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A vessel comprising a fluid-confining surface, a basal surface disposed below the fluid-confining surface, and a light guide. The light guide includes a first window partly defining the basal surface and a second window partly defining the fluid-confining surface, where the second window is disposed

A vessel comprising a fluid-confining surface, a basal surface disposed below the fluid-confining surface, and a light guide. The light guide includes a first window partly defining the basal surface and a second window partly defining the fluid-confining surface, where the second window is disposed above a lowest region of the fluid-confining surface. In other embodiments, an isolating structure is provided that substantially surrounds the light guide between the first and second windows.

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1. A vessel comprising: a fluid-confining surface;a basal surface disposed below the fluid-confining surface;a light guide including a first window which is part of the basal surface and is positioned to contact a surface on which the vessel is placed, and a second window which is part of the fluid-

1. A vessel comprising: a fluid-confining surface;a basal surface disposed below the fluid-confining surface;a light guide including a first window which is part of the basal surface and is positioned to contact a surface on which the vessel is placed, and a second window which is part of the fluid-confining surface and is disposed above a lowest point of the fluid-confining surface, the second window providing a convex boundary region in the fluid-confining surface and a complementary concave boundary region in the light guide; andan isolating structure including an air-filled jacket substantially surrounding the light guide between the first and second windows and configured to reduce loss of light admitted and projected back through the first window of the light guide. 2. The vessel of claim 1, wherein the fluid-confining surface and the basal surface are spatially distinct sections of the same surface. 3. The vessel of claim 1, wherein the isolating structure comprises an optically refractive layer adapted to adhere to the light guide, the optically refractive layer having a lower refractive index than a material of the light guide to which it adheres. 4. The vessel of claim 3, wherein the light guide comprises a polycarbonate monolith, and the isolating structure has a lower refractive index than polycarbonate. 5. The vessel of claim 1, wherein the second window comprises a partial spherical surface. 6. The vessel of claim 1, wherein the second window comprises a conical surface. 7. The vessel of claim 1, wherein the light guide further comprises a light pipe coupled between the first and second windows, and wherein a horizontal cross section of the light pipe is either elliptical or polygonal. 8. The vessel of claim 1, wherein the basal surface further comprises an anti-reflective coating. 9. The vessel of claim 1, wherein the second window defines a non-axial side region of the fluid-confining surface. 10. The vessel of claim 1, wherein the fluid-confining surface defines a space having Cs point symmetry. 11. The vessel of claim 1, further comprising a marking supported on the basal surface, the marking comprising two or more contrasting regions and distinguishing the vessel from among a plurality of otherwise similar vessels. 12. A system for indicating when to offer a beverage refill to a customer, the system comprising: a tabletop;a vessel comprising: a fluid-confining surface;a basal surface disposed below the fluid-confining surface; anda light guide including a first window which is part of the basal surface and is positioned to contact a surface on which the vessel is placed, and a second window which is part of the fluid-confining surface, and an isolating structure including an air-filled jacket substantially surrounding the light guide between the first and second windows and is disposed above a lowest point of the fluid-confining surface, the second window providing a convex boundary region in the fluid-confining surface and a complementary concave boundary region in the light guide, the basal surface configured, when the vessel is rested on the tabletop, to contact the tabletop in a first region directly below the first window and in a second region surrounding the first region, but to stay off the tabletop over a third region between the first and second regions;an illuminant configured to project light onto the basal surface;a detector responsive to light projected from the basal surface; andan image-receiving device operatively coupled to the detector and configured to indicate when an intensity of the light projected from the basal surface exceeds a threshold intensity. 13. The system of claim 12, further comprising an isolating structure substantially surrounding the light guide between the first and second windows and configured to reduce loss of light admitted and projected back through the first window of the light guide. 14. The system of claim 12, wherein the tabletop and the light guide are at least partly transparent in one or more overlapping infrared wavelength ranges, and the illuminant is configured to project substantially diffuse, infrared light onto the basal surface. 15. The system of claim 12, wherein the light guide is configured to promote total internal reflection of some of the light projected onto the basal surface.