Abstract:
The invention is a method and apparatus for creating a color and optical density selectable visible mark on an anodized aluminum specimen. The method includes providing a laser marking system having a laser, laser optics and a controller operatively connected to said laser to control laser pulse parameters and a controller with stored laser pulse parameters, selecting the stored laser pulse parameters associated with the desired color and optical density, directing the laser marking system to produce laser pulses having laser pulse parameters associated with the desired color and optical density including temporal pulse widths greater than about 1 and less than about 1000 picoseconds to impinge upon said anodized aluminum.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No.12/704,293, filed on Feb. 11, 2010, which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to laser marking of anodized aluminum articles. In particular it relates to marking anodized aluminum with a laser processing system. More particularly it relates to marking anodized aluminum in a durable and commercially desirable fashion with a laser processing system. Specifically it relates to characterizing the interaction between visible and infrared wavelength picosecond laser pulses and the anodized aluminum to reliably and repeatably create durable marks with a desired color and optical density. 
       BACKGROUND OF THE INVENTION 
       [0003]    Marketed products commonly require some type of marking on the product for commercial, regulatory, cosmetic or functional purposes. 
         [0004]    Desirable attributes for marking include consistent appearance, durability, and ease of application. Appearance refers to the ability to reliably and repeatably render a mark with a selected shape, color and optical density. Durability is the quality of remaining unchanged in spite of abrasion to the marked surface. Ease of application refers to the cost in materials, time and resources of producing a mark including programmability. Programmability refers to the ability to program the marking device with a new pattern to be marked by changing software as opposed to changing hardware such as screens or masks. 
         [0005]    Anodized aluminum, which is lightweight, strong, easily shaped, and has a durable surface finish, has many applications in industrial and commercial goods. Anodization describes any one of a number of electrolytic passivation processes in which a natural oxide layer is increased on metals such as aluminum, titanium, zinc, magnesium, niobium or tantalum in order to increase resistance to corrosion or wear and for cosmetic purposes. These surface layers can be colored or dyed virtually any color, making a permanent, colorfast, durable surface on the metal. Many of these metals can be advantageously marked using aspects of the instant invention. In addition, metals such as stainless steel which resist corrosion can be marked in this fashion. Many articles manufactured out of metals such these as are in need of permanent, visible, commercially desirable marking. Anodized aluminum is an exemplary material that has such needs. Marking anodized aluminum with laser pulses produced by a laser processing system can make durable marks quickly at extremely low cost per mark in a programmable fashion. 
         [0006]    Creating color changes on the surface of anodized aluminum with laser pulses has been known for several years. An article titled “Dry laser cleaning of anodized aluminum” by P. Maja, M. Autric, P. Delaporte, P. Alloncle, COLA&#39;99—5th International Conference on Laser Ablation, Jul. 19-23, 1999, Gottingen, Germany, published in Appl. Phys. A 69 [Suppl.], S343-S346 (1999), pp S43-S346, describes removing anodization from aluminum surfaces, however, note is taken of color changes which occur at laser energies below that required for removal of anodization from the surface. 
         [0007]    One mechanism which has been put forth to explain the change in optical density or color of metallic surfaces is the creation of laser-induced periodic surface structures (LIPSS). The article “Colorizing metals with femtosecond laser pulses” by A. Y. Vorobyev and Chunlei Guo, Applied Physics Letters 92, (041914) 2008, pp 41914-1 to 141914-3 describes various colors which may be created on aluminum or aluminum-like metals using femtosecond laser pulses. This article describes making black or gray marks on metal and creating a gold color on metal. Some other colors are mentioned but no further description is made. LIPSS is the only explanation offered for the creation of marks on metallic surfaces. Further, only laser pulses having temporal pulse widths of 65 femtoseconds are taught or suggested to create these structures. In addition, no mention is made as to whether the aluminum samples are anodized or have had the surface cleaned prior to laser processing. Further the article does not discuss possible damage to the oxide layer. 
         [0008]    When discussing laser pulse duration, the method of measuring pulse duration should be defined. Temporal pulse shape can range from simple Gaussian pulses to more complex shapes depending upon the task. Exemplary non-Gaussian laser pulses advantageous for certain types of processing are described in U.S. Pat. No. 7,126,746 GENERATING SETS OF TAILORED LASER PULSES, Sun et al inventors, which patent has been assigned to the assignees of the instant invention and is hereby incorporated by reference. This patent discloses methods and apparatus to create laser pulses with temporal profiles that vary from the typical Gaussian temporal profiles produced by diode pumped solid state (DPSS) lasers. These non-Gaussian pluses are called “tailored” pulses because their temporal profile is altered from the typical Gaussian profile by combining more than one pulse to create a single pulse and/or modulating the pulse electro-optically. This creates a pulse which the pulse energy varies as a function of time, often including one or more power peaks wherein the instantaneous power increases to a value greater than the average power of the pulse for a fraction of the pulse duration. This type of tailored pulse can be effective in processing materials at high rates without causing problems with debris or excessive heating of surrounding material. An issue is that measuring the duration of complex pulses such as these using standard methods typically applied to Gaussian pulses can yield anomalous results. Gaussian pulse durations are typically measured using the full width at half maximum (FWHM) measure of duration. In contrast to this, using the integral square method, as described in U.S. Pat. No. 6,058,739 LONG LIFE FUSED SILICA ULTRAVIOLET OPTICAL ELEMENTS, inventors Morton et al, allows complex pulse temporal shapes to be measured and compared in a more meaningful manner In this patent, pulse duration is measured using the formula where T(t) is a function which represents the temporal shape of the laser pulse. 
         [0000]    
       
         
           
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         [0009]    Another problem with reliably and repeatably producing marks with desired color and optical density in anodized aluminum is that the energy required to create very dark marks with readily available nanosecond pulse width solid state lasers is enough to cause damage to the anodization, an undesirable result. “Darkness” or “lightness” or color names are relative terms. A standard method of quantifying color is by reference to the CIE system of colorimetry. This system is described in “CIE Fundamentals for Color Measurements”, Ohno, Y., IS&amp;T NIP16 Conf, Vancouver, Conn., Oct. 16-20, 2000, pp 540-545. In this system of measurement, achieving a commercially desirable black mark requires parameters less than or equal to L*=40, a*=5, and b*=10. This results in a neutral colored black mark with no visible grayness or coloration. In U.S. Pat. No. 6,777,098 MARKING OF AN ANODIZED LAYER OF AN ALUMINUM OBJECT, Inventor Keng Kit Yeo describes a method of marking anodized aluminum articles with black marks which occur in a layer between the anodization and the aluminum and therefore are as durable as the anodized surface. The marks described therein are described as being dark grey or black in hue and somewhat less shiny than unmarked portion using nanosecond range infrared laser pulses. In addition, the aluminum is required to be cleaned of all surface particles, for instance particles remaining after polishing, prior to anodization. Making marks according to the methods claimed in this patent are disadvantageous for two reasons: first, creating commercially desirable black marks with nanosecond-range pulses tends to cause destruction of the oxide layer and secondly, cleaning of the aluminum following polishing or other processing adds another step in the process, with associated expense, and possibly disturbs a desired surface finish by further processing. 
         [0010]    What is desired but undisclosed by the art is a reliable and repeatable method of making marks on anodized aluminum in both black or grey or in color that does not require an expensive femtosecond laser or disturb the oxide layer in the process or require cleaning following surface preparation. In addition, no information is supplied on how to repeatably create various colors on anodized aluminum surfaces, nor has the effects of bleaching or damage to the anodization layer been thoroughly investigated. What is needed then is a method for reliably and repeatably creating marks having a desired optical density or grayscale and color on anodized aluminum using a lower cost laser, without causing undesired damage to the overlaying oxide or requiring cleaning prior to anodization. 
       SUMMARY OF THE INVENTION 
       [0011]    An aspect of this invention is to mark anodized aluminum articles with visible marks of various optical densities or grayscale and colors. These marks should be durable and have commercially desirable appearance. This is achieved by using picosecond laser pulses to create the marks. These marks are created at the surface of the aluminum underneath the oxide layer and are therefore protected by the oxide. The picosecond laser pulses create commercially desirable marks without causing significant damage to the oxide layer, thereby making the marks durable. Durable, commercially desirable marks are created on anodized aluminum by controlling the laser parameters with which create and direct picosecond laser pulses. In one aspect of this invention a laser processing system is adapted to produce laser pulses with appropriate parameters in a programmable fashion. 
         [0012]    Exemplary laser pulse parameters which may be selected to improve the reliability and repeatability of laser marking anodized aluminum include laser type, wavelength, pulse duration, pulse repletion rate, number of pulses, pulse energy, pulse temporal shape, pulse spatial shape and focal spot size and shape. Additional laser pulse parameters include specifying the location of the focal spot relative to the surface of the article and directing the relative motion of the laser pulses with respect to the article. 
         [0013]    Aspects of this invention create durable, commercially desirable marks by darkening the surface of the aluminum beneath the anodization with optical densities which range from nearly undetectable with the unaided eye to black depending upon the particular laser pulse parameters employed. Other aspects of this invention create colors in various optical densities in shades of tan or gold, likewise depending upon the particular laser pulse parameters employed. Other aspects of this invention create durable, commercially desirable marks on anodized aluminum by bleaching or partially bleaching dyed or colored anodization with or without marking the aluminum beneath. 
         [0014]    To achieve the foregoing with these and other aspects in accordance with the purposes of the present invention, as embodied and broadly described herein, a method for creating a color and optical density selectable visible mark on an anodized aluminum specimen and apparatus adapted to perform the method is disclosed herein. The invention is a method and apparatus for creating a color and optical density selectable visible mark on an anodized aluminum specimen. The method includes providing a laser marking system having a laser, laser optics and a controller operatively connected to said laser to control laser pulse parameters and a controller with stored laser pulse parameters, selecting the stored laser pulse parameters associated with the desired color and optical density, directing the laser marking system to produce laser pulses having laser pulse parameters associated with the desired color and optical density including temporal pulse widths greater than about 1 and less than about 1000 picoseconds to impinge upon said anodized aluminum. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1 . Laser processing system 
           [0016]      FIG. 2 . Mark made with prior art nanosecond pulses 
           [0017]      FIG. 3 . Mark made with picosecond pulses 
           [0018]      FIG. 4 . Beam waist 
           [0019]      FIG. 5 . Grayscale marks on anodized aluminum 
           [0020]      FIG. 6 . Marks on anodized aluminum 
           [0021]      FIG. 7 . Dyed, visible marked anodized aluminum 
           [0022]      FIG. 8 . Dyed, IR marked anodized aluminum 
           [0023]      FIG. 9 . Graph showing visible laser pulse thresholds 
           [0024]      FIG. 10 . Graph showing IR laser pulse thresholds 
           [0025]      FIG. 11 . Image data converted to laser parameters 
           [0026]      FIG. 12   a - i.  Color anodization being applied to an aluminum article 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0027]    A goal of this invention is to mark anodized aluminum articles with visible marks of various optical densities and colors, durably, selectably, predictably, and repeatably. It is advantageous for these marks to appear on or near the surface of the aluminum and leave the anodization layer substantially intact to protect both the surface and the marks. Marks made in this fashion are referred to as interlayer marks since they are made at or on the surface of the aluminum beneath the oxide layer that forms the anodization. Ideally the oxide remains intact following marking in order to protect the marks and provide a surface that is mechanically contiguous between adjacent marked and non-marked regions. Further, these marks should be able to be produced reliably and repeatably, meaning that if a mark with a specific color and optical density is desired, a set of laser parameters is known which will produce the desired result when the anodized aluminum is processed by a laser processing system. It is also contemplated that such marks created with a laser processing system be invisible. In this aspect, the laser processing system creates marks which are not visible under ordinary viewing conditions, but which become visible under other conditions, for example when illuminated by ultraviolet light. It is contemplated that these marks be used to provide anti-theft marking or other special marks. 
         [0028]    An embodiment of the instant invention uses an adapted laser processing system to mark anodized aluminum articles. An exemplary laser processing system which can be adapted to mark anodized aluminum articles is the ESI MM5330 micromachining system, manufactured by Electro Scientific Industries, Inc., Portland, Oreg. 97229. This system is a micromachining system employing a diode-pumped Q-switched solid state laser with an average power of 5.7 W at 30 K Hz pulse repetition rate, second harmonic doubled to 532 nm wavelength. Another exemplary laser processing system which may be adapted to mark anodized aluminum articles is the ESI ML5900 micromachining system, also manufactured by Electro Scientific Industries, Inc., Portland, Oreg. 97229. This system employs a solid state diode-pumped laser which can be configured to emit wavelengths from about 355 nm (UV) to about 1064 nm (IR) at pulse repetition rates up to 5 MHz. Either system may be adapted by the addition of appropriate laser, laser optics, parts handling equipment and control software to reliably and repeatably produce marks in anodized aluminum surfaces according to the methods disclosed herein.) These modifications permit the laser processing system to direct laser pulses with the appropriate laser parameters to the desired places on an appropriately positioned and held anodized aluminum article at the desired rate and pitch to create the desired mark with desired color and optical density. A diagram of such an adapted system is shown in  FIG. 1 . 
         [0029]      FIG. 1  shows a diagram of an ESI MM5330 micromachining system adapted for marking articles according to an embodiment of the instant invention. Adaptations include the laser  10 , which, in an embodiment of this invention is a diode pumped Nd:YVO 4  solid state laser operating at 1064 nm wavelength, model Rapid manufactured by Lumera laser GmbH, Kaiserslautern, Germany. This laser is optionally frequency doubled using a solid state harmonic frequency generator to reduce the wavelength to 532 nm or tripled to about 355 nm, thereby creating visible (green) or ultraviolet (UV) laser pulses, respectively. This laser  10  is rated to produce 6 Watts of continuous power and has a maximum pulse repetition rate of 1000 KHz. This laser  10  produces laser pulses  12  with duration of 1 to 1,000 picoseconds in cooperation with controller  20 . These laser pulses  12  may be Gaussian or specially shaped or tailored by the laser optics  14  to permit desired marking. The laser optics  14 , in cooperation with the controller  20 , direct laser pulses  12  to form a laser spot  16  on or near article  18 . Article  18  is fixtured upon stage  22 , which includes motion control elements which, in cooperation with the controller  20  and laser optics  14  provides compound beam positioning capability. Compound beam positioning is the capability to mark shapes on an article  18  while the article  18  is in relative motion to the laser spot  16  by having the controller  20  direct steering elements in the laser optics  14  to compensate for the relative motion induced by motion of the stage  22 , the laser spot  16  or both. 
         [0030]    The laser pulses  12  are also shaped by the laser optics  14  in cooperation with controller  20 , as they are directed to form a laser spot  16  on or near article  18 . The laser optics  14  directs the laser pulses&#39;  12  spatial shape, which may be Gaussian or specially shaped. For example, a “top hat” spatial profile may be used which delivers a laser pulse  12  having an even dose of radiation over the entire spot which impinges the article being marked. Specially shaped spatial profiles such as this may be created using diffractive optical elements. Laser pulses  12  also may be shuttered or directed by electro-optical elements, steerable mirror elements or galvanometer elements of the laser optics  14 . 
         [0031]    The laser spot  16  refers to the focal spot of the laser beam formed by the laser pulses  12 . As mentioned above the distribution of laser energy at the laser spot  12  depends upon the laser optics  14 . In addition the laser optics  14  control the depth of focus of the laser spot  12 , or how quickly the spot goes out of focus as the plane of measurement moves away from the focal plane. By controlling the depth of focus, the controller  20  can direct the laser optics  14  and the stage  22  to position the laser spot  16  either at or near the surface of the article  18  repeatably with high precision. Making marks by positioning the focal spot above or below the surface of the article allows the laser beam to defocus by a specified amount and thereby increase the area illuminated by the laser pulse and decrease the laser fluence at the surface. Since the geometry of the beam waist is known, precisely positioning the focal spot above or below the actual surface of the article will provide additional precision control over the spot size and fluence. 
         [0032]    Picosecond lasers, which produce laser pulse widths in the range from 1 to 1,000 picoseconds, are the preferred lasers for reliably and repeatably creating marks on anodized aluminum.  FIG. 2  is a microphotograph showing a mark created on anodized aluminum  30  using prior art laser with &gt;1 nanosecond pulses. The anodization shows clear signs of cracking  32  in the mark area  34 , an undesirable result.  FIG. 3  shows the same color and optical density mark  38  on the same type of anodized aluminum  36  made with a picosecond laser showing no cracking. Picosecond lasers mark anodized aluminum articles with a commercially desirable black without causing damage to the oxide layer. Commercially acceptable black is defined as a mark having CIE chromaticity of L*=40, a*=5, and b*=10 or less. Another advantage of using picosecond lasers is that they are much less expensive, require much less maintenance, and typically have much longer operating lifetimes than prior art femtosecond lasers. In addition, aspects of the instant invention do not require cleaning of the aluminum surface prior to anodization to create commercially desirable marks. 
         [0033]    An embodiment of the instant invention performs marking on anodized aluminum under the anodization. For the interlayer marking to happen, the laser fluence, defined by: 
         [0000]    
       
      
       F=E/s  
      
     
         [0034]    where E is laser pulse energy and s is the laser spot area, must satisfy Fu&lt;F&lt;Fs, where Fu is the laser modification threshold of the substrate, aluminum in this case, and Fs is the damaging threshold for the surface layer, or anodization. Fu and Fs have been obtained by experiments and represents the fluence of the selected laser at which the substrate and surface layer start to get damaged. For 10 ps pulses, our experiments show that Fu for Al is −0.13 J/cm 2  for ps green and −0.2 J/cm 2  for ps IR, and the Fs is −0.18 J/cm 2  for ps green and −1 J/cm 2  for ps IR. Varying the laser fluence between these values creates marks of varying color and optical density. Different pulse durations and laser wavelengths would each have corresponding values of Fu and Fs. The actual thresholds for a given set of laser parameters are determined experimentally. 
         [0035]    Laser parameters associated with a particular color or optical density can also be determined by methods other than empirical. For example, laser parameters may be determined by running computer simulations of laser/material interactions. Other sources of information regarding laser/material interactions such as textbooks, laser manuals or other technical literature may be accessed and appropriate laser parameters determined by extrapolation therefrom. By directing the laser processing system to produce laser pulses with the proper laser parameters and precisely controlling the laser fluence, marks of desired color and optical density can be reliably and repeatably created on anodized aluminum articles. 
         [0036]    An embodiment of this invention precisely controls the laser fluence at the surface of the aluminum article by adjusting the location of the laser spot from being on the surface of the aluminum article to being located a precise distance above or below the surface of the aluminum.  FIG. 4  shows a diagram of a laser pulse focal spot  40  and the beam waist in its vicinity. The beam waist is represented by a surface  42  which is the diameter of the spatial energy distribution of a laser pulse as measured by the FWHM method on the optical axis  44  along which the laser pulses travel. The diameter  48  represents the laser pulse spot size on the surface of the aluminum when the laser processing system focuses the laser pulse at a distance (A-0) above the surface. Diameter  46  represents the laser pulse spot size on the surface of the aluminum when the laser processing system focuses the laser pulses at a distance (0-B) below the surface. 
         [0037]    In addition to commercially desirable black, marking articles with grayscale values is also useful.  FIGS. 5 and 6  show a series of grayscale marks made on anodized aluminum made by an embodiment of this invention. The optical density of the marks range from nearly indistinguishable from the background to fully black. According to an aspect of the instant invention, each grayscale mark can be identified by a unique triplet of CIE colorimetry values, L*, a*and b*. An aspect of the instant invention associates each desired grayscale value with a set of laser parameters that reliably and repeatably produce the desired grayscale value mark on anodized aluminum upon command Note also that the marks which may seem indistinguishable to the naked eye can become visible when illuminated with other than broad spectrum visible light, for example ultraviolet light. 
         [0038]      FIG. 5  shows black marks  60 ,  62 ,  64 , and  66  made on anodized aluminum  70  by an embodiment of this invention. These marks  60 ,  62 ,  64 , and  66  have CIE chromaticities ranging from less than L*=40, a*=5 and b*=10, to totally transparent making them commercial desirable marks. Another feature of these marks is that since they are underneath undamaged anodization, they have uniform appearance over a wide range of viewing angles. Marks made using prior art methods tend to have wide variation in appearance depending upon viewing angle due to damage to the anodization layer. In particular, when marking with prior art nanosecond pulses, applying enough laser pulse energy to the surface to make dark marks causes damage to the anodization which causes the appearance of the marks to change with viewing angle. Marks made by an aspect of the instant invention do not damage anodization regardless of how dark the marks are, nor do they change in appearance with viewing angle. These improved marks were made with the following laser parameters: 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Laser parameters for color and grayscale marking 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Laser Type 
                 DPSS Nd:YVO 4   
               
               
                   
                 Wavelength 
                 532 nm 
               
               
                   
                 Pulse duration 
                 10 ps 
               
               
                   
                 Pulse temporal 
                 Gaussian 
               
               
                   
                 Laser power 
                 4 W 
               
               
                   
                 Rep Rate 
                 500 KHz 
               
               
                   
                 Speed 
                 25 mm/s 
               
               
                   
                 Pitch 
                 10 microns 
               
               
                   
                 Spot size 
                 10-400 microns 
               
               
                   
                 Spot shape 
                 Gaussian 
               
               
                   
                 Focal Height 
                 0-5 mm with 0.5 mm step 
               
               
                   
                   
               
             
          
         
       
     
         [0039]    The marks  60 ,  62 ,  64 ,  66  range in optical density from virtually unnoticeable  60  against the unmarked aluminum to full black  62 . Grayscale optical densities  64 ,  66  between the two extremes are created by moving the focal spot closer to the article, increasing the fluence and thereby creating darker marks. The height of the focal spot above the surface of the aluminum varies from zero, in the case of the darkest optical density mark  62 , increasing by 500 micron increments for each mark  64 ,  66  from right to left in  FIG. 4 , ending at 5 mm above the surface for the lightest mark  60 . Note that marks  64  created with focal spot located 4.5 to 1.5 mm above the surface of the aluminum show tan or golden colors and marks created with focal spot one mm  62  and  66  or less appear gray or black. Maintaining this precise control over the laser focal spot distance from the work surface in addition to maintaining other laser parameters within normal laser processing tolerances permits laser marks with desired color and optical density to be made on anodized aluminum. In addition, the darkest mark exhibits a CIE chromaticity of less than L*=40, a*=5, and b*=10, making it a commercially desirable black mark. 
         [0040]    Another aspect of the instant invention determines the relationship between marks with colors other than grayscale and picosecond laser pulse parameters. Colors other than grayscale can be produced on anodized aluminum in two different ways. In the first, a gold tone can be produced in a range of optical densities. This color is produced by changes made at the interface between the aluminum and the oxide coating. Careful choice of laser pulse parameters will produce the desired golden color without damaging the oxide coating.  FIG. 5  also shows various shades of gold or tan created by an aspect of the instant invention. 
         [0041]    Laser marking of anodized aluminum can also be achieved by an aspect of the instant invention which uses IR wavelength laser pulses to mark the aluminum. This aspect creates marks of varying grayscale densities by varying the laser fluence at the surface of the aluminum in two different manners. As discussed above, grey scale can be achieved by varying the fluence at the surface by positioning the focal spot above or below the surface of the aluminum. The second manner of controlling grey scale is to vary the total dose at the surface of the aluminum by changing the bite sizes or line pitches when marking the desired patterns. Changing bite sizes refers to adjusting the rate at which the laser pulse beam is moved relative to the surface of the aluminum or changing the pulse repetition rate or both, which results in changing the distance between successive laser pulse impact sites on the aluminum. Varying line pitches refers to adjusting the distance between marked lines to achieve various degrees of overlapping.  FIG. 6  shows an aluminum article  74  with an array of marks  72 . These marks  72  are arranged in an array of six columns and four rows. The six columns represent six Z-heights of the focal spot above the surface of the aluminum ranging from 0 (top row) to 5 mm (bottom row). The four rows represent pitches of 5, 10, 20 and 50 microns reading from left to right. As can be seen from  FIG. 6 , varying the Z-height of the focal spot and varying the pitch of the laser pulses can predictably produce graylevels of any desired optical density from less than CIE L*=40, a*=5, and b*=10 to nearly transparent, thereby producing commercially desirable marks on anodized aluminum. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Laser pulse parameters for grayscale IR marking 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Laser Type 
                 DPSS Nd:YVO 4   
               
               
                   
                 Wavelength 
                 1064 nm 
               
               
                   
                 Pulse duration 
                 10 ps 
               
               
                   
                 Pulse temporal 
                 Gaussian 
               
               
                   
                 Laser power 
                 2.5 W 
               
               
                   
                 Rep Rate 
                 500 KHz 
               
               
                   
                 Speed 
                 50 mm/s 
               
               
                   
                 Pitch 
                 5, 10, 20, 50microns 
               
               
                   
                 Spot size 
                 55-130 microns 
               
               
                   
                 Spot shape 
                 Gaussian 
               
               
                   
                 Focal Height 
                 0-5 mm with 1 mm step 
               
               
                   
                   
               
             
          
         
       
     
         [0042]    A second type of marking which may be applied to anodized aluminum using picosecond laser pulses is alterations in color contrast caused by bleaching of dyed anodization. On a microscopic scale, anodization is porous, and will readily accept dyes of many types. Referring again to  FIG. 3 , this microphotograph of anodized aluminum shows the porous nature of surface. Laser pulses used to mark dyed anodized aluminum can, depending upon the wavelength and pulse energy, bleach the dye as it marks the aluminum, making the anodization transparent and thereby reveals the marks on the aluminum underneath. With higher fluence, simultaneous dye bleaching and marking of the aluminum beneath the anodization layer with black, grey scale, or colors presented in previous section is possible. Less energetic pulses can partially bleach the anodization dyes rendering it translucent and thereby partially coloring the underlying aluminum marks. Finally, longer wavelength pulses can mark the aluminum with commercially desirable black or grey scale colors without bleaching the anodization.  FIG. 7  shows a dyed anodized aluminum article with marks made with visible (532 nm) laser pulses. Note that the dye in the anodization is bleached in the areas subjected to laser pulses.  FIG. 8  shows the same type of dyed anodized aluminum article with marks made with IR (1064 nm) laser pulses. Note that the anodization is not bleached by the IR laser pulses and therefore does not reveal the aluminum color beneath beyond the translucency of the original oxide. 
         [0043]    Another aspect of this invention relates to laser marking anodized aluminum with colored anodization using picosecond lasers. Since anodization typically forms a porous surface, dyes can be introduced which alter the appearance of the aluminum. These dyes can either be opaque or translucent, allowing varying amounts of incident light to reach the aluminum and be reflected back through the anodization.  FIG. 7  shows an anodized aluminum article  80  with pink dye in the anodization and an array of marks  82  produced according to an aspect of the instant invention. Colors are created by bleaching the dye in the oxide layer as the aluminum underneath showed native (silver) color to a range of laser-marked colors from shades of tan, to gray and finally black. These shades are created by varying the fluence of the laser pulses at the surface of the aluminum. The four rows represent varying the pitch of the laser pulses from 10 to 50 microns and the columns represent varying the focal spot distance from the surface from 0.0 to 5.0 mm These laser parameters in all cases bleach the dye in the oxide overlaying the aluminum allowing the marks on the aluminum show through. The laser marks optical density range from transparent to CIE chromaticity less than L*=40, a*=5, b*=10. Laser parameters used to create these marks are given in Table 3. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Laser parameters for visible oxide bleaching 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Laser Type 
                 DPSS Nd:YVO 4   
               
               
                   
                 Wavelength 
                 532 nm 
               
               
                   
                 Pulse duration 
                 10 ps 
               
               
                   
                 Pulse temporal 
                 Gaussian 
               
               
                   
                 Laser power 
                 4 W 
               
               
                   
                 Rep Rate 
                 500 KHz 
               
               
                   
                 Speed 
                 50 mm/s 
               
               
                   
                 Pitch 
                 10 microns 
               
               
                   
                 Spot size 
                 10-400 microns 
               
               
                   
                 Spot shape 
                 Gaussian 
               
               
                   
                 Focal Height 
                 0-5 mm 
               
               
                   
                   
               
             
          
         
       
     
         [0044]    Bleaching of anodization dye is frequency dependent. As shown in  FIG. 7 , 532 nm laser pulses bleach anodization dye even at the lowest fluence. IR laser wavelengths, on the other hand, create marks on dyed anodized aluminum without bleaching the dye for most translucent dye colors.  FIG. 8  shows an anodized aluminum article  100  with pink dye with marks  102  made with IR laser pulses. The marks range from translucent to black and were made by altering the laser fluence by both changing the distance from the focal spot to the surface and by changing the pitch. The six columns represent changing the distance between the focal spot of the laser pulses and the surface of the aluminum from 5.5 mm (right) to zero (left). The four rows represent changing the laser pulse pitch from 10 to 50 microns. Laser parameters used to create these marks is shown in Table 4. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Laser parameters for IR colored anodization marking 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Laser Type 
                 DPSS Nd:YVO 4   
               
               
                   
                 Wavelength 
                 1064 nm 
               
               
                   
                 Pulse duration 
                 10 ps 
               
               
                   
                 Pulse temporal 
                 Gaussian 
               
               
                   
                 Laser power 
                 4 W 
               
               
                   
                 Rep Rate 
                 500 KHz 
               
               
                   
                 Speed 
                 50 mm/s 
               
               
                   
                 Pitch 
                 10 microns 
               
               
                   
                 Spot size 
                 10-400 microns 
               
               
                   
                 Spot shape 
                 Gaussian 
               
               
                   
                 Focal Height 
                 0-5 mm 
               
               
                   
                   
               
             
          
         
       
     
         [0045]    The relationship between bleaching anodization dye, marking aluminum and causing surface ablation for 532 nm (green) laser wavelengths is shown in  FIG. 9 . For 532 nm (green) laser pulses with parameters within those given in Tables 1, 2 and 3,  FIG. 8  shows the fluence thresholds in Joules/cm 2  for bleaching anodization (Fb), marking aluminum under the anodization (Fu), and surface ablation (Fs). For an aspect of the instant invention 532 nm laser pulses yield the values are Fb=0.1 J/cm 2 , Fu=0.13 J/cm 2 , and Fs=0.18 J/cm 2    FIG. 10  shows the fluence thresholds in Joules/cm 2  for 1064 nm (IR) laser pulses with parameters within those given in Tables 1, 2, and 3. For an aspect of the instant invention the fluence threshold values for 1064 nm laser pulses in Joules/cm 2  are Fu=0.2 J/cm 2 and Fs=1.0 J/cm 2 . Note that no threshold for bleaching anodization is available since IR wavelength laser pulses do not begin to bleach anodization until laser fluence is great enough to cause damage to the overlaying anodization. Note that the exact values for Fb, Fu and Fs will depend upon the particular laser and optics used. They must be determined experimentally for a given processing setup and article to be marked and stored in the controller for later use. 
         [0046]    In another embodiment of this invention, the programmable nature of the adapted laser processing system permits the marking of anodized aluminum articles with commercially desirable marks in patterns. As shown in  FIG. 11 , in this aspect a pattern  110  is converted into a digital representation  112 , which is decomposed into a list  114 , where each entry  116  in the list  114  contains a representation of a location or locations, with a color and optical density associated with each location. The list  114  is stored in the controller  20 . The controller  20  associates laser parameters with each entry  116  in the list  114 , which laser parameters, when sent as commands to the laser  10 , optics  14  and motion control stage  22  will cause the laser  10  to generate one or more laser pulses  12  which impinge aluminum article  18  at or near the surface  16 . These pulses will create a mark with the desired color and optical density. By moving the laser pulses  12  in relation to the aluminum article  18  according to the locations stored in the list as the marks are being created, marks of the desired range of colors and optical density are made on the anodized aluminum surface in the desired pattern. 
         [0047]    In another embodiment of this invention colored anodization is patterned over previously patterned marks to present additional colors and optical densities. In this aspect, a grayscale pattern is created on an anodized aluminum article. The article is then coated with a photoresist coating that can be developed by exposure to laser pulses. The grayscale patterned, photoresist coated article is placed into the laser processing system and aligned so that the system can apply laser pulses in registration with the pattern already applied to the article. The photoresist used is a type known as “negative” photoresist, where areas exposed to laser radiation will be removed and the unexposed areas will remain on the article following subsequent processing. The remaining photoresist protects the surface of the article from introduction of dyes, while the areas of the anodization which had been exposed and subsequently removed will be dyed the desired color. This anodization layer is designed to be translucent in order to allow light to pass through the anodization to the pattern below and be reflected back through the anodization and thereby create color patterns with selected color and optical density. This color anodization can also be bleached if necessary using techniques disclosed by other aspects of this invention to create a desired color with desired transparency. This color can be applied over areas of the underlying pattern or applied on a point-by-point basis down to the limits of resolution of the laser system, typically in the 10 to 400 micron range. This operation can be repeated to create multiple color overlays. In one aspect of this invention, the anodization color overlay is applied in a multiple color overlay grid, such as Bayer pattern. By designing the grayscale pattern to work with the color overlay grid, a durable, commercially desirable full color image can be created on the anodized aluminum article. 
         [0048]      FIGS. 12   a  through  12   i  show a sequence of steps used to create this color overlay for two colors. In  FIG. 12   a , an aluminum article  118  has a transparent anodization layer  120  and marks  122  previously applied according to other aspects of this invention. A negative photoresist  124  is applied to the surface of the transparent anodization  120 . In  FIG. 12   b,  laser pulses  126  expose areas  128 ,  130  of the photoresist  124 . In  FIG. 12   c  the unexposed resist  134  remains following resist processing, but the exposed resist has been removed leaving voids  132  in the processed resist layer  134 .  FIG. 12   d  shows the base anodization layer  120  with sections  136  where the anodization has been dyed with color beneath the voids  132  in the processed resist layer  134 . The intact processed resist  134  prevents the anodization from acquiring color anywhere except where the processed resist  134  has been removed  132 .  FIG. 12   e  shows the article  118  with base anodization  120  with color portions of anodization  136  in relation to previously applied marks  122  following removal of processed resist. 
         [0049]      FIG. 12   f  shows an article  118  with base anodization  120  including colored portions  136  and a second resist layer  138 .  FIG. 12   g  shows this second layer of resist  138  impinged by laser pulses  142  to cause area  140  to become exposed.  FIG. 12   g  shows the article  118  with base anodization  120  following processing to, dye the anodization beneath the removed resist  140 , and removal of the remaining resist  138 . This leaves the intact base anodization layer with colored areas  136 ,  144  over the previously marked areas  122 .  FIG. 12   i  shows subsequent laser pulses  146  being used to optionally bleach portions of the previously anodized and dyed portions of the aluminum article to create additional desired colors or optical densites. The processing described by this aspect of this invention results in a colored pattern being overlaid over a grayscale pattern, yielding marks with a wide range of durable, commercially desirable colors and optical densities in patterns which are programmable. 
         [0050]    In another embodiment of this invention, the color anodization may be created on the anodized aluminum article in particular patterns which yield the appearance of full color images when viewed. In this aspect, a pattern representative of an image is applied to the surface using techniques described herein. The color dyes are introduced in the manner illustrated in  FIGS. 12   a  through  12   i,  except that the pattern with which these dyes are introduced into the base layer of anodization is designed to convert the grayscale representation into full color. An example of such a pattern is a Bayer filter (not shown), which juxtaposes red, green and blue filter elements in a pattern such that the eye perceives the red, green and blue elements fusing into a single color with optical density related to the grayscale mark underneath the color anodization filters, thereby creating the appearance of a full color image or pattern. The resist may be negative or positive resist, and the patterns which expose the resist may be created by masks, such as used in circuit or semiconductor applications, or directly written by a electronic means or directly deposited by technologies such as inkjet or directly ablated by laser. 
         [0051]    It will be apparent to those of ordinary skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims.