Patent Publication Number: US-2009223290-A1

Title: Optically monitoring fullness of fluid container

Description:
BACKGROUND 
     A variety of technologies exist for measuring the relative fullness of a fluid container. For example, a dipstick can be physically dipped into a fluid container to determine the fluid level of the container. As another example, a float may be connected to a variable resistor that changes resistance as the float moves with the changing fluid level in the fluid container. As still another example, a fluid container, such as a measuring cup, may include a series of calibrated markings arranged along a sidewall of the fluid container, and the markings can correspond to the volume of fluid in the fluid container. 
     SUMMARY 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
     The relative fullness of a fluid container can be optically monitored. In particular, reference light can be directed at a bottom surface of the fluid container. The fluid container can be designed so that it varies the amount or pattern of reflected reference light in relation to the relative fullness of the fluid container. Accordingly, the relative amount or pattern of reference light reflected from the fluid container can be measured and correlated to the relative fullness of the fluid container. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a fluid-monitoring system for monitoring a relative fullness of a fluid container in accordance with an embodiment of the present disclosure. 
         FIG. 2  shows an example fluid container in accordance with an embodiment of the present disclosure. 
         FIG. 3  shows another example fluid container in accordance with an embodiment of the present disclosure. 
         FIG. 4  shows another example fluid container in accordance with an embodiment of the present disclosure. 
         FIG. 5  shows another example fluid container in accordance with an embodiment of the present disclosure. 
         FIG. 6  shows an example reflective identification-pattern on a bottom surface of a fluid container in accordance with an embodiment of the present disclosure. 
         FIG. 7  shows another example reflective identification-pattern on a bottom surface of a fluid container in accordance with an embodiment of the present disclosure. 
         FIG. 8  shows a fluid-monitoring system presenting a drink-refill message to a guest in accordance with an embodiment of the present disclosure. 
         FIG. 9  shows a fluid-monitoring system sending a drink-refill message to a computer in accordance with an embodiment of the present disclosure. 
         FIG. 10  shows a fluid-monitoring system sending a drink-identification message to another fluid-monitoring system in accordance with an embodiment of the present disclosure. 
         FIG. 11  shows a process flow of an example method of optically monitoring a relative fullness of a fluid container. 
         FIG. 12  shows a surface computing device capable of monitoring a relative fullness of a fluid container in accordance with an embodiment of the present disclosure. 
         FIG. 13  shows another surface computing device capable of monitoring a relative fullness of a fluid container in accordance with an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a nonlimiting example of a fluid-monitoring system  10  and fluid container  12 . As described in more detail below, fluid-monitoring system  10  can optically monitor a relative fullness of fluid container  12  by directing reference light at a bottom surface of the fluid container and measuring a relative amount or pattern of reference light reflected from the fluid container. The amount or pattern of reflected reference light changes based on the fullness of the fluid container. Therefore, the relative amount or pattern of reference light reflected from the fluid container can be correlated with a relative fullness of the fluid container. 
     Fluid-monitoring system  10  includes a surface  14 , a light source  16 , a sensor  18 , an image-generation subsystem  20 , an analysis subsystem  22 , and a communication subsystem  24 . 
     Surface  14  is orientated to support fluid container  12 . For example, surface  14  may be a tabletop, a bar top, a countertop, a dining table, a café table, a shelf, or virtually any other surface capable of supporting a fluid container. The surface may be orientated substantially horizontally, although other orientations are possible. While shown as a substantially planar surface, non-planar surfaces also may be used. 
     The size of a surface can be varied tremendously. For example, a surface may be sized to support a single fluid container, or a surface may be sized to support a plurality of different fluid containers. 
     Surface  14  may optionally be a display surface capable of presenting static or dynamic images. As a nonlimiting example, surface  14  may be a light-transmissive rear projection screen capable of presenting images projected from behind the surface. The fluid-monitoring system may utilize image-generation subsystem  20  for projecting images onto surface  14 . In such embodiments, surface  14  may include a clear glass or plastic layer, one or more diffusion layers, and/or an at least partially colored, tinted, or opaque layer. In some embodiments, surface  14  may not have any display functionality. 
     Light source  16  is positioned to direct reference light  26  at fluid container  12  from behind surface  14 . In other words, reference light travels through surface  14  before reaching fluid container  12 . 
     Light source  16  may project any suitable wavelength, including but not limited to infrared and visible wavelengths. The reference light projected from light source  16  may have a single wavelength or may be comprised of two or more different wavelengths. While light source  16  is illustrated as a single device, a plurality of different devices may be cooperatively used to project the reference light. Further, while light source  16  is illustrated as being located substantially directly below surface  14 , a light source may additionally or alternatively be located at one or more sides of surface  14 , or at virtually any other suitable position. Embodiments that include an image-generation subsystem may optionally utilize light source  16  to generate light for projecting images onto surface  14 . 
     Sensor  18  can detect reference light that is reflected from fluid container  12 . The sensor is operatively positioned behind surface  14 . In other words, reference light reflected from fluid container  12  travels through surface  14  before reaching sensor  18 . 
     In many conditions, a high percentage of reference light projected by light source  16  is transmitted through surface  14  without being reflected. However, an object at or near the surface may reflect the reference light. Reflected reference light may be detected by sensor  18 . In some embodiments, sensor  18  may be able to identify a position of an object at or near surface  14  based on the location from which reference light reflects. 
     The properties of an object can affect how much reference light is reflected by the object. As a nonlimiting example, an opaque object touching the surface may reflect a relatively high percentage of reference light back to the sensor, while a transparent object touching the surface may transmit a relatively high percentage of the reference light without reflecting the reference light to the sensor. 
     As described in more detail below, fluid container  12  can be configured to reflect two or more different relative amounts or patterns of reference light based on a relative fullness of the fluid container. In other words, the fluid container may reflect relatively more reference light when full than when empty, or vice versa. As a result, the amount or pattern of reflected light can be measured to determine a relative fullness of the fluid container. 
     While sensor  18  is illustrated as a single device, a plurality of different devices may be cooperatively used to measure reflected reference light. Further, while sensor  18  is illustrated as being located substantially directly below surface  14 , a sensor may additionally or alternatively be located at one or more sides of surface  14 , or at virtually any other suitable position. 
     By positioning both light source  16  and sensor  18  behind surface  14 , the fluid-monitoring system can serve as an unobtrusive device well suited for incorporation into a variety of different usage environments. Additionally, neither the light source nor the sensor physically interfere with use of the surface, as both are substantially hidden behind the surface. In some embodiments, an area behind the surface can be at least partially environmentally sealed, thus providing protection to the light source, sensor, and other components located behind the surface. Such protection may lessen potential damage that could be caused by spilled contents of the fluid container. 
     While described above in the context of monitoring a relative fullness of a single fluid container, it should be understood that a plurality of different fluid containers may be monitored by the same fluid-monitoring system and/or by a plurality of different fluid-monitoring systems cooperating with one another. 
     Communication subsystem  24  may include one or more wired or wireless interfaces for communicating with other devices. As nonlimiting examples, and as described in more detail below, the communication subsystem can send messages relating to a relative fullness of a fluid container or an identity of a fluid container. Such messages may be sent to any suitable message recipient, such as a computer that tracks beverage sales and service at a bar or restaurant. The communication subsystem may send messages via IEEE 802.11x, IEEE 802.15.x, IEEE 802.3, or other suitable communication technologies, in virtually any suitable format. 
     Analysis subsystem  22  can include analog and/or digital components for determining a relative fullness of the fluid container. In particular, the analysis subsystem can analyze a relative amount or pattern of reference light reflected from a fluid container, as measured by sensor  18 . Such analysis may include correlating the relative amount or pattern of reference light reflected from the fluid container with a relative fullness of the fluid container. To facilitate such a correlation, the analysis subsystem may be preconfigured to recognize certain magnitudes of reflected light, certain patterns of reflected light, and/or other characteristics of reflected light as corresponding with a particular fullness of a fluid container. The analysis subsystem may utilize one or more lookup tables or other such data structures for correlating a relative amount or pattern of reflected light with a relative fullness of a fluid container. 
     The analysis subsystem may optionally include a processing subsystem  28  and computer-readable media  30 . The processing subsystem may include one or more general processing units, application specific integrated circuits, or other devices capable of performing logical operations. The computer-readable media may include one or more volatile and/or nonvolatile memory devices for storing and/or temporarily holding instructions that can be executed by the processing subsystem to perform logical operations defined by the instructions. In other words, the computer-readable media may include instructions, that when executed by the processing subsystem, perform one or more routines that assist in the optical monitoring of one or more fluid containers. As a nonlimiting example, the computer-readable media may include instructions that correlate a relative amount or pattern of reference light reflected from a fluid container with a relative fullness of the fluid container. 
     The herein described instructions may include source code instructions, object code instructions, machine code instructions, system-level software instructions, application-level software instructions, instructions embedded in firmware, instructions embedded in hardware, or virtually any other type of executable instructions. 
     In some embodiments, a portion (i.e., some to all) of the processing subsystem and/or the computer readable media may be remotely located relative to surface  14 . As such, some of the instructions may be stored, temporarily held, and/or executed remotely. 
     Fluid container  12  defines a fluid-holding space  32  for holding a fluid. In the illustrated embodiment, fluid container  12  is a pint glass, and fluid-holding space  32  is partially filled with a clear liquid  34 , such as water. Liquid  34  may be referred to as a test fluid, because fluid container  12  and fluid-monitoring system  10  are cooperatively configured to test the level of the liquid within the fluid container. As used herein, a test fluid is any fluid that can be monitored by a fluid-monitoring system and fluid container. A fluid-monitoring system and/or a fluid container can be adapted to test a wide range of different fluids. 
     Furthermore, the herein described concepts may be applied to virtually any fluid container, including, but not limited to, beakers, measuring cups, beer steins, tankards, flagons, chalices, goblets, coffee cups, mugs, sake cups, shot glasses, teacups, Collins glasses, highball glasses, pony glasses, dinner glasses, coolers, pilsner glasses, tumblers, champagne flutes, cocktail glasses, sherry glasses, wine glasses, snifters, bottles, cans, bowls, punch bowls, and pitchers. 
     Fluid container  12  includes a light-transmissive bottom  36  and a light guide  38 . The light-transmissive bottom is configured to pass reference light to light guide  38 . As such, at least a portion of the light-transmissive bottom is constructed so that reference light originating at light source  16  may enter light guide  38  before being reflected to sensor  18 . In some embodiments, the light-transmissive bottom may be shaped so as to closely mate with surface  14 , thus creating a substantially gap-free path for reference light travelling from surface  14  to light-transmissive bottom  36 . As a nonlimiting example, the surface and the light-transmissive bottom may both be substantially flat. In some embodiments, the light-transmissive bottom may be constructed from a material that has an index of refraction that is similar to the index of refraction of the surface. It should be understood that the light-transmissive bottom may only account for a portion of the total bottom surface of the fluid container, and other portions of the bottom surface may be light absorbing and/or light reflecting. 
     Light guide  38  includes an end portion  40  and, in some embodiments, a guide section  42 . When included, the guide section is located intermediate the light-transmissive bottom and the end portion. The guide section directs reference light between the light-transmissive bottom and the end portion. The guide section is designed to limit the amount of reference light that escapes between the light-transmissive bottom and the end portion. In some embodiments, guide section  42  may include an internally reflective surface that helps keep reference light from escaping. In some embodiments, the guide section may be constructed from a material that encourages total internal reflection of reference light traveling through the guide section. 
     The length of the guide section can be selected to place end portion  40  at a desired level within fluid-holding space  32 . In some embodiments, the guide section can be substantially straight, as illustrated in  FIG. 1 . In other embodiments, the guide section can bend, twist, or otherwise deviate from a straight course. The course of the guide section may be selected to produce fluid containers with a functional shape and/or a pleasing aesthetic. 
     As can be seen in  FIG. 1 , end portion  40  of light guide  38  projects into fluid-holding space  32  of fluid container  12 . In the illustrated embodiment, the end portion projects substantially vertically into the fluid-holding space. In other embodiments, the end portion may project substantially horizontally into the fluid-holding space, or project at a skewed angle into the fluid-holding space. 
       FIG. 2  shows a cross-sectional view of fluid container  12  holding two different amounts of liquid  34 . On the left, liquid  34  is at a higher level  44  in the fluid container, and on the right, liquid  34  is at a lower level  46  in the fluid container. 
     As demonstrated on the left, end portion  40  of fluid container  12  is configured to pass reference light  26  to a test fluid (e.g., liquid  34 ) when the end portion is submerged in the test fluid. However, as demonstrated on the right, end portion  40  of fluid container  12  returns reference light  26  to light-transmissive bottom  36  when the end portion is not submerged in the test fluid (e.g., liquid  34 ). As such, the amount or pattern of reference light reflected by the fluid container changes as the fullness of the fluid container changes. The position, shape, and/or material of light guide  38  can be selected so that the amount or pattern of reference light that is returned to light-transmissive bottom  36  changes as a function of fluid container fullness. 
     End portion  40  may include two or more opposing reflection faces. As illustrated, the end portion includes reflection face  48  and reflection face  50 . As can be seen in  FIG. 1 , end portion  40  has a conical shape. As used herein, “opposing reflection faces” includes opposing sides of a substantially continuous conic surface, or another substantially continuous surface. In general, “opposing reflection faces” can include virtually any two serially reflecting surfaces. In other embodiments, an end portion may have a pyramidal, dome, wedge, or other suitable shape. 
     In the illustrated embodiment, reflection face  48  and reflection face  50  are orientated at approximately a right angle relative to one another and at approximately a forty-five degree angle relative to an optical axis of the light guide (e.g., the direction light travels through the light guide). These angles are nonlimiting. The angle of the reflection faces relative to the optical axis can be selected so as to promote total internal reflection when the end portion of the light guide is not submerged, but to allow light to pass out of the light guide when the end portion is submerged. Light may encounter the reflection faces from a range of different angles, and the angle of the reflection faces can be selected accordingly to promote total internal reflection under desired submersion conditions. 
     Total internal reflection occurs when light encounters a boundary between different materials at an angle greater than a critical angle. When light does not encounter the boundary at an angle greater than the critical angle, the light will be partially refracted and partially reflected at the boundary. However, refraction will stop and all light will be internally reflected if the critical angle is exceeded. The ratio of the refractive index of the less dense medium compared to the refractive index of the denser medium determines the critical angle at the boundary between the different mediums. 
     The refractive index of the light guide may be greater than the refractive index of air, which is very close to 1.0. As a nonlimiting example, the light guide may be constructed from glass or polycarbonate, which are characterized by refractive indices of approximately 1.5 to 1.6. As such, the critical angle at the reflective face may be approximately thirty-nine degrees to forty-two degrees. Therefore, in such an embodiment, if reference light encounters the reflection face at greater than approximately thirty-nine to forty-two degrees, the reference light will be totally internally reflected. It should be understood that other materials may be used to construct the light guide, and such materials may result in a different critical angle. 
     The refractive index of the light guide may be closer to the refractive index of one or more different test fluids than to the refractive index of air. As such, when reference light traveling through the light guide encounters an end portion that is submerged in a test fluid with a similar refractive index, light may pass out of the light guide into the test fluid. For example, assuming a refractive index of 1.3 for the test fluid and a refractive index of 1.5 for the light guide, the critical angle is approximately sixty degrees. Therefore, the reference light will not be totally internally reflected unless it encounters the reflection face at an angle greater than sixty degrees. In the illustrated embodiment, the reference light encounters the reflection face at approximately forty five degrees. Accordingly, total internal reflection does not occur. Relatively more reference light escapes from the light guide when the light guide is submerged. 
     Reference light that is totally internally reflected at the end portion of the light guide may return through the light guide back to the light-transmissive bottom. As such, the reflected reference light may be measured by a sensor of the fluid-monitoring system. Furthermore, the relative amount or pattern of reference light reflected from the fluid container can be correlated with a relative fullness of the fluid container. 
       FIG. 3  shows a cross-sectional view of another fluid container  52  holding two different amounts of a liquid  54 . On the left, liquid  54  is at a higher level  56  in the fluid container, and on the right, liquid  54  is at a lower level  58  in the fluid container. 
     Fluid container  52  includes a light-transmissive bottom  60  and a light guide  62 . In the illustrated embodiment, light guide  62  travels outside of a fluid-holding space  64  of the fluid container. Further, light guide  62  bends so as to project an end portion  66  substantially horizontally into fluid-holding space  64  of the fluid container. 
     End portion  66  is configured to return reference light to light-transmissive bottom  60  when the end portion is not submerged in a test fluid (e.g., liquid  54 ). For example, as shown on the left, reference light  68  passes from end portion  66  when the end portion is submerged in liquid  54 . However, as shown on the right, reference light  68  is totally internally reflected by end portion  66  when the end portion is not submerged in liquid  54 . The reference light is illustrated as taking a curved path through light guide  62  for simplicity. It should be understood that the reference light may reflect off the interior faces of the light guide when travelling through the light guide. 
     A fluid container may include a plurality of different light guides. For example,  FIG. 4  shows a cross-sectional view of a fluid container  70  that includes light guide  72  and light guide  74  projecting into fluid-holding space  76 . Fluid container  70  is holding three different amounts of a liquid  78 . On the left, liquid  78  is at a higher level  80  in the fluid container, in the middle, liquid  78  is at an intermediate level  82  in the fluid container, and on the right, liquid  78  is at a lower level  84  in the fluid container. 
     A light guide can be placed to monitor a particular fullness level of the fluid-holding space. If a usage scenario benefits from monitoring different levels, light guides can be calibrated to such levels. With added light guides, a fluid-monitoring system may more accurately monitor a relative fullness of a fluid container. For example, light guide  72  projects to a relatively high level, and light guide  74  projects to a relatively low level. As can be seen on the left, when light guide  72  and light guide  74  are both submerged, reference light  86  escapes both light guides into the test fluid (e.g. liquid  78 ). As shown in the middle, reference light  88  is reflected by light guide  72  when it is not submerged, but reference light  90  escapes light guide  74  because it remains submerged. As shown on the right, reference light  92  is reflected from both light guide  72  and light guide  74  when neither light guide is submerged. As such, fluid container  70  reflects at least three different amounts or patterns of reference light depending on a relative fullness of the fluid container. 
     It should be understood that while fluid container  70  is shown with two different light guides projecting to two different levels of the fluid-holding space, a fluid container may be constructed with virtually any number of different light guides. 
     Furthermore, the light guides can be positioned at locations other than those shown in the presented example embodiments. For example, one or more light guides may be positioned adjacent a sidewall of the fluid container or integrated into a sidewall of the fluid container. Furthermore, as shown in  FIG. 3 , one or more light guides may be positioned to project substantially horizontally into the fluid-holding space. 
       FIG. 5  shows a cross-sectional view of another fluid container  94  holding three different amounts of a liquid  95 . Fluid container  94  includes a light-transmissive bottom  96  and a light guide  98  including an end portion  100 . Light guide  98  is configured to vary a ratio of reference light returned to the light-transmissive bottom and reference light passed to the test fluid responsive to a change in a submersion level of the end portion in the test fluid. In other words, more reference light is returned to the light-transmissive bottom as the fluid container is emptied. As shown in  FIG. 5 , more of the light guide becomes exposed to air as the fluid container empties. Therefore, more reference light is totally internally reflected by the light guide as the fluid container is emptied. Accordingly, relatively more reference light is returned to light-transmissive bottom  96 . This provides a somewhat analog measurement of a fullness of fluid container  94  throughout the range covered by end portion  100  of light guide  98 . Such analog measurements may be possible with any light guide having an end portion that covers a range of different fluid levels within a fluid-holding space of a fluid container. 
     The herein described fluid containers may be constructed using conventional manufacturing techniques. The light guides can be molded or otherwise integrated into the fluid containers in any suitable manner. There need not be any moving parts or electronics, thus making the fluid containers inexpensive, durable, and compatible with standard washing and handling methods. 
     In some embodiments, a fluid container may include an identifier so that the fluid container can be identified by a fluid-monitoring system. As a nonlimiting example, a bottom surface of the fluid container may include a reflective identification-pattern. 
       FIG. 6  shows a light-transmissive bottom surface  102  of a fluid container  104 . Fluid container  104  includes a light guide  106  configured to return reference light to the light-transmissive bottom when the light guide is not submerged in a test fluid. The fluid container also includes a nonlimiting example of a reflective identification-pattern  108 . In the illustrated embodiment, reflective identification-pattern  108  includes a pattern of thirty-two hexagonal markers. Each marker can be configured for either high or low reflectivity. For purposes of illustration, high reflectivity is indicated by white hexagonal markers and low reflectivity is indicated by black hexagonal markers.  FIG. 7  shows a different fluid container  110  that includes a different reflective identification pattern  112 . In some embodiments, one or more of the markers with relatively high reflectance may be configured with high retro-reflectivity. 
     A sensor of a fluid-monitoring system can recognize an identification-pattern of reference light reflected from the bottom surface of a fluid container. An analysis subsystem of the fluid-monitoring system can analyze the identification-pattern to identify a particular fluid container and/or distinguish a particular fluid container from other fluid containers. For example, embodiments of a fluid-monitoring system that include a processing subsystem may include computer-readable media including instructions that, when executed by the processing subsystem, identify a fluid container by analyzing a reflective identification-pattern of the fluid container. 
     In the illustrated embodiments, the reflective identification-pattern includes thirty-two different markers, each of which can be configured with high or low retro-reflectivity. In other words, there are 2 32  (i.e., 4,294,967,296) different possible reflective identification-patterns using the illustrated arrangement.  FIGS. 6 and 7  show just two of the 4,294,967,296 different reflective patterns that can be created by changing the retro-reflectivity of the individual hexagonal markers. 
     It should be understood that the illustrated identification patterns are nonlimiting examples. Other identification patterns may use fewer or more markers, markers having different shapes and/or sizes, and/or markers in different patterns. In some embodiments, one or more markers may be positioned so as to establish an orientation of the other markers. A reflective-identification pattern can be pattern matched and correlated to a fluid container using a lookup table. Additionally or alternatively, at least some of the markers may represent a digit in a binary number, and the monitored reflections at each marker can be used to set a digit corresponding to a marker to either 0 or 1. Virtually any identification pattern that is distinguishable by a sensor of the fluid-monitoring system can be used. Furthermore, a fluid container may be identified using other techniques. For example, a shape of a bottom surface of a fluid container can be used to identify that fluid container. 
     As described above, a fluid monitoring system can be used to monitor a fluid level of a fluid container. This ability can be used in a variety of different usage environments. As a nonlimiting example, a fluid-monitoring system can be used by a bar or restaurant to help monitor the drinking progress of one or more guests. 
     Drink sales are an important aspect of the hospitality business and often account for a large fraction of profits. In addition, guest perception of service is largely driven by the timing at which drink refills are offered. Prompt service not only increases a guest&#39;s desire to return, it also may allow for extra table turns. Bars and restaurants will often train staff to watch for the right moment to offer a refill. If a refill is offered too early, a guest may feel unnecessarily pestered. Waiting until the guest has finished a drink may allow the guest to enter the mindset that the time for drinking has passed. Well trained staff strive to wait until a drink is almost, but not completely, finished in order to increase the probability of a refill order. However, it is very difficult for even the most well trained staff to monitor the drinking progress of all guests at all times. 
     The herein described fluid-monitoring system allows a bar or restaurant to automatically offer refills to a guest when the guest is almost finished with a beverage. The bar or restaurant may further automatically adjust the guest experience based on drink consumption levels. For example, variables such as background music, temperature, and/or offers of other products and/or services can be tailored to the monitored drinking progress of one or more guests. 
     A fluid-monitoring system that includes a display surface can be used to provide an intuitive interface for ordering food and drinks, while also serving as the physical table upon which these items are placed. During a visit, the fluid-monitoring system can be used to entertain guests or provide other services, including advertising and shopping. The fluid-monitoring system may also track the drinking progress of the guests, and automatically offer drink refills when appropriate, or alert wait staff that it is the appropriate time to offer a refill. 
     As a nonlimiting example, a bar may determine that its guests appreciate refill offers when there is one inch of beer left in a pint glass. As such, the bar may utilize pint glasses with light guides that project into the fluid-holding space of the pint glass at a predetermined drink-refill level. For example, if the bar wishes to know when one inch of beer is left, the bar may set a height of the light guide at approximately one inch. In this way, the light guide changes the overall reflectance of the pint glass when beer falls below the one inch level. A fluid-monitoring system can monitor the fullness of the pint glass and generate a drink-refill message responsive to the relative fullness of the fluid container dropping below a predetermined drink-refill level (e.g., one inch). The fluid-monitoring system may include instructions for generating such a message. 
       FIG. 8  shows a nonlimiting example of a fluid-monitoring system  114  causing a display surface  116  to present a drink-refill message  118  to a guest  120 . Guest  120  is drinking from a fluid container  122  in the form of a pint glass. The fluid-monitoring system has recognized that a relative fullness of the pint glass has dropped below a predetermined drink refill level because the pint glass is reflecting a relatively high amount of reference light. As such, the fluid-monitoring system is able to promptly ask the guest if a refill is desired, thus providing the guest with excellent service. If the guest desires another drink, the guest can use the touch-screen capabilities of the fluid-monitoring system to order another drink. 
     In some scenarios, it may be more appropriate to give a guest face-to-face service. As such, a fluid-monitoring system may include instructions that cause a communication subsystem to send a drink-refill message to a message recipient. For example,  FIG. 9  shows fluid-monitoring system  124  sending a drink-refill message  126  to a wait staff coordination computer  128 . Such a message can alert wait staff that a guest  130  at table  4  is almost ready to finish a beverage and may desire another beverage. The wait staff coordination computer additionally may be configured to display a textual and/or graphical representation of the drink-status of a plurality of different beverages throughout an establishment. 
     In some embodiments, a wait staff coordination computer can be used to keep track of wait staff performance, sales trends, and/or other metrics. Such data can be used to determine, among other things, how service responsiveness, locations of tables, times of day, food and drink specials, and/or other factors affect drink sales. 
     As another example, a waiter may carry a communicator that receives drink-refill messages, thus providing the waiter with notifications when a guest may desire another beverage. As still another example, the fluid-monitoring system may display a discrete drink-refill message on a display surface, thus providing subtle notification to the wait staff that the guest may be ready for another beverage. 
     The fluid-monitoring system may send other types of messages to a variety of different message recipients. For example, a fluid-monitoring system may include instructions that cause a communication subsystem to send a drink-identifier message to another fluid-monitoring system and/or to a wait staff coordination computer. For Example,  FIG. 10  shows a guest  132  moving from a first fluid-monitoring system  134  to a second fluid-monitoring system  136 . The first fluid-monitoring system sends a drink-identification message  138  to at least the second fluid-monitoring system. In this way, when guest  132  places a fluid container  140  on the second fluid-monitoring system, the second fluid-monitoring system can identify the fluid container and obtain information about the fluid container, information about the contents of the fluid container, or information about the guest from the first fluid-monitoring system or from a central repository. 
       FIG. 11  shows a process flow  150  of an example method of optically monitoring a fullness of a fluid container. At  152 , the method includes directing reference light at a bottom surface of the fluid container. At  154 , the method includes measuring a relative amount or pattern of reference light reflected from the fluid container. At  156 , the method includes correlating the relative amount or pattern of reference light reflected from the fluid container with a relative fullness of the fluid container. 
     The method may optionally include, at  158 , identifying the fluid container by analyzing a reflective identification-pattern of the fluid container. If identified, the method may further include, at  160 , sending a drink-identifier message to a message recipient. 
     The method may optionally include, at  162 , generating a drink-refill message responsive to the relative fullness of the fluid container dropping below a predetermined drink-refill level. At  164 , the drink-refill message can optionally be displayed. At  166 , the drink-refill message can optionally be sent to a message recipient. 
     A variety of different devices can serve as a fluid-monitoring system. A surface computing device is a nonlimiting example of such a device.  FIGS. 12 and 13  show nonlimiting examples of surface computing devices capable of optically monitoring the relative fullness of one or more fluid containers. 
       FIG. 12  shows a schematic depiction of an embodiment of a surface computing device  200  utilizing an optical touch sensing mechanism. Surface computing device  200  comprises a projection display system having an image-generation subsystem  202 , optionally one or more mirrors  204  for increasing an optical path length and image size of the projection display, and a display screen  206  onto which images are projected. 
     Image-generation subsystem  202  includes an optical or light source  208  such as the depicted lamp, an LED array, or other suitable light source. Image-generation subsystem  202  also includes an image-producing element  210  such as the depicted LCD (liquid crystal display), an LCOS (liquid crystal on silicon) display, a DLP (digital light processing) display, or any other suitable image-producing element. Display screen  206  includes a clear, transparent portion  212 , such as a sheet of glass, and a diffuser screen layer  214  disposed on top of the clear, transparent portion  212 . In some embodiments, an additional transparent layer (not shown) may be disposed over diffuser screen layer  214  to provide a smooth look and feel to the display surface. The display screen can serve as a surface for supporting one or more fluid containers. 
     Continuing with  FIG. 12 , surface computing device  200  further includes an analysis subsystem  216  comprising computer-readable media  218  and a processing subsystem  220 . Further, surface computing device  200  may include a communication subsystem  222  configured to conduct one-way or two-way communication with other devices. Communication subsystem  222  may be configured to conduct wired or wireless communications with other device in any suitable manner. 
     To sense objects placed on display screen  206 , surface computing device  200  includes an image capture device  224  configured to capture an image of the entire backside of display screen  206 , and to provide the image to analysis subsystem  216  for the detection of objects appearing in the image. Diffuser screen layer  214  helps to avoid the imaging of objects that are not in contact with or positioned within a few millimeters of display screen  206 , and therefore helps to ensure that only objects that are touching display screen  206  are detected by image capture device  224 . 
     Image capture device  224  may include any suitable image sensing mechanism. Examples of suitable image sensing mechanisms include but are not limited to CCD and CMOS image sensors. Further, the image sensing mechanisms may capture images of display screen  206  at a sufficient frequency to detect motion of an object across display screen  206 . Display screen  206  may alternatively or further include an optional capacitive, resistive or other electromagnetic touch-sensing mechanism, as illustrated by dashed-line connection  225  of screen  206  with analysis subsystem  216 . 
     Image capture device  224  may be configured to detect reflected or emitted energy of any suitable wavelength, including but not limited to infrared and visible wavelengths. To assist in detecting objects placed on display screen  206 , image capture device  224  may further include an additional optical source or emitter such as one or more light emitting diodes (LEDs) configured to produce infrared or visible light. Light from LEDs  226  may be reflected by objects placed on display screen  206  and then detected by image capture device  224 . The use of infrared LEDs as opposed to visible LEDs may help to avoid washing out the appearance of projected images on display screen  206 . 
     LEDs  226  may be positioned at any suitable location within surface computing device  200 . In the depicted embodiment, a plurality of LEDs  226  are placed along a side of display screen  206 . In this location, light from the LEDs can travel through display screen  206  via internal reflection, while some can escape from display screen  206  for reflection by an object on the display screen  206 . In alternative embodiments, one or more LEDs may be placed beneath display screen  206  so as to pass emitted light through display screen  206 . 
     LEDs  226  can be used to direct reference light at a bottom side of a fluid container, and image capture device  224  can measure a relative amount or pattern of reference light reflected from the fluid container. In this way, a relative fullness of the fluid container can be optically monitored. 
       FIG. 13  shows a schematic depiction of another embodiment of a surface computing device  300  that utilizes an optical touch sensing mechanism. Surface computing device  300  comprises a projection display system having a wide-angle image-generation subsystem  302  and a display screen  306  onto which images are projected. Image-generation subsystem  302  includes a light source  308  and an image-producing element  310 . Display screen  306  includes a transparent glass structure  312  and a diffuser screen layer  314  disposed thereon. Display screen  306  may serve as a surface for supporting one or more fluid containers. 
     Continuing with  FIG. 13 , surface computing device  300  includes an analysis subsystem  316  comprising computer readable media  318  and processing subsystem  320 . Further, surface computing device  300  includes a communication subsystem  322  configured to conduct one-way or two-way communication with other devices. 
     Surface computing device  300  further includes a plurality of image capture devices, depicted as  324   a - 324   e , and an optical emitter such as an LED array  326  configured to illuminate a backside of display screen  306  with infrared or visible light. Image capture devices  324   a - 324   e  are each configured to capture an image of a portion of display screen  306  and provide the image to analysis subsystem  316 , and to assemble a composite image of the entire display screen  306  from the images. In the depicted embodiment, image capture devices  324   a - 324   d  are positioned generally beneath the corners of display screen  306 , while image capture device  324   e  is positioned in a location such that it does not pick up glare from LED array  326  reflected by display screen  306  that may be picked up by image capture devices  324   a - 324   d . In this manner, images from image capture devices  324   a - 324   e  may be combined by analysis subsystem  316  to produce a complete, glare-free image of the backside of display screen  306 . This allows detection of an object such as a fluid container placed on display screen  306 . Display screen  306  may alternatively or further include an optional capacitive, resistive or other electromagnetic touch-sensing mechanism, as illustrated by dashed-line connection  325  of screen  306  with analysis subsystem  316 . 
     It will be appreciated that the embodiments described herein may be implemented, for example, via computer-executable instructions or code, such as programs, stored on computer-readable storage media and executed by a computing device. Generally, programs include routines, objects, components, data structures, and the like that perform particular tasks or implement particular abstract data types. As used herein, the term “program” may connote a single program or multiple programs acting in concert, and may be used to denote applications, services, or any other type or class of program. Likewise, the terms “computer” and “computing device” as used herein include any device that electronically executes one or more programs, including, but not limited to, surface computing devices, personal computers, servers, laptop computers, hand-held devices, microprocessor-based programmable consumer electronics and/or appliances, etc. 
     It should be understood that the configurations and/or approaches described herein are exemplary in nature, and that these specific embodiments or examples are not to be considered in a limiting sense, because numerous variations are possible. The specific routines or methods described herein may represent one or more of any number of processing strategies. As such, various acts illustrated may be performed in the sequence illustrated, in other sequences, in parallel, or in some cases omitted. Likewise, the order of any of the above-described processes may be changed. 
     The subject matter of the present disclosure includes all novel and nonobvious combinations and subcombinations of the various processes, systems and configurations, and other features, functions, acts, and/or properties disclosed herein, as well as any and all equivalents thereof.