EP0840882A2

EP0840882A2 - Colour measuring method and device

Info

Publication number: EP0840882A2
Application number: EP97925130A
Authority: EP
Inventors: Laurence Caisey-Bluteau; Johan Aubert
Original assignee: LOreal SA
Current assignee: LOreal SA
Priority date: 1996-05-23
Filing date: 1997-05-23
Publication date: 1998-05-13
Also published as: JPH11509931A; WO1997044642A3; WO1997044642A2; FR2749077B1; AU3037597A; FR2749077A1; JP2006276030A; US20020051097A1; US6362849B1; EP1510799A1; AU729262B2; CA2224891A1

Abstract

The invention features a colour measuring device comprising: an acquisition system (2, 6) comprising a video-camera (2), and processing means (5) arranged to determine on the basis of the camera-supplied signals the trichromatic components (X, Y, Z) of the colour of the object in a reference colorimetric system (XYZ) using a transfer matrix (M) for switching from the colorimetric system linked to the said acquisition system to the reference colorimetric system and a correction function Η of non-linearities of the said acquisition system.

Description

COLOR MEASURING METHOD AND DEVICE.

The present invention relates to a device and method for measuring color.

It is known to use to measure the color of an opaque object a colorimeter comprising a light source to illuminate the object, an optical device for analyzing the light reflected by this object and calculation means for determining, from of signals delivered by the optical analysis device, the trichromatic components X, Y and Z of the color of the object in a reference colorimetric system, for example that adopted in 1931 by the International Commission on Lighting (CIE) .

The optical analysis device comprises means for dividing the light reflected by the object into three beams each passing through a filtering system and ending in an associated photoelectric cell.

This type of known colorimeter has the disadvantage of requiring direct contact of the optical analysis device with the object whose color is to be measured, and of not being suitable for carrying out a measurement at a distance of the color of an object. or a measurement of the color of a non-opaque object diffusing or absorbing light while allowing a part to pass, such as the skin or certain plastics or make-up products.

To overcome these drawbacks, attempts have been made to measure color by means of a video camera.

However, to the knowledge of the applicant company, there is not yet a device for remote measurement of the color of an object, in particular a non-opaque object, which is capable of allowing precise, reliable and rapid measurement of the color while remaining relatively low cost. The object of the present invention is in particular to propose a new device for measuring the color of any type of object, using a video camera, which allows precise, reliable and rapid measurement of the color while being relatively inexpensive and easy to use.

The invention achieves this by means of a color measurement device, comprising: - an acquisition chain including a video camera, said acquisition chain being capable of delivering signals representative of the trichromatic components of the color of an object , placed in the field of observation of the camera, in a colorimetric system linked to said acquisition chain, and

- processing means arranged to determine, from said signals, the trichromatic components of the color of the object in a reference colorimetric system using a pass matrix of the colorimetric system linked to said acquisition chain towards the colorimetric system of reference and a function for correcting the non-linearities of said acquisition chain, said pass matrix and said correction function being calculated by an iterative process from known trichromatic components in the reference colorimetric system of three primary colors and at least two gray levels and corresponding trichromatic components of the same colors and gray levels in the colorimetric system linked to said acquisition chain, obtained from the observation by said video camera of said primary colors and of said levels of Grey.

In a preferred embodiment of the invention, the device further comprises a display chain for restoring all or part of the image observed by the video camera after processing, by said correction function, of said signals delivered by said chain of 'acquisition.

Preferably, said display chain comprises a display device with cathode ray tube.

Advantageously, the device comprises a light source for illuminating the object placed in the field of observation of the camera, this source having a continuous emission spectrum I (λ) chosen so as to approximate a reference illuminant D (λ).

The camera comprising a set of optical filters of spectra FR (λ), FV (λ) and FB (λ) to decompose on the sensors of this camera the image observed into primary color images, the source is advantageously filtered by one or several filters whose resulting filtering function F (λ) is chosen so as to minimize the error of the differences between the products D (λ) x (λ), D (λ) y (λ), D (λ) z ( λ) and a linear combination of the products

F (λ) .I (λ) .FR (λ), F (λ) .I (λ) .FV (λ), F (λ) .I (λ) .FB (λ) where x (λ), y (λ) and z (λ) are the spectral trichromatic coordinates in the reference colorimetric system.

Preferably, said reference illuminant is the CIE spectrum illuminator D65 (λ).

Another subject of the invention is a method for measuring the color of an object from an acquisition chain including a video camera capable of delivering signals representative of the trichromatic components, in a colorimetric system linked to said chain d acquisition of the color of an object placed in the field of observation of the camera, characterized in that it comprises the steps consisting in:

- place successively or simultaneously in the field of observation of the camera three primary colors and at least two gray levels, the trichromatic components of said primary colors and said gray levels being known in a reference colorimetric system, - calculating by a process iterative, from said trichromatic components in said reference color system and corresponding trichromatic components in the color system linked to said acquisition chain, obtained from the observation by said video camera of said primary colors and of said gray levels, a matrix of passage from the colorimetric system linked to the acquisition chain to the reference colorimetric system and a function for correcting the non-linearities of said acquisition chain, - determining, using said passage matrix and said correction function thus calculated , the trichromatic components in the colorimetric reference system of the color of an object placed in the field of observation of the camera. Advantageously, in this method, the color of the object is also displayed by means of a display chain after correction of the non-linearities of the acquisition chain.

Advantageously, in this process, the non-linearities of the display chain are also corrected. In a particular example of implementation of the method according to the invention, the display chain comprises a display device with cathode ray tube. In a particular example of implementation of the method according to the invention, a function for correcting the non-linearities of the display chain is determined by:

- displaying two zones of the same color but with luminances which may be different, the color of one of the zones being obtained by a juxtaposition of pixels of distinct control levels and the color of the other zone being obtained by a set of pixels with the same control level corresponding to the average of the control levels of the pixels of the other zone, - making the luminances of the two zones equal for an observer by acting on the control level of the pixels of one of the zones.

From the value of the control levels of the pixels of each of said zones, information is deduced before and after equalization of the luminances information for the calculation of said function of correction of the non-linearities of the display chain.

Preferably, one of the zones is screened. Advantageously, this area which is screened comprises one screen out of two which is black.

In the case where the display chain comprises a cathode-ray tube display device, during a transition in the control level of the pixels of the same frame causing a variation in luminance between at least one pixel of this frame and the pixel immediately according to the direction of scanning of the frame, the level of control of said immediately following pixel is advantageously chosen as a function of the speed of variation of the control signal of the electron beam reaching the pixels located on the same frame when the level control of said pixels varies.

To determine the correction to be made to take into account the speed of variation of the control signal of the electron beam reaching the pixels located on the same frame, one can advantageously proceed as follows.

Two zones of the same color are displayed, but with luminances which may be different, the color of one of the zones being obtained by a juxtaposition on the same frame of pixels of distinct command levels and the color of the other. zone being obtained by a set of pixels of the same control level, then the luminances of the two zones are made equal for an observer by acting on the level of control of the luminance of the pixels of one of the zones.

We deduce from the value of the control levels of the luminance of the pixels of each of said zones before and after equalization information for the calculation of the correction to be made to take into account the speed of variation of the control signal of the electron beam reaching pixels located on the same frame when the pixel control level varies. Preferably, the area formed by the juxtaposition of pixels of distinct luminance comprises one pixel out of two, in the direction of scanning, the luminance is controlled at a different level from that of the previous pixel.

Another subject of the invention is a method of correcting the response of a display device comprising frames of pixels, according to which during a transition of the control level of the pixels of the same frame resulting in a variation in luminance between at least one pixel of this frame and the immediately following pixel, having regard to the scanning direction of the frame, the control level of said immediately following pixel is chosen as a function of the speed of variation of the luminance of the pixels located on the same frame when the level of control of said pixels varies.

Advantageously, a function for correcting the non-linearities of this display device is determined by:

- displaying two zones of the same color but with luminances which may be different, the color of one of the zones being obtained by a juxtaposition of pixels of distinct control levels and the color of the other zone being obtained by a set of pixels with the same level of control,

- by making the luminances of the two zones equal for an observer by acting on the level of control of the pixels of one of the zones, and

by deducing from the value of the control levels of the pixels of each of said zones information for the calculation of said function for correcting non-linearities of the display device. In a particular embodiment of this method, the area composed of pixels of distinct control levels is screened.

Advantageously, this screened area includes one frame out of two which is black. In another particular mode of implementing the method, the zone composed of pixels of distinct control levels comprises one pixel out of two, at each frame, whose command level is different from that of the previous pixel on this frame.

Furthermore, in a particular example of implementing a method according to the invention, the aforementioned iterative process comprises the steps consisting in:

- calculate an approximate pass matrix from the known trichromatic components of said primary colors and said gray levels and an approximate correction function, - calculate a new approximate correction function using the approximate pass matrix thus calculated and the components trichromatic known from said gray levels and by interpolating the missing values, - start again the calculation of the approximate pass matrix and the approximate correction function until reaching a fixed convergence threshold.

As a variant, the iterative process comprises the steps consisting in: calculating an approximate correction function from the known trichromatic components of said primary colors and of said gray levels and of an approximate passage matrix,

- calculate a new approximate passage matrix using the known trichromatic components of said gray levels and by interpolating the missing values,

- repeat the calculation of the approximate correction function and the approximate pass matrix until a fixed convergence threshold is reached.

Advantageously, the inhomogeneity of the illumination by the source of the object and the optical aberrations of the camera are also corrected by measuring the luminance of the image at various points and comparing it with the luminance as a reference point, for example the center of the image.

The invention will be better understood on reading the detailed description which follows, of an example of non-limiting embodiment of the invention, and on examining the appended drawing in which: FIG. 1 is a schematic view of a color measurement device according to an exemplary embodiment of the invention,

FIG. 2 is a modeling of the acquisition chain,

FIG. 3 is a modeling of the processing leading to the determination of the trichromatic components in the reference colorimetric system,

FIG. 4 is a flowchart illustrating the iterative calculation process of the pass matrix and of a function for correcting non-linearities in the acquisition chain,

- Figure 5 is a modeling of the processing leading to the determination of trichromatic components corrected in the colorimetric system linked to the display chain, - Figure 6 is a modeling of the processing of the input signals in a cathode-ray tube display device ,

- Figures 7 and 8 show two test patterns used for calibrating the display chain and, - Figures 9 and 10 schematically illustrate the variation in the luminance of the pixels of the same frame as a function of the variation in the level of control of these pixels, respectively without and with advance correction to take account of the speed of variation of the control signal of the electron beam reaching the pixels when the level of control varies.

FIG. 1 shows a device 1 in accordance with an exemplary embodiment of the invention, making it possible to remotely measure the color of an object O.

The device 1 comprises a video camera 2, a microcomputer 3 and a cathode-ray tube display device 4. The camera 2 comprises three sensors 8, 9 and 10 of the CCD type, capable of respectively delivering electrical signals in analog form VR, V _V and V _B representative of the levels of red, green and blue at each point of the image as observed and analyzed by camera 2, in a manner known per se.

A light source 7 makes it possible to illuminate the object 0 placed in the field of the camera 2. Advantageously, a light source with xenon is used which makes it possible to approach the spectrum Dg ₅ (λ) defined by the CIE.

The microcomputer 3 comprises a central unit

5 and a three-way analog / digital converter 6, connected to the central unit 5 and making it possible to convert the analog signals V _R , V _v and V _B delivered by the camera 2 into a digital form.

The signals converted into digital form and respectively denoted R, G and B are then sent to the central unit 5.

In the particular example described, the signals R, G and B are each coded on 8 bits and the data acquired by the camera are at each point of the image constituted by a triplet (R, G, B) of three integers between 0 and 255.

Of course, it is not going beyond the ambit of the invention to code the signals on a different number of bits.

The camera 2 and the converter 6 constitute an acquisition chain which has been modeled in FIG. 2. The light spectrum reflected at each point of the object O observed by the camera is the product of the spectrum of the light source I (λ) and the reflectance spectrum

Re (λ) at this point.

The optical system of camera 2 comprises three optical filters with respective spectra FR (λ), FV (λ) and FB (λ) to decompose on the CCD sensors 8, 9 and 10 the image observed by the camera into color images primary red, green and blue. All the electronic components of the acquisition chain introduce noise which can be broken down into an AC component and a DC component BR, BV and BB respectively for each of the channels.

The low and high digitalization levels of each channel 11, 12 or 13 of the converter 6 are adjustable.

The adjustment of the low level of digitalization of each channel is carried out so as to eliminate the continuous component of the noise and one proceeds with the camera cover on the lens of the latter, gradually increasing the low level of digitalization until the least bright area on the image corresponds to a digital signal, at the output of the converter, equal to unity for each channel.

The high digitization level is adjusted by placing the brightest object likely to be observed in front of the camera's field of view. Advantageously, a white surface is used. the high level of digitalization is adjusted until the brightest area observed on the image corresponds at output to the maximum value of the digital signal minus one unit, that is to say the value 254 for each channel in the example described. If necessary, the light intensity of the source 7 is reduced if such a high level adjustment is not possible.

The attenuation of the alternative noise can be carried out by means of several successive acquisitions of the same image.

The digital signals (R, G, B) delivered by the converter 6 are processed in the central unit 5 to determine at each point of the image observed by the camera 2 the trichromatic components in a reference colorimetric system, such as the CIE XYZ color system. This processing corrects the non-linearities of the acquisition chain, i.e. essentially the non-linearity of the CCD sensors 8, 9, 10 as well as that of the converter 6. This treatment also advantageously corrects the inhomogeneity of the illumination of the object 0 by the source 7 and the optical aberrations of the camera 2. Of course, this is not beyond the scope of the invention by choosing as a reference colorimetric system a reference colorimetric system other than the XYZ colorimetric system of the CIE.

The processing performed by the central unit 5 has been modeled in FIG. 3.

We have designated by Y _R , Y _v and Y _B the components of the function Y of correction of the non-linearities of the CCD sensors 8, 9, 10 and of the converter 6 for each of the red, green and blue channels. The transition function from the color system linked to the acquisition chain to the reference color system can be written in the form of a matrix M.

The trichromatic components in the reference colorimetric system are designated by X, Y and Z.

We have designated by FX (i, j), FY (i, j) and FZ (i, j) the functions for correcting the inhomogeneity of the illumination of the object by the source 7 and of the optical aberrations of the camera 2, as a function of the coordinates i, j of each point considered on the image.

The calculation of the coefficients of the matrix M as well as the calculation of the components Y _R , Y _v and Y _B , will now be described with reference to the algorithm shown in Figure 4. First, we acquire in step 14 a first set of data, used for further processing by placing successively or simultaneously in the field of observation of the camera n colorimetric samples corresponding to distinct gray levels, including the trichromatic components (X, Y, Z) in the XYZ reference color system are known.

Then we acquire by placing in the next step 15 successively or simultaneously three color samples corresponding to three primary colors, the trichromatic components of which in the reference color system are known, a second set of data used for further processing. The trichromatic components in the colorimetric reference system can be measured using a spectrocolorimeter.

These primary colors can for example be red, green and blue or magenta, cyan and yellow. In general, they can be arbitrary provided that they form a base in the colorimetric reference system.

Preferably, these primary colors are chosen so that the triangle that they form in the chromatic plane of the reference colorimetric system encompasses all of the colors to be measured.

The colorimetric samples used are preferably opaque and of uniform color, therefore measurable by a conventional colorimeter or spectrocolorimeter. The measurement of the various primary grays and colors is preferably carried out in the center of the field of observation of the camera 2 and over a sufficiently small surface, preferably representing only about 10% of the total surface covered by the field of observation of the camera, and which can be considered as uniformly illuminated by the source 7.

Preferably, in order to acquire the different grays and primary colors, the position of the source 7 and that of the camera 2 relative to the samples, as well as the type of lighting used, come as close as possible to the configuration of measurement of the colorimeter or spectrocolorimeter used to determine the trichromatic components of the color of the colorimetric samples used in the reference colorimetric system.

The acquisition chain delivers in step 14 signals representative of the trichromatic components of the n gray levels in the form of triplets of values simplified denoted REx, RE ₂ , ..., RE _n , and the corresponding triples in the reference colorimetric system are respectively also denoted in simplified form XE _lf XE ₂ , ..., XE _n . The notation REi in fact designates a triplet of values (R, B, V) and the notation XEi the triplet of corresponding values (X, Y, Z).

The acquisition chain delivers in step 15 triplets of trichromatic components RC _lx RC ₂ and RC ₃ , the corresponding triplets of which in the reference colorimetric system are XC ^, XC ₂ and XC3 respectively, using the same type of simplified notation than previously.

In step 16, an iteration parameter k is initialized to the value 0 and in step 17, an approximate correction function Y, capable of varying at each iteration, of components Y _R , Y _v , Y is initialized _B

At iteration k = 0 we choose Y _R °, Y _v °, Y _B ° equal to the identity function. In step 18, an approximate matrix P of the transition from the reference colorimetric system XYZ to the colorimetric system linked to the acquisition chain by the formula is calculated for each iteration k:

P ^k = (Y ^k ) ([RCi RC ₂ RC ₃ ]) [XCj XC ₂ XC ₃ ] - 'In step 19, we calculate n values of the approximate correction function Y by the formula:

Y ^k (RE *) for c ranging from 1 to n In step 20, the missing values of the correction function Y are interpolated at the iteration k in the following manner.

For the component Y _R for example: if Y _R (ω _Q ) is known and if Y _R <ωι) is known then for ω ₀ <ω <û> ι _# we determine Y _R ^k (ω) by the formula: log (Y ^k _R (ω) / 255) = ((ω-ωι) / (ω ₀ -ω ₁ )). log (Y _R ^k (ωo) / 255) + ((ω-ωo) / (ωo-ωι)). log (Y _R ^k (ωι) / 255) We do the same for Y _v and Y _B.

In step 21, the correction functions Y _R ^k , Y _v ^k , and YB are normalized

255. Y _R ^k “o) / Y _R ^k (255) → Y _R ^k (ω>

255. Y _v ^k (ω) / Y _v ^k (255) → Y _v ^k (ω)

255 .Y _B ^k (ω) / Y _B ^k (255) → Y _B ^k (G>) for ω ranging from 1 to n.

In step 22, the error e is calculated by the formula:

for i going from 1 to n, with M = (P)

In step 23, the iteration parameter k is incremented.

In step 24, it is tested whether there is convergence of the error e or not.

If this is the case, we stop the iterative process in step 25 and otherwise we return to step 18 of calculation of a new approximate pass matrix P and of a new approximate function Y of correction of non-linearities .

In step 25, the final matrix M of the passage from the colorimetric system linked to the acquisition chain to the reference colorimetric system is calculated by the formula:

M≈M * = Y ^k ([XCι XC ₂ XC ₃ ]) [RC, RC ₂ RC ₃ ] ^" '

The computation of the passage matrix M and of the function Y of correction of non-linearities is thus carried out from an iterative method in which at each iteration an approximate passage matrix and a function of correction of non-linearities are calculated. approximate linearity. ^" Once the computation of the components Y _R , Y _v , and Y _B of the function Y and the computation of the matrix of passage M carried out, one determines the functions of correction of the inhomogeneity of the illumination of the object 0 by the source 7 and correction of the optical aberrations of the camera, that is to say the functions FX (i, j), FY (i, j) and FZ (i, j) mentioned above.

The functions FX (i, j), FY (i, j) and FZ (i, j) are calculated by acquiring the image of an object of uniform color occupying the entire field of observation of the camera and by determining the correction to be made so that all the points of the image have the same trichromatic components as the center of the image. More precisely, in the example described, the procedure is carried out by averaging the trichromatic components of a set of points belonging to a measurement window, this set of points being centered on a point of coordinates i, j. We thus obtain average values IMX (i, j),

IMY (i, j), IMZ (i, j) and the functions FX (i, j), FY (i, j) and FZ (i, j) are determined by the following formulas:

FX (ij) = IMX (t _ccnlre ,; centreVIMX (t /)

FY (*. /) = IMY (c „„, „, j ccntreVIMY (*. /)

FZ {ij) = IMZ (i "..u ,, j center) / IMZ (ti /) where Ce _n t _r e, Ce _n t _r e are the coordinates of the center of 1 ^" image. The transition from components (R, G, B) to components (X, Y, Z) as just described can introduce a bias in the case where the products D ₆₅ (λ) .x (λ), Ds ₅ (λ) .y (λ) and Dgs (λ) .z (λ) are not linear combinations of the products I (λ) .FR (λ), I (λ) .FV (λ), I ( λ) .FB (λ) which is the case most of the time. In order to reduce this bias, it is advantageous to filter the source with an optical filter F defined so as to minimize the error of the differences between the products.

D ₆₅ ⁽ λ ⁾ .x (λ), D ₆₅ (λ) .y (λ), D ₆₅ (λ) .z (λ) and a linear combination of the products

F (λ) .I (λ) .FR (λ), F (λ) I (λ) .FV (λ), F (λ) I (λ) .FB (λ).

This filter is used to optimize the measurement of color and is adapted to the acquisition chain.

It is understood that the particular embodiment of the invention which has just been described makes it possible to know precisely the components (X, Y, Z), in a colorimetric reference system, of an object placed in the field d observation of a video camera.

Objects, for example containers or labels moving at high speed in a production line, can thus be inspected without physical contact, in order to detect a variation in the color of these objects or surface defects.

It can also be interesting to faithfully reproduce by means of a display chain the color of the object observed by the camera.

Advantageously, both a measurement of the color is carried out and the color is faithfully reproduced by means of the display chain. However, without departing from the scope of the invention, one or the other can be carried out.

In the case where one is content to faithfully reproduce the color of the object by means of the display chain, it is not necessary to display for the user the trichromatic components of the color of the object in the reference colorimetric system, which have been calculated by the central processing unit 5 in the manner indicated above.

Sending the output signals (R, G, B) of the converter 6 directly to the input of the display chain would lead to an unsatisfactory rendering of the color of the object on the screen of the display device 4, in particular because of the non-linearities of the acquisition chain and those of the display chain and of the difference between the colorimetric systems linked respectively to the acquisition chain and to the display chain.

A first improvement is made by correcting the non-linearities of the acquisition chain as described above.

However, the quality of the color reproduction of the object can be improved even more when the non-linearities of the display chain are corrected.

This correction is carried out in the central unit 5. The processing has been modeled in FIG. 5 to pass from the trichromatic components (X, Y, Z) in the reference colorimetric system XYZ to the trichromatic components (R ', V', B ') at the entry of the display chain. We start by transforming the trichromatic components (X, Y, Z) previously determined into trichromatic components in the colorimetric system linked to the display device, by means of a passage matrix M ', then we apply to each of the components thus obtained a correction function Y not related to the position on the screen, of components Y _R , Y _v , and Y _B and a function for correcting the non-uniformity of the screen illumination FR (i, j), FV (i, j), FB (i, j) linked to the position on the latter. The processing of the digital signals (R ', V', B ') received by the display chain and leading to the production of an image on the screen of the display device has been modeled in FIG. 6.

The trichromatic components (R ', V', B ') for each point of the image are transformed into signals analog by means of a three-way digital / analog converter 26, 27 and 28. The analog signals at the output of the converter are processed in a manner known per se at 29 to each control a cathode beam intended to illuminate phosphorescent regions of the screen producing in response to the incident electron flow a light emission of determined color. The addition of the emission spectra R (λ), V (λ), B (λ) of the phosphorescent regions, respectively producing the

colors red, green and blue, leads to a resulting spectrum E (λ) at each point of the image.

This passage matrix M 'is calculated by knowing the trichromatic components (X _R. , V _R. , Z _R -), (Xv _/ Yv _/ Zv) and (X _B. , Y _B. , Z _B. ) In the XYZ colorimetric reference system of the colors corresponding respectively to the triplets (255,0,0), (0,255,0) and (0,0,255) as input for each point of the image.

The passage matrix M 'has as column vectors the coordinates in the reference colorimetric system of the three primary colors considered.

X _R • Xy <X _{B '} M' = Y _R. Y _v - Y _B -

Z _R "Zy Z ₃ "

To calculate the components Y _R , Y _V / and Y _B of the correction function Y, we can use, knowing the matrix M ", a screen colorimeter to measure the light emitted in the center of the image as a function of triplets successive and different input values (R ^" , V ', B'). We can still determine the components Y _R ,

Y _v ^" and Y _B ^' without screen colorimeter as follows.

We take into account the fact that the luminance W at each point of the screen is linked to the voltage V controlling the electron beam which reaches it by the following approximate formula: logCW / W ^) = k _! + k ₂ log (V / V _" „) + k ₃ (log (V / V _m „) ² + kt (logCVΛ /, ™)) ³ - -

There are then two domains for the input R ', V or B' values: [0.64] and [64.255].

For each area, the parameters are determined and k ₄ by the value of the luminance when the digital input signal is successively 0.32, 64.128 for the first domain and 32.64.128.255 for the second.

As an example, we suppose that we start by wanting to determine the component Y _R.

A test pattern is displayed on the screen as illustrated in FIG. 7, presenting in the center a red rectangle corresponding to the choice of (128,0,0) as input. We include the central rectangle in a raster rectangle with a line in two of red color corresponding to (255,0,0) in input and with a line in two of black color corresponding to (0,0,0) in input.

The digital input signal is then modified if necessary (R ′, 0.0) which controls the luminance of the central rectangle until the luminance of the latter and the luminance of the screened rectangle appear equal. We then deduce from the correction that had to be made to the input signal to obtain this equalization the value of the function Y _R (128).

We then start again by displaying in the center a rectangle of color (64,0,0) surrounded by a raster rectangle with a line out of two of red color corresponding to (Y _R (128), 0,0) and with a line out of two black in color, which makes it possible to calculate Y _R (64), etc.

The values of the parameters ^k i _f ^k ₂ , ^ ₃ and k _{4 are then determined} and the missing values of Y _R d • are calculated according to the preceding formula. t ι

We do the same to calculate Y _v and Y _B.

The non-uniformity of the screen illumination is mainly due to the dispersion of the electron beam which strikes the screen. In order to correct this inhomogeneity, we measure for each of the three channels of red, green and blue the luminance at different points on the screen.

For each red, green or blue channel, the corrective factors to be applied at each point of the screen are calculated.

The correction functions FR (i, j), FV (i, j) and FB (i, j) were noted for the red, green and blue channels respectively.

We consider that the screen is calibrated in the center, so it suffices to know the corrective factors

to be applied at each point of coordinates i, j of the screen surface outside the center to obtain a fully calibrated screen.

The illumination of the screen as a function of the coordinates i, j of a point outside the center and of the coordinates icent _r βf j _cen t _r e in the center of the screen varies approximately as the following function C (i, j) :

C (ij) '* * ((1 + ** where r denotes the radius of curvature of the screen. The light intensity for each rectangle of a calibration grid as shown in FIG. 8 is measured using a spectrocolorimeter or a screen colorimeter.

By comparing this intensity with the light intensity in the center, we can deduce a plurality of values for the function FR (i, j).

Knowing these values it is then possible to calculate the missing values of the function FR (i, j) by interpolation using a polynomial function of degree 3.

We get FV (i, j) and FB (i, j) in the same way.

Furthermore, the cathode-ray tube behaves like a low-pass filter and the value of the control signal of the electron beam reaching, on the same frame, one pixel depends on the value of the control signal of the previous pixel on this frame. - -

By way of example, FIG. 9 shows the evolution of the variation in the luminance of successive pixels situated on the same frame as a function of the variation of the control level. More precisely, we illustrate the case where the command level is equal to m ^ for the pixels located at the start of the frame and goes to m ₂ for the following pixels, the value m ₂ being less than mid

It is observed that the luminance W of the pixels, in response to the control level m _{2 /} does not suddenly decrease from W (mι) to W (m ₂ ) but in a substantially exponential manner.

In other words, if the command level of the pixel suddenly changes from the value mi to the value m ₂ , the luminance does not immediately reach the level W (m ₂ ).

The level of control is then advantageously corrected by anticipating the rise or fall time for each of the three channels of red, green and blue.

The correction is made for each frame in the scanning direction.

Assuming that one wishes for the same frame to go from a luminance W (mχ) corresponding to a control level mi to a luminance W (m ₂ ) corresponding to a control level m ₂ , the level is corrected in advance of the control signal of the pixels that one wishes to activate so that the latter reach the luminance level W (m ₂ ) more quickly.

Thus, the pixels are controlled with a control level 111 ₃ less than m ₂ , so as to obtain a faster decrease in luminance as illustrated in FIG. 10.

This predictive correction of the command level does not appear on the modeling represented in FIG. 5, for the sake of clarity of the drawing. To determine the time constant allowing to calculate the rise or fall time of the electron beam control signal, we can proceed by displaying on the screen a first rectangle of the same color which we control every other pixel of each frame making up this rectangle with command levels of different values.

We also display on the screen a second rectangle of the same color and with the same level of control for all the pixels. It is then possible to deduce from the values of the command levels of the pixels making up the two rectangles displayed on the screen the time constant and the correction to be made to the command levels of these pixels to effect the aforementioned predictive correction. Finally, the invention makes it possible to measure the color of opaque or non-opaque objects, flat or in relief, mobile or fixed, without contact with the latter, in transmission or in reflection, and more especially the color of the skin and the hair. . In an advantageous embodiment, the invention also makes it possible to restore the color of an object without excessive denaturing by means of a display device.

The invention also makes it possible to measure the color at each point of an object, even if the latter is of non-uniform color.

Of course, the invention is not limited to the embodiment which has just been described and it is possible in particular to use other display devices, such as a printer or a liquid crystal screen.

Claims

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CLAIMS 1 - Color measurement device, characterized in that it comprises:

- an acquisition chain (2,6) including a video camera (2), said acquisition chain being capable of delivering signals (R, G, B) representative of the trichromatic components in a colorimetric system linked to said chain d acquisition of the color of an object (O) placed in the field of observation of the camera, and - processing means (5) arranged to determine, from said signals (R, G, B), the components trichromatic (X, Y, Z) of the color of the object in a reference colorimetric system (XYZ) using a matrix (M) for passage from the colorimetric system linked to said acquisition chain to the reference colorimetric system and a correction function (T) for the non-linearities of said acquisition chain, said passage matrix (M) and said correction function (T) being calculated by an iterative process from the trichromatic components known in the colorimetric system of refer nce of three primary colors (XCx, XC _{2 /} XC ₃ ) and at least two gray levels (XEι, XE ₂ , ..., XE _n ) and their trichromatic components (RC _lf RC _{2 /} RC ₃ ; REi, RE ₂ , ... _/ RE _n ) in the colorimetric system linked to said acquisition chain, obtained from the observation by said video camera of said primary colors and of said gray levels.

2 - Device according to claim 1, characterized in that it further comprises a display chain (4) for reproducing all or part of the image observed by said video camera (2) after processing, by said function of correction of said signals (R, G, B) delivered by the acquisition chain (2,6).

3 - Device according to claim 2, characterized in that said display chain comprises a display device (4) with cathode ray tube. 4 - Device according to one of claims 1 to 3, characterized in that it further comprises a light source for illuminating the object placed in the field of observation of the camera, this source having an emission spectrum continuous I (λ) chosen so as to approximate a reference illuminant, of spectrum D (λ).

5 - Device according to claim 4, the camera comprising a set of optical filters of spectra FR (λ), FV (λ) and FB (λ) to decompose on the sensors of this camera the image observed by the camera into images of primary colors, characterized in that the source is filtered by one or more filters whose resulting filtering function F (λ) is chosen so as to minimize the error of the differences between the products D (λ), χ (λ) , D (λ) .y (λ), D (λ) .z (λ) and a linear combination of the products

F (λ) .I (λ) .FR (λ), F (λ) .I (λ) .FV (λ), F (λ) .I (λ) .FB (λ) where x (λ), y (λ) and z (λ) are the spectral trichromatic coordinates in the reference colorimetric system. 6 - Device according to claim 5, characterized in that said reference illuminant is the illuminant of spectrum Dfc ₅ (λ) of the CIE.

7 - Method for measuring color from an acquisition chain including a video camera (2) capable of delivering signals (R, G, B) representative of the trichromatic components in a colorimetric system linked to said chain of color acquisition of an object

(0) placed in the field of observation of the camera, characterized in that it comprises the steps consisting in:

- successively or simultaneously placing three primary colors and at least two gray levels in the field of observation of the camera, the trichromatic components of said primary colors and gray levels being known in a reference colorimetric system (X, Y, Z) , - -

- calculate, by an iterative process, from said trichromatic components in said reference colorimetric system and corresponding trichromatic components in the colorimetric system linked to said acquisition chain, obtained from the observation by said video camera of said primary colors and from said gray levels, a matrix (M) for passing from the colorimetric system linked to said acquisition chain to the reference colorimetric system and a function for correcting (T) the non-linearities of said acquisition chain,

- the trichromatic components, in the reference colorimetric system, of the color of an object placed in the field of observation of the camera being determined using said passage matrix and said correction function thus calculated.

8 - Method according to claim 7, characterized in that it further comprises the step of viewing by means of a display chain the color of the object after correction of the non-linearities of the acquisition chain .

9 - Method according to claim 8, characterized in that it further corrects the non-linearities of the display chain. 10 - Method according to one of claims 8 and

9, characterized in that the display chain comprises a display device (4) with cathode ray tube.

11 - Method according to claim 10, characterized in that during a transition of the control level of the pixels of the same frame causing a variation in luminance between at least one pixel of this frame and the immediately next pixel, having regard in the scanning direction of the frame, the control level of said immediately following pixel is chosen as a function of the speed of variation of the control signal of the electron beam reaching the pixels located on the same frame when the control level of said pixels varies . - -

12 - Method according to any one of claims 9 to 11, characterized in that one determines a function of correction of the non-linearities of the display chain by displaying two areas of the same color but with luminances likely to be different, the color of one of the zones being obtained by a juxtaposition of pixels of distinct command levels and the color of the other zone being obtained by a set of pixels of the same command level, making equal the luminances of the two zones for an observer by acting on the level of control of the pixels of one of the zones, and by deducing from the value of the levels of control of the pixels of each of said zones information for the calculation of said function of correction of the non-linearities of the chain.

13 - Method according to claim 12, characterized in that the area composed of pixels of distinct control levels is screened.

14 - A method according to claim 13, characterized in that said screened area includes a frame on two which is black.

15 - Method according to claim 12, characterized in that the area composed of pixels of distinct command levels comprises one pixel out of two, at each frame, whose command level is different from that of the previous pixel.

16 - Method according to any one of claims 7 to 15, characterized in that said iterative process comprises the steps consisting in: - calculating an approximate pass matrix from the known trichromatic components of said primary colors and of said gray levels and an approximate correction function,

- calculate a new approximate correction function using the approximate passage matrix thus calculated and the known trichromatic components of said gray levels and by interpolating the missing values, - start again the calculation of the approximate passage matrix and the approximate correction function until reaching a fixed convergence threshold.

17 - Method according to any one of claims 7 to 15, characterized in that said iterative process comprises the steps consisting in:

- calculate an approximate correction function from the known trichromatic components of said primary colors and said gray levels and an approximate passage matrix, calculate a new approximate passage matrix using the known trichromatic components of said gray levels and by interpolating missing values, - repeat the calculation of the approximate correction function and the approximate pass matrix until a fixed convergence threshold is reached.

18 - Method according to any one of claims 7 to 17, characterized in that one also corrects the non-uniformity of 1 • illumination by the source of the object and the optical aberrations of the camera by measuring the luminance of the screen at different points and comparing it with the luminance at a reference point.

19 - Method for correcting the response of a display device (4) comprising pixel frames, characterized in that during a transition of the control level of the pixels of the same frame causing a variation in the luminance between at least one pixel of this frame and the immediately following pixel, having regard to the scanning direction of the frame, the control level of said immediately following pixel is chosen as a function of the speed of variation of the luminance of the pixels located on the same frame when the level of control of said pixels varies. 20 - Method according to claim 19, characterized in that one determines a function for correcting the non-linearities of said display device (4) by displaying two zones of the same color but with luminances likely to be different, the color of one of the zones being obtained by a juxtaposition of pixels of distinct control levels and the color of the other zone being obtained by a set of pixels of the same level of control, by making the luminances of the two zones equal for an observer by acting on the level of control of the pixels of one of the zones, and by deducting from the value of the levels of control of the pixels of each of said zones information for the calculation of said non-linearity correction function of the display device.

21 - Method according to claim 20, characterized in that said area composed of pixels of distinct control levels is screened.

22 - A method according to claim 21, characterized in that said screened area comprises a frame in two which is black.

23 - Method according to claim 21, characterized in that said area composed of pixels of distinct command levels comprises one pixel out of two, at each frame, the command level of which is different from that of the previous pixel on this frame.