Abstract:
A method and apparatus for labeling plant products based on laser activation of a color-changing compound is disclosed. In the preferred embodiment, a nozzle sprays a coating of photosensitive material containing color-changing chemical component. An optional drying station is set up to optimize homogeneity and adhesiveness of the color-changing coating. A laser equipped with beam steering optics is used to image the desired mark on the plant product by inducing a change of color in the photosensitive coating, without contacting the plant product skin and at a high speed. An optional nozzle sprays a sealant coating after printing, for extended durability of the imaged label. In addition, an optical sensor detects the incoming plant product, determines its size and sends information for selecting the proper label to be imaged. An additional optical sensor can be placed at the end of the process to verify the quality and legibility of the imaged label. The apparatus described can be extended to multiple marking stations, which can be controlled by a central computer to allow for dynamic updating of the desired label(s).

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates in general to a method and apparatus for labeling the exterior of fresh plant products using color-change chemistry techniques. In particular, the invention concerns a method and apparatus for marking on plant products using an edible color-change based coating and a laser as a means for photo stimulation, without etching or burning the plant product peel or skin.  
       BACKGROUND OF THE INVENTION  
       [0002]     Labeling of plant products, such as fruit and vegetables, is an important practice in the plant product processing industry. The Produce Marketing Association (PMA) has established that about 80% of bulk and packaged plant product has some type of label. The information on the label may include the plant product type, inventory and pricing control, traceability data, and/or producer brand. Particularly common is an unique number assigned to every plant product sold in bulk known as price look up code (P.L.U.) In addition to serving as an identification code for grocery store inventory and check out procedures, the P.L.U. categorizes how the plant product was grown. Namely, conventionally grown plant products have a 4 digit P.L.U. number, whereas organically grown and genetically engineered plant products have a 5 digit P.L.U. For organically grown plant products, the 5 digit P.L.U starts with 9, and for genetically engineered plant products the 5 digit P.L.U. starts with 8. Although the P.L.U. is not a part of a regulatory system, it has become a standard not only for the United States plant product industry, but worldwide as well.  
         [0003]     For the market of bulk plant products, the most common labeling scheme is the use of pre-printed adhesive labels (hereafter referenced generally as “stickers”) that are mostly used in high-speed plant product processing lines. The literature contains many methods for performing this task, but a typical of method is disclosed in U.S. Pat. No. 6,257,294 B1, issued Jul. 10, 2001 to Weisbeck, and the references cited therein. There, an applicator head picks up a sticker from a roll and then the head rotates towards the plant product and applies the sticker to the surface by exerting some pressure. Stickers have an advantage in that they can be applied to irregular surfaces and plant products of different sizes and surface shapes. In the multiple disclosed systems, the applicator may be in the form of a blade, a piston, etc. Although the adhesive material employed is edible, the stickers, which are generally made of paper or vinyl, are not. Consequently, the stickers must be removed before consumption and, in some instances, only hot water and washing can get rid of stubborn ones. Another inconvenience of the sticker scheme is on the side of the packinghouses. Stickers that lack adhesive or were misplaced by the applicator end up on the conveyor apparatus causing numerous problems. Also, sticker rolls frequently jam the applicator or get out of position. As a consequence, packinghouses devote extensive manual labor to keep these types of systems operating. In addition, due to recent concerns about the safety of the food supply, the PMA has issued some guidelines for growers who wish to introduce in their labels information for tracing the plant products based on the identity of growers and place of production. This will require a dynamic labeling scheme with criteria that conventional stickers may not be able to satisfy.  
       DESCRIPTION OF THE RELATED ART  
       [0004]     U.S. Pat. No. 4,784,714, issued to Shibata in 1988, discloses a method where the label is printed just before it is glued onto a product. The method prints the sticker with a thermal printing head requiring, as a consequence, that the sticker consists of thermal paper. This apparatus is convenient for labeling packaged food, but it not practical for fresh fruit or vegetable labeling.  
         [0005]     Sunkist Growers, Inc. has been using ink stamp applicators for many years to label fruit where conventional stickers are not desirable. For example, this is the case for those lemon product customers that slice the fruit for drinks. The sticker cannot be cut easily and it is necessary to remove it. The ink applicator consists of a rubber stamp with the company&#39;s logo, and an ink repository that transfers the ink to the rubber stamp. As the lemon passes under the applicator, the label is transferred by contact. Although this labeling system has been operating for many years, it produces a poorly legible mark that is not very appealing, particularly as the size and shape of lemons can vary. It also lacks the dynamic and flexible labeling characteristics that the market requires.  
         [0006]     To overcome these disadvantages, alternate schemes have been proposed. For instance, U.S. Pat. No. 5,660,747 and No. 5,897,797 granted to Drouillard and Kanner in August 1997 and April 1999, respectively, where a high-power laser equipped with beam steering optics is aimed directly at the plant product with enough exposure to produce an etching on the outer layers of the plant product skin. The localized etching is produced in the shape of small dots to finally generate a legible mark in a way very similar to conventional dot matrix printers. Although this scheme provides the desired dynamic labeling, it requires very precise laser induced etching to prevent either burning or too light of a mark to be noticed. The laser induced etching is influenced by a combination of laser intensity, exposure time, laser-to-plant product surface distance, surface contour, and the particular peel characteristics of the printed plant product. In practice, it is extremely difficult to control all these factors. Differences in plant product size and shape, different peel textures even within the same commodity, in addition to external factors such as humidity condensation or industrial plant product coatings, significantly affect the degree of etching during printing. If overexposed, the resulting etching may potentially reduce the plant product market yield due to peel dryness, induced decay, reduced shelf life, or other similar factors. Additionally, the energy required to induce the desired etching to the fruit skin requires that a high power carbon dioxide (CO2) laser (20 W or more) must be used in this application.  
         [0007]     An edible color-changing material is described in U.S. Pat. No. 6,888,095, issued on May 3, 2005 to Khan. Several materials are disclosed there which are composed in general by an color-changing agent in the form of a metal salt, a polymer, or some other metal compounds, a binder agent, and a transporter in the form of a solvent. Such compounds are photosensitive to a specific wavelength in the infrared spectral region (10,600 nm) so when a CO2 laser—emitting at that wavelength-images a label onto the photosensitive compound, the compound reacts by changing color only in the irradiated areas creating a clear inscription. As disclosed in U.S. Pat. No. 6,888,095, the entire contents of which are incorporated herein by reference, different renditions of the color-changing material have been employed to inscribe on objects of uniform size and shape with stable, heat resistant exterior surfaces, for example, pharmaceutical products and food packages. The additives may be a polyhydroxy compound and a dehydrating agent, the agent typically being a metal salt of the type that removes OH groups from sugars, e.g. sucrose, starches, modified starches, cellulose, modified celluloses, etc. Examples of suitable metal salts are alkali metal, alkaline earth metal, iron oxide/salts and organometallics. When heated by the application of laser energy, the sugars will char or dehydrate, causing a color change. Other examples of materials that will give a color change by dehydration in the presence of a metal salt include: hydroxypropylcellulose, methylhydroxypropylcellulose, sodium carboxymethylcellulose and polyvinyl alcohol. Suitable metal salts for this purpose include: MgCl 2 , Mg(OH) 2 , CaO, FeO, Fe 2 O 3 , CaSiO 3 , Zn acetate, ZnO and alumino-silicates  
         [0008]     As explained in the patent, the elimination reaction alternatively may comprise dehalogenation, dehydrohalogenation or deacetylation, in which case the relevant functional group is a halogen atom or carboxyl group. Examples of additives for this purpose are vinyl polymers, typically in the present of a metal salt. Suitable polymers include: polyvinyl chloride (PVC), polyvinyl acetate, vinyl esters, vinyl chloride/acetate copolymer and vinyl chloride/maleate copolymer. Suitable metal compounds for this purpose include: ZnO, Zn salicylate, kaolin and CaSiO 3 . Other additives may undergo deetherification. Thus, for example, ethyl cellulose and a metal salt will give a color upon irradiation.  
         [0009]     The examples given in the patent are primarily of metal salt-induced elimination, but further embodiments include acid or base-induced dehydration, such that a color is generated using p-toluenesulphonic acid with PVOH (polyvinyl alcohol). Based on this information, other suitable materials will be known, or can be readily chosen or tested for their suitability, by those of ordinary skill in the art.  
         [0010]     The examples in the patent are suitable for products having a uniform and repetitive surface structure in view of their manufacture according to close industry tolerances. However, the application of the principles of the patent to plant products, particularly those having inconsistent and widely variable shapes, sizes and surface features, has not been contemplated.  
         [0011]     The method and apparatus described herein discloses a method that takes advantage of laser printing features like dynamic and sticker-less labeling, and overcomes the disadvantages held by the disclosed systems in U.S. Pat. No. 5,660,747 and No. 5,897,797, by mainly avoiding etching of the plant product skin, even where the plant products vary in size, shape or skin texture. The present invention uses instead an edible color changing material of the type described in U.S. Pat. No. 6,888,095 but is adapted to provide a mark or label on plant products having variable sizes, shapes and surface characteristics.  
       SUMMARY OF THE INVENTION  
       [0012]     In an exemplary and non-limiting embodiment of the present invention describes a method for labeling plant products based on laser activation of a color-changing compound. The method includes the steps of: conveying a plant product to plural locations, and detecting the presence of the plant product as it is conveyed. Then, a coating of a color change compound on at least a portion of a surface of the plant product as the plant product is conveyed. Then, the aforementioned coating is dried as the plant product is conveyed. Then, light is selectively applied to at least a portion of the surface coated with the color changing compound in order to create a desired label, mark or the like. Then, a protective wax coating is applied to the marked area.  
         [0013]     As a further feature of the invention, a coat of sealant is sprayed over the previously coated area of the plant product, either prior to application of the light or after development of the label, mark or the like.  
         [0014]     As yet another feature of the invention, the print quality of the mark, label or image is evaluated and an accept/reject category may be assigned. On the basis of that assignment, a plant product ejection to a predetermined location may be undertaken.  
         [0015]     As yet another feature of the exemplary and non-limiting embodiment of the present invention, the apparatus for labeling plant products based on laser activation of a color-changing compound may include a conveying system in the form of a spool, cup, belt, or the like providing an encoding pulse and constant or variable transportation speed.  
         [0016]     As yet another feature of the exemplary and non-limiting embodiment of the present invention, the apparatus for labeling plant products based on laser activation of a color-changing compound may include a drying unit consisting of a heating element and air blower.  
         [0017]     As yet another feature of the exemplary and non-limiting embodiment of the present invention, the apparatus for labeling plant products based on laser activation of a color-changing compound may include a vision system consisting of suitable light emitter, a suitable light detector, a processing unit, and a control unit.  
         [0018]     As yet another feature of the exemplary and non-limiting embodiment of the present invention, the apparatus for labeling plant products based on laser activation of a color-changing compound may include a CO2 laser with beam steering optics, and a laser control unit.  
         [0019]     As yet another feature of the exemplary and non-limiting embodiment of the present invention, the apparatus for labeling plant products based on laser activation of a color-changing compound may include a delivery system consisting in an actuated nozzle(s) attached to a tank containing color changing material and optionally an additional purging tank containing cleaning solvent; an actuated nozzle(s) attached to a tank containing a sealant; a control unit for the nozzles; and a structure to dry the sprayed coatings. The delivery system may include a heater unit to heat up the color changing material to facilitate the step of drying.  
         [0020]     As yet another feature of the exemplary and non-limiting embodiment of the present invention, the apparatus for labeling plant products based on laser activation of a color-changing compound may include a sealant system providing a protective coating or wax material or the like to the produced mark.  
         [0021]     As yet another feature of the exemplary and non-limiting embodiment of the present invention, the apparatus for labeling plant products based on laser activation of a color-changing compound may include a vision system consisting of a visible or infrared light emitter, a visible or infrared light detector, a processing unit, and a control unit. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0022]     For a better understanding of the invention, reference may be made to the accompanying drawings in which:  
         [0023]      FIG. 1  is a schematic view of an exemplary and non-limiting embodiment of the present invention;  
         [0024]      FIG. 2  is a flowchart of the logic of a preferred embodiment of the present invention.  
         [0025]      FIGS. 3A and 3B  illustrate cross-sections of a plant product with a coating of a color changing compound on an entire surface and on a portion of a surface, respectively.  FIG. 3C  depicts a label imaged on the plant product.  
         [0026]      FIGS. 4A-4C  are flow charts related to processes for compensating for various sizes and shapes of plant products. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     An exemplary and non-limiting embodiment of the present invention provides a method and apparatus for labeling plant products based on laser activation of a color changing compound placed on at least a part of the surface of the plant products. The plant product may be any suitable fruit, including non-citrus and citrus fruit, vegetable, legume, or the like, and will be generally referenced herein as a “plant product.”  FIG. 1  illustrates in schematic form a marking system  100  according to an exemplary embodiment of the present invention. The marking system includes a conveying system  114 , which may comprise one or more sections, where singled out fruit travels in a given direction  106 . In the illustrated exemplary embodiment of the invention, there is only a single section in the conveying system  114  and that section moves at a constant speed. However, as would be understood by one skilled in the art, where plural sections are involved, the conveyors in each section may travel at different speeds and the speed in each section may be controllable. The conveying system  114  can consist of at least one of a spool, cup, belt, and the like, and each section of a plural conveying section system can have different structures. Preferably, each section that is controlled to move independently at a predetermined fixed or variable speed is provided with a source of an encoding pulse  112 . In the illustrated system  100 , only a single source of a pulse  112  is provided in order to simplify the illustration and not by way of limitation. Multiple pulses and other synchronization techniques may be used, as would be known to those skilled in the art.  
         [0028]     A control unit  120  provides communication links to the rest of the marking system  100  in any suitable manner, preferably through Control Area Network (CAN), having an appropriate and conventional communication protocol. The control unit  120  also provides interfacing with the rest of the marking system  100  in any suitable manner, preferably by including a processor, memory, and software having at least one software instruction. The control unit  120  may be located close to the rest of the components of marking system  100  or in a remote location. The control unit  120  may be dedicated to a given marking system  100 , or may be centralized to serve multiple marking systems  100 . In addition, though the control unit  120  is depicted in  FIG. 1  as being a single unit, there is no requirement in the present invention that control unit  120  be assembled as an integrated whole or be assembled in such a way as to exclude components not shown, or shown outside the depicted block.  
         [0029]     In an exemplary and non-limiting embodiment of the present invention, a plant product detector  102  is provided at or proximate to the beginning of the system  100 , and may be at least one of a photodiode, a photosensor, a camera, a camera equipped with an optical filter, a CCD sensor, or any other suitable type of detector or combination of detectors. The plant product detector  102  can optionally be equipped with additional optics, including but not limited to lenses, polarizers, optical filters, a photometric unit such as a grate or prism, or the like. In an exemplary but non-limiting embodiment of the present invention, the plant product detector  102  is of the form of an area-scan camera equipped with an optical filter tuned to the frequency and/or wavelength of the light from illumination unit  104  that is reflected from the region under observation. One of ordinary skill in the art could readily utilize a line-scan camera for instance, with suitable changes to the software and optics of the marking system  100 , without departing from the spirit of the present invention. Similarly, one or more of other types of detectors with appropriate arrangements of software and optics could readily be implemented for use with the present invention.  
         [0030]     Illumination unit  104  may be at least one of a light-emitting diode (LED), a broad-spectrum lamp, a broad-spectrum lamp equipped with an optical filter, a laser, or any other suitable source of illumination, including combinations of illumination sources. The illumination unit  104  can optionally be equipped with additional optics (lenses, polarizers, or the like). Preferably illumination unit  104  is of the LED type. Additionally, it is preferred that intensity of light from illumination unit  104  is controlled by control unit  120 .  
         [0031]     In an exemplary and non-limiting embodiment of the present invention, a plant product detector  102  may be connected to a processor unit  108 , containing a memory  110 , and software having at least one software instruction. The processing unit  108  is adapted to continually receive data representing images of the conveying system  114  at the region under observation and generated by the plant product detector  102 , and includes software to determine when there is a plant product in the field of view of the plant product detector, the size of the plant product, and other information pertaining the plant product that may be pertinent to the location, size, content or type of label that is to be applied to the plant product. Memory  110 , which may be a RAM or ROM type storage, may be used to store all the label marks pertaining to a specific operation in such a manner that it can be accessed by processing unit  108  for retrieval of relevant data and/or instructions. Thus, a single label or a variety of different labels, constructed from a single data image or plural overlapping data images, may be created, as desired. Processing unit  108  selects a specific label and accesses the necessary printing data according to the plant product information identified, or according to printing data that is calculated, and sends the printing data to the laser marker  124  through the control unit  120 . Alternatively, the labels database contained in memory  108  can be sent at once to laser marker control unit  124 B or any other intermediate memory, in order to provide quicker access to the database. On the basis of the foregoing arrangement, as would be understood by one skilled in the art, multiple plant product detectors  102  and multiple illumination units  104  and the combination of them can be used to determine plant product label related features for application at a single or multiple marking stations, to track the same or different individual plant products  140 . For a single plant product  140 , a single or several images may be taken. There is no requirement in the present invention that the plant product detector  102  and related components  104 ,  108 , and  110  be of the aforementioned type. For example, in an operation where labels are always the same and plant products are of the same size, a photosensor can replace the plant product detector  102  and related components  104 ,  108 , and  110  for plant product tracking without departing from the spirit and scope of the present invention.  
         [0032]     In a preferred embodiment of the present invention, a delivery system includes at least one of a color-changing compound container  122 , a valve  130 , a nozzle  126 A, and source  150  of forced gas, preferably air for the sake of economy. Other types of gases may be used, as desired, where the environment for the application of the color changing compound necessitates use of such other gas. In the illustrated exemplary embodiment, the delivery system is located at a fixed distance  172  from the initial location  170  where the plant product detector  102  is stationed for identification of a valid plant product image in a manner known in the art. The control unit  120  keeps track of the speed of the one or more segments of the conveyor system  114  through the detection of one or more encoding pulses  112 . In the illustrated exemplary embodiment, where there is only a single conveyor section moving at a fixed speed and a single encoding pulse, the control unit  120  activates nozzle  126 A through valve  130  after a predetermined delay from the time that the pulse  112  is generated. The delivery system also includes forced air  150  whose pressure can be controlled so the amount of fluid sprayed by the nozzle  126 A can be controlled by regulating the time the valve  130  is open. Optionally, the spraying nozzle  126 A may be at least one of a single nozzle, and a combination of nozzles, a brush, a combination of the aforementioned, and any applicator that may provide a fluid on all or a desired portion of the plant product.  
         [0033]     Optionally, the spraying nozzle  126 A may be attached to an additional tank (not shown) containing flushing solvent for maintenance. The valve  130  may be controllably switched between the solvent tank and the color changing compound tank  122  in response to signals from control unit  120 . There is no requirement in the present invention that the delivery system includes a dedicated tank  122 . For example, the color-changing compound can be incorporated into the wax containers already used during normal operations without changing the scope and spirit of the present invention.  
         [0034]     The exemplary delivery system  100  also may include a forced gas blower  180 , preferably an air blower, to assist in shortening the drying time of the coated color-changing compound. As would be understood by those skilled in the art, typically, the mark quality improves if the color changing compound coating is completely dry prior to the laser applying the mark to the plant product.  
         [0035]     Optionally, a separate heater may be attached prior to the spraying nozzle  126 A. The heater (not shown) increases the color-changing compound temperature prior spraying to further assist in shortening the drying time of the aforementioned coated compound.  
         [0036]     In the exemplary embodiment of the present invention in  FIG. 1 , the marking sub-system includes at least one combination of a laser  124 A and a laser control unit  124 B. In the illustrated embodiment where there is only one marking sub-system, it is located at a predetermined distance  174  from the initial location  170  where the plant product detector  102  detected a valid plant product image. The control unit  120  keeps track of the speed of the conveyor system  114  or relevant section thereof, for example, through the use of encoding pulse  112 . Optionally, if plural conveyor sections are used, a separate pulse for each section can be employed. In this manner, the control unit  120  can activate the laser  124 A after a predetermined delay. In an exemplary and non-limiting embodiment of the present invention, a laser control unit  124 B receives encoded instructions from control unit  120  for laser triggering and for selecting the appropriate label from the label database in memory  110 . As would be understood by one skilled in the art, the laser control unit may itself have a processor and memory that can control the generation of an image based on a command from the control unit  120 . The laser can operate in one of a dot matrix mode or a continuous-wave, scribing mode. Other centralized or distributed control arrangements are encompassed by the present invention. In any event, the same or different labels, selected according to predetermined parameters for the plant products, may be applied in different sizes, colors or areas of the plant product in a controlled manner.  
         [0037]     On the basis of the foregoing logic and with reference to the arrangement in  FIG. 1 , as would be understood by one skilled in the art, multiple marking systems  124  may be used for a single plant product or for multiple conveying systems marking several independent plant products simultaneously. As already noted, the plant products may have variable shapes, sizes and surface contours. In one preferred embodiment, the laser  124 A includes beam steering optics to produce the mark on the plant product. Optionally, the mark can be produced with any other suitable image generator such as image projection or diffractive elements. In an exemplary embodiment of the present invention, the marker  124 A is of at least one of a CO2 type operating at 10600 nm with a maximum power of 10 W. It would be understood by someone skilled in the art that a different operating wavelength and power output may be used without departing of the scope and spirit of the present invention.  
         [0038]     In a preferred embodiment of the present invention, a sealant system includes at least one of a sealant compound container  128 , a valve  132 , a nozzle  126 B, and forced gas supply  150 , preferably one providing air. The sealant system may be located at a predetermined distance  176  from a location  170  where a plant product detector  102  identified a valid plant product image. In the illustrated exemplary embodiment, the control unit  120  keeps track of the conveyor system  114  speed on the basis of the encoding pulse  112 . As already noted, the speed may be steady or variable, and there may be one or plural conveyor sections that are commonly or independently monitored and controlled. In any of a variety of arrangements, the control unit  120  activates nozzle  126 B through valve  132  after a predetermined delay. The sealant system also includes forced gas source  150 , which preferably provides air, whose pressure can be controlled so the amount of fluid sprayed by the nozzle  126 B can be controlled by regulating the time the valve  132  is open. Optionally, the spraying nozzle  126 B may be at least one of a single nozzle, and a combination of nozzles, a brush, a combination of the aforementioned, and any other suitable applicator.  
         [0039]     Optionally, the spraying nozzle  126 B may be attached to an additional tank (not shown) containing flushing solvent for maintenance. The valve  132  may be controlled to switch between the solvent tank and the sealant compound tank  128  through control unit  120 . There is no requirement in the present invention that the sealant system includes a dedicated tank  128 . For example, the sealant compound can be pumped from wax containers that already are used during normal operations without changing the scope and spirit of the present invention.  
         [0040]     Optionally, the sealant system also includes at least one of a forced gas blower  180  to assist in shortening the drying time of the coated sealant compound.  
         [0041]     Optionally, a separate heater may be attached prior to the spraying nozzle  128 A. The heater (not shown) increases the sealant compound temperature prior spraying to further assist in shortening the drying time of the aforementioned coated compound. The mark quality and legibility remains longer if the sealant compound coating is completely dry prior to final packing of the plant product.  
         [0042]     In an exemplary and non-limiting embodiment of the present invention, a mark verification system includes at least one of a mark quality detector  152 , an illumination source  154 , a processing unit  158 , and a control unit  160 .  
         [0043]     In one exemplary embodiment of the present invention, a mark quality detector  152  may be at least one of a photodiode, a photosensor, a camera, a camera equipped with an optical filter, a CCD sensor, or any other suitable type of detector  152  or combination of detectors  152 . The mark quality detector(s)  152  can optionally be equipped with additional optics (lenses, polarizers, optical filters, a photometric unit such as a grate or prism, or the like). In a preferred embodiment of the present invention, the mark quality detector  152  is of the form of an area-scan camera equipped with an optical filter tuned to the illumination  154 . One of ordinary skill in the art could readily utilize a line-scan camera for instance, with suitable changes to the software and optics of the marking system  100  without departing from the spirit of the present invention.  
         [0044]     Illumination source  154  may be at least one of a light-emitting diode (LED), a broad-spectrum lamp, a broad-spectrum lamp equipped with an optical filter, a laser, or any other suitable illumination source or combination of illumination sources. The illumination source  154  can optionally be equipped with additional optics (lenses, polarizers, or the like). Preferably, illumination source  154  is of the LED type. Additionally, it is preferred that the intensity of illumination source  154  be controllable by control unit  160 , or centrally by a common control unit.  
         [0045]     In a preferred embodiment of the present invention, a mark quality detector  152  may be connected to a processor unit  158 , containing software having at least one software instruction. The processing unit  158  continually receives images from the region of the conveying system  114  under observation, as generated by the mark quality detector  152 , and includes software to determine the presence of a plant product in the detected image. The processing unit also is operative to determine at least one of a presence of a mark on the plant product, mark legibility assurance, and mark type verification. On the basis of the foregoing arrangement, as would be understood by one skilled in the art, multiple mark quality detectors  152  and multiple illumination sources  154 , and any combination of them, may be used to determine an appropriate plant product label having desired features for application at a single or multiple marking stations, and to track the same or different individual plant product(s)  140 . For a single plant product  140 , one or more images may be taken. There is no requirement in the present invention that the mark quality detector  152  and related components  154 , and  158  be of the aforementioned type, without departing from the spirit and scope of the present invention.  
         [0046]     In an exemplary embodiment of the present invention, the processing unit  158  includes software for assigning a category to the mark quality and legibility, which generally may be a reject/accept decision based on predetermined criteria. In an exemplary and non-limiting embodiment of the present invention, the processing unit  158  sends the encoded decision to the mark quality control unit  160 , which includes communication link with ejection system  116  and software for assigning the plant product to different ejection locations, according to the encoded decision.  
         [0047]      FIG. 2  illustrates a flowchart of the logic of an exemplary embodiment of the present invention as illustrated in  FIG. 1 . As already noted, however, the system arrangement is not limited thereto and there are many variations in the arrangement that can be envisioned by one skilled in the art, and the operation of the system would be defined by a logic based upon the principles in the following description. In  FIG. 2 , control begins at start block  200  and passes to first control block  202 , where the plant product detector  102  and related components detect the presence of the plant product  140 . A sequence in control block  230  is started at location  170 , Time=0. Simultaneously, control then proceeds to optional block  220  (as shown by the dotted line) where plant product relevant information is calculated. In the same optional branch, control passes to block  222  where a label, image, design, character or mark is selected from the database. Control then passes to optional block  224  where information related to the desired image is sent to the laser  124 . At the same time, control block  202  passes control to block  204 , which provides data and commands for the application of the color changing compound to at least a portion of the surface of plant product  140 , and is dried. For control block  204 , the sequence status is location  172 , Time=delay1, where delay1 is calculated from the conveying system speed and the fixed distance between positions  170  and  172 .  
         [0048]     After control block  204 , control passes to control block  206  where the label, character, image, design or mark is printed onto the color changing coating deposited on the plant product  140  by appropriate control of the laser. For control block  206 , the sequence status is location  174 , Time=delay2, where delay2 is calculated from the conveying system speed and the fixed distance between positions  170  and  174 .  
         [0049]     After control block  206 , control passes to control block  208  where the sealant compound is applied onto the plant product  140  over the label, character, image, design or mark and dried. For control block  208 , the sequence status is location  176 . Here, Time=delay3, where delay3 is calculated from the conveying system speed and the fixed distance between positions  170  and  176 .  
         [0050]     After control block  208 , control passes to optional control block  210  (as indicated by the dotted line) where the mark is verified for quality and legibility. After control block  210 , control passes to the optional accept/reject decision block  212 . If the mark is accepted the sequence status is location  178 , Time=delay4, where delay4 is calculated from the conveying system speed and the fixed distance between positions  170  and  178 . Then, the plant product may be directed to the normal operation locations or predetermined packing locations. If the mark is not accepted, control passes to control box  218  where the plant product is sent to a specific reject location, usually a wash station and ultimately back to the marking station. All the time delays aforementioned can be measured in physical time units or derived from conveyor encoded pulses.  
         [0051]     According to the foregoing description, a common conveyor or a plurality of conveyor sections operating at a constant speed is assumed. However, it would be understood by one skilled in the art that the conveyor may comprise plural sections each operating at a predetermined speed that is the same or a different speed, and each being individually controlled, in order to optimize the processing and throughput of the system or to handle processing from a plurality of sources. In such case, appropriate modification of the foregoing process would be made to detect a location of plant product at a particular conveyor section and control synchronization of speed and processing.  
         [0052]     Further, while a conveyor or conveyors that carry the plant products at arbitrary positions on the conveyor are described in the exemplary embodiment, such that a position and parameter detector is needed, one skilled in the art would understand that the plant product may be placed in holders at predetermined positions on the conveying mechanism such that the location of the plant product is pre-established and the control of various operations along the conveying mechanism would take place without the need for optical detectors.  
         [0053]     According to the foregoing description, the sealant coating is applied in block  208  after the printing step, but as would be understood by one skilled in the art, the sealant coating may be sufficiently transparent such that the printing of the label, character, design or other image may occur by transmitting the light beam through such coating. Thus, the sealant coating may be applied prior to the printing step, or may be applied both prior to and after the printing step. Moreover, the sealant step may be eliminated altogether.  
         [0054]      FIG. 3A  illustrates a cross-section of a plant product  10  with a coating  11  of a color changing compound on an entire surface and a coating of a sealant  12  over the colorant.  FIG. 3B  illustrates a cross-section of a plant product  10  with a coating  11  of a color changing compound on a portion of a surface and a coating of a sealant  12  over the entire surface. The Figures do not show the coatings drawn to scale, as would be understood by one skilled in the art.  FIG. 3C  illustrates a label developed on the plant product by laser development of a color changing coating.  
         [0055]     Plant products may be of various sizes, even for products of a given variety, and such products may vary widely in shape and surface texture. For example, oranges may have shapes that vary from perfectly round to oval or elliptical shapes, or even shapes with bulges or the like. Moreover, the sizes may vary within a certain range, yet the variation may have some affect on the focal plane of a laser. Of course, the surface of a citrus fruit, for example, may vary in texture, thereby having some impact on the manner in which the label may be applied. Thus, it would be advantageous to have the laser labeling system provide compensation for such variations, either on a group or individual piece by piece basis.  
         [0056]     One approach used for a compensation for variations in size, shape, and surface texture has been implemented. Size variation concerns only the actual distance measured from the top of the plant product to the laser printing head. To adjust for fruit size difference, the laser printing head is equipped with optics with large focal depth. In addition, on the basis of a detected actual distance by well known sensor or vision techniques, suitable signals are sent to the laser control unit to adjust intensity and adjust the laser internal marking speed setting. When there is a large variation in size, for instance lemons and grapefruit, the system may be located on a lifting mount that will change a distance uniformly for all plant products of a given type or size within a given lot or run. Thus, with reference to  FIG. 4A , in a first step S 40 A, an individual plant product is detected and in step S 41 A the distance from the laser source is determined, for example, using detector  102  in the exemplary system of  FIG. 1  or other desirably located detector. On the basis of the detected distance, in a step S 42 A, a determination is made of a distance by which the laser focal point or plane needs to be adjusted, if at all. The distance or adjustment quantity can be selected from a look-up table or similar conventional data retrieval technique in control unit  120 . In a step S 43 A, the focal length is adjusted, for example, by moving the laser  124 A mount, adjusting optics or even moving a plant product holder. Finally, in a step S 44 A, the process ends with the laser thereafter being controlled to produce the label.  
         [0057]     When there is a small to medium variation in size, the intensity of the laser may be adjusted along with a variation in laser internal marking speed setting, for an individual product or for all plant products of a given type or size within a given lot or run. Thus, with reference to  FIG. 4B , in a first step S 40 B, an individual plant product is detected and in step S 41 B the distance from the laser source is determined, for example, by detector  102 . On the basis of the detected distance, in a step S 42 B, a determination is made by control unit  120  of a value by which the intensity of the laser must be adjusted for that distance and, if necessary, the laser speed setting to ensure an appropriate clarity to the resulting image. The adjustment quantity can be selected from a look-up table or similar conventional data retrieval technique. In a step S 43 B, the intensity and laser speed are adjusted, for example, by adjusting appropriate control parameters for control unit  124 B. Finally, in a step S 44 B, the process ends with the laser intensity and scan, including direction and speed, thereafter being controlled to produce the label.  
         [0058]     Variations in shape similarly can be compensated for, at a more basic level, by use of the same optics with a large focal depth that is arranged to compensate for variations in plant product size due to the round shape of most fruits. If additional adjustment is required, delay times can be adjusted slightly to mark in the same general area, for instance at the thickest zone in a pear.  
         [0059]     With respect to variations in surface texture, citrus fruit presents a porous texture and the printing requires a fairly uniform coating. The viscosity and composition of the color changing material is modified to allow optimal spraying of the coating to fill the fruit pores. In the exemplary and non-limited embodiment of this invention, a color changing material similar to the ones described in U.S. Pat. No. 6,888,095 (hereafter referred as SWD material) was modified for the specific use in this embodiment. As received from the material manufacturer (Sherwood Technologies, Inc.), the SWD material is unsuitable for use in the exemplary embodiment of the invention. The liquid is comprised of a powder and denatured ethanol as a liquefying agent. Directly from the manufacturer it has a low viscosity (excessive ethanol) and relatively large particle size (the powder is too coarse). The excessive ethanol extends the drying time to an extent that requires very long drying time or unpractical high temperatures to adequately dry the SWD for optimal printing. The particle size diminishes the spray nozzle ability to atomize the SWD material sufficiently to apply it evenly on the surface of the plant product; it also contributes to clogging of the system once the viscosity is increased. Measured conditions of the liquid as supplied are a particle size of either &lt;70 micron or &lt;45 micron depending on how it is ordered; and a viscosity of 270 centistokes.  
         [0060]     In the tests conducted, it was found that in order to spray and dry the SWD in the exemplary embodiment of this invention the particle size must be reduced to &lt;10-microns (&lt;5 is optimal). The viscosity must also be increased to 525-550 centistokes. The process required to do this takes 5-7 days. First the particle size must be reduced. This is accomplished by placing the liquid in a vibratory tumbler with a hard; fine grain, ceramic grinding media that has been run on its own to polish the media. The SWD is added to the media with enough ethanol to reduce the viscosity to &lt;25 centistokes. At this low viscosity the powder grinds to &lt;10-micron particle size in 4-6 days. Higher viscosities will not allow the media to grind the particles below 30 microns. Following the grinding process the added ethanol must be evaporated off. To achieve this, an open topped beaker was setup along with an agitation motor, and a set of fans. Two points that must be monitored is the evaporation cannot be done during days of high humidity and the liquid must be vigorously agitated. Excess humidity causes the material to aggregate back into larger particle clusters. Slow moving areas on the surface of the liquid cause the formation of flakes in the liquid. The evaporation is continued until the viscosity of the liquid reaches a viscosity of 525-550 centistokes. After this is done the liquid is filtered through a 40-micron screen. In the tests conducted, it was found that the flakes of material are usually large (&gt;100 micron) thus this large opening screen allows the liquid to pass through and blocks any flakes that would otherwise clog the delivery system.  
         [0061]     Additional uniformity is achieved by adjusting the amount of material sprayed, for example, by adjusting the flow of liquid and air pressure in the sprayer nozzles, and also by adjusting the time the nozzle is active. Where there are variations among plant products in surface texture from lot to lot or even within the same lot, an automatic process could be used. For example, as illustrated in  FIG. 4C , an individual plant product may be presented to a detector  102  in step S 40 C and a surface texture may be detected in step S 41 C. Then a control parameter may be selected in step S 42 C on the basis of a look-up table or the like and control unit  120  may control one or more of the amount of material provided to the valve  132  for nozzle  126 B, the viscosity of the material based on a blending of different base and solvent materials, a duration of spraying, or the like. The actual adjustment of parameters for the relevant mechanisms in the spraying process by one or more of the variable parameters (nozzle, valve, etc.) would be conducted in step S 43 C. The process then ends in step S 44 C and the coating process proceeds. The resultant uniform coating may be controlled for each individual plant product or may be controlled for a group of products based on a detected surface texture value of one representative product.  
         [0062]     While the foregoing description is directed to certain exemplary embodiments, the invention disclosed herein is not limited thereto, but is to be defined by the appended claims.