commission
 Internationale d'Eclairage), have specified generally accepted standard
 illuminants having particular spectral power distributions, and color
 measurement devices usually contain means for approximating one or two
 such illuminants. Such means is often a rich physical light source with
 specific optical filters. The C, D55, D65, and D75 sources are frequently
 encountered, but others such as A, D60, F2, etc. may also be found in
 industrial applications.
 Similarly, since human observers may match color samples differently
 depending on the size of the color samples, standard spectral observers
 have been defined for 2 degree and 10 degree fields of view.
 Since human vision reduces many wavelength bands in a light spectrum into a
 three dimensional signal in the retina, color is conventionally expressed
 as colorimetric quantities having three values. Colorimetric systems in
 common use include for example CIE Tristimulus; CIE Chromaticity,
 Lightness; CIE L*a*b*; Hunter L,a,b; Hue Angle, Saturation Value and
 Dominant wavelength, Excitation purity, Lightness.
 Under given conditions of illumination and geometry, CIE tristimulus values
 may be calculated for the standard spectral observers using formulae which
 are defined by the CIE. These tristimulus values provide a base from which
 the other colorimetric quantities can be calculated using formulae defined
 by the pertinent standards authorities. Such formulae are occasionally
 revised, as the state of the art is improved. Some auxiliary colorimetric
 quantities are also of importance in appearance specifications. These are
 also derived from the tristimulus values, with definitions provided by the
 CIE and other standards authorities. They include for example tint;
 whiteness index; yellowness index and blue reflectance.
 The tristimulus values are calculated from the apparent reflectance or
 transmittance of an object, using the spectral power distribution of the
 illuminant for which the object's color appearance is to be evaluated.
 Conventionally, tristimulus values are defined as integrals but are
 normally evaluated as finite approximations:
 ##EQU4##
 where k is a normalization factor, S is the spectral power distribution of
 the target illuminant, and x, y, z, are the standard observer functions,
 tabulated at uniform wavelength intervals. In the case that the
 reflectance data is abridged or truncated, or measured at non-standard
 wavelength intervals, there are various recommended techniques for
 interpolation, extrapolation or resampling. Similar equations to (4a-4c)
 and corresponding methods are used in calculating tristimulus from
 transmittance spectra. Note that the spectral power distribution of the
 illuminant used in evaluations (4a-4c) need not be the same as the
 spectral power distribution of the source used to illuminate the sample
 during measurement of reflectance or transmittance. It is assumed that the
 reflectance and transmittance do not depend on the light source.
 Each industry tends to have a preferred colorimetric system, although there
 may be regional differences in such preference. For example, Hunter L,a,b
 is used widely in the papermaking industry in the U.S.A., but rarely
 elsewhere, as CIE L*a*b is preferred in the papermaking industry in most
 other regions, and is also used in the U.S.A. The CIE L*a*b values are
 defined (1976) for photopic conditions as follows:
 ##EQU5##
 where X.sub.n, Y.sub.n, and Z.sub.n are the tristimulus values for the
 illuminant. Photopic conditions exist when the ratios X/X.sub.n,
 Y/Y.sub.n, and Z/Z.sub.n all exceed 0.008856; otherwise either mesopic or
 scotopic conditions exist, and the equations used differ from (5a), (5b)
 and (5c), as described in ASTM test method E308-90, for example. These and
 other issues of colorimetry are well known per se, and are not further
 discussed. Measurement of color and evaluation of colorimetric quantities
 in photopic, mesopic, and scotopic conditions are contemplated by, and
 within the scope of the present invention.
 Auxiliary non-colorimetric quantities are of importance in some industries.
 For example, indices of brightness may be derived from the reflectance
 spectrum, whereas indices of opacity and transparency may be derived from
 the transmittance spectrum. Definitions of these and other
 non-standardized quantities are often industry-specific. However, in their
 respective fields of application, they are of equal importance to the
 standardized colorimetric quantities.
 The foregoing discussion pertains to describing the measured color of a
 sample. However, in the case that the sample is not opaque, it may be
 necessary to calculate the color which would be measured from a stack of
 samples which is thick enough to be effectively opaque. The transmittance
 of such a stack is obviously zero, so we are concerned only with its
 reflectance.
 Often, a sufficient number of substantially identical samples can be
 stacked, and the measurement made directly thereon. However, in other
 cases this may not be practical--for instance, if the measurement is made
 on a moving sheet during manufacture. There are several multi-flux models
 which allow calculation of the infinite stack reflectance from
 measurements of sample reflectance and transmittance, and some knowledge
 of the relative absorbing and scattering power of the sample. One which is
 in widespread use in sheet forming industries is the Kubelka-Munk two-flux
 model, for diffuse light fluxes in both directions. Another is the
 four-flux model, which incorporates directional light fluxes in addition
 to the diffuse light fluxes.
 If the quality specification for a translucent material is given in terms
 of the color of an infinite (or opaque) stack of samples, it is also
 necessary to perform the inverse calculations to derive a single-layer
 color from an infinite stack color. Similarly, these techniques can be
 used to calculate the color which would be measured from a sample of
 different thickness to the measured sample. In this case, the thickness
 need not be a multiple of the sample thickness, and may be less than or
 greater than the sample thickness. Since, in the general case, such
 calculation need not be for an opaque thickness, both reflectance and
 transmittance may be so calculated.
 The equations and methods of multi-flux models, including the four-flux and
 Kubelka-Munk two-flux models may be found in Volz, H. G., "Industrial
 Color Testing", VCH, Weinheim Germany, 1995, among others. These models do
 not incorporate fluorescence or other spectral transformations; they only
 model absorption and scattering phenomena.
 The difference in color between two samples, or between a sample and a
 color specification, can be evaluated on the basis of the available
 measurements. Customarily, a numerical expression of such a color
 difference is used to determine acceptability of manufactured items, by
 comparing that numerical value to the allowable maximum value. Depending
 on the number and type of color variables measured or specified, more than
 one method of evaluating color difference may be thus employed.
 As an example, a commonly used expression for color difference in a
 colorimetric system is the distance between the co-ordinates of the
 compared measurements. The CIE L*a*b* color difference is defined (1976)
 as:
EQU .DELTA.E*=[(.DELTA.L*).sup.2 +(.DELTA.a*).sup.2 +(.DELTA.b*).sup.2 ] (6)
 A refinement of (6) was promulgated in 1994, but is not yet in widespread
 use in industry. Analogous definitions exist for other colorimetric
 systems, and specialized methods for evaluating color difference exist in
 specific industries.
 It is possible for two different reflectance or transmittance curves to
 produce identical tristimulus or other colorimetric quantities under
 specific conditions of the illuminant and observation. However, if the
 illuminant or observer is changed, the colorimetric quantities will no
 longer match. This phenomenon is known as metamerism.
 To avoid source metamerism and field metamerism, the color specification
 for an object may be supplied in spectral form, as reflectance and/or
 transmittance curves. In the absence of fluorescence, reflectance curves
 are invariant with changes to the illuminant. Thus, if the reflectance and
 transmittance curves match for two samples under one illuminant, they will
 have matching tristimulus and other colorimetric values under all
 illuminants and observers.
 Instrument metamerism is the phenomenon whereby one color measurement
 device may indicate that a pair of samples match in color, while another
 color measurement device indicates a color mismatch. Instrument metamerism
 arises in spectrophotometric devices through differences in source
 spectrum, polychromator characteristics, number and wavelength of
 photodetector elements, and internal standards, among others.
 The color of a non-self-luminous opaque sample is commonly measured by
 means of spectrophotometers in which a sample is illuminated with a
 particular rich light source (one having significant energy at all visible
 wavelengths), usually filtered to approximate a standard illuminant, and
 the reflected light is measured at several wavelengths in the visible
 band. The sample may be continuously illuminated, using a constant light
 source, or intermittently, using a flashing source.
 In the case of a non-self-luminous translucent sample, the transmitted
 light may be measured additionally or alternatively to the reflected light
 by means of a detector on the opposite side of the sample to the
 illuminant. In other prior art apparatuses, the transmitted light can be
 reflected back through the sample by a suitable reflector opposite the
 illuminant such that the detector for transmitted light is on the same
 side as the illuminant and the detector for reflected light. By suitable
 means for alternating a reflective white backing with a non-reflective
 black backing, a device may use a single detector to measure reflected
 light and reflected light with doubly transmitted light alternately.
 Estimates of the single layer transmittance and of the infinite stack
 reflectance may then be derived by suitable calculations. For example, if
 the black backing is completely non-reflective, then the following
 Kubelka-Munk equation (given in Wendtland, W. W. and Hecht, H. G.,
 "Reflectance Spectroscopy", Wiley, New York USA, 1966) may be used to
 estimate the infinite stack reflectance:
 ##EQU6##
 where R.sub.white is the reflectance with white backing, R.sub.black is the
 reflectance with black backing, and R.sub.backing is the reflectance of
 the white backing.
 In practice, the reflectance is rarely calculated using (1). Instead, the
 reflected light is compared to the reflected light obtained when a
 reference sample of known reflectance is placed in the sample location and
 illuminated with the same source:
 ##EQU7##
 In all these cases of prior art, neither true reflectance nor true
 transmittance is measured. Rather, the measuring device measures the
 apparent reflectance and/or the apparent transmittance. This is a
 consequence of measuring all wavelength bands of the reflected or
 transmitted light while illuminating with a rich light source.
 The apparent reflectance of an infinite stack is often calculated from the
 apparent reflectance of a single layer, and inverse calculations are often
 performed for apparent reflectance targets, as disclosed by U.S. Pat. No.
 5,082,529. This adjustment typically uses methods based on the
 Kubelka-Munk two-flux model, even in cases where it is inappropriate (e.g.
 when the instrumental illumination contains a directional radiance, and is
 not purely diffuse).
 Whereas the true reflectance and transmittance at each wavelength is at
 most unity, the apparent reflectance and apparent transmittance may exceed
 unity due to fluorescence. The process of fluorescence involves absorption
 of light in a range of wavelengths termed the absorption band, and the
 emission of part of that absorbed energy as light in an emission band,
 containing longer wavelengths than the absorption band, but which may
 partly overlap the absorption band. The efficiency of absorption may vary
 at different wavelengths in the absorption band. Each wavelength in the
 absorption band can have a different efficiency of emission at each of the
 wavelengths in the emission band. If the incident light contains
 sufficient power in the absorption band of a fluorescent object, the light
 consequently emitted in its emission band, when combined with light
 reflected or transmitted in the emission band, can yield an apparent
 reflectance or transmittance in the emission band which is greater than
 unity. If there is little or no incident light in the emission band, the
 apparent reflectance or transmittance in that band may be much greater
 than unity. Note that, regardless of whether the light absorbed in a
 fluorescent relation is directional or diffuse, the emitted light will
 generally be diffuse.
 In this specification, we shall continue to use the terms "reflected" and
 "transmitted" to describe respectively the light excident from a sample on
 the same side as the illumination and on the opposite side, including the
 effects of fluorescent emission. Note that while the above mentioned
 multi-flux models incorporate absorption and scattering, they do not
 incorporate spectral transformations of the kind under discussion here.
 These processes can be expressed in the following way:
 ##EQU8##
 where E(.lambda.,.zeta.) is the apparent emissivity of the sample, being
 the ratio of light apparently reflected at wavelength .lambda. to the
 light incident at wavelength .zeta., and U(.lambda.,.zeta.) is the
 apparent transmissivity of the sample, being the ratio of light apparently
 transmitted at wavelength .lambda. to the light incident at wavelength
 .zeta.. The lower limit of each integration, min, is a wavelength below
 the fluorescence absorption band of the sample; in practical cases, this
 wavelength is generally 200 nm or higher. Matrix representation of
 emissivity and transmissivity provide finite approximations:
 ##EQU9##
 where E.sub.jk and U.sub.jk are respectively the apparent emissivity matrix
 and apparent transmissivity matrix, with elements defined for quantum
 relations between discrete wavelength bands, centered on specific sets of
 wavelengths .lambda..sub.j, .zeta..sub.k, and .DELTA..zeta..sub.k is the
 width of the wavelength band centered on .zeta..sub.k. For instance:
 ##EQU10##
 Thus, the light apparently reflected from a sample depends on the apparent
 emissivity matrix of the sample as well as on the light incident on the
 sample. In the same way, the light transmitted through a translucent
 sample depends on the apparent transmissivity of the sample as well as on
 the light incident on the sample.
 For non-fluorescent samples, the apparent emissivity E.sub.jk is nonzero
 only for elements where .lambda..sub.j =.zeta..sub.k, and these emissivity
 values are the reflectance values at those wavelengths. Similarly, the
 apparent transmissivity U.sub.jk of a non-fluorescent translucent sample
 is nonzero only for elements where .lambda..sub.j =.zeta..sub.k, and these
 transmissivity values are the transmittance values at those wavelengths.
 For fluorescent samples the emissivity and, if translucent, the
 transmissivity have nonzero values for some elements where .lambda..sub.j
 &gt;.zeta..sub.k.
 It is clear from (9a) or (10a) combined with (1) or (8) that for a
 fluorescent sample, there can be a difference between its apparent
 reflectance curves measured under different conditions of illumination.
 The degree to which the apparent reflectance curves differ depends on the
 degree to which the illuminants differ in their spectral power
 distribution in the fluorescence absorption and emission bands. The
 apparent transmittance of a translucent fluorescent sample will depend in
 an analogous way on the spectral distribution of illuminants, as is
 obvious from combining (9b) or (10 b) with (2).
 These phenomena give rise to fluorescent metamerism, in which samples which
 have identical apparent reflectance and apparent transmittance curves when
 measured with one rich illuminant can have non-identical apparent
 reflectance and apparent transmittance curves when measured with another
 rich illuminant.
 It is important to note for the purposes of this invention that, although
 the apparent reflectance and apparent transmittance of a sample will vary
 with the illumination used in the measuring device, the apparent
 emissivity and apparent transmissivity are invariant. Similarly, although
 addition of a fluorescent colorant to a substrate will cause changes
 .DELTA.R(.lambda.) and .DELTA.T(.lambda.) in its apparent reflectance
 R(.lambda.) and transmittance T(.lambda.) which will vary with the
 illumination S(.lambda.) used in the measuring device, the changes
 .DELTA.E(.lambda.,.zeta.) and .DELTA.U(S.lambda.,.xi.) caused in its
 apparent emissivity E(.lambda..xi.) and transmissivity U(.lambda.,.xi.)
 are invariant with illumination.
 When there are plural absorption-emission relations between different
 bands, it is possible for fluorescent cascades to exist. In this case, the
 emission band of a first fluorescent relation is partly or wholly in the
 absorption band of a second fluorescent relation. Thus, part of the light
 emitted as a result of absorption in the first absorption band may be
 emitted in the second emission band, even when there is no incident light
 in the second absorption band. Such cascades can involve more than two
 fluorescent relations, and be complex in nature.
 Methods whereby source metamerism and observer metamerism can be avoided in
 non-fluorescent materials are well-known. Most of these involve
 specifying, measuring, and controlling the reflectance spectrum of the
 material, rather than merely a set of colorimetric quantities. For
 example, U.S. Pat. No. 4,439,038 uses a least-squares approximation of the
 reflectance spectrum, while Shakespeare, J. and Shakespeare, T., "An
 Optimizing Color Controller", proc. TAPPI 1997 PCE&l at Birmingham Ala.,
 127-135, TAPPI Press, Atlanta USA, 1997 use a reflectance model to
 optimize colorimetric quantities in addition to the reflectance. U.S. Pat.
 No. 4,565,444 discloses methods whereby measurements of color are made
 across the entire width of a sheet without scanning by means of light
 pipes, or by providing illumination and detection across the entire sheet.
 U.S. Pat. No. 4,801,809 discloses a similar idea to U.S. Pat. No.
 4,565,444, but implements it differently. U.S. Pat. No. 5,082,529 also
 discloses measurement and control of reflectance, adding Kubelka-Munk-type
 adjustments for infinite stack calculations.
 In an attempt to quantify the effects of fluorescence, various modified
 spectrophotometers have been devised. In general, these employ additional
 rich light sources or optical filters to approximate each of plural
 specific illuminants, such as C, D65, F12 or intermittently removing some
 or all of the near ultraviolet from the approximation to an illuminant
 such as D65 or intermittently adding a rich ultraviolet illuminant to a
 specific illuminant such as C or D65.
 Each of these techniques partly addresses the issue of measuring
 fluorescent metamerism, but none copes with it in a satisfactory way.
 Equally, none provides an adequate model for color control in the presence
 of fluorescent metamerism or for color control which will avoid or
 minimize the effects of fluorescent metamerism.
 Removal of near-ultraviolet light from, and addition of near-ultraviolet
 light to an illuminant are equivalent in that they allow the apparent
 reflectance to be measured with different amounts of near-ultraviolet
 light in the illuminant. Thus, the sensitivity of apparent reflectance to
 near-ultraviolet light can be quantified. However, this technique
 completely fails to address fluorescence where both absorption and
 emission occur within the visible range. Similarly, it fails to address
 fluorescence where both absorption and emission occur within the
 near-ultraviolet range. Also, since rich near-ultraviolet sources are
 used, it does not distinguish between the different efficiencies in each
 quantum relation of a fluorescence from near-ultraviolet to visible. Thus,
 it cannot provide a model for addition or removal of near-ultraviolet
 light of different relative spectral distribution than that used in the
 measuring device. Another consequence is that it cannot provide a model
 for fluorescent cascades existing in any wavelength bands, whether
 near-ultraviolet or visible.
 From colorimetric data alone, it is difficult or impossible to deduce the
 amounts of different colorants present in a sample, even when the nature
 of the substrate and colorants is known. However, if reflectance and/or
 transmittance spectral data are provided in the visible range of
 wavelengths, it becomes possible in some cases to estimate the amounts of
 known non-fluorescent colorants present, provided the spectral responses
 of all colorants are quantified and the reflectance and/or transmittance
 of the substrate is known. The estimation can be performed, for example,
 by modification of the control calculations disclosed in the above
 mentioned article "An Optimizing Color Controller", so that the difference
 in reflectance or transmittance between the substrate and the sample is
 optimally fitted by scaled combination of normalized spectral responses of
 the colorants, hence providing the amounts of colorants present as said
 scale factors. A different method is disclosed in U.S. Pat. No. 4,977,522
 which omits consideration of the substrate, and hence applies only to
 opaque coatings such as paints. These estimation methods are unreliable if
 fluorescence is present to a significant degree either in the substrate or
 in the colorants even if the data covers the fluorescent absorption region
 as well as the fluorescent emission region, as a result of several of the
 issues discussed earlier.
 The discussion thus far has concentrated mainly on the measurement of color
 and related issues, and colorimetry is concerned only with the range of
 wavelengths visible to humans. However, in relation to the properties of
 reflectance, transmittance, and fluorescence, and their effects, the
 issues raised are not limited to those wavelengths, but are valid over a
 much wider range.
 Spectral reflectance and transmittance measurements both inside and outside
 the visible range are commonly used to determine the composition of
 samples. U.S. Pat. No. 5,250,811 discloses a method for analyzing the
 composition of a multilayer web by measuring spectral reflectance in the
 near infra-red region. This method employs polychromatic illumination, in
 a similar manner to the polychromatic illumination used in determining
 color by measurement of reflectance as discussed above, differing only in
 the wavelength range.
 U.S. Pat. No. 5,155,546 discloses a method employing spectral reflectance
 measurements in the visible region for analyzing the composition of rock
 samples. Also, U.S. Pat. No. 4,602,160 discloses an apparatus for
 measuring diffuse spectral reflectance and spectral transmittance in the
 infra-red region, and for analyzing those measurements to estimate the
 content of specific substances in a material. In these latter two
 disclosures, the sample to be analyzed is illuminated with monochromatic
 or nearly monochromatic light at each of several wavelengths bands one at
 a time, but the measurement of the reflected or transmitted light does not
 employ a monochromator, although it may employ a filter to exclude
 wavelengths outside the range to be measured which is substantially the
 same as the whole gamut of illumination bands. Thus, the reflected or
 transmitted light measured when the sample is illuminated at wavelength
 .xi. with a detector uniformly sensitive to wavelengths from
 .lambda..sub.min to .lambda..sub.max is given by:
 ##EQU11##
 A simple modification of these equations is required if the detector is
 differently sensitive to different wavelengths between .lambda..sub.min
 and .lambda..sub.max. For non-fluorescent samples, (101a) and (101b) give
 results substantially identical to (9a) and (9b), and for such samples, it
 is largely irrelevant whether the single monochromator is used in the
 illuminator or in the detector. The apparent reflectance and transmittance
 calculated using (1), (2) or (8) from measurements described by (101a) and
 (101b) are given by:
 ##EQU12##
 For non-fluorescent samples, the apparent reflectance measured in this way
 is clearly the true reflectance, R(.zeta.)=E(.zeta.,.zeta.), and the
 apparent transmittance is clearly the true transmittance,
 T(.zeta.)=U(.zeta.,.zeta.).
 For fluorescent samples, the reflectance or transmittance calculated from
 measurements of this type does not exceed unity, but it fails to
 distinguish between luminescent and non-luminescent contributions to the
 measurement. This deficiency reduces the amount of information which can
 be used to determine composition or other properties of the sample from
 the spectral measurements, and a significant fluorescent emission leads to
 an error in the calculated reflectance or transmittance. This error leads
 to an overestimation of the reflectance or transmittance in the
 fluorescent absorption band rather than in the fluorescent emission band,
 as would happen in the case of a device employing a detector monochromator
 with a rich light source. This systematic problem obviously introduces
 further sources of error in estimating properties or composition of the
 measured material when fluorescent substances are present.
 U.S. Pat. No. 3,904,876 discloses a method for determining the amount of
 ash in paper by measuring the absorption of one or more monochromatic
 X-ray beams. U.S. Pat. No. 4,845,730 discloses a method which combines
 infra-red absorption measurements at several wavelengths with an
 absorption measurement for a monochromatic X-ray beam and measurements of
 beta ray absorption in estimating the amounts of a base material and two
 or three other components present in a paper web. The measurements made
 according to these methods also are described by equations (101a) and
 (101b), except that a different essentially monochrome detector may be
 used for each monochrome illumination wavelength.
 U.S. Pat. No. 5,778,041 discloses a method which employs two polychromatic
 X-ray beams whose spectral power distribution differ in a particular way,
 and by measuring the amount of each beam absorbed in passing through a
 paper web, estimate the amounts of specific substances in that web. A
 different detector may be employed for each beam, but, monochromators are
 not employed either on the illuminator or on the detectors. However,
 filters may be used in controlling the spectral power distributions of the
 two illuminator beams.
 Prior art methods also exist for estimation of composition and other
 properties from reflectance and transmittance spectral measurements by
 reference to sets of calibration data measured on samples of known
 properties. These methods are used for reflectance, transmittance, and
 absorbance spectral measurements, obtained either with a monochromator on
 the illuminator or on the detector. U.S. Pat. No. 4,800,279 discloses a
 method using infra-red absorbance spectra of calibration samples of known
 physical properties to determine those infra-red wavelengths at which the
 absorbance correlates with a physical property to be quantified, and then
 estimate that property for a sample from its infra-red absorbance
 spectrum. U.S. Pat. No. 5,121,337 discloses a method for estimating
 unmeasured properties such as composition from spectral measurements on a
 sample, using a model fitted by least-squares fitting, principal
 components regression, or partial least-squares regression to spectral
 measurements and measurements of the desired property or composition for a
 set of calibration samples. U.S. Pat. No. 5,446,681 discloses a method
 which employs rule-based critera in addition to statistical procedures in
 the estimation of property or composition from spectral measurements on a
 sample and spectral measurements on a calibration set of known properties
 or composition.
 The above methods for analyzing spectral measurements to estimate
 composition or other physical properties, and for use of calibration data
 sets in such methods have a number of common features: i) the spectral
 data or a simple variant thereof such as its derivative is fitted as a
 combination of particular component spectral factors which are suitably
 scaled, (ii) the particular component spectral factors or known
 combinations thereof are associated with the physical properties or
 composition variables, (iii) the physical properties or composition
 variables are calculated using coefficients in a specific relation from
 the fitting parameters of the associated component spectral factors, and
 (iv) the particular component spectral factors and coefficients for
 relations are either known a priori or are derived from calibration data.
 The reliability of this class of analysis method depends on the extent to
 which the requisite component spectral factors can be discerned in the
 measurement, and the extent to which those patterns are invariant both
 within the calibration data set and between the calibration data and the
 measurements to be analyzed. The presence of significant amounts of
 fluorescence, and especially variation in that fluorescence can severely
 comprise the accuracy and reliability of such analyses based on
 spectrophotometric or spectroscopic measurements.
 SUMMARY OF THE INVENTION
 The object of the present invention is to provide an improved method and
 apparatus for measuring color. Another object of the present invention is
 to provide an improved method and apparatus for measuring composition and
 other properties.
 The method of the invention is characterized by comprising the steps of
 illuminating a sample of the material with at least three separate
 illumination bands singly or in combination, said illumination bands
 collectively substantially spanning at least the visible range, and
 measuring the light reflected or transmitted by the sample in each of the
 states of illumination with at least four light detector elements
 responsive to light in wavelength bands which collectively substantially
 span the visible range as an apparent emissivity or transmissivity.
 The apparatus of the invention is characterized by comprising at least one
 arrangement producing at least three separate illumination bands which
 collectively substantially span at least the visible range, at least one
 arrangement containing at least four light detector elements responsive to
 light in wavelength bands which collectively span substantially all of the
 visible range, means for coordinating the operation of the arrangement
 producing the illumination bands and light detectors such that when a
 light source illuminates a sample the sample, the detectors measure the
 spectrum of the light reflected or transmitted from the sample as an
 apparent emissivity or transmissivity.
 The basic idea of the invention is that the color of a material is measured
 by illuminating a sample of the material with at least three separate
 illumination bands singly or in combination, said illumination bands
 collectively substantially spanning the visible range and by measuring the
 light reflected or transmitted by the sample when it is illuminated with
 at least four light detector elements responsive to light in wavelength
 bands which substantially span the visible range. It is also beneficial
 that the width of the illumination bands differs in such a manner that the
 illumination bands are narrowest near the expected fluorescence absorption
 bands. According to a preferred embodiment, at least two separate
 illumination bands can be operated in such a manner that they illuminate
 the sample simultaneously.
 An advantage of the invention is that the measurement of the color is
 independent of fluorescent metamerism as well as of source and field
 metamerism. Further the color of a material may be characterized under all
 conditions of illumination. The color of the material may be controlled
 during manufacture. The means of measuring color is independent of the
 conditions of the illumination and thus describes the color under any
 particular conditions of illumination. Further the color may be controlled
 such that fluorescent metamerism, source metamerism and field metamerism
 may be avoided or minimized or specific metameric effects may be achieved.
 The measurement is reasonably simple and can be performed within a
 reasonably short time.
 A further advantage of the invention is that by measuring the emissivity or
 transmissivity of the sample, if the responses in apparent emissivity or
 transmissivity of the colorants are known, then the amounts of the known
 colorants present in the sample can be estimated in almost all cases. The
 estimation can be performed, for example, by a two-dimensional fitting by
 scaling normalized emissivity or transmissivity responses. If the data
 include the fluorescent absorption region and fluorescent emission region,
 then such estimation can include known fluorescent substances as well.
 Moreover, due to the potentially large amount of information in emissivity
 of transmissivity data, there in a degree of redundancy for measurement of
 fluorescent samples, so that it is possible to estimate the amounts of
 selected known colorants without necessarily knowing the nature of all
 colorants present. This can be accomplished for example by performing said
 fitting only in regions of the emissivity data where the responses of said
 selected known colorants have significant distinguishing features.
 Furthermore, this latter technique allows the amounts of known colorants
 to be estimated without necessarily knowing the emissivity or
 transmissivity of the substrate.
 Yet another advantage of the invention is that by measuring the apparent
 emissivity or transmissivity of a material, it is possible to distinguish
 between fluorescent and non-fluorescent components of spectra, and to
 characterize the fluorescent relations precisely. Accordingly, the
 composition and other properties of a sample may be determined with
 greater accuracy from emissivity or transmissivity measurements than from
 traditional spectral measurements, especially when fluorescent substances
 are present. Each component of a material such as a paper web colors the
 web, either in the visible region or in the infra-red region, or in
 another range of wavelengths. According to a preferred embodiment,
 illumination and detection bands in the infra-red region are used to
 determine the infra-red emissivity or transmissivity of a material, and
 the composition of the material is estimated from said measurements.
 A further advantage is that the measurement of emissivity and
 transmissivity allows a greater variety of signal processing techniques to
 be employed than is possible for prior art spectroscopic or
 spectrophotometric spectral measurements. Obviously, the spectrum measured
 using a single illumination condition can be subjected to similar
 techniques as those used in prior art. Equally, the spectrum of
 measurements at a single detector band made under each illumination
 condition may be subjected to similar techniques. Since plural such
 spectra are available for plural illumination conditions and for plural
 detector elements, more and different information is available compared to
 prior art measurements. Moreover, these signal processing techniques can
 be used on one or more apparent reflectance or transmittance or absorbance
 spectra calculated from the measured emissivity or transmissivity,
 providing the same information as would be available from said prior art
 measurements. However, it is also possible to employ image processing
 techniques or other two-dimensional signal processing methods directly on
 the emissivity and transmissivity. Any commonly known low-pass, high-pass,
 band-pass, smoothing, or noise suppression filtering may be used, and
 these may be either one-dimensional or two-dimensional. Also feature
 enhancement operations such as differentiation in one or both dimensions
 or convolution with a one- or two-dimensional transform may be used. The
 measurement of emissivity or transmissivity or a filtered or enhanced
 measurement may be transformed for feature extraction or other analysis by
 means of a one- or two-dimensional Fourier or Mellin or Wavelet or
 Wigner-Ville transformation, among others known to those skilled in the
 art of signal or image processing, as described for example in Poularikas,
 A. (ed.) "The Transforms and Applications Handbook", CRC Press, Boca
 Raton, Fla., 1996, or in Ifeachor, E., and Jervis, B. "Digital Signal
 Processing", Addison-Wesley, Wokingham UK, 1993, among others.
 For the purposes of this specification, the term "self-luminous" includes
 the properties of non-fluorescent luminescence, so that an entity
 described as "non-self-luminous" is understood to be neither luminous nor
 non-fluorescently luminescent. A non-self-luminous entity may, however, be
 transparent, translucent, or opaque, be fluorescent or non-fluorescent,
 and be specular, glossy, or matte.
 For the purposes of this specification, the term "visible" generally refers
 to the gamut of wavelengths to which the normal human retina is receptive.
 However, when considering the measurement of color of entities intended to
 be subject to instrumental optical telemetry, the term "visible" shall
 refer to the gamut of wavelengths to which such instruments are
 photosensitive, rather than to the aforesaid gamut of humanly-visible
 wavelengths. We thus contemplate use of the methods and apparatus invented
 herein in wavelength bands outside the visible band, and such use is
 within the scope of our invention. Also the term "color" refers not only
 to humanly-visible colors but also to colors sensed by an optical
 instrument. The limits for integration or summation in several of the
 cited equations would be modified as necessary to match the ranges of
 illumination and detection wavelengths used. Moreover, one or more
 quantities would be calculated from measurements which need not all be
 within the visible band, with observer functions for calculating scalar
 observations and suitable coefficients for calculating properties from
 said observations. The observer functions may be defined as sets of
 factors to be applied in linear or nonlinear relations with reflectance or
 transmittance spectra calculated at particular values or ranges of
 wavelengths for particular conditions of illumination, or in linear,
 affine, or nonlinear relations with emissivity or transmissivity for
 particular values or ranges of illumination and detection wavelengths. For
 example, an observation P.sub.1 may be calculated using a linear observer
 function defined as set of factors p.sub.1 in any of the following ways:
 ##EQU13##
 where in the case of (111a) and (111b), a particular illumination condition
 S.sub.1 (.lambda.) is specified for evaluation of observation P.sub.1.
 Alternative definitions may employ the measurement of light absorbed
 (defined as the complement of reflectance or transmittance or emissivity
 or transmissivity) rather than the measurement of light detected.
 Additionally or alternatively, observers may be defined using filtered or
 enhanced or transformed versions of the emissivity or transmissivity
 measurements, or using linear or affine or nonlinear combinations of
 emissivity and transmissivity. An unmeasured property Q may then be
 estimated using one or more coefficients q.sub.1 in a linear, affine, or
 nonlinear relation with one or more observations P.sub.i, such as:
 ##EQU14##
 The CIE L*a*b* color space discussed earlier serves as an example of a
 nonlinear relation, in which the observations are the CIE tristimulus
 values calculated using the standard colorimetric observer functions.
 Such observer functions and coefficients may be known a priori, or may be
 derived from measurements of emissivity or transmissivity or absorbance or
 reflectance or transmittance made on sets of calibration samples for which
 the properties to be estimated using observer functions are already known,
 or are measured by other means. Suitable observer functions and
 coefficients may be derived using chemometric methods such as
 partial-least-squares regression, as described in Hoskuldsson, A., "PLS
 Regression Methods", Journal of Chemometrics, volume 2, pages 211-228,
 1988, or using principal components regression or continuum regression or
 canonical correlation or other statistical techniques, as described for
 example in Basilevsky, A., "Statistical Factor Analysis and Related
 Methods", Wiley, New York, 1994, among others. For example, the
 proportions of certain constituents of the sheet may be thus inferred from
 infra-red measurements on the sheet using also a set of reference or
 calibration measurements. These reference calibration measurements
 comprise measurements of the emissivity or transmissivity at some or all
 combinations of illumination and detection wavelengths together with
 measurements of composition or other properties made by other means for
 each of several reference or calibration samples.

DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1 illustrates a web 1 the color and/or composition of which is
 measured. The web 1 is for example a paper web or a board web and is
 arranged to move in the direction of arrow A. An illuminator 2 is arranged
 to a measuring frame 4. The illuminator 2 comprises at least three light
 sources which emit light in wavelength bands which substantially span the
 visible range and preferably a range of shorter wavelengths. Thus the
 illuminator 2 produces light at the wavelength of several different
 illumination bands. Each illumination band illuminates the sheet 1 singly
 and in combination according to a predetermined sequence. A first detector
 3a measures the light reflected by the sheet 1 when it is illuminated. The
 illumination bands may be noncontiguous, contiguous, or overlapping in
 wavelength. A second detector 3b measures the light transmitted by the
 sheet 1 when it is illuminated. Each detector 3a and 3b comprises at least
 four detector elements responsive to light in wavelength bands which
 substantially span the visible range. Preferably the detectors 3a and 3b
 have at least one additional light detector element responsive to light in
 shorter wavelength bands. The operation of the light sources and light
 detectors 3a and 3b is controlled with means 5 for coordinating such that
 when a light source illuminates the sample, the detectors measure the
 spectrum of the light reflected and/or transmitted from the sample. It is
 also possible to control the detectors to measure also when the sample is
 not illuminated to evaluate the effects of extraneous illumination.
 The illuminator 2 is a source of light which can produce light of
 wavelengths within one or more substantially contiguous wavelength bands.
 The illuminator 2 may consist of a rich light source and a set of suitable
 optical filters or it may consist of plural distinct light sources each of
 which has suitable optical arrangements. The detectors 3a and 3b are
 sensitive to several wavelength bands independently.
 Illumination of the sheet 1 can be made with individual illumination bands
 separately or with plural bands simultaneously. All wavelength bands in
 the detectors 3a and 3b simultaneously or independently for each state of
 illumination. The wavelength bands in the detector 3a need not correspond
 to those in the detector 3b, and wavelength bands in the detectors 3a and
 3b need not correspond to illumination bands of the illuminator 2.
 In a first embodiment of a light source arrangement, the wavelength bands
 for the light sources are substantially equal. In a second embodiment,
 wavelength bands which are not all equal are chosen for the light sources
 to provide measurements of the greatest utility using the fewest light
 sources. This choice will depend on the anticipated usage and its
 requirements so that light sources can be concentrated in the expected
 fluorescence absorption bands.
 In an example of the second embodiment, when a near-white paper sheet is
 measured during manufacture, fluorescence occurs predominantly from
 near-ultraviolet to blue-visible wavelength bands. However, there may be
 more than one fluorescing constituent of the sheet, and each fluorescent
 constituent may differ from the others in its absorption and emission
 bands. Thus, in a simple case for measurement of color, one light source
 would span the near-ultraviolet from 300 nm to 400 nm, and another light
 source would span the visible range, from 400 nm to 700 nm. In this case,
 only two spectra need be measured to estimate the apparent emissivity or
 apparent transmissivity. A more sophisticated arrangement would employ a
 greater number of unequal bands for the light sources, such as 250-300 nm,
 300-350 nm, 350-400 nm, 400-500 nm, and 500-800 nm. In this latter case,
 five spectral measurements would be required to estimate the emissivity or
 transmissivity, which estimates would be of correspondingly higher
 spectral resolution than in the former case. In other cases for
 measurement of composition, the light source arrangement may employ bands
 of various bandwidths in the infra-red or other wavelength regions. Source
 bands need not be contiguous in wavelength, and bands used in measuring
 composition and bands used in measuring color may exist in the same light
 source arrangement.
 In an example of the first embodiment of an illumination band the range
 from 300 to 800 nm could be divided into illumination bands having a width
 of 20 nm, in which case the first illumination band .lambda..sub.1 would
 be 300 to 320 nm, the second illumination band .lambda..sub.2 320 to 340
 nm, the third illumination band .lambda..sub.3 340 to 360 nm etc., and the
 last illumination band .lambda..sub.25 would be 780 to 800 nm. If the
 expected fluorescence absorption bands are between 300 and 400 nm, the
 illumination bands could be combined in such a manner that each
 illumination band .lambda..sub.1 to .lambda..sub.5 would illuminate the
 object independently and the sixth state in the illumination sequence
 would be a combination of illumination bands .lambda..sub.6 to
 .lambda..sub.15 which would illuminate in a wavelength band of 400 to 600
 nm, and the seventh state would be a combination of illumination bands
 .lambda..sub.16 to .lambda..sub.25 which would illuminate in wavelength
 bands of 600 to 800 nm. In that case a sequence of only seven illumination
 states would be needed for achieving a very accurate result. If desired,
 illumination bands can be combined non-contiguously, and this can be
 accomplished without compromising the efficacy of measurement. For
 example, if fluorescent absorption at below 400 nm has emission which is
 entirely below 550 nm, then any or all illumination bands greater than 550
 nm may be used simultaneously with any or each illumination band below 400
 nm. This further reduces the number of illumination states required and
 speeds up the measurement process. An optimal illumination sequence can be
 chosen either from a priori knowledge of the fluorescent absorption and
 emission relations, or by measuring said relations with an illumination
 sequence in which each illumination band is used individually. The optimal
 sequence may also combine source bands for measuring color with source
 bands for measuring composition.
 In a first embodiment of a light detector arrangement, the wavelength bands
 of the detector elements are of substantially equal width, which need not
 be the same as the width of any light source wavelength band. In a second
 embodiment, the wavelength bands of the detector elements substantially
 match the wavelength bands of the light sources in one of the light source
 arrangements. In a third embodiment, wavelength bands which are not all
 equal are chosen to provide measurements of the greatest utility using the
 fewest detector elements.
 The invented method and apparatus measure the apparent emissivity and/or
 the apparent transmissivity of the sample, depending on the relative
 deployment of source arrangement and detector arrangement. Each row in the
 emissivity or transmissivity is measured as a single spectrum in the
 detector according to whether the detector is on the same side as or
 opposite the active light source. With reference to equations 11a and 11b,
 the wavelengths .lambda..sub.j are the central or average wavelengths of
 the detector elements, while the wavelengths .zeta..sub.k are the central
 or average wavelengths of the light sources.
 Using measurements from a light detector arrangement opposite a light
 source arrangement, the apparent transmissivity is:
 ##EQU15##
 where incident light intensity is preferably measured at a time when there
 is no sample in the measurement position. Those skilled in the art will
 immediately recognize that 12a requires a detector which provides absolute
 measurements of light energy in each channel when
 .lambda..sub.j.noteq..zeta..sub.k. Moreover, 12a requires an approximate
 correspondence between source wavelength bands and detector wavelength
 bands or groups of wavelength bands to provide the case
 .lambda..sub.j.noteq..zeta..sub.k. However, it is possible to use a
 detector which provides only relative measurements of light energy in each
 channel, and which does not need to have spectral bands which match the
 source wavelength bands. In this latter case, measurements from different
 channels need not be directly comparable, and are derived by comparing
 them with the measurements made using one or more reference samples of
 known transmissivity. Apparent transmissivities U.sub.jk, with
 .lambda..sub.j.noteq..zeta..sub.k, are calculable where a reference sample
 has a known nonzero apparent transmissivity in the same band relation:
 ##EQU16##
 when illuminated by a light source of wavelength .zeta..sub.k. If more than
 one reference sample has a nonzero apparent transmissivity in a particular
 band relation, then plural values based on the different references can be
 combined to estimate the apparent transmissivity. Apparent
 transmissivities U.sub.jk, with .lambda..sub.j =.zeta..sub.k, are also
 calculable using 12b.
 Similarly, using measurements from a light detector arrangement on the same
 side as a light source arrangement, the apparent emissivity is:
 ##EQU17##
 However, If there is no detector arrangement opposite the source
 arrangement, the incident light cannot be measured. Moreover, even if an
 opposite detector exists, its measurements may not be suitable for use in
 (13a), due to spectral resolution or calibration issues. In these cases,
 the apparent emissivity is calculated using measurements of reflected
 light from one or more references having known nonzero apparent emissivity
 values in the respective wavelength bands. Apparent emissivities E.sub.jk,
 with .lambda..sub.j.noteq..zeta..sub.k, are calculable where a reference
 sample has a known nonzero apparent emissivity in the same band relation:
 ##EQU18##
 when illuminated by a light source of wavelength .zeta..sub.k. If more than
 one reference sample has a nonzero apparent emissivity in a particular
 band relation, then plural values based on the different references can be
 combined to estimate the apparent emissivity. Apparent emissivities
 E.sub.jk, with .lambda..sub.j =.zeta..sub.k, are also calculable using
 13b.
 Equations 12a, 12b, 13a, 13b assume that the wavelength bands can be
 completely separated in the measurements of reflected and transmitted
 light. In practice, such ideal band separation is impossible, and optical
 components such as monochromators and polychromators with better band
 separation characteristics are more costly. However, if the band overlap
 characteristics of the components used in the instrument are approximately
 known, the measured light spectra can be deconvoluted. This method
 provides improved estimates of the spectra which would be obtained if the
 optical components were ideal. For example, if the band overlap matrices
 are known for the source bands and for the detector bands, then the
 apparent emissivity matrix can be deconvoluted:
EQU E.sub.corr =(B.sub.reflect B.sub.source).sup.-1 E.sub.meas (14a)
 where E.sub.meas is the measured emissivity matrix, E.sub.corr is the
 deconvoluted emissivity, B.sub.source is the source band interaction
 matrix, and B.sub.reflect is the band interaction matrix for the reflected
 light detector. The apparent transmissivity matrix may similarly be
 deconvoluted:
EQU U.sub.corr =(B.sub.transmit B.sub.source).sup.-1 U.sub.mea (14b)
 where U.sub.meas is the measured transmissivity matrix, U.sub.corr is the
 deconvoluted transmissivity, and B.sub.transmit is the band interaction
 matrix for the transmitted light detector. Deconvolution may alternatively
 be performed on apparent reflectance and apparent transmittance spectra
 derived from undeconvoluted emissivity and transmissivity matrices.
 The apparent emissivity and apparent transmissivity of a sample do not
 depend on the source used to illuminate the sample. Rather, they describe
 how the sample's appearance will change as the illuminant is varied. Thus,
 specification of the apparent emissivity and/or apparent transmissivity is
 a better characterization of color than mere specification of reflectance
 and/or transmittance under one or more particular conditions.
 Apparent reflectances and apparent transmittances for arbitrary known
 illuminants may be calculated from the apparent emissivity and apparent
 transmissivity, respectively. This calculation can employ (10a) or (10b),
 for example, in combination with (8) or (2). Also, colorimetric quantities
 and related non-colorimetric quantities can clearly be calculated from
 such apparent reflectances and apparent transmittances. Such calculations
 of apparent reflectance, apparent transmittance, and associated
 colorimetric and non-colorimetric quantities can clearly be performed for
 plural known illuminants. Similarly, the light measured by illuminating
 with monochrome or narrow-band illuminators while measuring with a single
 detector of known broad bandwidth can be calculated. This calculation can
 employ (101a) or (101b) for example. The apparent reflectance or apparent
 transmittance spectrum which would be measured by such a method can also
 be calculated, for example using (102a) or (102b), whence the apparent
 absorption can be calculated using (3).
 Since an apparent reflectance and transmittance may be calculated for
 arbitrary illuminants from the apparent emissivity and apparent
 transmissivity, it is possible to calculate the light which would be
 reflected or transmitted if the sample were measured using a
 spectrophotometer or spectroscope of known source characteristics. If the
 monochromator and detector characteristics of that spectrophotometer or
 spectroscope are also known, especially its band interaction matrix, it is
 then possible to calculate the reflectance or transmittance or absorbance
 which it would measure.
 By measuring the change in apparent emissivity and/or the change in
 apparent transmissivity of a sheet during manufacture, when the process
 conditions are changed, the effect of such process changes can be
 characterized in terms of effects on apparent emissivity and/or apparent
 transmissivity. Such characterization can further be parametrized in terms
 of values for a number of parameters and the values of process variables
 and the change effected on process conditions. Said parametrization can be
 purely phenomenological, or can include a mathematical model for the
 process. Thus, the expected change in emissivity and/or the expected
 change in transmissivity which would result from a change in process
 conditions can be subsequently computed from said parameters for different
 values of said process variables.
 From such a characterization, the effects of said process changes can be
 calculated for arbitrary known illuminants in terms of an effect on the
 apparent reflectance and/or apparent transmittance under such conditions
 of illumination. Furthermore, from such a characterization, the effects of
 said process changes can be calculated for arbitrary known illuminants in
 terms of an effect on colorimetric quantities or related non-colorimetric
 quantities under such conditions of illumination. Similarly, from such a
 characterization, the effects of said process changes can be calculated in
 terms of an effect on one or more composition quantities or other
 properties which can be estimated from the emissivity or transmissivity
 measurement.
 For example, if the expected change in emissivity resulting from a change
 in process conditions is .DELTA.E and the expected change in
 transmissivity is .DELTA.U, then the expected changes in reflected light
 and in transmitted light which would be measured using an illuminant S can
 be calculated:
 ##EQU19##
 A matrix representation of these equations is also possible. The ability to
 compute the change under arbitrary illuminants, including under each of
 plural illuminants is particularly valuable in color control, especially
 when multiple color targets are given for each of plural illuminants.
 By measuring color in a way which is independent of the illumination, it
 becomes possible to regulate the color of a material independently of the
 illumination. In this way, the color of the material may be regulated for
 all illuminants, by using a target which is independent of illumination.
 Alternatively, since reflectances and transmittances can be evaluated for
 each of plural illuminants, specific reflectance or transmittance color
 targets can be provided for each of plural conditions of illumination, and
 the color of the material may be regulated to provide the closest match to
 those targets. Alternatively, colorimetric targets may be provided for
 plural specific illuminants, and the color of the material regulated to
 provide the closest match to those targets. In a similar way, the process
 may be governed to regulate the values of one or more composition
 quantities which can be estimated from emissivity or transmissivity
 measurements.
 In practice, regulating the color measurement to achieve an exact match
 with the color target using the available means of modulating the process
 may not always be possible, or may be expensive,. This situation can occur
 even if only a single colorimetric target is supplied for a single
 illuminant. It is more common when multiple colorimetric and/or spectral
 targets are supplied for different illuminants, or when an
 illuminant-independent target is supplied. In these cases, a control
 algorithm which minimizes the difference between the color measurement and
 the color target, optionally using weighting factors on each component of
 the target, can provide the nearest match over the set of targets. The
 difference function to be minimized can also include the costs and usage
 of materials and other resources which are used by the means of modulating
 the coloring process, as well as limits on their allowed usage, in
 calculating an optimal control action.
 FIG. 2 illustrates how the measurement of the invention is arranged to
 control process 6. The process 6 may be for example a paper or cardboard
 or paperboard or tissue manufacturing process producing the web 1. The
 color or composition of the web 1 is measured with a measuring arrangement
 7. A color or composition measurement 8 yielded by the measurement
 arrangement 7 is led into a controller 9. A color or composition target 10
 is also fed into the controller 9. On the basis of the color or
 composition measurement 8 and color or composition target 10 the
 controller 9 controls the process 6 by means of a control signal 11.
 The controller 9 calculates a change for at least one manipulable process
 variable so as to reduce the difference between the color or composition
 measurement 8 and the color or composition target 10 and governs said
 manipulable process variables to accomplish the calculated change. The
 manipulated process variable may be a condition of processing of the
 material, especially conditions taken from the list: temperature;
 pressure; irradiation; mechanical agitation or electric potential.
 Further, the manipulated process variable may be a chemical condition to
 which the material or of one or more of its constituents is exposed,
 especially conditions taken from the list: pH; presence and concentration
 of cations or anions; presence and concentration of chemical activators or
 inhibitors or other agents for inducing or suppressing reactions; presence
 and concentration of catalysts, or other agents for facilitating
 reactions. For control of color, the manipulated variable may further be
 the combinatory proportion of a colorant in the material, especially
 colorants taken from the list: dyes; bleaches; brighteners; whiteners;
 fluorescence inhibitors; tinting agents; opacity agents; pigments,
 fluorescent colorants; fluorescent brighteners; fluorescent pigments;
 pre-colored feedstocks or feedstocks of different colors. For control of
 composition, the manipulated variable may further be the combinatory
 proportion of a feedstuff in the material, especially feedstuffs taken
 from the list: filters; sizing; coatants; recycled stocks or stocks of
 different constituent substances; or may be the concentration of a
 substance in a feedstuff.
 The color target 10 may be specified in at least one of the following ways:
 as desired values of emissivity; as desired values of transmissivity; as
 desired values of one or more apparent reflectance spectra with particular
 conditions of illumination; as desired values of one or more apparent
 transmittance spectra with particular conditions of illumination or as
 desired values of one or more sets of colorimetric quantities with
 particular conditions of illumination and observation. The controller 9
 may comprise means for supplying weighting factors for the color target 10
 specified in at least one of the following ways corresponding to one or
 more of the ways the color target is supplied: as weighting factors for
 emissivity values; as weighting factors for transmissivity values; as
 weighting factors for one or more reflectance spectra; as weighting
 factors for one or more transmittance spectra or as weighting factors for
 one or more sets of colorimetric quantities. Said weighting factors are
 used in calculating the difference between the color measurement 8 and the
 color target 10. The controller 9 may further comprise means for supplying
 quality acceptance ranges for the color target specified in at least one
 of the following ways, corresponding to one or more of the ways the color
 target 10 is supplied: as a range of acceptable emissivty values; as a
 range of acceptable transmissivity values; as one or more ranges of
 acceptable reflectance spectra; as one or more ranges of acceptable
 transmittance spectra or as one or more ranges of acceptable colorimetric
 quantities. Said ranges are used in calculating the difference between the
 color measurement 8 and the color target 10.
 Composition and other targets 10 may be specified according to the manner
 of estimation of their measurements 8. For example, composition targets 10
 may be specified as percentual or fraction content of a component in the
 material by mass or volume, or as an amount or mass of a component per
 unit area of the material. Targets 10 may also be supplied for one or more
 of the scalar observation quantities from which composition or color or
 other properties are estimated. The controller 9 may comprise weighting
 factors for these composition and other targets, and may further comprise
 means for supplying quality acceptance ranges for one or more such
 targets.
 In one embodiment of the invention the color or composition of a sample of
 the material is measured when the process 6 is in a first substantially
 steady state. Thereafter, a change is induced in a manipulable process
 variable causing the process to reach a second substantially steady state.
 The color or composition of a sample of the material is measured when the
 process is in the second substantially steady state. The difference
 between the color or composition measurements in the first and second
 steady states is scaled according to the size of the change induced in the
 manipulated variable to produce a normalized color or composition change
 and the effect of the manipulated variable is characterized in terms of
 the normalized color or composition change. The timing of changes in the
 color or composition of the material as the process moves from the first
 to the second steady state may be observed and the effect of the
 manipulated variable may be characterized in terms of the normalized color
 or composition change and its timing parameters.
 The drawing and the related description are only intended to illustrate the
 inventive concept. The details of the invention may vary within the scope
 of the claims. Therefore the possible combinations of this invention with
 prior art are many and varied. For example, the measurement of color or
 composition profile may be combined with art for separation of MD- and
 CD-components of variability, such as by deploying measurement devices at
 each of plural locations across the web, or by employing light pipes from
 a measurement device to convey light to and from each of plural locations
 across the web. The directional illumination or directional light
 detection may be combined with art for illumination or detection in an
 annulus or in one or more arcs. The characterization of the effect of
 process changes may be combined with prior art for process model
 identifications. The measurement of reflected light may be combined with
 prior art for alternating sample backings of different reflectance. The
 control of color or composition may be combined with prior art for optimal
 control, especially using methods for constrained optimization.
 Furthermore, the control of color or composition according to this
 invention may be combined with art for control of properties in the
 cross-machine direction, such as disclosed in WO 98/32916. Similarly, the
 set of calibration samples shown in FIG. 1 with broken lines used in
 determining the factors and coefficients for estimating properties from
 the emissivity and/or transmissivity measurements or from a calculated
 apparent reflectance and/or transmittance and/or absorbance may be
 provided within the apparatus for regular recalibration, or may be
 furnished externally from time to time for intermittent recalibration.
 Additionally or alternatively, calibration samples may be measured using
 other means, and those measurements or factors and coefficients determined
 therefrom may be supplied to the means for determining properties from the
 emissivity or transmissivity measurements. The samples may be chosen with
 properties which facilitate the calibration process, for example by
 optimizing the amount of information which can be deduced by the
 statistical method used in determining the factors and coefficients.
 Suitable techniques are given for example in Box, G., and Draper, N.,
 "Empirical Model-Building and Response Surfaces", Wiley, New York, 1987,
 among others. Moreover, similar techniques can be applied to
 characterizing the effect of manipulating process variables on measured
 properties, in that optimally selected sets of process disturbances can be
 used which facilitate the identification and parametrization of said
 process effects.
 Moreover, as the art of color measurement and control advances, common
 conventions and recommendations may be revised by the generally accepted
 standards authorities in the art. For example, new instrument geometries
 or colorimetric formulae may be adopted in a future standard. This
 invention anticipates such revisions, and should be understood to cover
 such, where extension of this specification to provide such coverage would
 require mere substitution of the new term in place of an existing like
 term, or would require merely adding the new term to a list of existing
 like terms.
 In an embodiment for measurement of reflected light, a light detector
 arrangement is located on the same side of the sample as the light source
 arrangement. In an embodiment for measurement of transmitted light, a
 light detector element is located on the opposite side of the sample to
 the light source arrangement. In one embodiment for measurement of both
 reflected and transmitted light, a light source arrangement is located on
 one side of the sample, and light detector arrangements are located on
 both sides of the sample. In another embodiment for measurement of both
 reflected and transmitted light, light source arrangements are located on
 both sides of the sample, and a light detector arrangement is located on
 one side of the sample. In yet another embodiment for measurement of both
 reflected and transmitted light, light source arrangements are located on
 both sides of the sample, and light detectors are located on both sides of
 the sample.
 In an embodiment for measuring color or composition of a moving sheet, the
 apparatus comprising one or more light source arrangements and one or more
 light detection arrangements is deployed in a scanning apparatus which
 traverses the sheet in a direction substantially perpendicular to the
 direction of movement of the sheet. In another embodiment, plural
 apparatuses each comprising one or more light source arrangements and one
 or more light detection arrangements are employed in plural locations in a
 non-scanning apparatus which is positioned across the path of the moving
 sheet. In a variation on this embodiment, the apparatus may scan over the
 moving sheet with a scan distance which is substantially equal to the
 separation between the light paths of the plural arrangements, so that
 substantially the whole width of the sheet is measured. In a further
 embodiment, light pipes are used to convey light from one or more light
 source arrangements to each of plural locations across the sheet, and
 light pipes are used to convey light reflected from or transmitted through
 the sheet at each of plural locations across the sheet to one or more
 light detector arrangements, where said source arrangements and detector
 arrangements need not be equal in number and need not be situated across
 the path of the moving sheet.
 In the above mentioned embodiments, a light source arrangement may be
 constructed so that the sample is illuminated in any practical geometrical
 relation to the sample. Preferably, the sample is illuminated
 substantially according to one of the following geometries:
 perpendicularly to the sample (0 geometry); from one or more directions,
 each subtending an angle of approximately 8 degrees from the perpendicular
 (8 geometry); from one or more arcs, each subtending an angle of
 approximately 8 degrees from the perpendicular (8 geometry); from an
 annulus subtending an angle of approximately 8 degrees from the
 perpendicular (8 geometry); from one or more directions, each subtending
 an angle of approximately 45 degrees from the perpendicular (45 geometry);
 from one or more arcs, each subtending an angle of approximately 45
 degrees from the perpendicular (45 geometry); from an annulus subtending
 an angle of approximately 45 degrees from the perpendicular (45 geometry)
 or diffusely, with incident light substantially at all angles (d
 geometry).
 In the above mentioned embodiments, a light detector arrangement may be
 constructed so that the transmitted or reflected light is detected in any
 practical geometrical relation to the sample. Preferably, the light is
 detected substantially according to one of the following geometries:
 perpendicularly to the sample (0 geometry); from one or more directions,
 each subtending an angle of approximately 8 degrees from the perpendicular
 (8 geometry); from one or more arcs, each subtending an angle of
 approximately 8 degrees from the perpendicular (8 geometry); from an
 annulus subtending an angle of approximately 8 degrees from the
 perpendicular (8 geometry); from one or more directions, each subtending
 an angle of approximately 45 degrees from the perpendicular (45 geometry);
 from one or more arcs, each subtending an angle of approximately 45
 degrees from the perpendicular (45 geometry); from an annulus subtending
 an angle of approximately 45 degrees from the perpendicular (45 geometry);
 diffusely, with detected light at substantially all angles (d geometry) or
 diffusely, with detected light substantially at all angles, except the
 angle of directional illumination (t geometry). The various geometrical
 embodiments may be combined, so that either illumination or detection or
 both may be performed in plural geometries with respect to the sample,
 simultaneously, individually or independently.