CA1169140A

CA1169140A - Method and apparatus for measuring and controlling the color of a moving web

Info

Publication number: CA1169140A
Application number: CA000398458A
Authority: CA
Inventors: John W. Mactaggart
Original assignee: Sentrol Systems Ltd
Current assignee: Sentrol Systems Ltd
Priority date: 1982-03-16
Filing date: 1982-03-16
Publication date: 1984-06-12

Abstract

Abstract of the Disclosure Apparatus for measuring and controlling the color of a moving web in which a circular variable bandpass filter having a passband varying substantially continuously with angular displacement through the optical spectrum is interposed in the optical path between the web and a detector and is rotated to produce a detector output which periodically scans the optical spectrum. In one disclosed embodiment, the detector outputs at the various wavelengths are weighted to produce X, Y and Z tristimulus values while, in another disclosed embodiment, dye flows are so controlled as to minimize the total square error between the measured reflectance spectrum and the desired reflectance spectrum.

Description

' ` 1169140 ¦ Title of the Invention AND ~PPARATUS FOR MEASURING A~D
CO~TROLLING THE COLOR OF A MOVING WE~

Field of the Invention ~.

5 ~ Thi~ invention relates to a method and apparatus for meaqur~ng and controlling color and, especially, to . a method and apparatus for measuring and controlling the color of a moving web.

Backg~ound of the IDvention In general, systems o~ the prior art ~or controlling the dyeing o a moving web operate by measuring the tristimu-lus value~ X, Y and Z o~ l~ght réflected from a moving por-tion o~ the web. The tristimulus values, which are roughly ¦ equiv~lent to the "red," "green" and "blue" ComponentJ, ~.
15 1 respectively, o the reflccted light, are either measured slmultaneously by different detectors, as in De Rcmlgis ¦ U. S. Patent No. 3,936,189, or successively using a fllter ¦ wheel or the like as in Lodzinski U. S. Patent No. 4,019,819, The trist~mu1us values X, Y and Z are either used directly 20 ¦ or control purposes or are ~irst converted to other coordin-2tes such as Hunter coordinates L, a and b.

. ~
While three- or our-filter colorimeters o the type do3cribcd above are common ln the art and are adoquate or ., I .

. ~ -2-r/, ' ! ordinary control applications, they suffer serious drawbacks.
Fisat, tho X, Y and Z tri~tlmulus outputs are only ind~ica-tive of the perceived color of the web under the illuminant u~ed in the colorimeter. A color "match" obtained in ter~
of tristimulu~ values using a standard illuminant aoes not neces3arily lndicate a match with an illuminant having a - dif~erent spectral composition, and, in general, it i~
impossible to predict the color properties of a mater~al with a given illuminant if only it~ tristimulus values X, 10l Y and Z are known. Further, if the actual spectral curves of the illuminant or detector used in the colorimeter differ from those for which the filters were designed, the tristlmu-lu~ values obtained will not necessarily even indicatc the color properties of the material under a standard illum~nant.
While LodzinsXi does suggest, as an alternative, using a relatively large number of narrow-band filters so as to approximate an abridged spectrophotometer, he suggests no practical implementation of this proposal in an on-line system.
Another defect of control ~ystems of the prlor art arises from the nonlinearLty of the relationship betwecn the tristimulus values X, Y and Z and the dye concentratlons to be controlled. While this nonlinearity is relatively insignificant at low dye concentrations, it increases wLth dye concentration so that, when relatively saturated color3 are being sought, the nonlinearity is substantial. As a result, in practical systems, the relationship between X, Y
and Z and the dye concentrations must be linearized about ~ome nominal setpoint to maXe the computation tractable.

_3_ 1 ~69 ~40 This need for linearization is obviously disadvantageou~, since not only does the operating point vary about the setpoint, but the setpoint itself is often changed, necessi-tating a recomputation of the linearized equation.

Mccarty ~. S. Patent No. 3,601,589 disclose~ a system for selecting pigments to match a given surface coating in which an in~tial pigment formulation is generated in advance of actual mixing by selecting those concentrations which minimize the total square error between the measured reflectance of the coating being matched and the computed reflectancc of the pigment formulation. ~owever, the actual mixing proccss itself is controlled by sampling the mixture with a colorimeter and using L, a and b coordin~tes computed from the colorimeter output to correct the ~nitial pigment ~ -formulation.

Summa_y_of the Inventio_ One object of my invention is to provide a color measurement and control system which insures a color match under an arbitrary illuminant.

Anothcr object of my invention is to provido a color measurement and control system which does not roquire matching of the spcctral curvcs of its variou~ optical com-ponents .

116g~0 ' Still another object of my invention is to provide a color measurement and control system which ~s relatively insensitive to changes in opcrating point.
A further object of my invention is to provide a 5 color measurement and control system which permits the independent control of four or more dyes.

Other and further objects will be apparent from tho following description.

In one aspect, my invention contemplates apparatus 10 for measuring the optical reflectance of a surface such as that of a moviny web in which a first predetermined optical path couples a portion of the surface to a light sourcc of predetermined spectral content whlle a second predetermined optical path couples the same surface portion 15 to a detactor. Disposed in at least one of the paths i5 a bandpass filter hDving a passband varying subitantially continuously through the cptical spectrum with the point of incidence of the optical path on the filter. Varying the point of incidence of the optical path on the filter produce~
20 an output from the datcctor which scans tho optical spcctrum. c Preferably, the resolution of the optical system ~n such as to permit successive dotector outputs representing about 180 differcnt wavolengths oach spaced about 1.7 nanometers apDrt.
Preferably the continuous filter is a circular variable filtor 25 which inters~cts thc optical path at an off-center location and which is rotated to scan periodically the optical spectrum.
In another aspect, my invention contemplates an '~
on-linc system for controlling the application of a plurality of colorants to a continuously formed material in which the reflectance of a portion of the material containing the colorants is measured at a plurality of wavelengths. The flow of the colorants to the material is then adjusted so as to minimi7e the sum of the squares of the deviations of the measured reflectances from predetermined desired réflectances. Preferably the reflectance measurements of the material are obtained by using a circular variable ; bandpass filter in the manner described in the preceding paragraph.

lS Brief Descri~tion of the Drawinqs In the accompanying drawings to which reference i3 made in the instant specification and in wh~ch like reference characters are used to indicate like parts in the various views:

FIGURE 1 is a sid~ elevation, with parts shown in section, of the sensing head of my color measurement and control system.

~ 1~9140 .:

FIGURE 2 is an o~lique view of the circular variable filter used in the head shown in FIGURE l.

FIGURE 3 is a fragmentary section of the head shown in FIGURE l, takcn along linc 3-3 thereof.
FIGURE 4 is a schematic view of a tristimulus color measurement system incorporating the head shown in FIGURE l.
FIGURE 5 is a flowchart of a program for controlling the operation of the system shown in FIGURE 4.
FIGURE 6 is a schematie view of a eolor control 10 system incorporating the head shown in FIGURE 1.
FIGURES 7a and7b are a flowchart of a program for controlling the operation of the system shown in FIGURE 6.
FIGURE 8 is a graph illustrating the matching of the individual dye absorption spectra to the ~eb reflectance spec-15 trum measured by the system shown in FIGURE 6.

Descri~tion of the Preferred Embodiments Referring now to thc drawings, the sensor portion of my syqtem, indicatcd generally by tho referenee character lO, is adapted to measure the color of a web 12 20 of paper or the like. The sonsor portion 10 includes an C
optical sensing head, indicatcd generally by the reference character 14, disposcd above thc web 12 and an optical shoo, indicated generally by the reference character 16, disposed 1169~40 , below the head. Any suitable me2ns (not ~hown) may be provided for mounting the head and shoe for movement out of associated relationship with the web 12.

Thc optical sensing head 14 includes a housing 18 to which a top plate 20 is secured by any suitable means such as by screws 22 into sealing engagement with a gasket 24 extending around the top of the housing. Top plate 20 carries a mounting stud 26 adapted to be secured to tha head support ~not shown). I provide the housing 18 with respective access openings 28 and 30 normally closed by covers 32 and 34 which cngage gaskets 36 to seal the access openings 28 and 30.

The base 38 of tho housing 18 i8 provided with an opening 40 over which a window 42 is secured. For example, a framo 44 carrying the window is adapted to be threaded onto a flange on the bottom 38 around opening 40 and into sealing engagement with a gasket 46.

The scnsing hcad 14 includcs a light-integrating spherc, indicated generally by the roference character 48, located inside housing 18 and made up of a lowcr half 50, formed with an opening 52 which registers with the wlndow 42, and with an upper half 54 secured in operative rolation-ship with the lowor half in any suitablo manner.

1 ~69 140 I secure rospective bulb-mounting tubes 56 over openings in the upper sphere half 54. Caps 58 assembled on the tubes 56 hold bulbs 60 and 62 and their associated mounts in position in the tubes 56 to direct light into thc interior of the sphere 48. I provide the lower sphare half 50 with a pair of light deflectors 64 and 66 and pro-vide the upper sphere half 54 with light deflectors 68 and 70 for ensuring proper distribution of light from the sources 60 and 62 within the sphere while at the same time preventing the detector to be described hereinbelow, from being directly illuminated by the sources 60 and 62. While any suitable sources may be employed, preferably I employ two 50-watt tungsten-filament quartz-iodine lamps supplied by a constant-current xource for the lamps 60 and 62.

I form an opening 72 in the upper sphere half 54 through which reflected light from a spot portion of the web 12 ix directed onto a detector. More specifically, a lens 76 disposed inside a tube 7~, the lower end of which adjoins the opening 72, focuses light from the spot portion 20 of the web 12 onto 3 pho~odetector 78 positioned at the upper end of the tubc 74. In~erposed betwcen thc detector 7a and the upper end of the tube 74 i~ a circular variable filter indicated gcncrally by thc refercncc numeral 30.
As shown in FIGURE 2, filter 80 comprises a substratu 82 t~9~
.:~

having an interference filter coating 84 on one side thereof.
In a manner known in the art, the thickness of the inter-ference filter coating 84 that is applied to the substrate 82 varies with angular displacement about the axis of the filter 80. As a result, there is a corresponding angul~r dependence of the center wavelength that is passed by any particular angular segment of the filter coating 84. Thus, in the embodiment shown, the th;cXness to of the thinnest, or 0 , coating segment is such as to pass a wavelength of about 400 nanometers, while the thickness of the 360 segment (not shown in FIGURE 2) is such as to pass a wavelength of about 700 nanometers. ~etween these two extremes, the _, thickness - and hence passband wavelength - vary linearly with angular displacement, the thickness tl80 of the 180 scgment, for example, being such as to pass a wavelength of about 550 nanometers.

Filter 80 is mounted on a shat 86 of a suitable motor such as a stepper motor 88 which rotates the filter 80 to vary the wavelength transmittcd to the detector 78.
A posltion encodcr 90 coupled to the motor shaft 86 provides a parallel digital output L on a line or channel 108 indicating the particular two-degree angular segment of the filter 80 that intercepts the optical axis. Prcferably, to limit the circumferential extent of the f~lter 80 that is ~r~

1 ~691~0 .

"seen" by the detector 78 at any particular instant, an optical slit-for~ing member 92 is disposed between the filter 80 and detector 78. Preferably the width d of the slit is such as to subtend about 2 at its average spacing r from the axis of the filter 80.

Shoe 16, which supports the web 12 as it moves past the head 14, comprises a housing 104 within which i8 disposed a rotatable block 102. Normally, during the color measurement or control phase of operation, block 102 ls 80 oriented within housing 104 as to position a suitable stand-ard reflecting surface 94 beneath the web 12. Block 102 also supports three additional rcflecting surfaces 96, 98 _ .
and 100 which are rotated into position beneath the web 12 during calibration. These surfaces 96, g8 and 100 may com-prise, for example, a standard "white" reflecting surface, a standard "black" reflecting surface and an additional reflecting surface for calibrating the response of the detector 78.

Referring now to FIGURE 4, I show a system, indicated generally by thc refercnce numeral 105, in which scanning head 14 supplies inputs to a computer whlch generates and displays the X, Y and Z tristimulus values of thc light reflcctad from the web 12. More particularly, a digital computer 1~0 of any suitable type known to the 1 169 :~4~

.

art, such as a genoral purpose microcomputer~ receives one data input from line 108, which carrics thc signal L
indicating the angular position of the filter 80. A line 106 from the head 14, carrying the detector output IREFL
which is proportional to the reflected light intensity, feeds an additional data input to computer 110 through an analog-to-digital converter ~ADC) 112. Computer 110 provides a suitable digital output to an X display 114, a Y display 116 and a Z display 118. Displays 114, 116 and 118 may be of any suitable type known to the art such as, for example, segmental digital displays, strip chart recorders, or thc like. In addition, the X, Y and Z outputs appear on respec-tiva lines 113, 115 and 117, whlch may provide inputs to a suitable control system (not shown) for regulatlng the application of dyes to the web.

Referring now to ~IGURE 5, I show a program which may bc uscd by the computer 110 to generate tristimulus values X, Y and Z from the outputs L and IREFL of the scanning head 14. The program shown may typically be a sub-routine that is repeatedly entercd during the mcasuromentphase of operation between poriods of calibration. More particularly, after tho subroutine i3 entered at block 120, an index K representing the number of completa revolutions of the filter 80 por averaging period is initializcd at zcro ~block 122). Aftar the tristimulus values X, Y and Z are 1 ~69 1'10 :~

also initialized at zero tblock 124), the index K i8 incremented by 1 (block 126) while a second index I, indicat-ing the particular 2 angular segment of the filter 80, is initialized at 7.ero (block 128).

S The index I is then incremented by 1 (block 130), and the position encoder signal L is interrogated to determine if it s equal to the index I (blocks 132 and 134), the motor 88 being either run continuously or stepped.

2 each traverse of blocks 132 and 134. The subroutine waits nntil the position signal L matches the index I and then interrogates the detector output IREFL (block 136).
The subroutine then uses the signal REFL to update the tristimulus values X, Y and Z by adding to the pre-viously stored value3 quantitie3 proportional to the product of the detector output IREFL and tho particular tristimulus value of the wavelength corresponding to the index I (blocks 138, 140 and 142). The subroutine continues along blocks 130 to 142 for each value'of tho index I until the index reaches 180, at which point the subroutine leaves the loop (block 144) and test~ whether tha index K has reachod a predotermined value, in this ,^.
caso 10 (block 146). If the index K is less than 10, the subroutine returns to block 126 where it incroments K by 1 and updato3 the tristimulus values X, Y and Z for another revolution of the filter 80. This process is continued until K reache3 10, at which ppint the subroutine feeds the finally computed tristimulus values X, Y and Z to the :l t69 1~0 displays 114, 116 and 118 (bloc~ 146). The subroutine then returns (block 150) to the main program (not shown), which typically may immediately re-entcr the subroutine shown in FIGURE 5.

Referring now to FIGURE 6, I show an embodiment of my invention in which the absorption spectra of the dyes . .
are fitted by the method of least squares to the measured spectrum of the web to generate flow correction signals.
In this system, indicated generally by the reference numeral 152, I use a computer 54 which may be similar or identical to the computer 110 shown in PIGURE 4. Computer 154 receives the position signal L from the head 14 directly through a suitable input port. An analog multiplex circuit 156 receiving the IREFL signal from head 14 as one analog input ~ -provides a selected analog input to analog-to-digital con-verter (ADC) 158 in accordance with an address signal pro-vided by the computer 154. ADC 158 provides a multi-bit digital output to an additional input port of computer 154.

Computer 154 provides flow control signals FCl, FC2 and FC3 via respectivo channels 160, 162 and 164 to digital-to-analog converters 166, 168 and 170. Converters 166, 168 and 170 control respective pumps 172, 174, and 176 controlling raspective dye lines 178, 180 and 182 leadlng from die supplies 184, 186 and 188. Lines 178, 180 and 182 . .

1 169~0 ., , feed a single spray head 190 which applies the dye from the supplies 184, 186 and 188 to the web 12 moving past the head 190. Head 190 is, of course, located upstream from measuring head 14 to permit the head 14 to measure changes effected by adding dye to the web 12. Respecti~e fl~w meters 192, 194 and 196 in dye lines 178, 180 and 182 provide measured flow inputs E1, F2 and F3 to the analog multiplex circuit 156 via respective lines 198, 200 and 202.

The re)ationship between the measured reflectance Ri of the web 12 at a given wavelength ~ i and the respective dye concentrat~ons cl, c2 and C3 as indicated by the dyc flows Fl, F2 and F3 is closely approximated by the followlng equation: ;

Ri ' Irefl/IOi = ROi/eXP ~Xilcl + Xi2C2 + Xi3 C3 + ei) where i is an index ranging from 1 to 180, Irefl ls the measured intensity of reflected light from the web 12 a~
indicated by the ~ignal IREFL; Io~ is the previously determined intensity of light incident on the same wob por-tion at wavelength ~it Roi is the reflectance of theundyed sheet at wavclength ~ i ; Xil , Xi2 ~3 predetcrmined constants; and ei is a random ~arror term reflecting such factors as deviations in actual undyed sheot .

1 169:~40 reflectance, deviations in actual aye strength or composition, and the like.

Equation tl) may be restated in terms of the following equation:

Yi = -ln (~i/ROi) = Xilc1 + Xi2C2 + Xi3C3 + ei (2) Equation (2) may be re-expressed in matrix notation as follows:

Y = Xc + e (3) 10 wherc Y i8 a column vector with clemcnts Yl, Y2, ... Y180;
C i5 a column vector with elements el, e2, ... el80;
c is a column ve~ctor with elemcnts cl, c2, ci and x i~ a 180 x 3 matrix with elements Xij .

Standard regression theory tells us that an "estimatea" or "effective" dye conccntration vector c~, that is, the quantity that minimizes the square crror (Y - Xc~)'(Y - Xc) (4) where ~Y - Xc)' is the transpose of (Y - Xc), i3 given by the expresRion .. , I :1~169~40 . .

C ~ lx~x)~lx~Y ~s~

Or, more simply, the concentration vector Ce can be expressed as ~ Ce ~ AY ~6) 5 1 where ~X'X)~ ~' ~7) ¦ In the system contemplated, the "effective~ dye concentrations obtained in thia manner are compared with 1I previously determined desired concentrations to establlsh ,, 10 ~¦ concentration "errors" due to the factors mentioned above.
The re~pective actual dye concentratlons as ind$cated by the dye flow meters are then offset by amounts equal to ¦ those concentration errors to generate corrected concen-¦ trations of dyes to be applied to the web. These correctedlS , dye concentrations minimize the total square error between ¦ the measured reflectance spectrum and the des1red reflec-¦ tance spectrum of the web.

~o see that this 18 the case, let u~ deflne cd as ! a three-dimensional column vector of the theoretical zo I donirod dyo concnntrationn, ann~ming no orror voctor o e as the quantity defined by the equation ~c = Ce ~ Cd t8) and ck as the quantity defined by the equation Ck = c - ~c (9) where e, as stated before, represents the actual dye concentrations corresponding to the dye flows Pl, F2 and F3.

If we now change the actual dye coneentrations to Ck, then the new value of Y is given by the exprsssion 10Yk = Xek + a tl0) On the other hand, tho theoret~eal desired coneentrations Cd would, assuming an error voctor e of zero, result in a "desired" value of Y of . Yd Xed tll) ' 15The difference, or error, between theso two ; quantitles is Yk - Yd = XCk ~ e - XCd tl2) ~.

Applying equations t3), t8) and t9), this reduces to Yk Yd Xe ~ e - Xee 20= Y - Xee tl3) 1 169 14~

, where Y is the value obtained with the original actual concentrations c. Since, however, we have already minimized the square o the right-hand side of equation (13) by our selection of Ce , we have minimized the square of the "error" expression on the left side as well.

Referring now to FIGURES 7a and 7b, I shcw a program for controlling the application of dye from supplies 184, 186, 188 in accordance with the color of the dyed web 12 as sensed by the head 14. LiXe the progra~
~hown in FIGURE 5, the program shown in FIGURES 7a and 7b may typically be rcpcatedly entered as a subroutine between successive calibrations of the system 152. After entry at block 204, the subroutine initializes to zero an indax ~ indicatlng the numbcr of revolutions of the filter 80 lS (blocX 206), as well as the quantities C~l), C~2), and C~3) corresponding to the components of the "effective" dye concentration vector Ce ~blocX 2Q8). The subroutine then increments K by 1 ~blocX 210) and initializes the index I, corrosponding to the index i in equations ~1) to ~13) 20 ~blocX 212). Next, tho subroutinQ 'ncrements the index ~, I by 1 ~blocX 214) and waits ~blocks 216 and 218) until the position signal L from the head 14 matches the indcx I.
When this occurs, thc subroutine suitably addrcsscs the Iq , .

1 169 1'10 multiplex ci~cuit 15G to input the measured light intensity IREFL (block 220).

After it has obtained the reflected light intensity signal IREFL, the subroutine divides this 5 quantity by a previously stored value IO(I) (corresponding to Ioi) indicating the incident light intensity at that wavelength to generate a signal R(I) (corresponding to Ri) indlcating the reflectance of the web 12 at the wavelength indicated by the index I (block 222). The subroutine 10 divides the quantity R(I) by a previously stored quantity RO(I) ~corresponding to Roi) indicating the reflectance of an undyed web portion at that wavelength and takes the nega- _, tive logaritl~ of the quotient to obtain a quantity Y~II
~corresponding to Yi) that varles linearly with dye concen-tration (block 224). The subroutine then enters a loop ~blocXs 226 to 232) in which it revises the previou~ly stored quantities C(l), C~2) and C~3) by adding to the~
terms proportional to the computed quantity Y~I). In block 230, the quantity A(I,J) corresponds to tho element 20 Aij of the least-square optimization matrix dafinod in equatlon ~7). c The subroutine next interrogates I to determino whether it has reachcd 180 and, if not, returns to block 214 to obtain and proccss the measurcd light intensity IREFL at 1 1691~

the next wavelength I, the subroutine reiterating blocks 214 to 234 for each value of the index I. When this loop has been traversed for all values of the $ndex up to 180, the subroutine leaves the loop (blocX 234) and interrogates the index K to determine whether it has reached a predetermined quantity, for example 10 (block 236).
If not, the subroutine returns to block 210 and repeats the éntire sequence (blocks 210 to 236~ for another revolu-tion of the filter 80.
After a suitable averaging interval of ten filter revolutions in this case (block 236), the subroutine initializes a timer (not shown) internal to the computer 154 to define a time interval for the control oper-ation (block 238). During thi~ period, the subroutine first genarates respective concentration error signals CEl, CE2 and CE3 corresponding to the components of the error vector ~c by subtracting from the respective estimated concentrations C(l), C(2) and C(3) the quantities C01, C02 and C03 corresponding to the elements of the des$red concen-tration vector cd (block 240). The subroutine then gener-ates respective flow control signals FCl, FC2 and FC3 by multiplying the concentration error signals CEl, CE2 and CE3 by previously determined coefficient~ -Gl, -G2 and -G3 ~block 242). After th$s, the subroutine generates a suitable address signal to obtain the flow inputs Fl, F2 and F3 (block 244) and generates respective target flow values FlT~

1169~Q

F2~, and F3T by adding to the respective measured flow signals Fl, F2 and F3 the respcctive flow correction signals FCl, FC2 and FC3 ~block 246 ) ., The subroutinc then entars a loop (blocks 248 to 254) in which it continually interrogates the measured flcw values Pl, F2 and F3 and generates flow-control signals FCl, FC2 and FC3 on the basis of the difference between the measured flow values and the target flow values previously generated. These continually recomputed correction signals FCl, PC2, and FC3 are provided to the digital-to-analog con-verters 166, 168 and 170 controlling the pumps 172, 174 and 176. ~t the end of the interval determined by the timer in _, .
block 238, at a point when t~le mcasured flow values Fl, F2 : and F3 have convcrged upon tar~et flow values FlT, F2T and F3T, the ~ubroutine leaves the loop (block 254) and returns~block 256) to the program (not shown) calling the subroutine.
As mentioned above, typically the cubroutine shown in EIGURES 7a and 7b is repeatedly re-entered between succes-sive calibration periods of tho apparatu~ 152.

In the system shown in FIGURE 6, the flow correction signals FCl, FC2 and FC3 are generated by fit-ting the dye absorption spectra to thc measurcd wcb reflec-tance spectrum to obtain the "cffectivc" dye concentrations C(l), C(2) and C(3). Ilowever it will bc apparent to those 2,~ .

1 :169 1 ~ O

skilled in the art that the computational steps involved are commutative and that one could alternatively compare the measured rcflcct~nce spectrum with a desired spectrum and then fit the dye absorption spectra to the error spes:-5 trum thus obtained.

Referring now to FIGURE 8, I show a graph illustrating the matching of the individual absorption spectra of the dyes to the measured reflectance spectrum of the dyed web 12. In FIGURE 8, the abscissa represents the 10 wavelength in nanometers while the ordinate represents the negative of the logarithm of the measured reflectance, as it i8 this quantity which i9, to a first approximation, linearly dependent on the dye co ncentration. In FIGURE 8, curve 258 corresponds to thc measured reflectance of the dye of lS dyed wob 12, while curve~ 260, 262 and 264 correspond re-spectively to thc absorption spectra of the individual dyes, weighted by the estimated dye concentrations C(l), C ~2) and C (3) obtained by the subroutine shown in FIGURES
7a and 7b~, In thi~ graph, it i5 assumed that the reflect-20 ancc of an undyed web is independent of the wavelength )~
80 that the spcctrum corrcspond~ng to the sum of the curves 260, 262 and 264 rcpresents a least-squarc approximation of the actual reflectance curve 258.

Whilc thc system 152 shown in FIGURE 6 omploy~

~ 69~

three dyeq, it should be emphasized that my sy~tem i~
¦ readlly adaptable to control the simultaneous application of a greater number of dyes if more accurate color match~ng I is desired. Indeed, one of the ~alient advantages of my 5 ' dye control ~ystem employing lea~t-square optimization is ¦I that it is not limited to only throe dyes as are systems based on measurement of the tristimulus values X, Y and Z.
In my system, for example, a color mi6match occurring over one portion of the visible spectrum can be correctea by 1~ I using an additional dye that is selectively ab~orptive in that portion of the spectrum without affect~ng the color match elsewhore.

It will be seen that I have accomplished the ¦ object~ of my invention. My color measurement and control lS I system doe~ not require matching of the spectral curve~ of I it~ various optlcal component~, and i9 relatively in~ensi-¦l tive to changes in operating point. Finally, my system por-mit~ the independent control of four or more dyes.

It will be understood that certain feature3 and 20 ¦ subcomblnations are of utillty and may be employed without reference to other features and subcombination~. ~hi3 1~
' contemplated by and is within the scope of my clalms. It 18 further obvious that various changes may be made in dotalls within the scope of my claims without departing fr~m -24_ ' 1.1~91~ .

~

the spirit of my invention. It ~B, therefore, to be understood that my invention is not to be limited to the -.
~peclfic details shown and described.

t~a*ing thu~ described my invention what I s:laim 18:

Claims

The Claims

1. Apparatus for controlling the application of a plurality of colorants to a continuously formed web including in combination a light source, a detector, means for providing a first optical path between said source and said web, means for providing a second optical path between .

said web and said detector, a circular variable bandpass filter disposed in one of said paths, said filter having a passband varying substantially continuously with angular displacement through the optical spectrum, means for rotat-ing said filter to produce an output from said detector representing the measured reflectance spectrum of said web, means for generating correction signals such as to minimize the square of the deviation of said reflectance spectrum from a predetermined desired spectrum, and means responsive to said correction signals for adjusting the flow of said colorants to said web.

2. Apparatus for controlling the application of a plurality of colorants to a continuously formed web including in combination a light source, a detector, means for providing a first optical path between said source and said web, means for providing a second optical path between said web and said detector, a bandpass filter disposed in one of said paths, said filter having a passband varying substantially continuously through the optical spectrum with the point of incidence of said one optical path on said filter, means for varying the point of incidence of said one optical path on said filter to produce an output from said detector representing the measured reflectance spectrum of said web, means for generating correction signals such as to minimize the square of the deviation of said reflectance spectrum from a predetermined desired spectrum, and means responsive to said correction signals for adjusting the flow of said colorants to said web.

3. An on-line system for controlling the application of a plurality of colorants to a continuously formed material including in combination means for measuring the reflectance of a portion of said material containing said colorants at a plurality of wavelengths, means for generating correction signals such as to minimize the sum of the squares of the deviations of said measured reflectances from pre-determined desired reflectances, and means responsive to said correction signals for adjusting the flow of said color-ants to said material.

4. Apparatus as in Claim 3 in which said colorants are transparent colorants.

5. Apparatus as in Claim 3 in which said colorants are dyes.

6. A method of controlling the application of a plurality of colorants to a continuously formed material including the steps of measuring the reflectance of a portion of said material containing said colorants at a plurality of wavelengths, generating correction signals such as to minimize the sum of the squares of the deviations of said measured reflectances from predetermined desired reflectances, and adjusting the flow of said colorants to said material in response to said correction signals.

7. A method as in Claim 6 in which said colorants are transparent colorants.

8. A method as in Claim 6 in which said colorants are dyes.

9. Apparatus for measuring the optical reflectance of the surface of a moving web including in combination a light source of predetermined spectral content, a detector, means for providing a first predetermined optical path between said source and said surface, means for providing a second predetermined optical path between said surface and said detector, a circular variable bandpass filter disposed in one of said paths, said filter having a passband varying substantially continuously with angular displacement through the optical spectrum, and means for rotating said filter to produce an output from said detector representing the measured reflectance spectrum of said web.

10. Apparatus for measuring the optical reflectance of the surface of a moving web including in combination a light source of predetermined spectral content, a detector, means for providing a first predetermined optical path between said source and said surface, means for providing a second predetermined optical path between said surface and said detector, a bandpass filter disposed in one of said paths, said filter having a passband varying substantially continuously through the optical spectrum with the point of incidence of said one optical path on said filter, and means for varying the point of incidence of said one optical path on said filter to produce an output from said detector representing the measured reflectance spectrum of said web.

11. Apparatus for measuring the optical reflectance of a surface including in combination a light source of predetermined spectral content, a detector, means for providing a first predetermined optical path between said source and said surface, means for providing a second predetermined optical path between said surface and said detector, a bandpass filter disposed in one of said paths, said filter having a passband varying substantially continu-ously through the optical spectrum with the point of incidence of said one optical path on said filter, means for varying the point of incidence of said one optical path on said filter to produce an output from said detector repre-senting the measured reflectance spectrum of said surface, and means responsive to said detector for generating weighted averages of said measured spectrum corresponding respectively to the X, Y and Z tristimulus values of the reflected light from said surface.

12. Apparatus for measuring the optical reflectance of a surface including in combination a light source of predetermined spectral content, a detector, means for providing a first predetermined optical path between said source and said surface, means for providing a second predetermined optical path between said surface and said detector, a bandpass filter disposed in one of said paths, said filter having a passband varying substantially continu-ously through the optical spectrum with the point of incidence of said one optical path on said filter, means for varying the point of incidence of said one optical path on said filter to produce an output from said detector representing the measured reflectance spectrum of said surface, means for sampling the output of said detector at each of a plurality of angular positions of said filter, and means for obtaining a weighted sum of said sampled detector outputs.

13. Apparatus for measuring the optical reflectance of a surface including in combination a light source of predetermined spectral content, a detector, means for providing a first predetermined optical path between said source and said surface, means for providing a second predetermined optical path between said surface and said detector, a bandpass filter disposed in one of said paths, said filter having a passband varying substantially continu-ously through the optical spectrum with the point of inci-dence of said one optical path on said filter, means for varying the point of incidence of said one optical path on said filter to produce an output from said detector representing the measured reflectance spectrum of said surface, and means responsive to said detector for generating a weighted average of said measured spectrum.

14. Apparatus for measuring the optical reflectance of a surface including in combination a light source of predetermined spectral content, a detector, means for providing a first predetermined optical path between said source and said surface, means for providing a second predetermined optical path between said surface and said detector, a circular variable bandpass filter dis-posed in one of said paths, said filter having a passband varying substantially continuously with angular displacement through the optical spectrum, and means for rotating said filter to produce an output from said detector representing the measured reflectance spectrum of said surface.

.

15. Apparatus for measuring the optical reflectance of a surface including in combination a light source of predetermined spectral content, a detector, means for providing a first predetermined optical path between said source and said surface, means for providing a second predetermined optical path between said surface and said detector, a bandpass filter disposed in one of said paths, said filter having a passband varying substantially continuously through the optical spectrum with the point of incidence of said one optical path on said filter, and means for varying the point of incidence of said one optical path on said filter to produce an output from said detector repre-senting the measured reflectance spectrum of said surface.