Optical scanning apparatus for detecting faults in transparent material wherein the plane of incident light is arranged at the breuster angle to the normal to the surface

An optical scanning apparatus for transparent, substantially flat material comprises a light source (21) and a mirror wheel (22) illuminated by the light beam. The mirror wheel generates a scanning beam which executes a periodic scanning movement in a scanning plane extending obliquely to the surface of the material. The scanning beam generates a scanning light bead on the surface of the material with the scanning light bead moving along a scanning line. A photoelectric light receiving device is arranged at the angle of reflection. The plane of incidence (11) is arranged at the Brewster angle relative to the normal (12) to the surface of the material (13) The light of the scanning beam (14) which executes the periodic scanning movement in the scanning plane (11) is linearly polarized parallel to the surface of the material or perpendicular to the scanning plane (11).

The invention relates to an optical scanning apparatus for transparent, 
substantially flat, plate-like or sheet-like material, the apparatus 
comprising a light source, a light deflecting device, such as a mirror 
wheel, illuminated by the light beam generated by the light source, with 
the light deflecting device generating a scanning beam which executes a 
periodic scanning movement in a scanning plane extending obliquely to the 
surface of the material and with the scanning beam generating a scanning 
light bead on the surface of the material, with the scanning light bead 
moving along a scanning line and with a photoelectric light receiving 
device arranged at the angle of reflection. 
Known optical scanning apparatuses (for example DE-OS 25 52 331, DE-OS 33 
14 620) serve to detect faults which absorb or scatter light, deviations 
of the work surface from the flat state and also deviations of the web 
perpendicular to its plane from a normal position, and to make such faults 
accessible to systematic electrical measurement by appropriate signals at 
the output of the light receiving device. The light receiving device can 
consist of a single photomultiplier or of an arrangement of several 
individual photoreceivers alongside one another. Surface arrays or 
position sensitive individual photoreceivers can also be used in order to 
detect deviations of a reflected beam to several sides from a normal 
position. 
Problems in measuring the flatness of the surfaces of transparent material 
webs however occur because the scanning beam incident on the surface is 
reflected both at the front side and also at the rear side of the material 
web, so that two received signals are superimposed in the photoelectric 
light receiving arrangement and no unambiguous fault signal can any longer 
be obtained, particularly with different deviations in the flatness of the 
front side and rear side of the web. 
The object underlying the present invention is thus to provide an optical 
scanning device of the initially named kind with which the deviations in 
flatness of the front and rear surfaces of the material can be optically 
derived pointwise and detected in the form of electrical signals. 
In order to satisfy this object the present invention provides an 
arrangement which is characterised in that the scanning plane is arranged 
at the Brewster angle relative to the normal to the surface of the 
material; in that the light of the scanning beam which executes the 
periodic scanning movement in the scanning plane is linearly polarised 
parallel to the surface of the material; and in that light reflected from 
the upper side and from the lower side of the web material is 
differentially detected by the light receiving arrangement by determining 
angular deviations between the beams reflected from the upper side and 
from the lower side. 
As a result of this construction two laterally displaced reflected beams 
occur the direction of reflection of which is influenced by deviations of 
the upper or lower surface of the material from the flat state. If the two 
reflected beams are for example parallel to one another as a result of 
surfaces which extend exactly parallel to one another, then they fall at 
the same point on a position sensitive light receiving arrangement, which 
can be evaluated as a measure for the parallelism of the two beams. As a 
result of the position sensitivity of the light receiving arrangement it 
can also be deduced whether the two displaced reflected beams which extend 
parallel to one another deviate as a whole from a predetermined normal 
reflection direction. 
If the flatness of the two surfaces however deviate somewhat from one 
another then the two reflected beams from the front and rear surfaces 
subtend an angle to one another so that they impinge at different points 
on the position sensitive light receiving arrangement which can be 
evaluated for determination of the angle and thus also to detect the 
relative deviations from flatness. With this method it is thus only the 
difference in the deviations of the two surfaces from a state of flatness 
which can be determined. 
The optical scanning apparatus of the invention is used in particularly 
advantageous manner in combination with a classical scanning apparatus 
which operates in refraction or transmission in as much as a second 
scanning beam which executes a scanning movement in a steeper scanning 
plane impinges on the scanning line; and in that a further photoelectric 
light receiving arrangement is respectively arranged at the angle of 
reflection .beta. of the second scanning beam and/or in its extension. 
It this arrangement it is of advantage for the two scanning beams present 
in the two scanning planes are generated using a partially transmitting 
mirror from the same light source and the same light deflecting device. 
In particular provision should be made that the light receiving 
arrangements have at least two and preferably however several individual 
photoreceivers arranged in a specific direction alongside one another. 
It is particularly expedient when the individual photoreceivers are 
arranged as an areal array. The use of a position sensitive individual 
photoreceiver is also possible. 
The present invention also comprises a method of measuring the deviations 
of the surface of a transparent material from the state of flatness using 
a scanning device with the method being characterised in periodic light is 
used which is periodically alternately polarised parallel and 
perpendicular to the scanning plane; and in that the output signals of the 
light receiving arrangement are separately evaluated within each period in 
which a specific state of polarisation exists. 
In a preferred arrangement in which the material is continuously moved 
perpendicular to the scanning line and parallel to its plane, the above 
method is preferably further characterised in that the material is 
continuously advanced perpendicular to the scanning line and parallel to 
its plane, characterised in that the direction of polarisation is 
periodically changed with a frequency which is so high that practically at 
each point of the material one measurement takes place with the one 
direction of polarisation and one measurement takes place with the other 
direction of polarisation; and in that the deviations of both the lower 
side and also of the upper side from flatness are derived from the two 
measurements and indeed either summed or individually. 
In these embodiments the light incident on the measuring location on the 
material is thus polarised parallel to the surface during a first period. 
In this case a first part of the incident light beam will be reflected at 
the upper surface and a second part at the lower surface of the material. 
The two reflected beams can either be separately detected by the light 
receiving arrangement or as a sum with defined components from the upper 
and lower surface. 
During the next measuring period light is used which is polarised 
perpendicular to the scanning plane. As a result of the arrangement at the 
Brewster angle no light is reflected at the upper surface but instead the 
entire quantity of light penetrates at the angle of refraction fully into 
the interior of the material web. This light is reflected if the rear side 
of the web is coated at the rear or lower surface of the material and 
reaches the light receiving arrangement which can thus also form an 
electrical signal representative of the angular tilting of the deviation 
from a state of flatness of the rear side or other error of the material. 
A particularly advantageous embodiment of the invention is thus to be seen 
in the fact that switching is effected in a rapid sequence to and fro 
between the two polarisation directions and that the light receiving 
arrangement is synchronised with this switching process via an electronic 
evaluation circuit in such a way that the two measurement procedures are 
separated in the electronic evaluation circuit. The rapid change-over 
between the two directions of polarisation can for example be realised by 
a rapidly rotating polarisation filter. One can also use two cyclically 
operating light sources with directions of polarisation arranged 
perpendicular to one another whose light beams can be deflected into the 
same beam path. 
The use of light polarised parallel to the surface for fault inspection in 
film emulsions is admittedly already known (US-PS 37 34 624) however, an 
attempt is made there to suppress the beam component which is reflected at 
the lower surface of the emulsion so that, in contrast to the invention, 
it is not used for the measurement of angular faults of the upper and 
lower surfaces relative to one another. 
Whereas the angle of incidence of the scanning beam in a classical fault 
scanning apparatus lies at approximately 10.degree. the angle of incidence 
of the polarised scanning beams, i.e. the Brewster angle, lies for example 
at 56.degree.. 
In the simplest case the generation of polarised light can take place by 
the use of a linear polarisation filter in unpolarised light. 
It is however also possible to use to circularly polarised light and to 
convert this into a suitably directed linearly polarised light by a 
quarter wave plate. 
Furthermore it is possible from the outset to use a polarised light in one 
of the two preferential directions and to rotate its direction of 
polarisation periodically through 90.degree. by a half wave plate. The 
polarised light can also be generated by filtering circularly polarised 
light. 
In the event that the scanning apparatus of the invention is combined with 
a classical scanning apparatus, in which the light incidence at the 
Brewster angle is effected by a partially transmitting mirror, then the 
larger optical path in the scanning beam incident at the Brewster angle is 
not problematic because the angular tilting errors which are to be found 
here are mainly very much larger in area than defects in the material 
which are to be found, for example scratches. The scanning beam, which is 
incident at the Brewster angle, may therefore also be somewhat defocussed 
on impinging onto the surface of the work material. 
The Brewster angle is computed in known manner in accordance with the 
formula: 
EQU tan.alpha.=(n.sub.1 /n.sub.2) 
where n.sub.1 is the refractive index above the material, i.e. the the 
refractive index of air (=1), whereas n.sub.2 is the refractive index of 
the material.

In accordance with FIG. 1 a laser 21 illuminates a mirror wheel 22 rotating 
in the direction of the arrow via a beam broadening optical system 29 and 
a plane deflecting mirror 30. The mirror wheel 22 forms a scanning beam 14 
in a sector-like scanning beam plane 11 and the scanning beam generates a 
sharp light bead 16 on the surface of a transparent web-like material 13 
arranged beneath the mirror wheel. This sharp light bead 16 periodically 
scans the surface along a scanning line 15 in the direction of the arrow. 
The material 13 is continuously advanced in the direction of the arrow L 
and indeed at a speed such that in practice all points of the work surface 
are detected once by the light bead 16, through the linewise scanning of 
the surface of the material 13 transverse to the direction of advance. A 
linear polariser 31 is arranged close to the mirror wheel 22 within the 
plane of incidence 11 and polarises the light coming from the laser 21 
linearly parallel to the scanning line 15. The angle of incidence .alpha. 
which the scanning plane 11 includes with the normal 12 to the material 13 
is the same as the Brewster angle. 
A light receiving arrangement 17 is provided at the angle of reflection 
.alpha. to the normal 12 and includes a strip-like concave mirror 32 which 
gathers the light reflected from the surface of the web material 13 and 
forms an image of the surface of the mirror wheel 22 on a photoreceiver 
array via a strip-like plane deflecting mirror 33. The photoreceiver array 
comprises a central single photoreceiver 23 and further individual 
photoreceivers 24, 25, 26 and 27 arranged around it. 
When the surface of the web material 13 is undisturbed, i.e. faultfree, the 
sharp light bead 16 located on the surface of the mirror wheel 22 is 
imaged onto the central single photoreceiver 23, so that the attached 
electronic evaluation circuit 28 transmits a signal representative of an 
undisturbed faultfree web surface. 
If the surface of the web material 13 is in contrast tilted in one or the 
other direction at the position where the light bead 16 instantaneously 
strikes it, i.e. if it deviates in one or other direction from the desired 
flatness, then the image of the light bead on the mirror wheel 22 reaches 
one of the adjacent individual photoreceivers 24, 25, 26 or 27, whereupon 
the electronic evaluation circuit 28 forms a corresponding angular 
deviation signal, from which conclusions can be drawn concerning the 
nature and the degree of unevenness of the web surface at this position. 
FIG. 1a shows to a greatly enlarged scale, a cross-section through the 
board-like or sheet-like material 13 of FIG. 1, with the plane of cutting 
standing perpendicular to the scanning line 15. The polarisation direction 
of the scanning beams 14 extends parallel to the scanning plane 11 and the 
surface of the material 13. 
The light intensity of the scanning beam 14 incident on the surface of the 
material 13 is designated with 100%. 15% of the light intensity is 
reflected at the angle of reflection .alpha. at the surface of the 
material 13.85% of the light intensity will be refracted at the angle of 
refraction .beta. into the interior of the material 13 where it then 
strikes the lower or rear surface of the material 13. There 72.3% of the 
light intensity leaves the material 13, whereas 12.7% is reflected back at 
the angle of reflection .beta. to the upper surface of the material 13. Of 
this light 10.8% of the original light intensity emerges from the upper 
surface of the material 13. The further reflections indicated only as an 
arrow within the material 13 are negligible from the point of view of 
their intensity. 
In FIG. 1a it is assumed that the upper surface and the lower surface of 
the material 13 extend exactly parallel to one another at the position of 
incidence of the incident scanning beam 14, so that the two emergent beams 
extend parallel to one another with intensities of 10.8 and 15% 
respectively. They would be united by the optical arrangement of FIG. 1 
onto the central individual photoreceiver 23. 
If the two surfaces have errors in flatness while retaining precise 
parallelism then the two reflected beams with 10.8 and 15% of the 
intensity will be deflected onto one of the side disposed individual 
photoreceivers 24, 25, 26 or 27. The fact that only one single individual 
photoreceiver is illuminated by one light point thus states that the front 
and rear surface extend precisely parallel to one another at the light 
impingement positions. 
If the front and rear surfaces of the material 13 are not precisely 
parallel then an angle arises between the reflected beams with 10.8 and 
15% light intensity, which for example leads to one beam impinging on the 
central individual photoreceiver 23, while the other beam impinges on the 
side photoreceiver 25. From this conclusions can be drawn regarding the 
angular divergence and thus the deviation from parallelism of the two 
surfaces of the material 13 at the positions of light incidence. The 
corresponding evaluations are effected in the electronic evaluation 
circuit 28, the practical embodiment of which can be, as shown in the 
block circuit diagram of FIG. 3, which will be described below in 
conjunction with FIG. 2. 
In the following figures the same reference numerals are used to designate 
components which have counter-parts in FIG. 1. 
A telecentric beam path is provided in the embodiment of FIG. 2, in 
distinction to FIG. 1. The light transmitted by the mirror wheel 22 
through the polarisation filter 31 is first deflected via a plane 
deflecting mirror 35 onto a strip-like concave transmitter mirror 34, the 
focal point of which lies on the surface of the mirror wheel 22. In this 
way a scanning beam 14 is generated, which on rotation of the mirror wheel 
22 is displaced parallel to itself. During this the scanning beam 14 again 
runs through the scanning plane 11 which extends at the Brewster angle 
.alpha. to the normal to the material 13. 
The strip-like concave mirror 32 at the receiver side is so arranged that 
it concentrates the received light onto the central individual 
photoreceiver 23 of the photoreceiver array when the surfaces at the 
position of light incidence extend parallel to one another and are 
arranged in a desired plane which corresponds to the arrangement of the 
plane material 13 in FIG. 2. The photoreceiver array 23 to 27 is thus 
located at the focal point of the strip-like concave mirror 32. 
Furthermore it is indicated in FIG. 2, in distinction to FIG. 1, that the 
polarisation filter 31 is rotatable about an axis which coincides with the 
central scanning beam 14' and indeed by a motor 36 which for example 
drives the holder of the polarisation filter 31, which is provided with an 
outer toothed ring, via a pinion 37, so that the polarisation filter 
executes a rapid rotary movement. The motor 36 is controlled by the 
electronic evaluation circuit 28 and transmits an angular position signal 
to the electronic evaluation circuit 28 which makes it possible for the 
evaluation circuit 28 to recognise the instantaneous direction of 
polarisation of the scanning beam 14 and to synchronise with the received 
signals from the photoreceiver array 23, 24, 25, 26, 27. 
A further light receiving arrangement 38 is provided in the extension of 
the scanning beam 14 behind the material web 13 which receives the total 
light passed through the web material 13 and converts it into a 
corresponding electrical signal. The photoelectronic light receiving 
arrangement 38 can be constructed in similar manner to the photoelectric 
light receiving arrangement 17 operating in reflection, this is however 
not illustrated in order to simplify the illustration. 
As a result of this construction, when the polarisation filter 32 generates 
light with a polarisation direction extending parallel to the scanning 
line 15, light will enter in reflection in accordance with FIG. 1a into 
the light receiving arrangement 17 and in transmission (72.3% in FIG. 1) 
into the light receiving arrangement 38. Light receiving arrangement 38 
will respond, with a construction corresponding to that of the light 
receiving arrangement 17 (position sensitive), to wedge errors of the 
material at the position of light incidence. 
An evaluation circuit for the received signals is shown in FIG. 3 for the 
case in which the light is polarised parallel to the scanning line 15. The 
received signals of the light receiving arrangement 17 are transmitted 
within the electronic evaluation circuit 28 on the one hand to a position 
sensitive detector circuit 51 and to an error detector circuit 52 via a 
low pass filter 49 and a recursive filter 50, respectively. The position 
sensitive detector circuit 51 delivers at its output a signal which 
corresponds to the sum of the flatness error (angular tilting) at the 
upper and lower surface of the material web in the region of the point of 
light incidence Error signals appear at the output of the fault detector 
circuit 52 which are representative of defects within the material 13 or 
on its upper side. 
The further light receiving arrangement 38 is connected via a low pass 
filter 49' to a position sensitive detector circuit 51 so that a signal 
representative of the wedge angle between the two surfaces at the position 
of light incidence appears at the output of the detector circuit 51. 
The evaluation circuit of FIG. 3 is used when light is used which is always 
polarised parallel to the surface of the material 13, i.e. when the 
polarisation filter 31 is not rotating. 
If now the polarisation filter 31 in FIG. 2 is set rotating, so that light 
which is alternately polarised parallel and perpendicular to the scanning 
plane comes into action, then evaluation is expediently effected in 
accordance with the upper circuit diagram in FIG. 6. The output signals 
are applied within the evaluation circuit 28 to a polarisation direction 
selection stage 54 which is controlled by the angular position signal 
derived from the motor 36. Thus a first output signal appears at the 
output 55 for light which is polarised parallel to the scanning plane 11. 
A further output signal is available at the output 56 which corresponds to 
light incidence on the individual photoreceivers 23 to 27 when the light 
is polarised perpendicular to the scanning plane 11. The two outputs 55, 
56 are applied via low pass filters 49", 49'" to a position sensitive 
detector 51' which operates as follows: 
A fault signal 49" corresponding to the sum of the flatness errors of the 
front and rear surface is transmitted from the low pass filter 49" to the 
position sensitive detector 51' The low pass filter 49'" delivers only a 
signal representative for the flatness error of the coated rear side of 
the material 13. In the position sensitive detector circuit 51' these 
signals are so processed that a signal appears at a first output 57 which 
is representative of the flatness error of the rear surface of the 
material 13 and so that a signal appears at the output 58 which is 
representative for the flatness error of the upper surface of the material 
13, which can be derived by difference formation from the signals coming 
from the low pass filter 49" on the one hand, and from the low pass filter 
49'" on the other hand. The difference signal obtained in this way can 
also be multiplied by a correction factor. 
In this manner it is not only possible to measure the flatness error of the 
upper and lower surface of the material 13 (FIG. 3 upper circuit diagram), 
but it is also possible to separately measure the absolute lack of 
flatness of both the upper and lower surfaces. 
FIG. 4 schematically shows the combination of an optical scanning apparatus 
in accordance with the invention operating with polarised light at the 
Brewster angle with a classical scanner. 
A transmitting scanning device such as is shown in FIGS. 1 and 2 first 
generates a primary scanning beam 14' which executes a scanning movement 
perpendicular to the plane of the drawing of FIG. 3. This primary scanning 
beam 14' impinges at an angle .beta. onto the surface of the web material 
13 which is relatively steep (for example 10.degree.) and is in any case 
substantially smaller than the Brewster angle .alpha.. A photoelectric 
light receiving arrangement 18 is located at the reflection angle .beta. 
comprising a strip-like concave mirror 39 perpendicular to the plane of 
the drawing, a deflecting mirror 40 and a photoreceiver 41 consisting of 
one or more individual photoreceivers. 
A further photoelectrical light receiving arrangement 19 is furthermore 
provided in the extension of the primary scanning beam 14' and has a 
strip-like concave mirror 42, a plane deflecting mirror 43 and also a 
photoreceiver 53 consisting of one or more individual photoreceivers. 
Faults of the web material 13 in reflection or transmission can be 
determined in the classical manner with the arrangement described here. 
A partially transmitting mirror 20 is located within the primary scanning 
beam 14' and the deflects a part of the transmitted light to a plane 
deflecting mirror 44 which reflects the incident light at an angle to the 
scanning line 15 of the primary scanning beam 14' in such a way that the 
second plane of incidence 11 which is formed in this way impinges onto the 
surface of the web material 13 at the Brewster angle .alpha.. 
The photoelectric light receiving arrangement 17 is again arranged at the 
reflection angle .alpha. in analogous manner to the arrangement of FIGS. 1 
and 2. 
Moreover, a further photoelectric light receiving arrangement 45 is 
provided in the extension of the plane of incidence 11 beneath the web 
material 13 and again includes a strip-like concave mirror 46, a 
strip-like plane deflecting mirror 46 and a photoreceiver 48 consisting of 
one or more individual photoreceivers. In this way one can, using one and 
the same scanner, determine errors in the classical manner and can also 
detect angular tilting of both surfaces of web material 13, both as a sum 
and also separately, which will now be explained in the following with 
reference to two practical embodiments in FIGS. 5 and 6. 
Providing the polarisation direction of the scanning beam 14 in FIG. 4 
extends parallel to the scanning plane 11 and to the surface of the 
material 13 the following errors can be determined with the arrangement of 
FIG. 5: 
The sum of the flatness errors of the upper and lower surfaces of the 
material 13 at the position of light incidence can be determined by means 
of the light receiving arrangement 17 via the low pass filters 49 and the 
position sensitive detector circuit 51. This fault determination 
corresponds to the upper switching stage in FIG. 3. 
The light receiving arrangement 18 transmits a signal via a recursive 
filter 50 to a fault detector circuit 52 at the output of which there 
appears a signal representative of local defects in the material 13. 
Alternatively, or at the same time, the electrical output signal of the 
light receiving arrangement 19 can be applied via a recursive filter 50 to 
a further fault detector circuit 52' and a signal appears at its output 
which is representative of local defects which make themselves noticible 
in transmission. 
The output signal of the light receiving arrangement 45 is applied via a 
low pass filter 49' to a position sensitive detector circuit 51, at the 
output of which there appears a signal, in similar manner to the bottom of 
FIG. 3, which is representative for wedge errors of the material 13 at the 
position of light incidence. 
If the polarisation direction of the scanning beam 14 is continuously 
switched to and fro in the manner shown in FIG. 2 between the two 
directions of polarisation perpendicular and parallel to the scanning 
plane 11 then a circuit in accordance with FIG. 6 is expedient. 
The upper part of the block circuit diagram has already been described in 
connection with FIG. 2. In addition the output signal of the light 
receiving arrangement 18 is applied in the embodiment of FIG. 4 via the 
recursive filter 50 and the fault detector circuit 52, and the signal of 
the light receiving arrangement 19 of the same embodiment is applied via a 
recursive filter 50' to fault detector circuit 52'; in order to 
additionally detect local defects of the material to which the relevant 
light receiving arrangements respond, analogously to the two middle 
circuits of FIG. 5.