Measuring table for densitometric analysis of printed sheets

A measuring table for facilitating the accurate densitometric measurement of a measuring field strip on a paper sheet is disclosed. The table carries a removable hold-down plate that is positioned between side stop rails and a registering rail. The hold-down plate is held in place on the table by cooperating electromagnets and ferromagnetic metal disks and holds a sheet of paper in place between the plate and the table's surface. The hold-down plate also includes a measuring slot so that a densitometer positioned on the upper surface of the plate is afforded optical access to the measuring field strip imprinted on the paper sheet.

FIELD OF THE INVENTION 
The present invention is directed generally to a measuring table for 
printed sheets. More particularly, the present invention is directed to a 
measuring table for densitometric analysis of printed sheets. Most 
specifically, the present invention is directed to a measuring table for 
densitometric analysis of printed sheets in which the table is provided 
with a hold-down plate that covers the sheet being measured. The table 
includes spaced registering bars and a transparent hold-down plate which 
is positionable between these bars. The hold-down plate is held on the 
measuring table by electromagnets or the like and protects the sheet being 
measured. A densitometer is freely supported on the hold-down sheet and is 
afforded unobstructed optical access to the sheet through a measuring slot 
formed in the hold-down plate. 
DESCRIPTION OF THE PRIOR ART 
Measuring tables that are equipped with traveling densitometers. and which 
are intended to measure a printed sheet are generally known in the art, as 
may be seen, for example in the German "ROLAND NACHRICHTEN 40." In that 
arrangement, a printed sheet that carries an imprinted measuring field 
strip is registered against a stop on the measuring table. An 
electromotively driven measuring head is caused to move across the sheet 
and transfers the measured values from the measuring field strip to a 
micro-computer. It is important that the measuring head or heads of the 
densitometer are maintained at a fixed distance from the printed sheet and 
this is usually accomplished by supporting the densitometer head by means 
of a roller or rollers which contact the sheet. However, as the 
densitometer travels over the surface of the sheet, the rollers produce 
creases in the paper. These creases change the distance between the 
densitometer head and the surface of the paper. Such changes in the 
distance between the head and the measuring field strips result in the 
production of inexact and incorrect measured values. The direct contact of 
the rollers which support the densitometer on the printed sheet may also 
soil or damage the printed sheet. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a measuring table for 
printed sheets. 
Another object of the present invention is to provide a measuring table for 
densitometric measurements of printed sheets. 
A further object of the present invention is to provide a measuring table 
having a hold-down plate. 
Yet another object of the present invention is to provide a measuring table 
having spaced registering bars. 
Still a further object of the present invention is to provide a measuring 
table having a feed-in slot for the paper sheet. 
As will be set forth in greater detail in the description of the preferred 
embodiment, the measuring table in accordance with the present invention 
includes a hold-down plate formed of a transparent material and sized to 
be placed between registering bars. The hold-down plate is releasably held 
on the table by suitable means such as electromagnets. A horizontal 
measuring slot is placed in the hold-down plate so that a movable 
densitometer can be afforded clear optical access to a printed measuring 
field strip carried on the paper web. 
Since the movable densitometer contacts only the upper surface of the 
hold-down plate and not the printed sheet itself, there is no possiblity 
of the sheet becoming creased or curving up at the measuring points. Thus 
the distance between the densitometer and the field strip on the sheet 
remains constant and no errors due to distance change are possible. 
The measuring table and hold-down plate in accordance with the present 
invention are suitably sized and the elongated measuring slot is similarly 
sized so that access to the measuring field strip is possible regardless 
of the positioning of the strip on the sheet. All of the width of the 
sheet is accessible to the densitometer. 
The hold-down plate is held on the measuring table by the cooperation of 
ferromagnetic disks in the plate and electromagnets in the table. During 
positioning of the sheet on the table, the electromagnets are shut off but 
once the sheet is in place, they are actuated. Since the paper sheet is 
positioned on the table between the hold-down plate and the top of the 
table, it is held in position during the travel of the densitometer. Thus, 
the sheet cannot shift during the measurement taking and inexact 
measurements due to paper shifting are eliminated.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring initially to FIG. 1, there may be seen a preferred embodiment of 
a measuring table for printed sheets in accordance with the present 
invention. A table top 1 of a generally well known measuring desk used for 
densitometric analysis of printed sheets is provided with a lower or 
inferior horizontal registering bar 2. This bar 2 is secured to the 
surface of table 1 in any suitable manner and is not intended to move. 
Lower registering bar 2 is sized to be longer than the maximum sheet width 
to be measured. A right side stop rail 3 is fixed to the top of table 1 
and extends vertically upwardly from the right end of lower registering 
bar 2. A similar left side stop rail 4 extends vertically upwardly from 
the left end of the lower registering bar 2. Right and left stop rails 3 
and 4 are spaced from each other a distance "a" which is also greater than 
the maximum width of the sheet to be measured. A registering rail 6 is 
removably positioned between right and left stop rails 3 and 4 adjacent 
the lower registering bar 2. The registering rail 6, the lower registering 
bar 2 and the right and left stop rails 3 and 4 are all, in the preferred 
embodiment, made of metal and are preferably all of the same thickness. 
A hold-down plate, whose purpose is to hold the sheet on the top of table 
1, is positioned between the right and left side stop rails 3 and 4, and 
the registering rail 6, as may be seen in FIG. 1. Hold-down plate 7, is in 
the preferred embodiment, made of a transparent material such as 
plexiglass. A horizontal measuring slot 8 is formed in the lower portion 
of hold-down plate 7 and, as may be seen in FIG. 2, extends through plate 
7. In the preferred embodiment, measuring slot is approximately 8 mm wide 
and extends generally across the width of hold-down plate 7 before 
terminating approximately 20 mm before the side edges of the hold-down 
plate 7. 
A plurality of ferromagnetic metal circles or disks 9 are countersunk in 
the bottom surface of hold-down plate 7, as is shown in FIGS. 1 and 2. 
These disks 9 extend inwardly into the plate 7 and are located generally 
in the upper third of the hold-down plate 7. In the preferred embodiment 
there are shown three such circles or disks 9 spaced at a distance "b" 
from each other. It will be understood however, that the number and 
spacing of disks can vary. Each ferromagnetic metal disk 9 overlies a pole 
surface of a d.c. current annular magnet 11 which is located in the 
surface of table 1, as may be seen in FIG. 3. When the electromagnets 11 
are actuated, they cooperate with the ferromagnetic metal disks 9 to 
secure the hold-down plate 7 on the surface of the table 1. 
A feed slot 12 passes through table 1, as may be seen in FIG. 2 and 
underlies the registering rail 6. This feed slot 12 is wider than the 
maximum width of the sheet to be measured. In use, the registering rail 6 
is lifted off the surface of the table and the sheet is passed through the 
feed slot 12 onto the surface of table 1. Once the sheet has been 
positioned under the hold-down plate 7, current is supplied to the 
electromagnets 11. Plate 7 is thus securely held in place and acts to 
clamp the sheet between the table 1 and the hold-down plate 7. The 
measuring field strip is positioned so that it is aligned with the 
horizontal measuring slot 8. A densitometer is positioned on the upper 
surface of the hold-down plate 7, in optical communication with the 
measuring field strip through measuring slot 8. Thus the densitometer can 
measure the densitometric color valves without having any physical contact 
between the densitometer's support rollers and the paper sheet itself. 
Accordingly, the inexact readings previously caused by sheet creases and 
the like are eliminated through the use of the measuring table for paper 
sheets in accordance with the present invention. 
While a preferred embodiment of a measuring table for paper sheets has been 
fully and completely set forth hereinabove, it will be obvious to one of 
ordinary skill in the art that a number of changes in, for example, the 
specific materials used, the sizes of the components, the number of 
magnets and the like could be made without departing from the true spirit 
and scope of the invention and that the invention is hence to be limited 
only by the following claims.