Printing of documents on substrates, such as paper, are well-known.  Newer forms of printing now include digital three-dimensional manufacturing, also known as digital additive manufacturing.  This type of printing is a process of making a three-dimensional solid object of virtually any shape from a digital model.  Three-dimensional printing is an additive process in which one or more printheads eject successive layers of material on a substrate in different shapes.  Three-dimensional printing is distinguishable from traditional object-forming techniques, which mostly rely on the removal of material from a work piece by a subtractive process, such as cutting or drilling. 
The production of a three-dimensional object with these printers can require hours or, with some objects, even days.  One issue that arises in the production of three-dimensional objects with a three-dimensional printer is consistent functionality of the inkjets in the printheads that eject the drops of material that form the objects.  During printing of an object, one or more inkjets can deteriorate by ejecting the material at an angle, rather than normal, to the printhead, ejecting drops that are smaller than an inkjet should eject, or by failing to eject any drop at all.  An inkjet suffering from any of these operational deficiencies is known as an inoperative inkjet.  If the operational status of one or more inkjets deteriorates during three-dimensional object printing, the quality of the printed object cannot be assessed until the printing operation is completed.  Consequently, print jobs requiring many hours or multiple days can produce objects that do not conform to specifications due to inoperative inkjets in the printheads.  Once such objects are detected, the printed objects are scrapped, restorative procedures are applied to the printheads to restore inkjet functionality, and the print job is repeated.  An apparatus that enables detection of defective objects while printing would enable restorative procedures to be applied during object printing so printing that can produce a properly formed object could continue.  In this manner, product yield for the printer is improved and its printing is more efficient.