Apparatus for automatically detecting and evaluating the characteristics of prints

An apparatus for automatically detecting and evaluating print characteristics comprising an optical detection and comparison means. In particular comparison is made using reflector means which is at the same time a background area for a print in its test position.

The invention relates to an apparatus for automatically detecting and 
evaluating one or more characteristics of prints by comparison of 
optically obtained data of a print to be tested with a standard print. 
A similar apparatus is known from French patent specification 2 359 467. In 
this known apparatus a comparison is made on the basis of reflection data 
of specific portions of a print with standard values for these data which 
have been recorded in a memory means. A decision about the tested 
characteristics is taken along the lines of a suitable composed 
combination value of test data and a correspondingly composed combination 
value of standard data. Data processing takes place in a computer. 
In addition such an apparatus is known from German Auslegeschrift 2 001 
049. In this known apparatus the reflection of the surface of a print is 
detected by simultaneously scanning said print and a reference surface. 
The reflection of the reference surface is used for calibrating, for each 
run, a preamplifier which amplifies the electric output signal of the 
photoelectric transducer responsive to reflected light. In this case a 
comparison is made of reflection data from specific portions of a print 
with standard values for these data which are recorded in a memory means. 
The comparisons carried out in the known apparatuses by their nature have 
not been directed to seeking equality but to a classification of data 
being collected in relation to one or more test limits associated with 
standard data. 
The use of a comparison area when scanning the prints to be tested, as in 
the apparatus known from German Auslegeschrift 2 001 049, is an inspection 
mode which eliminates, to a great extent, chance errors resulting from the 
reflection measurement and in particular such errors resulting from 
temperature fluctuations and instabilities in the photoelectric detection 
means. However, the apparatus known from German Auslegeschrift 2 001 049, 
appears to have insufficient reproducibility in spite of this measure. In 
addition the measure is time consuming. 
SUMMARY OF THE INVENTION 
An object of the present invention is an extension of the number of 
selection grades applied for the evaluation of prints. 
A further object of the invention is an optimal use of the light source 
needed for illuminating the prints to be tested. 
Another object of the invention is to improve the signal/noise ratio. 
Again a further object of the invention is to introduce in particular novel 
rejection top limits. 
Another object of the invention is to implement an optical calibration of 
the measurements being carried out without considerably delaying the 
evaluation process. 
According to the invention an apparatus for automatically detecting and 
evaluating one or more characteristics of prints is characterized by a 
light source in a first focussing optical system, a reflector means 
forming the test field background, and a light sensitive detection means 
in a second focussing optical system, whereby said first focussing optical 
system and said second focussing optical system are directed to said test 
field having their optical axes set at equal, but opposite angles with 
respect to the normal of the test field. 
In the apparatus according to the invention print testing is arranged in a 
reflection configuration so that for example holes in a print (and 
therefore spacial interruptions in the solid body of the substrate of the 
print) can be easily detected using a rejection top limit by having the 
reflector means present. 
Further the apparatus according to the invention can incorporate the 
reflection of the portion of the plane carrying the prints to be tested in 
between two successive prints enabling a pseudocontinuing standardisation 
without loss of time. The plane carrying the prints to be tested, then 
could advantageously be the reflector means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 a configuration is shown with a first optical system with axes AB 
and BC and a second optical system with axes CD, DE and EF. The axes BC 
and CD are set at equal but opposite angles of about 10.degree. with the 
normal point C on the surface of a drum 25. This drum 25 is part of a 
conveyor means passing the prints to be tested through the apparatus and 
in particular across the position defined by point C. The drum 25 is 
rotating around a shaft 26 in the direction as indicated in the figure. 
Point C is on a line parallel to shaft 26 of drum 25, said parallel line 
crossing the drum surface. Scanning of a passing print 29 takes place 
along said line. 
A first optical system comprises a light source 1 having an astigmatic 
condensor system consisting of lenses 2, 3 and 4, a cylindrical lens 5 
joined with a Fresnel lens 6, and finally an optical element 7 operative 
as a concave mirror. The first optical system images the filament of light 
source 1 at the surface of drum 25 where position C is located. 
The second optical system along the axes CD, DE and EF comprises a lens 
system 9, mirrors 10 and 11 and an ocular system 13. The second optical 
system images the narrow illuminated strip at position C on drum 25 onto a 
photoelectric converter element 14 in a uniform and linear manner. For 
example the photoelectric converter element 14 may be a so-called 
solid-state line scanner, type RL 64A of Reticon Corporation. This 
converter element comprises 64 tightly packed photodiode elements in a 
single array. The diode-elements of such a converter element are read in 
sequence. For enabling measurement of the dark current in each of the 
converter elements a shutter 12 is arranged in the second optical system. 
A fixed lens and mirror means 15 and a hinged mirror means 16 permit 
observations with the converter element 14 of diffuse light reflection in 
position C. 
When mirror 11 is carried out as a dividing mirror (for example as a cold 
light mirror) a second photoelectric converter means 21 can be put to use 
along axes EG and GH via a plane mirror 18 and a lens system 19, 20 with a 
stop slit for detecting a print edge when passing along position C. 
The surface of drum 25 may be used as a reflector means to cause saturation 
of one or more of the diode elements in the photoelectric converter 
element 14 when the radiation intensity of lamp 1 and the transmission 
characteristics of the optical systems have been properly adjusted. This 
adjustment has to be done in such a manner that the presence of a print in 
position C at least when this print is a sound one, does not cause 
saturation of any diode element. Holes and dog-ears respectively in the 
tested print can be easily detected in this manner. 
When the adjustment of the lamp, the transmission of the optical systems 
and the photoelectric converter element have been selected such that the 
surface reflection value of drum 25 is within the range of measurement of 
photoelectric converter element 14, this reflection value can be used as a 
reference for calibrating the complete apparatus in the time between the 
testing of two prints. 
In FIG. 1 a second drum 30 is shown which is part of the conveyor means and 
feeds prints to drum 25, for example. 
When the drum 25 is constructed of disc like parts which present an open 
space below the supporting face for the prints in the direction of the 
shaft 26, the reflector means can have a special embodiment, viz. in the 
form of an optical element 17 as shown in detail in FIG. 2. 
Optical element 17 comprises an elongated rectangular prism 27 and a plano 
convex cylindrical lens 28 being arranged onto the prism 27 opposite the 
rectangle. Optical element 17 is mounted in such a way that it is directed 
according to the normal to point C onto the supporting face of drum 25 for 
prints to be tested, the area as illuminated by the first optical system 
being located in the focal plane of lens 28. In this construction light 
being focussed at position C is reflected by reflector means 17 in the 
direction of axis CD of the second optical system. 
The embodiment as shown in FIG. 1 comprises an additional mirror 23 which 
can reflect the light as focussed by the first optical system, when in 
position 24 (being shown in dotted lines) to a reference surface 22 which 
can be a white standard, for example. The same reflect 23 in the position 
24 can mirror the reference surface 22 into the second optical system so 
that the reference surface 22 can enable an absolute calibration of the 
apparatus. 
The reference surface 22 is arranged on the inside of a window pane 8 
separating the space comprising the parts of the optical systems from the 
space in which the conveyor means 25, 30 are located. 
It appears that the apparatus as described above distinguished itself by 
its reproducibility, that is to say, a second test of a lot of prints in 
the apparatus will produce substantially identical results as a first 
test. This is the case even without recalibration during the test of the 
lot. A recalibration in between lots of 10 000 prints for example by means 
of the white standard 22 and the movable mirror 23 in position 24 appears 
to be sufficient. 
The separation of the optical systems for illumination and detection 
respectively enables an optimal construction of both systems, each with 
respect to its aim, which results in a relatively weak light source as far 
as the illumination is concerned so that heat development is limited as 
much as possible. 
Moveover, the spacial separation between the first optical system and the 
second optical system results in the heat sensitive parts of the 
electronic components, such as the photoelectric converter means 14 and 21 
experiencing no substantial influence from the development of heat by lamp 
1. Although the two optical systems are rather complicated, it appears 
that both systems are still very stable. It is advantageous that the 
calibration of the apparatus takes place in substantially the same way as 
a measurement. 
When the optical systems, as is the case here, have been folded, one 
optical system has an even number of folds (C, D, E, F) and the other 
system has an odd number of folds (A, B, C) to space apart the light 
source and the photo-electric converter means as much as possible. 
it is noted that the data processing apparatus in which the electric output 
signals delivered by the photo-electric converter element 14 and possibly 
by the photo-electric converter means 21, derives its timing control from 
the run of drum 25 which is therefor detected separately from the test of 
the prints being transferred on the drum.