1.  Field of the Invention
The present invention relates to a device for the optical measurement of a printed sheet and a method for operating the device.  The device includes a measurement table with a movable measurement head. 
Printed sheets are placed on a measurement table and scanned by a measurement head of the measurement table, for the purpose of monitoring print quality during an ongoing printing operation.  Over the course of time, the optical system of the measurement head may become soiled, thus necessitating regular cleaning.  The cleaning of the measurement head is a manual process that has many disadvantages. 
On one hand, the avoidance of damage to the measurement head during the cleaning process is dependent on the operator's care.  For example, there is a danger of the optical system being scratched, which might easily occur if it is cleaned inexpertly. 
On the other hand, the operator is distracted from the running print job when he or she carries out the cleaning operation.  However, it is desirable for the print job to be monitored continuously by the operator, in order for the operator to be able to intervene immediately in the event of a malfunction to limit the amount of waste. 
European Patent EP 0 064 024 B1, corresponding to U.S. Pat.  No.  4,505,589, for example, discloses a measurement table with the difficulties described above. 
Such measurement tables require the removal of sample sheets from the delivery of the printing press in order to place them on the measurement table to carry out the measurement.  Consequently, the measurement carried out with the aid of the measurement table positioned adjacent the press is performed offline. 
There are also measurement devices that monitor print quality in-line in the printing press.  German Published, Non-Prosecuted Patent Application DE 10 2004 021 600 A1, corresponding to U.S. Patent Application Publication No.  US 2007/0113748, for example, describes an in-line measurement device including a measurement bar along which the measurement head is movable.  The measurement bar may be open at the bottom or may be provided with a transparent cover at its underside. 
If the measurement bar is open at the bottom, air is blown into the measurement bar.  The air forms an air flow that emerges from the open underside and prevents dirt from entering the measurement bar.  However, the air stream does not provide thorough cleaning. 
If there is a transparent cover, it is cleaned manually through the use of a cleaning tool having a shape which matches the shape of the cover at least to reduce the risk of scratching the cover.  The cleaning tool is not ideal, however, because there remains a risk of damage and because the operator's attention is no longer focused on the running print job.