Patent Grant
12145386
This application is a U.S. National Stage Application filed under 35 U.S.C. § 371 of International Application No. PCT/EP2021/066753, filed Jun. 21, 2021, which claims the benefit of European Application No. EP 20181963.8, filed Jun. 24, 2020. Both of these applications are hereby incorporated by reference in their entireties.
The invention relates to a method for preparing a laser marking system to create a colored laser marking on a specimen and to a method for creating a colored laser marking on a specimen comprising a surface.
Creating visible patterns on surfaces using laser irradiation is a rapidly growing technology with many applications in object identification, customization, and authentication [Liu et al. 2019]. Laser marking is an environmentally friendly, low maintenance process with no consumables, dyes, or pigments. While mostly a monochromatic method, some materials exhibit a range of colors when treated with laser, as a result of complex physicochemical phenomena. Among such materials are stainless steel and titanium, some of the most important industrially metals. Despite the great potential, the industrial adoption of color laser marking is almost non-existent due to its challenging characterization. In the absence of such a characterization, the relationship between the device's design space (laser parameters) and performance space (e.g. marked colors) is unknown. This relationship is too complex to capture with physics-based methods [Nánai et al. 1997]. Instead, the current practice finds design parameters that lead to “interesting colors” by trial and error measurements. These primary colors are then used to mark simple motives and logos. This brute-force color gamut exploration scales poorly with the laser marking high-dimensional design space, resulting in neglecting some design parameters.
The coloration of different substrates using laser irradiation is an active field of research with a long history [Birnbaum 1965]. There are many color formation mechanisms employing different laser sources and different materials; see [Liu et al. 2019] for a recent review. Surface oxidation is one of these mechanisms where the heat (generated by a laser) facilitates the reaction of materials with oxygen. Oxidation-induced colors are believed to stem from multilayer, heterogeneous mixture of structural colors (based on thin-film interference) [Del Pino et al. 2004] on one hand, and the traditional pigment-based color of oxides [Langlade et al. 1998] on the other hand. Despite a handful of initial efforts [Veiko et al. 2013], predicting the structure and composition of oxide layers is extremely difficult due to the complex thermodynamics of the laser marking process [Nänai et al. 1997]. Even with known material composition, predicting the surface color requires a challenging light-matter interaction model most likely based on an electromagnetic simulation [Auzinger et al. 2018].
For some popular metals, such as stainless steel and titanium, oxidation-based color laser marking has been extensively studied. This spans a range of laser-marked metals' behaviors, from electromechanical [Lawrence et al. 2013] to corrosion resistance properties [ŁeRcka et al. 2016]. Further related to this invention is a class of studies focused on the effect of various process parameters on the marked colors [Laakso et al. 2009]. Most of these works [Adams et al. 2014; Antończak et al. 2013, 2014] rely on sampling and marking the process parameter space uniformly. As laser marking is time and material consuming, these methods cannot cope with the dimensions of the design space and end up ignoring a large portion of process parameters. It is worth noting that some empirical color discovery methods [Veiko et al. 2016] try to find different laser parameters that lead to the same color. But that color needs to be known beforehand. Moreover, these methods are restricted to interference-based colors within a limited range of laser energy and a limited number of parameters.
Formulating design problems based on multi-objective optimization and solving them by computing the Pareto front is known in the field of computer graphics. Notable examples are simplifying procedural shaders [Sitthi-Amorn et al. 2011] or minimizing power consumption in real-time rendering [Wang et al. 2016]. In computational fabrication, exploring the performance space of a process has attracted recent attention. With the advent of 3D additive technologies, these efforts are mainly focused on exploring the mechanical properties of 3D printed microstructures. As an example, Schumacher et al. [2015] precompute microstructures' performance space defined by mechanical metamaterial families in order to accelerate their heterogeneous topology optimization. They first populate the performance space by perturbing the initial designs (in the design space) and then fill the unpopulated regions of the performance space through either interpolation or inverse optimization. For a similar purpose, Zhu et al. [2017] combines a discrete, random perturbation of designs near the gamut boundaries with a continuous optimization that further expands the gamut by refining existing designs. In a more general-purpose method, Schulz et al. [2018] further emphasizes the importance of exploring the performance gamut's hypersurface (or Pareto front) instead of its hypervolume. A Pareto front captures a set of solutions in the performance space that are compromising different, potentially conflicting objectives. Although these methods serve as important sources of inspiration, it is not possible to rely on any of them as they depend on closed form, smooth characterization functions that connect the design and performance spaces. For example, the method of Schulz et al. [2018] requires a smooth (twice differentiable) forward characterization of the process and works only with continuous design parameters.
The object of the invention is to provide a method for preparing a laser marking system to create a colored laser marking on a specimen and to a method for creating a colored laser marking on a specimen comprising a surface such that the color laser marking is equipped with a high level of versatility.
The problem is solved by the method according to claim 1 and the method according to claim 10. The further dependent claims provide preferred embodiments.
According to the invention a method for preparing a laser marking system to reproduce a laser-marked color image on a specimen comprises the following steps:
The method provides a device characterization which is the prerequisite for any color reproduction system including laser marking. In the absence of an analytical function that maps laser marking parameters to marked colors, a data-driven method according to the invention is performed. The method provides a black-box model of the process ruling out a physics-based prediction of the laser-induced composition of oxides. This invention introduces the first systematic color discovery algorithm for laser marking systems. The present method is a non-exhaustive performance space exploration of a laser marking system. Preferably the surface layer is a metallic surface layer. However, the present method may also be applied on surface layers or specimen which are made of non-metallic materials. The present method may be applied for any kind of laser system and any kind of specimen.
Preferably step dd) involves a multi-objective optimization. Unlike a typical optimization, multi-objective optimization problems are evaluated based on multiple criteria. Very often, these criteria are in conflict. In the present case for example, some marked colors may be saturated but leave thick traces and lower the resolution. Hence, instead of a single optimal solution, there exists a set of optimal solutions, known as Pareto optimal solutions or Pareto set. The projected Pareto set into the performance space is called Pareto front. A Pareto front captures a set of solutions in the performance space that are compromising different, potentially conflicting objectives. A member of the Pareto front is not dominated by any other point in the performance space in all performance criteria. In other words, it is more performant than all other points in at least one criterion. Preferably a dense set of Pareto-optimal solutions to the color laser marking problem with the above objectives is uncovered.
Preferably step dd) employs non-dominated sorting genetic algorithm (NSGAII) which is a sorting algorithm based on the performance point's presence in multi-level Pareto fronts.
According to a preferred embodiment the laser system comprises at least one pulsed laser and at least one scanning device. Preferably by the scanning device a laser spot is movable relative to the specimen. Alternatively, or cumulatively by the scanning device the specimen is movable relative to the laser spot. The laser spot is preferably an area of the laser beam impinging on the specimen. With other words, it is conceivable that the specimen is in a fixed position and the laser beam is moved relative to the specimen. The scanning device could preferably be a galvanometric scanner, a movable mirror, or a similar device by which the direction of the reflected laser beam can be controlled. Alternatively, it could be conceivable that the laser beam is not moved, and the specimen is moved relative to the laser beam. The scanning device could therefore preferably be a x-y-stage or a x-y-z-stage. It could be also conceivable that the laser beam as well as the specimen are movable by a scanning device.
Further, the laser and the scanning device(s) are preferably controlled by the control unit. The control unit may advantageously also be connected with the at least one detection device in order to receive the measured data which are then preferably used by the evaluation device. It is also conceivable that more than one laser beam is used for the color laser marking. Depending on the used method to create a color laser marking the type of pulsed laser may be chosen. The pulsed laser could therefore preferably be a nano-second laser a picosecond-laser or a femto-second laser.
According to a preferred embodiment a design point comprises at least one laser parameter selected form: the frequency of the laser pulses, the power of a laser pulse, the width of a laser pulse, the speed of the laser beam relative to the specimen along a vector while marking, the line count, which defines the numbers of lines in a cluster representing the marked sample, the distance between the lines within a cluster representing the marked sample, the number of times a vector is marked. Preferably the dimensionality of the design space is set by the number of laser parameter represented by a design point. Thus, a design space could be 7 dimensional incase all seven of the aforesaid laser parameters are comprised in the design space. The design space may however be adjusted according to the specific needs. It could therefore be any number and any combination of the aforementioned laser parameters. It is conceivable that the design space comprises further parameters which might influence the formation of colors. Such further parameters depend on the actual used method for creating a color on the specimen. Preferably a design point comprises the parameter focal distance of the laser beam, type of medium gas, the ambient temperature. The medium gas is the ambient gas surrounding the specimen during the laser marking. This medium gas could be for example air. A preferred method for color marking is laser induced oxidation of the surface layer of the specimen. In such a method the type of the ambient medium gas is important in view of presence of oxygen and the amount of the present oxygen. The design space may therefore preferably comprise the relevant parameters which might influence the resulting color in the laser marking process.
According to a further preferred embodiment the performance criteria in step dd) comprise at least one of: chromaticity, hue spread, resolution, performance space diversity, design space diversity, color repeatability, color uniformity. Preferably the performance criteria in step dd) comprise all of the aforesaid criteria. According to a further preferred embodiment the criteria color repeatability, color uniformity are pruned. It is conceivable that further performance criteria are considered. The type, the number and the combination of the used performance criteria are preferably adjusted on the used specimen the laser system and further influencing factors.
The chromaticity may be preferably described by:
fC(a*,b*)=√{square root over (a*2+b*2)}
where a* and b* are the color coordinates of the CIELAB color space [Wyszecki and Stiles 1982]. Marked colors with large chroma produce more saturated color images. Preferably the hue spread (fHS) ensures the presence of high-chromaticity colors at all hue angles. The resolution may preferably be evaluated by measuring the thickness of a line marked by a set of given laser parameters. The resolution (fR) may be preferably described by:
where t is the line thickness. This criteria is preferably due to the preferred use of a line-based halftoning method. The performance space diversity (fPSD) is preferably measured for each point in the performance space as the reciprocal of the distance to its closest two neighbors and is given by:
where p is the point to be scored and P its respective population in performance space. The design space diversity (fDSD) is preferably measured analogously as the performance space diversity except in the design space.
According to a preferred embodiment the performance criteria in step dd) comprise at least one of: chromaticity, resolution, performance space diversity, design space diversity. Preferably performance criteria in step dd) comprise all of the aforesaid criteria. Preferably the hue spread is not performance criteria. Preferably wherein the performance points are projected in to a Commission Internationale de l'Éclairage CH (CIECH) space. It is advantageous that the CIECH space is split into a first number of circular sectors. Preferably the circular sectors en bloc form a hue wheel. The performance points within each circular sector of the hue wheel are advantageously evaluated regarding said performance criteria. Advantageously said evaluation is iterated for a preset iteration number, wherein in each iteration the number of sectors forming a hue wheel is altered. Consequently, for each iteration the area of the sectors is different. Preferably within each circular sector, the performance points are evaluated using a non-dominated sorting algorithm based on all said performance criteria except the hue spread. It is advantageous that this evaluation is repeated each time with a randomly chosen number of sectors, and with a random angular offset. After each iteration, every performance point is assigned to a potentially different Pareto front. At the end of the preset number of iterations, every single performance point is characterized by its front frequency vector that represents the frequency of its presence in the first front, second front and so on. Preferably each performance point is characterized by a frequency vector, which represents the presence in a certain Pareto front. By this procedure a gamut with a balanced hue spread is achieved. Thus, the performance criterion hue spread may be evaluated effectively by evaluating all remaining performance criteria multiple times.
According to a further preferred embodiment an additional evaluation regarding the achromatic properties of the performance points is performed by performing step b) using the performance criteria in step dd) lightness, resolution, performance space diversity, design space diversity. Thus, in the resulting first gamut both, the chromatic and the achromatic axes are covered.
According to a further preferred embodiment the method further comprises the step: selecting a set of primary colors from the first gamut, wherein the selected primary colors form a second gamut. The extraction of the primary colors is concerned with selecting a set of colors that generates the maximum color gamut through the use of the preferred marking method for example a halftoning method. While, unlike printers, the number of primaries is not strictly limited, a smaller number of primaries lead to improved marking time as they cause fewer switching delays of the laser system. Not all colors in the explored gamut can be considered for primary extraction. Thus, before primary extraction, the gamut is preferably pruned by excluding colors that: 1) don't satisfy a specified resolution requirement, 2) reveal a low repeatability, and 3) exhibit non-uniformity.
According to a further preferred embodiment the data relating to the design space and the performance space of the first gamut is stored in a database. This has the advantage that for a given laser system and a certain type of specimen the method for preparing the laser marking system preferably has to be done once. For the same type of specimen the relevant data regarding the first gamut may be retrieved from the database before further steps regarding the marking are implemented.
The object of the present invention is also solved by a method for creating a colored laser mark on a specimen comprising a surface.
The method for creating a colored laser mark on a specimen comprising a surface layer may comprise the single features or combinations of the features described above for the method for preparing a laser marking system to create a colored laser and vice versa. Further, the same advantages may apply for the method for creating a colored laser mark on a specimen comprising a surface layer as described above for the method for preparing a laser marking system to create a colored laser and vice versa.
The method for reproducing a laser-marked color image on a specimen comprising a surface layer comprises the following steps:
In step a) the database is searched if for the specific type of the specimen data related to the first gamut and/or second gamut is present. Such data is obtained by performing the method for preparing a laser marking system according to one of the above-mentioned embodiments. Such a verification is preferably done by a control unit. The user provides the control unit the specification of the specimen in particular the specification of the surface layer of the specimen. Preferably the surface layer is a metallic surface layer. However, the present method may also be applied on surface layers or specimen which are made of non-metallic materials. The present method may be applied for any kind of laser system and any kind of specimen.
According to a preferred embodiment the laser system comprises at least one pulsed laser and at least one scanning device. Preferably by the scanning device a laser spot is movable relative to the specimen. Alternatively, or cumulatively by the scanning device the specimen is movable relative to the laser spot. The laser spot is preferably an area of the laser beam impinging on the specimen. With other words, it is conceivable that the specimen is in a fixed position and the laser beam is moved relative to the specimen. The scanning device could preferably be a galvanometric scanner, a movable mirror, or a similar device by which the direction of the reflected laser beam can be controlled. Alternatively, it could be conceivable that the laser beam is not moved, and the specimen is moved relative to the laser beam. The scanning device could there-fore preferably be a x-y-stage or a x-y-z-stage. It could be also conceivable that the laser beam as well as the specimen are movable by a scanning device. Further, the laser and the scanning device(s) are preferably controlled by the control unit. The control unit may advantageously also be connected with the at least one detection device in order to receive the measured data which are then preferably used by the evaluation device. It is also conceivable that more than one laser beam is used for the color laser marking. Depending on the used method to create a color laser marking the type of pulsed laser may be chosen. The pulsed laser could therefore preferably be a nano-second laser a picosecond-laser or a femto-second laser.
According to a further preferred embodiment the color management workflow is a halftoning workflow. By using multi-color halftoning through a color reproduction workflow the reproduction of arbitrary images is enabled and not only uniform colors. Further a preview of the images before the marking is enabled. Therefore, for the laser marking a halftoning technique is implemented. By using a halftoning technique the visual impression of a continuous tone image is reproduced by taking advantage of the low-pass filtering property of the human visual system. The halftoning method aims at creating bilevel images conveying the visual illusion of a continuous tone image. Groups of colored and white pixels are printed with certain ratio and structure so that, when viewed by the eye, give the impression of continuous color. In color halftoning, a given number of color layers are halftoned separately. The final color halftone is the result of the color mixing of different halftone layers by overlaying them on top of each other. Existing color halftoning methods for printers are for example clustered dot and blue noise dithering. Herby a halftone layer is created for each color separately. The final color-halftone image is formed by the superposition of all the layers, wherein the screen dot layers partially overlap. In the laser marking an overlap of the colors and the superposition of the layers is not preferred. Preferably, the color management workflow is a juxtaposed halftoning workflow. Accordingly, it is preferred that for the laser marking a juxtaposed halftoning technique is implemented. The preferred juxtaposed halftoning technique relies advantageously on discrete line geometry, which provides subpixel precision for creating discrete thick lines. Preferably the continuous tone color image is converted into a set of binary images each corresponding to a primary color. The binary images are synthesized in the form of lines and places them next to each other without overlapping.
According to a further preferred embodiment the color management workflow comprises the steps
In step aa) the third gamut is preferably created from the second gamut and/or the first gamut under the condition a halftoning method is used. Preferably the third gamut is generated by halftoning a set of primary colors. The forward color prediction model predicts the color of several thousands of halftones spanning the space of the relative area of primaries in each halftone, known as area coverages. The third gamut surface is then fitted to this volumetric point cloud and is later used in step bb) for gamut mapping. Preferably the Yule-Nielsen (YN) prediction model to predict the multi-color, juxtaposed halftones of laser primary colors. In the gamut mapping the color space of the input image is translated to the color space of the third gamut. The color space is typically be displayed as a volume of achievable colors. In step cc) the color separation builds on the forward prediction model to compute the particular primaries and their area coverages that reproduce a given color (inside the color gamut).
The method for creating a colored laser mark on a specimen comprising a metallic surface may comprise the single features or combinations of the features described above for the method for preparing a laser marking system to create a colored laser and vice versa
According to a further preferred embodiment the laser marking is based on laser induced oxidation of the surface layer of the specimen. Preferably this applies to the method for creating a colored laser mark on a specimen comprising a metallic surface and/or the method for preparing a laser marking system to create a colored laser. In this approach the laser induces heating which leads to formation of a transparent or semitransparent oxide film on the surface of the specimen. A with light illumination can be reflected from the top and bottom surfaces of the oxide film. A constructive interference of the reflected beams makes the surface appear a certain color, which is determined by film thickness, refractive index of the oxide, and the order of interference [Liu et al. 2019].
Preferably, the laser marking is based on laser induced structuring of the surface layer of the specimen. Preferably this applies to the method for creating a colored laser mark on a specimen comprising a metallic surface and/or the method for preparing a laser marking system to create a colored laser. In this approach laser induced periodic surface structures (LIPSSs) are produced on the surface of the specimen by the laser system. The LIPSS act as a grating to give rise to iridescent colors due to the optical diffraction effect. The colors are not caused by pigments but originate from material surface micro/nanostructures, namely, structural colors.
Preferably, the laser marking is based on the laser induced generation of micro/nanoparticles on the surface layer of the specimen. Preferably this applies to the method for creating a colored laser mark on a specimen comprising a metallic surface and/or the method for preparing a laser marking system to create a colored laser. In this approach surface structures are induced by the laser system. The surface structures which excite the surface colors are randomly distributed without regularity, and the color does not vary with the viewing angle. Surface plasmon resonance (SPR) effects arising from metallic nanostructures and nanoparticles are the main causes for this type of coloring.
Preferably the laser marking is based on laser induced plasmonic colors on metals. Metal nanoparticles exhibit scattering properties due to excited plasmons that depend on their shape, size, composition and the host medium. There are various techniques known which render plasmonic colors including laser interference lithography. By plasmonic colors precious metals such as gold and silver and also metals like copper and aluminum may be marked.
The method for creating a colored laser mark on a specimen comprising a surface layer and/or the method for preparing a laser marking system to create a colored laser marking may however also be applied in combination with other laser induced color marking methods. Other mechanism may enable marking on a wide range of metals and even non-metals. Due to the fact that the actual marking process is preferably treated as a black box and the method is a data driven method it is adaptable to various other laser induced color marking methods.
Preferably the specimen comprises at least a top surface layer made of a metal on which the laser marking is performed. It is also conceivable that the specimen is made completely of a metal. Advantageously the surface layer and/or the entire specimen is made of stainless-steel titanium or a similar metal. Preferably this applies to the method for creating a colored laser mark on a specimen comprising a metallic surface and/or the method for preparing a laser marking system to create a colored laser.
Further advantages, aims and properties of the present invention will be described by way of the appended drawings and the following description.
In the drawings:
This invention provides means to equip color laser marking with the same level of versatility found in color printers. Assuming a blackbox model of the difficult device characterization, a measurement-based, data-driven performance space exploration is designed. Different performance criteria are explored including the color gamut and marking resolution by consecutive marking and measuring. For this, the process's Pareto front is uncovered by formulating a multiobjective optimization problem and solving it using an evolutionary method augmented by a Monte-Carlo approach. The optimization explores the hidden corners of the 7-dimensional design space in search of useful parameters that lead to a dense set of diverse, high-resolution colors. This invention goes far beyond the state of the art color image marking by introducing a complete color management workflow that takes an input image and laser-marks the closest approximation on metal surfaces. The color reproduction workflow adopts the principles of halftone-based color printing. It extracts a number of primary colors from the explored gamut and reproduces input colors by juxtaposing the extracted primaries next to each other in a controlled manner. The fabricated color images enjoy high resolution, introduce no significant artifact, and demonstrate accurate color reproduction. The invention provides therefore a discovery method that automatically finds the desired design parameters of a black-box fabrication system and the first color-image reproduction workflow for laser marking on metals.
Device characterization is the prerequisite for any color reproduction system including laser marking. In the absence of an analytical function that maps laser marking parameters to marked colors, one must rely on data-driven methods. In a first attempt, one can sample the design space, mark and measure the sampled design points, and construct a look-up table. This exhaustive strategy is subject to the curse of dimensionality given the relatively large number of parameters involved in color laser marking. The fact that function evaluations require actual marking and measuring further slows down the process. Additionally, the non-smooth color response to laser parameters renders interpolation schemes ineffective. This is shown in
wherein the steps bb) to dd) are iterated 9 for a preset iteration number, wherein in each iteration 9 the offspring design space 10a of the previous iteration is used in step bb), wherein in each iteration the measured performance space is combined 15 with the performance space of the previous iteration 9 such that in step dd) the combined performance space 13a is used. Preferably in each iteration the design space 10, 10a is combined with the design space 10, 10a of the previous iteration 9 such that in steps dd), ee) and ff) a combined design space 10b is used. Preferably the design space is initially populated with randomly chosen design points. Preferably the population size is in the range of 50 to 500, more preferably in the range of 75 to 250. Preferably the evaluation device is a computer, a processor unit, or a similar device. Preferably the method 1 is executed completely or on part by a control unit. The control unit might be a processor, a computer or a similar device.
The laser system 100 is depicted in
A design point 11, 11a comprises at least one laser parameter 12 selected form: the frequency of the laser pulses, the power of a laser pulse, the width of a laser pulse, the speed of the laser beam relative to the specimen along a vector while marking, the line count, which defines the numbers of lines in a cluster representing the marked sample, the distance between the lines within a cluster representing the marked sample, the number of times a vector is marked. A design point (11, 11a) may further comprise the parameter focal distance of the laser beam, type of medium gas (in the present case air), ambient temperature. Generally, the design space might comprise relevant parameters which influence the formation of a color on a specific specimen. The performance criteria in step dd) comprise at least one of: chromaticity, hue spread, resolution, performance space diversity, design space diversity, color repeatability, color uniformity. In the present case the criteria color repeatability and color uniformity are, however, pruned.
The method 1 for preparing a laser marking system 100 provides a non-exhaustive performance space 13 exploration of the laser marking system 100. Qualitatively speaking, the performance criteria favors diverse, saturated and high-resolution colors: the fundamental requirements for color images. For solving this problem a multi-objective optimization is casted. Unlike a typical optimization, multi-objective optimization problems are evaluated based on multiple criteria. Very often, these criteria are in conflict. In the present case, for example, some marked colors may be saturated but leave thick traces and lower the resolution. Hence, instead of a single optimal solution, there exists a set of optimal solutions, known as Pareto optimal solutions or Pareto set. The projected Pareto set into the performance space is called Pareto front. A member of the Pareto front is not dominated by any other point in the performance space in all criteria. In other words, it is more performant than all other points in at least one criterion. The goal is to uncover a dense set of Pareto-optimal solutions to the color laser marking problem with the above objectives. To this end, a multi-objective evolutionary method is adopted, which is a successful tool for finding Pareto optimal solutions [Fonseca et al. 1993]. The method, called non-dominated sorting genetic algorithm (NSGAII) [Deb et al. 2002] is well suited to our model-free characterization function, with both discrete and continuous parameters. At the heart of this method, is a evaluation or sorting method based on the members' presence in multi-level Pareto fronts. The NSGA-II non-dominated sorting is insufficient for the present specific problem due to our hue diversity objective. Thus, a preferable Monte-Carlo approach is considered on top of the non-dominated evaluation and introduce a new evaluation method based on front frequencies. This method is called Monte-Carlo, multi-objective, genetic algorithm or MCMOGA for short.
In order to preferably adopt halftoning for color reproduction a set of primary colors that covers both achromatic and chromatic axes is favorable. In a divide and conquer strategy, the—chromatic and achromatic (black and white) explorations may be separated, starting with explaining the former. High chromatic performance requires saturated colors corresponding to larger radii in the CIECH color space shown in
The performance criteria (1) to (4) are measured in the performance space while the performance criterion (5) is measured in the design space. The performance criteria (1) to (3) are the qualities which are directly sought from laser marked images. The performance criterion (4) improves the convergence rate and criterion (5) helps avoiding local extrema by promoting solo points in the performance space 14.
The method 1, navigates the laser's design space 10, 10a, 10b in directions that lead to a dense Pareto set, i.e., the set of designs (laser parameters) that improves the above performance criteria. It is started with a random population in the design space 10, mark it and measure its performance, then iteratively evolve it into a larger population with as many Pareto optimal solutions as possible. At each iteration, represented schematically in
As in almost any genetic method, a fitness measure should be assigned to each member of the population. Fitter solutions are selected and used to create the next generation. Given the difficulty of assigning a single fitness value to multi-criterion objectives, the non-dominated evaluation method [Deb et al. 2002] evaluates the members of a population according to their presence in multi-level Pareto fronts. It starts with finding the first non-dominated front, i.e., all solutions or performance points 14 in a population that belong to the Pareto front. This is done by comparing each performance points' 14 performance objective by objective to every other performance point 14 in the population. If a performance point 14 is more performant than all other performance points 14 in at least one criterion, it is labeled as a first-front performance point 14. The second non-dominated front is computed by temporarily discarding the first front and repeating the above procedure. This procedure is continued until all members of the population are labeled with their respective fronts. This results in a number of disjoint subsets making up the whole population, each with its front label. Note that, in the spirit of the Pareto concept, members inside the same front are not sorted.
Considering the hue spread objective the method is not favorable. The hue spread criterion helps the color gamut grow in all angular directions in a balanced manner. Without the hue spread criterion, the method may explore some specific hue angles more than others resulting in a non-uniform growth of the chromaticity gamut. For achieving a gamut 2 with balanced hue spread a single solution cannot be evaluated but rather in combination with other solutions. This can quickly lead to nontrivial computation: for 10 angular samples in a population of 200, (20010)≅1016 evaluations.
This combinatorial explosion can be avoided by resorting to a Monte-Carlo method. The performance criteria in step dd) comprise at least one of: chromaticity, resolution, performance space diversity, design space diversity. Preferably all of said performance criteria are used. The performance points 14 are projected in to a CIECH space, wherein a the CIECH space is split into a first number of circular sectors 15 forming a hue wheel 16. Thus, the hue wheel 16 splits the CIECH space into a random number of circular sectors 15 (
This procedure is schematically shown in
Further, an additional evaluation regarding the achromatic properties of the performance points 14 is performed by performing step b) using the performance criteria in step dd) lightness, resolution, performance space diversity, design space diversity. Thus, in the chromatic exploration, the lightness values (CIE L*) are discarded. Two separate explorations are performed for black and white colors on the lightness axis. For the black colors, minimize the chromaticity criteria is minimized, thereby encouraging low chromaticity colors. Additionally, Hue spread performance criteria is replaced with a lightness minimization. Exploring white colors is the same as the black colors except the lightness performance criteria is maximized.
After performing the method 1 for preparing a laser marking system 100 the performance space 13 of the laser marking system 100 is explored. Then the performance space 13 can be exploited for color image reproduction. This is done in this embodiment by adopt the principles of halftone-based color printing for color laser marking. For this, a number of primary colors is found that meet the resolution requirement and produce the largest second color gamut. Afterwards a color management workflow is built that takes input color images and marks the closest approximation using the selected color primaries.
Thus, the method 1 may further comprise the step selecting a set of primary colors from the first gamut 2. The selected primary colors form then a second gamut. The primary extraction is concerned with selecting a set of colors that generates the maximum color gamut through halftoning. While, unlike printers, the number of primaries is not strictly limited, a smaller number of primaries lead to improved marking time as they cause fewer switching delays of the laser. Not all colors in the explored first gamut 2 can be considered for primary extraction. Thus, before primary extraction, the first gamut 2 is pruned by excluding colors that: 1) don't satisfy the specified resolution requirement, 2) reveal low repeatability, and 3) exhibit nonuniformity.
Similar to the gamut exploration described above the achromatic and the chromatic primaries are extracted separately. First the explored chromaticity gamut of the laser marking system composed of a discrete set of colors is explored. The convex hull of this set in the CIExy chromaticity space is found where x=X/(X+Y+Z), y=Y/(X+Y+Z) [Wyszecki and Stiles 1982]. The reason for applying the convex hull in the CIExy is that, unlike CIEa*b* or CIECH, it is a linear space under halftoning. Colors inside the convex hull can be reproduced through halftoning with high accuracy. The colors in the convex set give the largest area and therefore the largest chromatic gamut. In order to reduce the number of primaries, those members of the convex set that don't contribute to the gamut area significantly may further be excluded. The achromatic primary extraction selects the darkest and the brightest colors with negligible chromaticity from within the black and white explored gamuts, respectively.
The data relating to the design space 10, 10a, 10b and the performance space 13, 13a of the first gamut 2 and/or data related to the second gamut are stored in a database 109. The database 109 may be connected to the control unit 108 and/or the evaluation unit 107.
The present invention comprises also a method 20 for creating a colored laser mark on a specimen 105 comprising a metallic surface 105a comprising the following steps:
In step a) the database 109 is searched for the specific type of the specimen data related to the first gamut 2 and/or second gamut is present. Such data is obtained by performing the method 1 for preparing a laser marking system 100 according to one of the above-mentioned embodiments. Such a verification is preferably done by the control unit 108. The user provides the control unit 108 the specification of the specimen 105, in particular the specification of the surface layer 105a of the specimen 105. The retrieved data comprises the data regarding the first and/or the second gamut which is matched to the used laser system and the specific specimen 105. The method is schematically shown in
Preferably the color management workflow 18 is a juxtaposed halftoning workflow. Accordingly said method 20, wherein the color management workflow 28a) comprises the steps:
The control data is then sent to the laser and step e) 28 may be performed.
A color management workflow ensures color reproducibility across different imaging devices. A real strength of the current method is to enable reproduction of arbitrary images and not only uniform colors using multi-color halftoning through a color reproduction workflow. It also enables a preview of the images before marking. The classic example is printing where the input images, from a camera for example, are printed as accurately as possible.
In
Color halftoning converts a continuous tone color image into a set of binary images, each corresponding to one of the printer's inks. The discrete-line juxtaposed halftoning [Babaei and Hersch 2012] synthesizes these binary images in the form of lines and places them next to each other without overlapping. In the original method designed for bitmap printers, using digital lines [Reveilles 1995] allows for subpixel thickness, low computational complexity, and, importantly to us, continuity. A continuous laser path ensures less switching delays, and therefore, faster marking with lower graininess caused by the two ends of each marked vector. As the original juxtaposed halftoning is designed for raster devices, the resulting raster images need to be transformed into vector representation suitable for our laser device. For this purpose, a naive line (a discrete line with unit thickness) as a mask and slide it on each halftone layer corresponding to each laser primary is used (
The color prediction model has two roles in the color management workflow. First, it constructs the third color gamut generated by halftoning a set of primaries. It predicts the color of several thousands of halftones spanning the space of the relative area of primaries in each halftone, known as area coverages. The gamut surface is then fitted to this volumetric point cloud and is later used for gamut mapping 30. Second, the forward model is used in the color separation step 31 that computes the area coverages of the primaries for any input colors to be reproduced.
The Yule-Nielsen (YN) prediction model is used to predict the multi-color, juxtaposed halftones of laser primaries. The Yule-Nielsen equation [Yule and Nielsen 1951] predicts the CIEX color coordinate (Xt) of a juxtaposed halftone as:
where Xi is the CIEX value of the i-th primary, and ai is its area coverage. The same equation applies for predicting CIEX and CIEY color coordinates. The exponent n, called the Yule-Nielsen n-value is a tuning parameter.
Color separation builds on the forward prediction model to compute the particular primaries and their area coverages that reproduce a given color (inside the third color gamut). As the YN model is not analytically invertible, color separation is carried out by optimization:
where c is the target color in the CIELAB color space and a is the optimization variable, i.e. the vector of area coverages of q primaries. As the CIEDE2000 color-difference formula [Sharma et al. 2005] is used for the distance metric, the modeled color using the YN model (YN(a)) should be converted to CIELAB from CIEXYZ (denoted by function Lab in the equation above. This equation searches for an area coverage vector that, after being marked, results in the minimum distance to the target color. As different primaries are juxtaposed, their relative area coverages should sum up to 1 and be non-negative.
The laser marking is based on laser induced oxidation of the surface layer (105a) of the specimen (105) or laser induced structuring of the surface layer (105a) of the specimen (105) or the laser induced generation of micro/nanoparticles on the surface layer (105a) of the specimen (105).
In the following different analyses and evaluations of both gamut exploration and image reproduction are presented. The laser marking system 100 is depicted in
In
For each point in the design space 10, 10a, 10b, its performance points 14 are measured in order to decide how to use that point in our exploration framework. All performance criteria, apart from the design space diversity, can be evaluated by measuring the thickness of a marked line cluster and the color of a marked patch. This is performed in two stages. First, for measuring the cluster's thickness, a first detection device 106 in the form of a hand-held digital microscope (Reflecta DigiMicroscope USB 200) is used. In a second step, the thickness of a given cluster is used to mark its corresponding patch by juxtaposing multiple clusters within the desired area. Both hue and chromaticity, the pillars of the performance space 13 exploration, are computed from CIELAB, a perceptual color space. Therefore, a colorimetric calibration [Hong et al. 2001] for measuring the color of marked patches is performed. The colorimetric calibration connects camera RGB signals to CIEXYZ coordinates through a form of regression. The CIEXYZ values then can be converted to the CIELAB coordinates using a set of well-known, analytical transformations [Wyszecki and Stiles 1982]. For training the regression, 121 printed color patches are used, with known spectra measured with an X-Rite i7 spectrophotometer, and obtain the ground-truth CIEXYZ values assuming D65 illumination. The same printed patches are captured with a second detection device 106 in form of Nikon D750 DSLR camera (with macro lens Tamron SP 90 mm F/2.8 Di) obtaining raw RGB signals that have been corrected for spatial and temporal light fluctuations. It needs to be pointed out that this setup is a possible experimental setup. As already pointed out the at least one detection device 106 could be connected to and controlled by the control unit 108. The above described measurements could then be performed automatically.
The colorimetric calibration shows high accuracy on a test set of 16 printed patches with an average ΔE00=2.26 and maximum 5.00. This calibration is therefore used to estimate the CIELAB color of marked patches. The structural nature of oxide colors causes a significant change in their appearance depending on the viewing and illumination geometry. It is observed that laser-marked colors appear most saturated at specular and near-specular geometries. Therefore, inspired by previous work on metallic prints [Pjanic and Hersch 2013], the color reproduction is confined to non-diffuse geometries. To this end, the stainless steel substrate is illuminated with a large, diffuse area-light tilted approximately 45° from the substrate's normal and captured with the camera with a similar angle.
For evaluating the proposed gamut exploration algorithm, multiple runs are performed while discarding different objectives during different runs to show the objective's effect on the exploration behavior. For a fair comparison, it is always started with the same randomly generated initial population. All generations have the same population size of 100. For the hue wheel, the random number of circular sections is limited between 4 and 72 while the random angular offset a is between 0 and 2π. The stopping threshold for MCMOS is set to 0.001%.
In
In
In
Marking high-quality images requires a set of diverse, saturated colors which are placed next to each other at a high spatial resolution. This criterion is defined by fR where design parameters that mark thin line clusters encouraged. A comparison of two explorations with equal number of iterations, one with (
The proposed color reproduction pipeline is evaluated by quantitative analysis and also a variety of full-color marked images. After the pruning step a total of 6 primary colors is obtained including a black and white primary. Four chromatic primaries are shown in
In the pruning stage, the spatial and temporal repeatability is checked by marking the candidate primaries at four different locations of the substrate and compare their colors pairwise. Primaries having an average ΔE00 higher than 4 among all comparisons are discarded. Also for the progressive primary discarding, the number of primaries is reduced until the gamut area drops by more than 10%. For resolution pruning, the colors with thickness 40±5 μm are kept. Additionally, colors with thickness around 20 μm are considered as juxtaposing two of them results in the target resolution.
For testing the accuracy of the Yule-Nielsen model, the primaries and also 92 test patches are marked with diverse area coverages of primaries. The resulting average ΔE00 error is 2.25 (Std=0.96, Min=0.50, Max=4.26) that demonstrates the high accuracy of the forward model. In
In the following, the question is studied of whether the marking parameters are transferable when using different marking settings or substrates. First a set of parameters reported in literature [Antończak et al. 2013] are marked and show the results in
In
In the following table color differences are reported when marking on the present setup a general set of parameters in different circumstances:
The table shows the repeatability errors of color laser marking in form of ΔE00 mean (and standard deviation). The general set, consisting of 89 design points, is chosen to represent different colors in the explored gamut. A pure repeatability test is seen, on AISI 304 alloy, using the same marking settings leads to acceptable but not satisfactory accuracy. When the marking settings are changed by using a thicker substrate (2 mm), and therefore exiting the focal plane, the repeatability worsens. Finally, using a different alloy of stainless steel (AISI 430) results in the worst repeatability.
For comparison, in Table 1, the result of the same experiments performed using the 6 extracted primaries are shown. Significantly higher accuracy in the pure repeatability experiment were observed as the primaries have been pruned against this circumstance. Interestingly, the primaries show acceptable repeatability when marked out of focus indicating that the extracted primaries are robust against some perturbations. However, the larger deviation when using a new substrate suggests that the primaries cannot be used for marking images on new substrates accurately. In
An iteration of the gamut exploration with a population size of 100 takes around 30 minutes. This includes marking the single clusters, measuring their thicknesses with a handheld microscope, marking the corresponding patches with proper distances of clusters, and finally capturing them with the colorimetric camera. The manual measurement of cluster thicknesses is the bottleneck as it takes approximately 20 minutes. Computing a new generation using the MCMOGA takes only a few seconds in Matlab. The marking time of an image is a function of the number of vectors (after halftone vectorization) and the marking speed of different primaries. A larger number of vectors causes more switching delays, making the marking time highly dependent on the image content in addition to its size. For example, the two marked images in the top row, and right side of the bottom row of
In this invention a computational framework is presented that enables a novel application of laser marking: color image reproduction. This method first characterizes the device using an evolutionary exploration of its performance space and then exploits that space for marking high-resolution, colorful images. A clear limitation of this method is the significant change of appearance from diffuse to non-diffuse configurations as shown in
In
All the features disclosed in the application documents are claimed as being essential to the invention if, individually or in combination, they are novel over the prior art.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20181963 | Jun 2020 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/066753 | 6/21/2021 | WO |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2021/259826 | 12/30/2021 | WO | A |
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| 20130088729 | Fu et al. | Apr 2013 | A1 |
| Number | Date | Country |
|---|---|---|
| WO 2019170679 | Sep 2019 | WO |
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| Number | Date | Country | |
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| 20230264505 A1 | Aug 2023 | US |
