Patent Publication Number: US-8534825-B2

Title: Radiant heater for print media

Description:
TECHNICAL FIELD 
     This disclosure relates generally to imaging devices that print images on media, and, more particularly, to heaters used to condition the media during printing operations. 
     BACKGROUND 
     In general, inkjet printing machines or printers include at least one printhead unit that ejects drops of liquid ink onto recording media or an imaging member for later transfer to media. Different types of ink may be used in inkjet printers. In one type of inkjet printer, phase change inks are used. Phase change inks remain in the solid phase at ambient temperature, but transition to a liquid phase at an elevated temperature. The printhead unit ejects molten ink supplied to the unit onto media or an imaging member. Once the ejected ink is on media, the ink droplets quickly solidify. 
     The media used in both direct and offset printers may be in sheet or web form. A media sheet printer typically includes a supply drawer that houses a stack of media sheets. A feeder removes a sheet of media from the supply and directs the sheet along a feed path past a printhead so the printhead ejects ink directly onto the sheet. In offset sheet printers, a media sheet travels along the feed path to a nip formed between the rotating imaging member and a transfix roller. The pressure and heat in the nip transfer the ink image from the imaging member to the media. In a web printer, a continuous supply of media, typically provided in a media roll, is entrained onto rollers that are driven by motors. The motors and rollers pull the web from the supply roll through the printer to a take-up roll. As the media web passes through a print zone opposite the printhead or heads of the printer, the printheads eject ink onto the web. Along the feed path, tension bars or other rollers remove slack from the web so the web remains taut without breaking. 
     Regardless of the type of media used, media heating helps transfer the ink more efficiently to the recording media. In web-fed printers, media heaters typically comprise one or more radiant heaters that are positioned along the media pathway. These heaters raise the temperature of the moving web. Adjusting the power supplied to the heaters controls the output of the radiant heaters. The printing system typically includes a thermal sensor positioned adjacent the media pathway to detect the temperature of the moving web and provide the detected temperatures to a controller. The controller may compare the detected temperatures to temperature thresholds to adjust the power provided to the heaters to maintain the temperature of the media web in appropriate temperature ranges at different locations along the feed path. 
     Existing radiant heaters used in printers generate heat using high-temperature lamps with one typical lamp having a filament configured to heat to 1200° C. with a surface temperature of 800° C. In operation, these lamps emit radiant energy with a range of wavelengths including portions of the visible spectrum at approximately 0.7 μm through portions of the infrared spectrum at 1.5 μm to 2.5 μm. Some of these lamps are relatively energy inefficient, and require separate reflector elements to redirect radiant energy toward the print media to bring the print media to an appropriate temperature. The energy consumption of the radiant heaters is one factor affecting the operating cost of the printing device. Thus, improvements to radiant heaters that can heat print media while reducing the power usage of printing devices are desirable. 
     SUMMARY 
     In at least one embodiment, a radiant heater for heating a print medium in a printer has been developed. The radiant heater includes a ceramic foam substrate having a first edge and a second edge, an electrical conductor bonded to the ceramic foam substrate, and a cover layer bonded to the electrical conductor. The electrical conductor has a first electrical resistance in a first heating zone formed proximate the first edge and the second edge of the ceramic foam substrate and a second electrical resistance in a second heating zone between the first edge and the second edge to enable radiant energy at a first power density in the first heating zone and radiant energy at a second power density in the second heating zone to be emitted by the cover layer, the first power density being greater than the second power density. 
     In at least one other embodiment, a solid ink printer has been developed. The printer includes a media handling system configured to transport a continuous media web along a media pathway through the imaging device, the media pathway having a first edge and a second edge, a solid ink printing system positioned along the media pathway, a web heating system positioned along the media pathway, and a web heating controller. The solid ink printing system is configured to print images on the continuous media web moving along the media pathway. The web heating system is positioned along the media pathway at a location that enables the web heating system to heat the continuous media web after the solid ink printing system has printed an image on the continuous media web, the web heating system being configured to heat the continuous media web to a web heating temperature. The web heating system includes at least one radiant heating unit positioned adjacent the media pathway a pair of radiant heaters configured within the housing to emit radiant energy in accordance with a variable radiant power signal. The at least one radiant heating unit includes a housing adjacent to the media pathway. The housing has an opening proximate the media pathway. The pair of radiant heaters are configured to be positioned selectively in the housing to any one of a plurality of positions between and including a fully open position in which the pair of radiant heaters are positioned side by side in the opening of the housing to direct radiant energy towards the media pathway and a retracted position in which the pair of radiant heaters are positioned inside the housing and facing each other, a view factor of the pair of radiant heaters with respect to the media pathway being different for each position in the plurality of positions. Each radiant heater includes an electrical conductor bonded to a substrate, the electrical conductor forming a plurality of heating zones, a panel driver operatively connected to the pair of radiant heaters to enable the pair of radiant heaters to be positioned in at least one of the plurality of positions in response to a variable view factor signal, and at least one temperature sensor configured to detect a temperature of the continuous media web moving along the media pathway and to generate a temperature signal indicative of the detected temperature of the continuous media web. The at least one heating zone is configured to emit radiant energy at a first power density towards the first edge and the second edge of the media pathway, and at least one other heating zone configured to emit radiant energy at a second power density towards a central portion of the media pathway. The web heating controller is operatively connected to the panel driver and configured to generate a selected radiant power signal and the variable view factor signal for operation of the panel driver to position at least one radiant heater to heat the continuous media web to the web heating temperature. The web heating controller is configured to generate at least one of the radiant power signal signals and the variable view factor signals in accordance with the temperature signal generated by the at least one temperature sensor. 
     In at least another embodiment, a radiant heater panel has been developed. The radiant heater panel includes an electrical conductor having a first electrical resistance, and a cover layer configured to emit heat generated by an electrical current flowing through the electrical conductor. The emitted heat has a wavelength in a predetermined range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a phase change imaging device for printing onto a continuous media web. 
         FIG. 2  is a front view of a radiant heater panel. 
         FIG. 3  is a plan view illustrating heater element artwork used in the radiant heater panel of  FIG. 2 . 
         FIG. 4  is a cross-sectional view of a portion of the heater panel of  FIG. 2 . 
         FIG. 5  is drawing of a radiant heating unit including two heater panels in a deployed configuration. 
         FIG. 6  is a drawing of the radiant heating unit of  FIG. 5  with the two panels in an intermediate configuration. 
         FIG. 7  is a drawing of the radiant heating unit of  FIG. 5  with the two panels in a retracted configuration. 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the environment for the system and method disclosed herein as well as the details for the system and method, the drawings are referenced throughout this document. In the drawings, like reference numerals designate like elements. As used herein the term “printer” refers to any device that is configured to eject a marking agent upon an image receiving member and include photocopiers, facsimile machines, multifunction devices, as well as direct and indirect inkjet printers, laser printers, thermal printers, LED printers, and any imaging device that is configured to form images on a print medium. As used herein, the term “process” direction refers to a direction of travel of an image receiving member, such as an imaging drum or print medium, and the term “cross-process’ direction is a direction that is perpendicular to the process direction along the surface of the image receiving member. 
     The term “artwork” refers to the size, shape, pattern, and arrangement of one or more electrical conductors formed in a heater panel. The electrical conductor generates heat in response to an electrical current flowing through the conductor. The configuration of the artwork may vary in different locations through the heater panel to change the power density of radiant energy emitted from the heater panel at each location. The term “power density” refers to an amount of radiant power emitted from a given area of a heater. For example, a 1 cm 2  section of a heater that emits 10 Watts of power has a power density of 10 watts/cm 2 . As used herein, a “view factor” is defined as the proportion of the radiant energy emitted by a radiant heater that reaches a print medium in relation to the total amount of radiant energy emitted by the radiant heater. 
     As shown in  FIG. 1 , the phase change ink printing system includes a web supply and handling system  60 , a printhead assembly  14 , a fixing assembly  50  and a web heating system  100 . The web supply and handling system  60  may include one or more media supply rolls  38  for supplying a media web  20  to the imaging device. The supply and handling system is configured to feed the media web in a known manner along a media pathway in the imaging device through the print zone  18 , and past the web heating system  100 , and fixing assembly  50 . To this end, the supply and handling system may include any suitable device  64 , such as drive rollers, idler rollers, tensioning bars, etc., for moving the media web through the imaging device. The system may include a take-up roll (not shown) for receiving the media web  20  after printing operations have been performed. Alternatively, the media web  20  may be fed to a cutting device (not shown) as is known in the art for cutting the media web into discrete sheets. 
     The printhead assembly  14  is appropriately supported to eject drops of ink directly onto the media web  20  as the web moves through the print zone  18 . In alternative embodiments, the printhead assembly  14  may be configured to emit drops onto an intermediate transfer member (not shown), such as a drum or belt, for subsequent transfer to the media web. The printhead assembly  14  may be incorporated into either a carriage type printer, a partial width array type printer, or a page-width type printer, and may include one or more printheads. As illustrated, the printhead assembly includes four page-width printheads for printing full color images comprised of the colors cyan, magenta, yellow, and black. 
     The solid ink supply  24  supplies ink to the printhead assembly. Since the phase change printer  10  is a multicolor device, the ink supply  24  includes four sources  28 ,  30 ,  32 ,  34 , representing four different colors CYMK (cyan, yellow, magenta, black) of phase change ink solid ink. Alternative embodiments of the printing system  10  may be configured to print ink having a single color, or to print various ink colors other than the CYMK colors, including spot colors and clear inks. The phase change ink system  24  also includes a solid phase change ink melting and control assembly or apparatus (not shown) for melting or phase changing the solid form of the phase change ink into a liquid form, and then supplying the liquid ink to the printhead assembly  14 . 
     Once the drops of ejected ink form an image on the moving web, a fixing assembly  50  fixes the ink image to the web as the web passes through the assembly  50 . In the embodiment of  FIG. 1 , the fixing assembly  50  comprises at least one pair of fixing rollers  54  that are positioned in relation to each other to form a nip through which the media web is fed. The pressure in the nip presses the ink drops into the media web and spreads the ink on the web. Although the fixing assembly  50  is depicted as a pair of fixing rollers, the fixing assembly may be any suitable type of device or apparatus, as is known in the art, which is capable of fixing the image to the web. 
     A controller  40  operates and controls the various subsystems, components and functions of the printer  10 . The controller  40  may be implemented as hardware, software, firmware or any combination thereof. In one embodiment, the controller  40  comprises a self-contained, microcomputer having a central processor unit (not shown) and electronic storage (not shown). The electronic storage may store data necessary for the controller such as, for example, the image data, component control protocols, etc. The electronic storage may be a non-volatile memory such as a read only memory (ROM) or a programmable non-volatile memory such as an EEPROM or flash memory. Of course, the electronic storage may be incorporated into the inkjet printer, or may be externally located. The controller  100  is configured to orchestrate the production of printed or rendered images in accordance with image data received from the image data source (not shown). The image data source may be any one of a number of different sources, such as a scanner, a digital copier, a facsimile device, etc. Pixel placement control is exercised relative to the media web  20  in accordance with the print data, thus, forming desired images per the print data as the media web is moved through the print zone. 
     The web heating system  100  comprises one or more radiant heating units  200  that direct radiant energy onto the web  20 . The media web absorbs the radiant energy emitted from the units  200  at a color temperature suitable for heating the chosen media type, including a 3.0-4.0 um range for paper. Radiant heating units  200  may be positioned anywhere along the media pathway for emitting radiant energy toward the media web. In the embodiment of  FIG. 1 , radiant heating units  200  are positioned downstream from the printhead assembly  14  in order to heat the media web  20  prior to fixing the image to the web at the fixing assembly  50 . Mid-heating is a term that describes this type of heating. In other embodiments, radiant heating units  200  may also be positioned to heat the media web prior to reaching the print zone (preheating) and/or downstream from the printhead assembly (post-heating). Any suitable number of radiant heating units may be employed. The web heating system  100  in the depicted embodiment includes three radiant heating units  200  positioned upstream from the printhead assembly that preheat the media web prior to printing and two radiant heating units successively positioned thereafter that heat a front side F of the media web  20 . The web heating system  100  also includes another radiant heating unit that is positioned to heat the backside B of the media web. 
     In operation, web heating system  100  may heat the media web to any suitable temperature depending upon a number of factors including web speed, web type, ink type, position along the media pathway, etc. For example, when heating the media web, the web heating system may be configured to heat the media web and ink layers to approximately 65 to 70 degrees C. prior to fixing ink images to the web. The web heating system may include one or more noncontact IR temperature sensors  108  as known in the art for measuring the temperature of the moving web  20  at one or more locations associated with the web. Temperature sensors  108  may be non-contact type sensors, such as thermopile or similar IR sensors. In one embodiment, a temperature sensor  108 A that is provided along the media pathway upstream from the radiant heating units  200  of the web heating system detects the temperature of the web prior to the web passing the radiant heating units. Another temperature sensor  108 B may also be provided along the media pathway downstream from the radiant heating units  200  to detect the temperature of the web after the heating units heat the web. Each of the temperature sensors  108 A and  108 B may measure the temperature of the media web at various positions in the cross-process direction. These temperature measurements enable the heating controller  110  to identify whether portions of the web are inside or outside of the operational temperature range. In any case, the temperature sensors  108  are operable to relay signals indicative of the one or more measured temperatures to the web heating controller  110 . Knowing temperatures before and after the heating unit enables the controller to adjust the view factor angle as the web passes the heating units  200  to control the exit paper temperature accurately. 
     Once the heater units have reached temperatures that are sufficient to heat print media, a relatively significant delay may occur between an adjustment of electrical power supplied to the panels and a corresponding change in the radiant power output of the panels. The web heating system  100  of the present disclosure includes a dual gain control system that regulates the radiant output of the panels by adjusting the delivery of electrical power to the panels (low gain control). The system  100  also controls the amount of radiant energy that reaches the media web from the panels by varying the view factor of the panels relative to the media web (high gain control). As described below, the view factor of the radiant panels to the web may be varied by adjusting the distance, angle and/or orientation of the panels of a heating unit with respect to the media web. View factor adjustments, thus, involve physical movement of the panels with respect to the media web. Therefore, depending on the method of moving the panels, view factor adjustments may be performed relatively quickly to facilitate rapid adjustments of the amount of radiant energy that reaches the media web. 
     Another development that facilitates the delivery of heat to a web is the construction of a heater panel that generates heat having a particular wavelength.  FIG. 2  depicts a front view of such a radiant heater  204  that is suitable for use in a radiant heater unit.  FIG. 2  depicts radiant heater  204  that is configured to radiate heat onto a media web  224 . The example embodiment of radiant heater  204  is electrically connected to a three-phase electrical power source  240 . The radiant heater  204  includes three different electrical conductors, shown schematically as conductors  244 A- 244 C, that generate heat in response to electrical current from one of the phases in the three phase power source  240  flowing through each conductor. The conductors  244 A- 244 C may also be referred to as heating elements. Each of the conductors  244 A- 244 C is electrically connected to one phase of the three-phase power supply  240  through electrical leads  242 A- 242 C, respectively. Using conductor  244 A as an example, the conductor is depicted as a line that undulates across a width of the radiant heater  204  between connectors  246 A and  246 B that couple the conductor  244 A to the electrical leads  242 A. The conductor  244 A traverses the width of the heater  204  three times between the connectors  246 A and  246 B. Conductors  244 B and  244 C are configured in a substantially identical fashion to their respective connectors in radiant heater  204 . 
     Referring to  FIG. 2  and  FIG. 3 , radiant heater  204  is configured to vary the heat generated by the heater in different heating zones arranged in the heater  204 . In the example of  FIG. 2 , each of the electrical conductors  244 A- 244 C passes through heating zones  208 A,  208 B,  212 A,  212 B, and  216 . The electrical resistance of each of the electrical conductors  244 A- 244 C is determined, at least in part, by the number of bends in the conductor in each heating zone. The electrical power source  240  applies an electrical current through each of the conductors  244 A- 244 C to enable the conductors to emit radiant energy at different predetermined power densities in the different heating zones. 
     The heating zones  208 A,  208 B,  212 A,  212 B, and  216  are arranged as seen in  FIG. 2  to enable the heater  204  to heat the media web  224  in a more uniform manner. When heated by conventional media heaters, the areas of the media web  224  near the outside edges  232  and  236  to cool more quickly than areas near the center of the media web  224 . Large differences in web temperature near the edges of the web may result in gloss image changes near the edge of the media, which may result in negative effects on image quality. To address the uneven web temperature, the shape and configuration of the conductors  244 A- 244 C are varied in the different heating zones  208 A,  208 B,  212 A,  212 B, and  216  to enable the radiant heater to radiate heat at a selected power density in each heating zone. The number of bends in the electrical conductor provided by the artwork configuration of the electrical conductor in each heating zone produces different electrical resistances in the conductor at the different zones in the heater panel. Specifically, the power densities in the outer zones  208 A and  208 B are higher than the power densities produced in zones  212 A and  212 B, which are greater than the power density produced in zone  216 . Consequently, the heater watt density (flux) is increased near the edges and varies the amount of heat generated in different sections of the heater to address losses that may occur at different areas of the material being heated. 
     In  FIG. 2 , the power densities of the heating zones  208 A- 208 B and  212 A- 212 B enable these zones to deliver an amount of radiant energy to portions of the print medium near either edge of the print medium in the cross-process direction sufficient to heat those portions of the media to a temperature within an operating temperature range. The portions of the continuous media  224  web near edges  232  and  236  radiate heat more quickly than the central portions of the media web passing over heater zone  216 , and therefore tend to have lower temperatures than the central portions of the media web. The power density of radiant energy delivered to the edges of the print medium  224  enables the temperature over the width of the print medium  224  to be more uniform, with one embodiment maintaining a temperature range of 65° C. to 70° C. across the width of the print medium  224 . The higher power density also reduces the effects of convection buoyancy losses and view factor losses at the edges of the print medium. 
     As described above, the power density of heat emitted by the conductor in each heating zone is determined by the artwork of the electrically conductive heating element in each heating zone.  FIG. 2  includes an area  302  around conductor  244 A that is depicted in more detail in  FIG. 3 . As described below, the radiant heater  204  is formed from a plurality of layers, and  FIG. 3  depicts only the configuration of the conductor  244 A through heating zones  208 B,  212 B and  216  for clarity. 
     In  FIG. 3 , conductor  244 A is arranged in a sinusoidal pattern extending through the heating zones  208 B,  212 B, and  216 . The pattern in the conductor  244 A presents minor images  304  and  306  of the conductor that are electrically connected at juncture  318 . Section  308  of the conductor  244 A has the densest arrangement of sinusoidal traces in the heating zone  208 B. The bends in this dense arrangement of the conductor  244 A increases the electrical resistance of the conductor in this area, which enables the conductor section  308  to emit a larger portion of heat per unit area than the other heating zones in which the conductor has fewer turns and/or narrower traces. 
     The artwork of the conductor  244 A in the heating zone  212 B is arranged with sinusoidal traces having a lower density, and a correspondingly lower power density in the heating zone  212 B. In the configuration of  FIG. 3 , the conductor section  308  has a higher electrical resistance than conductor section  312 . Heating zones  208 A and  212 A are configured in a substantially identical manner to heating zones  208 B and  212 B, respectively. 
     Heating zone  216  includes conductor section  316 , which has the lowest relative density of sinusoidal traces, and the corresponding lowest power density. Conductor section  316  also has a lower electrical resistance per unit of length than either conductor section  308  or  312 , but a greater overall electrical resistance because the conductor length is longer in this section. The reader should note that while heating zone  216  has the lowest power density of the heating zones depicted in  FIG. 3 , the total level of radiant power (watts) that heating zone  216  emits may be larger than heating zones  208 B and  212 B due to the larger size of the heating zone  216 . 
     The arrangement of conductor  244 A seen in  FIG. 3  is illustrative of one configuration of an electrical conductor in the radiant heater  204 , but other configurations are used in other embodiments. For example, in other embodiments, the conductor is arranged in a variety of different repeating patterns through each heating zone, including squared, sawtoothed, and crossing patterns. Any arrangement of the conductor that generates heat with an appropriate power density may be used. Additionally, while conductor  244 A is arranged differently in three distinct heating zones in  FIG. 3 , other embodiments have more or fewer heating zones. In another embodiment, the conductor  244 A is arranged with a continuously varying artwork pattern that generates a correspondingly continuous power density gradient across the width of the radiant heater. 
     In one operational configuration, three-phase power source  240  supplies a one phase of a three-phase 480V electrical signal to each of the conductors  244 A- 244 C. The heating zones  208 A and  208 B have a combined surface area of approximately 29.5 cm 2 , and the segments of the electrical conductors  244 A- 244 C in heating zones  208 A and  208 B are configured to have an electrical resistance of 9.2Ω. Heating zones  212 A and  212 B have a combined surface area of approximately 32.6 cm 2  with the segments of the electrical conductors  244 A- 244 C in those zones having a resistance of 8.5Ω. The central heating zone  216  has a surface area of 403.9 cm 2  with the segments of the electrical conductors  244 A- 244 C in those zones having a resistance of 84Ω. Since each of the conductors  244 A- 244 C forms a single series circuit with the power source  240 , heating zones  208 A- 208 B emit a total of 142.2 watts of radiant power, heating zones  212 A- 212 B emit a total of 131.8 watts of radiant power, and heating zone  216  emits a total of 1297.7 watts of radiant power. Consequently, heating zones  208 A- 208 B emit radiant energy with a power density of 4.8 watts/cm 2 , heating zones  212 A- 212 B have a power density of 4.0 watts/cm 2 , and heating zone  216  has a power density of 3.2 watts/cm 2 . Thus, in this embodiment, heating zone  216  has the highest total radiant power output, while heating zones  208 A- 208 B that direct radiant energy proximate to the edges of a print media have the highest power density. 
     The radiant heater  204  in  FIG. 2  is formed from multiple layers of material that are formed into a panel.  FIG. 4  depicts a cross-sectional view of a portion of the radiant heater panel  204  taken along lines  264  in  FIG. 2 . The radiant heater  204  includes a mineral wool backing  278 , aluminum reflector member  280 , ceramic support substrate  282 , the electrically conductive heating element  244 C, and a fiberglass cover layer  292 . A first layer of epoxy  288  bonds the ceramic foam substrate  282  to the heating element  244 C, and a second layer of epoxy  290  bonds the fiberglass cover layer  292  about the heating element  244 C. As seen in  FIG. 4 , the epoxy layers  288  and  290  bond to each other and fill in gaps formed around the heating element  244 C. When an electrical current is applied to the heating element  244 C, the heating element heats the fiberglass cover layer  292  and the radiant heater  204  radiates heat toward media web  224 . 
     Support substrate  282  is embodied here as a ceramic foam panel. Ceramic foam is a porous material with numerous air pockets formed through the ceramic foam to form an efficient thermal insulator. The air in the ceramic foam and the foam itself both have low specific heat and low thermal conductivity. In the embodiment of  FIG. 4 , an aluminum heat reflector  280  is positioned next to the ceramic foam layer  282  to reflect heat generated in the radiant heater  204  toward the heater element  244 C, fiberglass cover layer  282 , and media web  224 . The mineral wool layer  278  is a fibrous material that serves as a thermal insulator positioned next to the aluminum reflector  280 . In combination, the reflector plate  280 , ceramic foam layer  282  and mineral wool layer  278  insulate one side of the heating element  244 C to reduce the amount of heat that radiates away from the media web  224 . In the example embodiment of  FIG. 4 , the top surface  276  of the mineral wool layer  278  is approximately 400° C. cooler than the bottom surface  294  of the fiberglass cover layer  292  when the radiant panel  204  heats to an operational temperature. The ceramic foam substrate  282 , aluminum reflector  280 , and mineral wool  276  are illustrative of one configuration for containing heat within the panel  204 . Alternative embodiments include different materials and insulator configurations that are suitable for use in the operating temperature range of the radiant heater. 
     The heating element  244 C generates heat in the radiant heater  204  when an electric current passes through the heating element. Epoxy layer  288  bonds the heating element  244 C to the substrate layer  282 . In the example embodiment of  FIG. 4 , the heating element  244 C is formed from a metallic alloy that is commercially available under the Inconel® brand name. Other materials suitable for use as electrically conductive heating elements may be used in alternative radiant heater configurations. As described above, the shape and configuration artwork of the heating element  244 C changes between different heating zones in the radiant heater  204 . In some embodiments, the thickness of the heating element  244 C also varies to adjust the heat output of the heating element in different heating zones within the radiant heater panel  204 . 
     A fiberglass cover layer  292  is bonded to the heater element  244 C and substrate layer  282  by epoxy later  290 . This fiberglass layer  292  absorbs and radiates the heat generated by the conductors of the heater panel. In one embodiment, the epoxy later  290  permeates a porous fiberglass material to form a fiberglass-epoxy matrix for the fiberglass cover layer  292 . A fiberglass mesh such as a fiberglass scrim cloth is one form of fiberglass that forms a matrix with the epoxy. The fiberglass cover layer  292  emits heat through a bottom surface  294  with wavelengths of greater than 3.0 μm. In the embodiment of  FIG. 4 , the majority of the generated heat has a wavelength in the 3.0 μm to 4.0 μm range that corresponds to the infrared portion of the electromagnetic spectrum. Various materials commonly used in print media, such as paper, water, and wax, efficiently absorb wavelengths of heat in the infrared range of the electromagnetic spectrum. In other embodiments, the epoxy layer  290  is formed from a dark or black colored epoxy that permeates the fiberglass cover layer  292  and gives the fiberglass cover layer  292  a dark or black color. The dark or black color promotes emission of radiant heat energy with the selected wavelengths. 
     The radiant heater  204  emits radiant energy concentrated at wavelengths that heat print media efficiently and selectively concentrate the radiant energy on portions of the print media that lose heat more quickly. Thus, radiant heater  204  heats print media to an operational temperature range more efficiently than previously known heaters, and the radiant heater  204  operates with a lower electrical energy input than previously known heaters since the print medium  224  absorbs a portion of the radiant energy emitted from radiant heater  204  that is sufficient to heat the medium to an operating temperature. 
     With reference to  FIG. 2 , thermocouples  248  are bonded between one of the electrical conductors  244 A,  244 C and the cover layer  292  to generate electrical signals that correspond to the temperature of the heater  204 . In operation, a heater controller monitors the thermocouples  248  to identify the temperature and corresponding radiant power level of the heater  204 . The heater controller increases or decreases the voltage level applied to the heater  204  to increase or decrease, respectively, the total radiant power output of the heater  204 . The variable voltage applied to the panels is a variable radiant power signal, and the heater controller selects different variable voltage levels to apply low gain control to the heater panel. The thermocouples  248  are used in conjunction with various media temperature sensors, such sensors  108 A and  108 B shown in  FIG. 1 , to enable this feedback control of the heater. 
     The configuration of the radiant heater  204  depicted in  FIG. 2  is merely exemplary, and alternative heaters may employ greater or fewer heating zones with larger or smaller surface areas, power densities, and total radiant power output levels. Various alternative embodiments of radiant heater  204  include one or more heating zones energized with different electrical currents to produce selected levels of radiant power. Other alternative embodiments form a continuous power density gradient across the radiant heater instead of providing discrete heating zones. While radiant heater  204  is depicted using three electrical conductors that are connected to a three-phase electrical power source, alternative embodiments generate radiant energy using one or more conductors in response to receiving both alternating current (AC) having different phases and direct current (DC) electrical signals. 
       FIG. 5-FIG .  7  depict a radiant heating unit  200  with heater panels  402 A and  402 B arranged in three different positions. Radiant heating unit  200  includes two radiant heaters  402 A and  402 B, a panel driver  410 , shown here as actuators  404  and gas springs  408 , a drive link  412 , drive path  416 , and rotatable arms  420 . A print medium, shown here as a continuous media web  424 , travels in a process direction P past the radiant heating unit  200 . The embodiment of radiant heater  204  from  FIG. 2  may be used for the radiant heaters  402 A and  402 B, with the heating zones configured to deliver heat to the side edges of the media web  424  being arranged parallel to the process direction P. Drive link  412  includes two arms that are each rotatably engaged to actuator  404  and gas spring  408  at a first end and to a slidable member  414  at a second end. The slidable members  414  engage heaters  402 A and  402 B, and are configured to move along the drive path  416  in directions  432  and  430 . Actuators  404  may be pneumatic, hydraulic, or electromechanical devices configured to move the drive link  412  and slidable members  414  along the drive path  416  in direction  430 . Gas springs  408  are configured to generate a compressing force that urges the drive link  412  along the drive path  416  in direction  432 . Thus, actuators  404  and gas springs  408  apply opposing forces to the drive link  412 , and panel driver  410  may operate the actuators  404  and gas springs  408  to move heaters  402 A and  402 B to various positions with respect to the continuous media web  424 . In the configuration of  FIG. 5 , radiant heaters  402 A and  402 B are arranged in parallel to the continuous media web  424 . This configuration produces the maximum view factor for the heater unit  200  with substantially all of the radiant energy emitted from the radiant heaters  402 A and  402 B being directed towards continuous media web  424 . 
     In operation, heating unit  200  receives variable radiant power signals and variable view factor signals from a controller, such as heater controller  110  described above. The variable view factor signal may be an electrical signal that directs actuators  404  to exert a predetermined amount of force in direction  430 . The predetermined amount of force counteracts the forces exerted by gas springs  408  in direction  432 . In the configuration of  FIG. 5 , the variable view factor signal may direct the actuators  404  to exert no force. In the configuration of  FIG. 6 , actuators  404  exert a force sufficient to raise drive links  412  to an intermediate position along drive path  416 . The slidable member  414  pulls one end of each radiant heater  402 A and  402 B to the intermediate position. In response to the movement of slidable member  414 , the opposite ends of heaters  402 A and  402 B move with rotatable arms  420  in directions  436  and  440 , respectively. 
     As seen in  FIG. 6 , radiant heaters  402 A and  402 B are positioned at an acute angle with respect to the continuous media web  424 . This angle directs a fraction of the radiant energy emitted towards the media web  424  in the configuration of  FIG. 5  away from the media web. This action reduces the view factor presented by the heating unit. In  FIG. 7 , the controller operates the actuators to retract the radiant heaters  402 A and  402 B to a position that is substantially perpendicular with respect to continuous media web  424 . In the retracted position the view factor cancels each panels radiant energy allowing the delivered power to be reduced to simmer at roughly 15 to 25% of the actual run power controlled by the thermocouple imbedded in the panel. The simmer power establishes the correct panel temperature prior to unfolding to reduce the time necessary to achieve the correct surface temperature for running. The panel duty cycle is controlled from the IR sensor on the paper during run. The panel device moves to retracted or closed position fast enough to prevent the web from achieving  300 C surface temperature when the web is not moving. Additionally, the rotatable arms  420  enable radiant heaters  402 A and  402 B to slide together in directions  436  and  440 , respectively. 
     In the configuration of  FIG. 7 , a minimal amount of radiant energy reaches the continuous media web  424  as the heater panels  402 A and  402 B shield most of the emitted radiant energy to establish a minimal view factor with respect to the continuous media web  424 . The controller uses the variable view factor signal to operate the actuator  404  in the panel driver  410  to position the radiant heater panels  402 A and  402 B at various intermediate positions between the configurations of  FIG. 5  and  FIG. 7 . Panel driver  410  is configured to move heater panels between any of the configurations shown in  FIG. 5-FIG .  7 , as well as to various intermediate positions. 
     The embodiment of heater unit  200  depicted in  FIG. 5-FIG .  7  is illustrative of only one heater unit configuration. Various alternative embodiments may use a single heater panel, or use three or more heater panels in various configurations. Some embodiments may use heaters having different surface areas, heating zones, power outputs, and power densities in a single heater unit. The view factor may be also be adjusted in different ways. In one alternative embodiment, the view factor may be adjusted by moving the heaters in a linear direction to position the panels closer or farther from the media web. In another embodiment, a shielding member may be selectively positioned between a portion of the heaters and the media web to block a portion of the radiant energy from reaching the media web. Alternative mechanical drive units and drive link arrangements known to the art may be used to adjust the positions of the heaters. 
     It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems, applications or methods. For example, while the heater panels and radiant heater units depicted above are shown in the context of an inkjet printer, the foregoing heaters are suitable for heating print media to various operating temperatures in various embodiments of printers other than inkjet printers. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements may be subsequently made by those skilled in the art that are also intended to be encompassed by the following claims.