Abstract:
A low ink sensing system is combined with an ink cartridge detection system to enable a more efficient ink jet printer. An ink container which supplies ink to an associated printhead is modified by the incorporation of two light directing elements, in the preferred embodiment, a curvilinear prism-like structure and a curvilinear roof mirror, into a transparent wall of the container housing. The cartridge, comprising the ink container and associated printhead, is mounted on a scan carriage. Periodically, the carriage is conveyed to a sensing station comprising a pair of light sources and a commonly used photosensor. A first light source is energized and a beam of light is directed to a location where the curvilinear roof mirror would be positioned if the cartridge is present. If the cartridge is absent, lack of a reflected return signal is sensed, indicating a cartridge has not been inserted. Print operation is halted until a cartridge is inserted. If a cartridge is properly inserted, the curvilinear roof mirror returns most of the incident light to the photosensor which generates a signal indicating the presence of the cartridge. A second light source is then energized and directed towards the curvilinear prism-like structure, which is either immersed in ink or exposed to air within the interior of the container. If the latter, light is internally reflected by the curved surfaces back to the photosensor. If a print operation has been in progress, and the ink level has fallen, the common photosensor detects either a strong or weak redirected light component and initiates a status check and generates appropriate displays of low ink level or out of ink warnings.

Description:
BACKGROUND AND MATERIAL DISCLOSURE STATEMENT 
     Cross reference is made to allowed patent application Ser. No. 09/305,990 to Altfather et al. (hereinafter “Altfather”), which is herein incorporated in its entirety for its teachings, and for which there is common assignment with the present application to the Xerox Corporation. 
     The present invention relates to ink jet recording devices and, more particularly, to a system for detecting the presence of an ink supply container and also for detecting when the level of ink in the container is at or below a predetermined level. 
     Ink jet recording devices eject ink onto a print medium such as paper in controlled patterns of closely spaced dots. To form color images, multiple groupings of ink jets are used, with each group being supplied with ink of a different color from an associated ink container. 
     Thermal ink jet printing systems use thermal energy selectively produced by resistors located in capillary filled ink channels near channel terminating nozzles or orifices to vaporize momentarily the ink and form bubbles on demand. Each temporary bubble expels an ink droplet and propels it toward a recording medium. The printing system may be incorporated in either a carriage type printer or a page-width type printer. A carriage type printer generally has a relatively small printhead containing the ink channels and nozzles. The printhead is usually sealingly attached to an ink supply container and the combined printhead and container form a cartridge assembly which is reciprocated to print one swath of information at a time on a stationarily held recording medium, such as paper. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath, so that the next printed swath will be contiguous therewith. The procedure is repeated until the entire page is printed. In contrast, the page-width printer has a stationary printhead having a length equal to or greater than the width of the paper. The paper is continually moved past the page-width printhead in a direction normal to the printhead length at a constant speed during the printing process. Moving carriage type ink jet printers must either carry the ink container along with the printhead or provide a flexible ink supply line between the moving printhead and a stationary ink container. Page-width printers have an ink supply container located outside the print zone and directly connected to the print-bar ink channels. 
     For either a partial width printhead on a moving carriage or for a page-width print-bar, it is desirable to have a low ink level warning to alert a user to replace or refill the ink container so that the ink does not run out during a print job. Presently, for some applications (such as plotting), some users choose to install new print containers prior to starting an extensive printing job because it is less costly to replace a questionable container rather than lose one or more colors in the output prints. It is also important to ensure that the ink supply container is in the proper location; e.g., fluidly connected to the associated printhead. In some instances, an out of ink container may be removed but a replacement container neglected to be inserted. Printer operation with the container removed could potentially damage the associated printhead. 
     Various prior art methods and devices are known. One that is of note here is U.S. Pat. No. 5,997,121 to Altfather et al., which discloses a low ink sensing system combined with an ink cartridge detection system to enable a more efficient ink jet printer. An ink container which supplies ink to an associated printhead is modified by the incorporation of two light directing elements, in the preferred embodiment, a faceted prism and a roof mirror, into a transparent wall of the container housing. The cartridge, comprising the ink container and associated printhead, is mounted on a scan carriage. Periodically, the carriage is conveyed to a sensing station comprising a pair of light sources and a commonly used photosensor. A first light source is energized and a beam of light is directed to a location where the roof mirror, would be positioned if the cartridge is present. If the cartridge is absent, lack of a reflected return signal is sensed, indicating a cartridge has not been inserted. Print operation is halted until a cartridge is inserted. If a cartridge is properly inserted, the roof mirror returns most of the incident light to the photosensor which generates a signal indicating the presence of the cartridge. A second light source is then energized and directed towards the faceted prism, which is either immersed in ink or exposed to air within the interior of the container. If the latter, light is internally reflected by the prism facets back to the photosensor. If a print operation has been in progress, and the ink level has fallen, the common photosensor detects either a strong or weak redirected light component and initiates a status check and generates appropriate displays of low ink level or out of ink warnings. 
     Also of note is U.S. Design Pat. No. 425,110 to Dietl et al. for an Ink Tank. Provided therein is the ornamental design for an ink tank, as shown and described. 
     Therefore, as discussed above there exists a need for a technique which will solve the problem of providing a printer which can sense it&#39;s ink cartridge and whether that cartridge has ink inside it. Thus, it would be desirable to solve this and other deficiencies and disadvantages with an improved apparatus. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an ink container comprising a housing, and a curvilinear light directing element on a wall of that housing for directing light received there away from the wall of the housing. 
     More particularly, the present invention relates to an ink container for use in a liquid ink printer comprising a housing defining a chamber for storing a supply of liquid ink. The invention further comprises an arched roof mirror comprising a first and a second curvilinear reflector on the exterior of a wall of the housing. The first curvilinear reflector substantially completely reflects light received there toward the second curvilinear reflector. The second curvilinear reflector substantially completely reflects light received there away from the wall of the housing on a light path offset from and parallel to the light path of the light received at the first reflector. 
     Further, the invention relates to a sensing system for detecting a presence of an ink container and a level of ink therein comprising a first curvilinear light directing element forming part of the ink container and a light source having output beams directed toward the first curvilinear light directing element when in an ink container detect mode. The system further comprises a first photosensor for detecting a presence or absence of light directed from the first curvilinear light directing element and for generating an output signal indicative thereof and a second curvilinear light directing element forming part of the ink container, the light source having output beams directed toward the second curvilinear light directing element when in a low ink level detect mode. Finally, the system also comprises a second photosensor for detecting light directed from the second curvilinear light directing element, the level of detected light and, hence, the level of the photosensor output being representative of a presence or absence of the ink level adjacent the interior surface of the second curvilinear light directing element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a perspective view of an ink jet printer which incorporates the ink container and low ink level sensing system of the present invention. 
     FIG. 2 is a cross-sectional view through the ink cartridge shown in FIG.  1 . 
     FIG. 3 is an algorithm which is used to sequence the checks to determine presence or absence of a container as well as level of ink within the container. 
     FIG. 4 is a block diagram of the control circuitry for controlling operation of the sensing system. 
     FIG. 5A is a cross-section of a curvilinear reflective element within the cartridge showing the prism container with a sufficient level of ink. 
     FIG. 5B is a cross-section of the curvilinear reflective element of FIG. 5A showing the reflection path in a low ink environment. 
     FIG. 5C is a cross-section of a duo-curvilinear reflective roof mirror element within the cartridge. 
     FIG. 5D is a three dimensional profile of one preferred embodiment arrangement of the invention where the two curvilinear elements are shown stacked one on top of the other rather than side by side. 
     FIG. 5E is an alternative preferred embodiment utilizing ellipses to provide a duo-curvilinear profile. 
     FIG. 6 is a plot of low ink sensing output signals versus volume of ink depleted from a cartridge. 
     FIG. 7 illustrates a perspective view of a full color ink jet printer which incorporates the ink containers and low ink level sensing system of the present invention. 
     FIG. 8 is an algorithm for the FIG. 7 embodiment which is used to sequence the presence or absence of a container and the low ink sensing sequentially. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a perspective view of a thermal ink jet printer  8  which incorporates a preferred embodiment of the ink container and low ink detection system of the present invention. Printer  8  is exemplary only. The invention can be practiced in other types of thermal ink jet printers as well as other reproduction devices such as piezoelectric printers, dot matrix printers and ink jet printers driven by signals from a document Raster Input Scanner. Printer  8  includes an ink jet printhead cartridge  10  mounted on a carriage  12  supported by carriage rails  14 . The carriage rails are supported by a frame  15  of the ink jet printer  8 . The printhead cartridge  10  includes a container  16  shown in detail in FIG. 2, containing ink for supply to a thermal ink jet printhead  18  which selectively expels droplets of ink under control of electrical signals received from a controller  50  (FIG. 4) of the printer  8  through an electrical cable  20 . Container  16  comprises a housing  17  having a wall  17 A seating reflective elements  21  and  22 , shown in further detail in FIG.  2 . Container  16  is fluidly, but detachably connected, to printhead  18  and can be replaced when the ink is depleted therefrom. Alternatively, the entire cartridge can be replaced upon each depletion depending upon the particular system requirements. The printhead  18  contains a plurality of ink channels which carry ink from the container  16  to respective ink ejecting orifices or nozzles. When printing, the carriage  12  reciprocates back and forth along the carriage rails  14  in the direction of the arrow  23 , the entire width traverse constitutes a scanning path. The actual printing zone is contained within the scanning path. As the printhead cartridge  10  reciprocates back and forth along a print path and past a recording medium  24 , such as a sheet of paper or a transparency, droplets of ink are expelled from selected ones of the printhead nozzles towards the sheet of paper. Typically, during each pass of the carriage  12 , the recording medium  24  is held stationary. At the end of each pass, the recording medium  24  is stepped in the direction of the arrow  26 . For a more detailed explanation of the operation of printer  8 , reference is hereby made to U.S. Pat. No. 4,571,599 and U.S. Pat. No. Reissue 32,572, which are incorporated herein by reference in their entirety for their teaching. 
     Also shown in FIG. 1 is an optical sensing assembly  30 . Referring to FIGS. 1 and 2, assembly  30  includes a housing  31  within which are mounted a first light source  34 , a second light source  36 , a first photosensor  37 , and a second photosensor  38  located between the two light sources and commonly used therewith as will be seen. In an alternative preferred embodiment only one photosensor is used and it is shared between the two light sources. The light sources are electrically connected to a power source while the photosensor  37  and  38  output is electrically connected into the system controller circuits as will be seen. Container  16 , in a preferred embodiment, is designed as a two compartment unit. Assembly  30  is mounted in the carriage path so that, as container housing wall  17 A moves into a position opposite the assembly  30 , the light from light source  34  is directed toward light directing element  21 , and light from light source  36  is directed toward light directing element  22 . Photosensor  37  is positioned to detect light directed from element  21  and photosensor  38  is positioned to detect light directed from element  22  in the manner described in further detail below. 
     FIG. 2 includes a cross-sectional view of the printhead cartridge  10  along the line  2 — 2  of FIG.  1  and shows the housing  17  and the printhead  18  attached to the container. The printhead  18  is fluidly but detachably connected to the container  16 . The housing  17  is made of a lightweight but durable plastic, which in a preferred embodiment, is polypropylene. Housing  17  has an air inlet  32  and an ink outlet  35  formed within wall  17 B. The air inlet  32  provides for the transfer of air between the interior of housing  17  and the ambient. Ink outlet  35  provides for fluid transfer of ink contained in the ink container  16  from the interior of the housing  17  to the ink jet printhead  18 . Manifold  37  directs filtered ink from the ink outlet  35  into printhead  18  and to the ink ejecting orifices for ejecting ink onto the recording medium  24 . 
     Housing  17  defines an interior space partitioned into a first chamber  40  and a second chamber  42  by a dividing member  44 . The dividing member  44  extends from one side wall of the housing  17  to an opposite side wall of the housing and essentially divides the housing into the first chamber  40  and the second chamber  42  such that the second chamber  42  is larger than the first chamber  40 . 
     The first chamber  40  contains an ink retaining member  46  typically made of a foam material to hold liquid ink. Liquid ink  48 , stored in the second chamber  42 , is transferred from the second chamber  42 , which is substantially free of ink retaining material, to the ink retaining material  46  through an ink inlet  41  defined by the dividing member  44 . A fill port  49  allows for filling the cartridge with ink. 
     The ink  48  passes into the ink retaining material  46  through the ink inlet  41  and ink is released through ink outlet  35  as necessary to supply the printhead  18  with ink for printing. To maintain a proper amount of ink in the ink retaining material  46  for supply to the printhead  18 , the housing  17  includes a mechanism for transferring ink from the second chamber  42  to the first chamber  40  by maintaining a proper amount of air pressure above the liquid ink  48  for filling the material  46  with ink when necessary. This mechanism includes a directing member  63 , which defines, with the dividing member  44 , an air transfer passageway  62  having a vent inlet  64  coupled to a vent outlet  66  for pressurizing the second chamber  42  to a static (no flow) condition. The directing member  63  does not extend from one side-wall to an opposite side-wall as does the dividing member  44 , but instead forms a vent tube. 
     The construction of the container  16  compartments as described to this point is exemplary. There are other known ways of constructing an ink supply container with dividing sections while maintaining an appropriate back pressure to the printhead nozzle. See, for example, the container described in U.S. Pat. No. 5,138,332 and in U.S. Pat. No. 5,742,312, both of which are incorporated by reference. For purposes of the present invention, it is understood that the container is constructed so that, during operation, ink moves from chamber  42  to chamber  40  through the passageway between the two compartments under pressure conditions established by techniques well known to those skilled in the art. Of interest to the present invention is the modification made to the ink container  16  by introducing the arch member  21  and arch roof mirror  22  to the wall  17 A defining the rear of chamber  42 . 
     Referring particularly to FIG. 2, in a preferred embodiment, light directing element  21  is a reflector integrally formed in the bottom half of wall  17 A and made of the same light transmissive material as the wall; e.g., polypropylene. Polypropylene, or other hydrophilic materials are preferred. There are two shape type alternatives for a preferred embodiment: a curvilinear shape is constructed with just one curve  21 ; or in the alternative, a duo-curvilinear shape with two curves  21 A,  21 B. These may be thought of as curved facets or curvilinear reflective members. In either case, the shape extends into the interior of compartment  42 . The curvilinear light directing element or member  21  may have an elliptical shape or even a semicircular so as to direct light from a light source  34  to a photoreceptor  37 . The duo-curvilinear shape embodiment of light directing element  21  has curved surfaces  21 A,  21 B angled generally toward each other so as to direct and focus light from an emitter  34  to a receptor  37 . Curved surface facets  21 A,  21 B may also be connected by facet surface  21 C, which itself may be straight or curved, the exact shape being unimportant so long as there is no blockage of the light path. 
     Light directing element  22  is also formed as part of wall  17 A. In a preferred embodiment, element  22  may be one of two shape types. It may have a curvilinear shape constructed of just one curve. In the alternative, it may be a duo-curvilinear shape with two curves  22 A,  22 B extending into the interior of compartment  42  and angled towards each other and connected by surface  22 C. Either arrangement may be described as a curved facet or facets. Element  22  may be made more reflective by placing reflective films, foils or tapes  22 D,  22 E on surfaces  22 A,  22 B, respectively. However, in a preferred embodiment reflective films may no longer be needed because the light gathering action of the curvilinear surfaces renders such unnecessary. In this arrangement described above, light may be directed and focused from an emitter  36  to a receptor  38 . 
     It will be appreciated from the above that only a portion of wall  17 A need be transmissive; e.g., the portion accommodating reflective element  21 . Further, while the preferred embodiment has the reflective elements constructed integrally with the housing wall, the elements could be separately positioned adjacent the interior surface of wall  17 A. 
     OPERATION OF SENSING SYSTEM 
     The sensing system of the present invention, which is considered to comprise the combination of reflective elements  21 ,  22  and the optical assembly  30 , is designed to be enabled to perform an ink container presence and a low ink level check following a specific events such as the start of a print job or after the printing of a certain amount of prints. To perform the checks, the printer follows an algorithm that requires the ink container to be positioned adjacent assembly  30  and then sequenced through a series of detection steps. FIG. 3 is one embodiment of an algorithm that can be used. FIG. 4 shows control circuitry for implementing the ink container and ink level sensing system. A main controller  50  conventionally includes a CPU, a ROM for storing complete programs and a RAM. Controller  50  controls the movement of carriage  12  as well as other printer functions described below. 
     When a line recording operation is performed, each resistor associated with a jet in printhead  18  is driven selectively in accordance with image data from a personal computer P/C  52  or other data source sent into controller  50 . Controller  50  sends drive signals to the printhead heater resistors causing ink droplets to be ejected from the jets associated with the heated resistor thus forming a line of recording on the surface of the recording medium  24 . With continued operation of the printhead, ink contained in chamber  42  of container  16  gradually becomes depleted until a level is reached which has been predetermined to constitute a low ink level. 
     For purposes of description, the sensing system will be considered as being activated, first at the beginning of a print job, and at a later time following a preset period of printer operation. 
     OPERATION AT START OF PRINT JOB 
     Referring to FIGS. 1-4, image signals from the P/C  52  to controller  50  initiate a start print sequence. Carriage  12  is moved to sensing station  41  so as to position housing wall  17 A of container  16  adjacent and facing the optical assembly  30 . Under control of controller  50 , a power source  56  first energizes light source  36 . Source  36 , in a preferred embodiment, is an LED with a peak wavelength in the range of 830 to 940 nm. A beam of light is directed towards housing wall  17 A and, if a container is present, light is reflected from surfaces  22 A,  22 B of roof mirror  22  and redirected so as to impinge on photosensor  38 . The two reflections allow the beam to be stepped vertically downward to avoid a higher than acceptable angle of incidence at the detector. The output signal from photosensor  38  is sent to logic circuitry within controller  50 , which determines that the signal is within a preset range. The controller then sequences to power the second light source  34 . 
     If a container  16  is not present, the light output of source  36  will not be reflected back to photosensor  38 . The lack of output from the photosensor will be recognized in the computer as a “container missing” status. The printer will be disabled, and a warning display will be activated at P/C Display  55  informing the user that a) printing of the color associated with the missing tank will be prevented and b) the correct container should be installed to prevent potential damage to the printhead. 
     In a preferred embodiment, light source  34  is also an LED with characteristics similar to source  36 . Source  34  emits a beam of light which is transmitted through wall  17 A and is incident on curved surface area  21 A of light directing element  21 . FIG. 5A is a cross section of light directing element  21  and a schematic reproduction of the assembly  30  showing the path of the light beam when the light directing element, here a curvilinear prism like structure is still immersed in ink and, hence, the level of ink exceeds a preset low level. 
     The low ink detection is enabled by application of the principle of total internal reflection. Total internal reflection occurs when a ray, passing from a higher to a lower index of refraction (from N to N′), has an angle of incidence whose sine equals or exceeds N′/N. The critical angle I C  is expressed by the equation: 
     
       
         I C =arc sin N′/N  (1) 
       
     
     As shown in FIG. 5A, the output beam of LED  34  passes through wall  17 A which, being polypropylene and with an index of refraction of approximately 1.492, is almost completely transparent to the light, allowing approximately 96% of the light incident thereon to pass through and be incident on curved surface area  21 A at an angle of incidence of about 45° (at the center of the curve  21 A). Since the back side of curved surface area  21 A is immersed in ink with an index of refraction of about 1.33, and the critical angle is not reached, approximately 99% of the incident light will be transmitted into the ink and at an angle of refraction of about 51.4° and only approximately &lt;1% will be reflected to curved surface area  21 B. Since the interior facing side of curved surface area  21 B is also immersed in ink, &gt;99% of the 1% will also be transmitted into the ink. Only a very small amount (approximately 0.01%) of the original incident energy will be reflected towards the photosensor  37 . The output signal from the photosensor at controller  50  will register a low light level falling outside a low ink level preset range set in controller memory. The controller will compare this signal to a previous status signal to determine whether a container, previously identified as being in a low ink situation, has been replaced or refilled. A status log is then set, or reset, to a “not empty” level, and the printhead drive circuit  61  in controller  50  is enabled to send drive signals to the printhead to initiate a print sequence. The low ink level threshold for this embodiment has been set at 20% of the container  16  fill level. 
     FIG. 5C is a cross section of light directing element  22  and a schematic reproduction of the assembly  30  showing the path of the light beam when the light directing element, here a duo-curvilinear arched roof mirror  22  is in position and thereby in combination, allows indication of the presence of an ink container  16 . The essence of the present invention is directed to an improvement of allowed patent application Ser. No. 09/305,990 to Altfather et al. (hereinafter Altfather) and its parent, U.S. Pat. No. 5,997,121, incorporated herein by reference for its teaching. In the present invention, curved or arched surfaces are utilized to both reflect light, but more importantly focus and concentrate light which would otherwise be scattered. This improves system design and operation margins by virtue of light concentration and the generally improved sensitivity to light realized thereby. It also allows a tolerance for misalignment of the ink container  16  and housing  17  relative to the printer generally, and the optical sensing assembly  30  more particularly. 
     Here in the present invention, as schematically depicted in FIG. 5C, light from emitter  36  is directed towards arch roof member  22  and curvilinear surface  22 B. This curvilinear surface focuses and reflects light towards curvilinear surface  22 A which further focuses the light and reflects it towards receptor  38 . Hence, this is a dual curvilinear type of preferred embodiment. For comparison, dashed line  500  outlines a typical arrangement as found in an Altfather embodiment. Center-line light rays  502  are still met with a 45 degree reflection angle at surfaces  22 B and  22 A (or if provided reflective foils  22 E and  22 D, respectively). However, all other rays  503  find differing angles as provided by surfaces  22 B and  22 A acting to gather and focus the light. This allows a tolerance for rotational misalignment of the container  16  in an orientational manner as indicated by line  504 . 
     There are many arrangements of curvature which will allow operation of the present invention as will be apparent to one skilled in the art. In the duo-curvilinear arrangement depicted, one approach is an arc, the radius of which is two times the distance from the emitter  36  (or receptor  38 ) to the center-line reflection point. As will be understood by those skilled in the art, there are many other arrangements of both emitter location, receptor location, and curvilinear element shape and location which may be accommodated with the application of software such as a ray-tracing program (or other design means). For example, a single ellipse shape (a mono-curvilinear example) may be utilized, or in another alternative, two curves may be used as above but separated by a flat facet between. The essence of the invention is to accomplish both the focusing of the emitter source light while also redirecting that light, and is understood to encompass the above cited examples and other derivatives as would be apparent to those skilled in the art. 
     FIG. 5D is a three dimensional profile of a preferred embodiment arrangement of the invention. Here the two duo-curvilinear elements, the arch roof mirror  22  (with reflective film  22 D and  22 E) for sensing the presence of the ink container  16 , and arch member  21  for sensing the ink level, are shown stacked one on top of the other in wall  17 A, rather than the side by side arrangement depicted in FIG.  2 . This arrangement can facilitate the use of a single emitter for both the ink container presence sensing and the ink level sensing functions. 
     FIG. 5E provides an alternative preferred embodiment utilizing dual ellipses to provide a duo-curvilinear profile. The purpose of the first ellipse  504  with given foci A 1   505  and A 2   506 , is to provide an elliptical reflector  22 B so arranged as to redirect the illumination from emitter  36  (coincident with focus A 1   505 ), towards the second elliptical reflector  22 A, The second elliptical reflector  22 A having given foci B 1   507  and B 2   508 . The destination of the reflected illumination is a point on elliptical reflector B coincident with focus A 2   506 . 
     Elliptical reflector  22 B is best able to redirect the illumination towards the desired point, A 2   506 , when one of reflector A&#39;s foci is established at the point of origin for the illumination (emitter  36 ) and the other is established at a point on the surface of the second reflector  22 A. This takes advantage of the ellipse&#39;s property that directs a ray emanating from a focus towards the other focus after specular reflection off the ellipse interior surface. That property is: the tangent at any point on an ellipse is at equal angle from the lines which connect the point and the two foci. 
     The purpose of the second ellipse  509  with foci B 1   507  and B 2   508 , is to provide second elliptical reflector  22 A so arranged as to redirect the illumination from the first elliptical reflector  22 B, originating at a point coincident with focus B 1   507 , towards a detector  38 , which is at a point coincident with focus B 2   508 . 
     Elliptical reflector  22 A is best able to redirect the illumination towards the desired point when one of it&#39;s foci is established at the point of origin for the illumination, the surface of elliptical reflector  22 B, and the other is established at a point on the detector  38 . The great appeal in this preferred embodiment is the light concentrating properties from a such a duo-curvilinear arrangement will allow the elimination of any need for reflective foils. Once again, the ellipse property is taken advantage of to achieve this preferred embodiment profile. 
     The equation for an ellipse is . . .                           
     We can determine the values of the each ellipse&#39;s dimensions, a A , b A  and a B , b B  by using the following facts . . . 
     1) Ray A 1 B 1  is parallel to ray A 2 B 2 ←design intention 
     2) Ray A 1 B 1  is perpendicular to ray B 1 A 2 ←design intention 
     3) The distances A 1 B 1 , B 1 A 2  and A 2 B 2  are all free to be chosen by the system designer. 
     4) The distance A 1 A 2  is twice the distance from the ellipse center to a focus. 
     5) The distance from the center to either focus is {square root over (a 2 −b 2 )}←property of ellipse 
     6) The sum of distances from any point on the ellipse to the foci is  2   a .←property of ellipse 
     With some algebra we can see that . . .                      A   1          B   1       +       B   1          A   2         =       2        a   A       →             a   A     =       1   /   2          (         A   1          B   1       +       B   1          A   2         )                                   
          from                 fact                   6)               1   )                       A   1          B   1   2       +       B   1          A   2   2           =         A   1          A   2       =     2            a   A   2     -     b   A   2                       from                 fact                   5)                 2   )                       A   1          B   1   2       +       B   1          A   2   2           =     2            a   A   2     -     b   A   2                                         A   1          B   1   2       +       B   1          A   2   2         =       4        [       a   A   2     -     b   A   2       ]                     squaring                 both                 sides                                     A   1                     B   1   2       +                  B   1                     A   2   2         =                4   [                       (         1   /   4            (         A   1          B   1       +       B   1          A   2         )     2       -     b   A   2                   ]                       
          substituting                   eqs.  1)                                     A   1          B   1   2       +       B   1          A   2   2         =       (         A   1          B   1   2       +     2        A   1          B   1          B   1          A   2       +       B   1          A   2   2         )     -     4        b   A   2                                         1   /   2          (       A   1          B   1          B   1          A   2       )         =     b   A                     solving                 for                   b   A                                              
     In a similar manner, we can solve for the values a B , and b B . 
     It will be apparent to those skilled in the art that a single curvilinear profile may be realized in a manner similar to the above by proper placement of the foci for a single ellipse. This placement will be where the first and second foci are coincident with the emitter  36 , and detector  38 , respectively. The resultant ellipse where it intersects the ink tank  16  will then delineate a preferred profile for a curvilinear reflector  22 . Of course, as will be apparent to one skilled in the art, the tank to emitter/detector distance will be small and place them in close proximity for a preferred embodiment. 
     To summarize the operation of the sensing system thus far, the presence of an ink container is confirmed. Further, it has been confirmed that the ink within the container is above preset levels, and therefore, a print job can be started. The ink level sensing system operation will now be described at a second time set to occur following some predetermined operational time. 
     OPERATION DURING PRINTING JOB 
     As printer  8  begins to print a print job corresponding to image input signals from P/C  52 , ink is drawn from the foam in compartment  40  (FIG. 2) thereby reducing the saturation of the foam. A flow path is created that allows ink from compartment  42  to replenish the foam. Thus, the level of ink in compartment  42  gradually falls during usage of the printer. A low ink check can be initiated at the end of each print job or after some predetermined number of pixels, e.g., 7×10 6  pixels printed for any one color since the last check. For purposes of illustration, it will be assumed that a print job has been concluded drawing down the ink level in compartment  42  to a point below a predetermined trip point level represented by dotted line  80 . A low ink level sensing procedure is initiated at this point. 
     Continued printing is interrupted and, as previously described, carriage  12  is moved to a position so that the housing wall  17 A and light directing element  21  is opposite the sensing assembly  30 . The controller again sequences through activation of light sources  34 ,  36  (the container detection may be omitted). FIG. 5B shows the effect of the low ink level on the light beam. Light from source  34  passes through wall  17 A and is incident on curved surface area  21 A at about 45°. Since the ink level has dropped below the 20% fill level, ink is no longer in contact with the back surface of curved surface area  21 A which is now exposed to air with an index of refraction of 1.0. The angle of 42.9° is exceeded by the incident light on the facet; therefore, none of the incident light is transmitted through the surface. The rays are totally reflected back into the denser media resulting in total internal reflection (TIR) of the beam. All of the incident energy is reflected towards curved surface area  21 B. Since the back of that facet is also exposed in air, all of the energy is now directed back towards photosensor  37 . About 92% of the incident energy (minus any absorption) is returned to impinge on photosensor  37 . The output signal from the photosensor is recognized by controller logic as being within a preset low ink level range. The controller performs a status check to see if the change from a previous station status is from “not empty” to “empty”. Since this is the case for the instant example, the status log memory in controller  50  is set to “empty” status and a low ink level signal is generated and may be displayed at P/C display  55 . The low ink signal can be used, depending on the system requirements, to merely display a low ink level to an operator, to halt print operation until a cartridge refill or replacement is performed or, in the preferred embodiment, to allow operation to continue but with a modified “low ink” status. As shown in FIGS. 3 and 4, the controller sends a signal to P/C  52  which displays an appropriate warning defining the ink container that has just been checked is low on ink. Each ink container contains a remaining quantity of ink which can be correlated into a number of pixels (or drops) remaining. This number may be different for each ink color. The low ink signal generated in the controller logic enables counter  60  to begin counting the number of pixels (drops) ejected from the printhead jets and the drawing down of ink within the ink tank. When the pre-established number of pixels have been counted, the ink tank is defined as out of ink, and printing is automatically disabled. The termination occurs before the tank is completely exhausted (level of about 2-5%) in order to insure that the printhead and its ink channel lines are not emptied, a condition which would jeopardize the reliability of the printhead. During the time between the first detection of low ink and declaration of out of ink, increasingly urgent messages may be displayed at the P/C display. It is understood that the pixel value of the remaining ink is dependent upon the frequency of the low ink checks. 
     The above scenario posited a condition wherein light directing element  21  was either completely immersed in ink or completely free of ink. In between these two cases is a transition represented by a monotonically increasing light level to the signal from LED  34  as the ink level gradually exposes more and more of curved surface area  21 A to air. FIG. 6 shows a plot of ink, in milliliters (ml), delivered to the printhead against sensor output in volts. For the first 70% of ink delivered, the sensor current is low, and the voltage output across a comparison circuit in controller  50  is high. Between 70 and 75% depletion, a rapid transition occurs as the LED  34  output beam begins to be totally internally reflected from curved surface areas  21 A and  21 B of light directing element  21  thus increasing the output current from sensor  37  and causing a rapid voltage drop in the circuit. 
     The invention may be used in other types of ink jet printing systems including full color printers. FIG. 7 shows a full color scanning type of printer. Referring to FIG. 7, a thermal ink jet printer  70  is shown. Several ink supply cartridges  72 ,  73 ,  74 ,  75 , each with an integrally attached thermal printhead  76  to  79 , are mounted on a translatable carriage  77 . During the printing mode, the carriage  77  reciprocates back and forth on guide rails  78  in the direction of arrow  81 . A recording medium  79 , such as, for example, paper, is held stationary while the carriage is moving in one direction and, prior to the carriage moving in a reverse direction, the recording medium is stepped a distance equal to the height of the stripe of data printed on the recording medium by the thermal printheads. Each printhead has a linear array of nozzles which are aligned in a direction perpendicular to the reciprocating direction of the carriage. The thermal printheads propel the ink droplets  81  toward the recording medium whenever droplets are required, during the traverse of the carriage, to print information. The signal-carrying ribbon cables attached to terminals of the printheads have been omitted for clarity. The printer  70  can print in multiple colors, wherein each cartridge  72  to  75  contains a different color ink supply. For a representative color printer and additional control details, see for example, U.S. Pat. No. 4,833,491, the disclosure of which is incorporated herein by reference. 
     According to the invention, each of the ink containers forming part of cartridges  72 - 75  are of the same construction as the cartridge shown in FIG. 2, and for the purposes of the invention, each cartridge has an ink container having two prism reflectors formed in the wall facing outward. One reflector is associated with cartridge presence detection and the other with low ink detection. Cartridge  72  is shown having an ink container  80  with reflective members  82 ,  84 . Cartridges  73 - 75  have similar containers and reflective members not specifically called out for ease of description. As in the single cartridge embodiment, a sensing assembly  90  includes a housing  92  within which are mounted a first light source  94  and a second light source  96  and a photosensor  98  located between the two light sources. 
     In operation and referring to FIGS. 4,  7  and  8 , image signals from P/C  52  to controller  50  initiate a start print sequence. Carriage  77  is moved so as to position the cartridge  72  with first ink container  80  opposite the sensing assembly  90 . Under control of controller  50 , power source  56  is caused to sequentially energize light sources  94 ,  96  while measuring the output of photosensor  98 . The sequencing and detection operation for cartridge  72  is the same as that previously described for cartridge  10 . Source  96  is first energized to check that the cartridge is present (reflections from roof mirror  84  to the photosensor is within range), source  94  is turned on, and the ink level in the container system is determined after making comparisons with the previous status. (Reflections from light directing element  82  front surface are sensed by photosensor  98 ). Once cartridge  72  is serviced, carriage  77  is moved to position the next cartridge  73  in position to be sensed. The preceding process is enabled for each cartridge until all cartridges have been confirmed as being in place and all ink levels in the assembly ink containers are either within the acceptable levels or appropriate low ink level warnings have been displayed at the P/C. 
     While the embodiment disclosed herein is preferred, it will be appreciated from this teaching that various alternative modifications, variations or improvements therein may be made by those skilled in the art. A more efficient arrangement is possible for example (not shown) where a single light source is associated and utilized with both of the reflective elements  21 ,  22  in FIGS. 1 and 2. 
     In another example, while the FIG.  1  and FIG. 7 embodiments show the ink container mounted on a scanning carriage which is periodically moved to a detection station, the ink containers may be positioned in a fixed location and connected to the scanning printhead via a flexible ink supply line. For the FIG. 1 embodiment, container  16  would be fixed in position opposite optical assembly  30  and connected to printhead  18  via a flexible tube. For the FIG. 7 embodiment, four optical assemblies would be located outside the print zone opposite from an associated ink container, each of the ink containers connected to the respective printhead cartridge via flexible ink couplings. For the case of a full width array printhead of the type disclosed, for example, in U.S. Pat. No. 5,221,397, a remote ink container is connected to an ink manifold which connects ink with the plurality of input modules which are butted together to form the full width array. One or more optical assemblies would be located opposite the modified ink container.