Process and device for the optical testing of a surface

A process and device for the optical testing of a surface in which the surface can be illuminated substantially from above by at least one upper light and from the side by at least one lower light, at a sharp angle with respect to the surface. The light reflected and/or scattered by the surface is recorded in a test period by at least one light-sensitive receiver and at least one actual image which is compared with at least one desired image is produced. To permit an improved test provision is made for the surface to be recorded by the light-sensitive receiver at least twice during the test period. During the recording periods in question the surface is illuminated by an at least intermittent illumination from above and/or from the side in a different manner in order to obtain at least two actual images with different illumination of the surface.

BACKGROUND TO THE INVENTION 
The invention relates to a process for the optical testing of a surface in 
which the surface can be illuminated substantially from above by means of 
at least one upper light and from the side by means of at least one lower 
light, at a sharp angle with respect to the surface. In such a process the 
light reflected and/or scattered by the surface is recorded in a test 
period by at least one light-sensitive receiver and at least one actual 
image which is compared with at least one desired image is produced. The 
invention relates particularly, but not exclusively to the optical testing 
of the printed surface of a compact disc, upon which the following 
description concentrates. Nevertheless, the suitability of the invention 
for other surfaces will be recognised. 
CDs have recently become increasingly popular as sound media for home use 
because of the high potential sound quality, and as pure data media in 
data processing because of the high potential data density. They are 
therefore a mass-market product, but have to meet high to very high 
quality demands, particularly when used as pure data media. 
Generally speaking a CD comprises a circular disc, multi-layered in 
cross-section, with a central tap hole for fitting and centring in a 
player. From the bottom, i.e. viewed from the read side of the player, 
upwards the CD consists of a transparent plastic layer which contains all 
the data in the form of pits, a thin metal layer, generally of aluminium, 
for metallizing the plastic layer, and a thin lacquer layer which is 
usually hardened by UV light, for protecting the metal layer. The imprint, 
the so-called label, to provide information to the consumer, is then 
applied to the UV lacquer layer by means of known printing processes. 
In the radial direction, in a CD several coaxial, circular regions which 
move outwards from the tap hole can be distinguished. Directly at the tap 
hole is the region used to fit the CD in the player. Adjoining this is a 
region in which the so-called ident code, by means of which the CD can be 
unequivocally identified, is impressed. There then follows the region used 
for the actual data storage. If the CD is recorded up to its maximum 
storage capacity the region ends directly on the outer edge region. 
Otherwise the so-called lead-out or a reflective strip is arranged between 
the edge region of the CD and the data region. 
In the manufacturing process a polycarbonate blank is initially produced by 
the die-casting method, wherein all data are already impressed by the die. 
One surface of the blank is then provided with the metallic reflective 
layer by the sputter process and sealed with the lacquer layer. In this 
process the CD is centrifuged in order to achieve a uniform distribution 
of the lacquer layer and as thin a layer as possible. The label is then 
printed on the CD. 
In principle the label is of no importance to the function of the CD 
because a CD is read from the underside. For the consumer, however, a 
perfect label is often a criterion for perfect playback of the CD so that 
it is necessary to produce a faultless print on the surface of the CD. In 
contrast the UV lacquer layer on a CD must always be perfect because 
otherwise there is the risk of the underlying metal layer oxidizing 
prematurely, which could cause reading errors. Processes with which the 
surface of the CD, i.e. the label and optionally also the UV lacquer 
layer, can be tested are therefore required. As a test of each CD must be 
performed after manufacture it is necessary to incorporate the test 
process for the upper side of the CD into the continuous production 
process. This means that there is often only a limited testing time 
available for the test process. 
Generally speaking the surface of the CD is printed in any manner. Colour 
surfaces, pictures, inscriptions or the like can be applied with the most 
varied colour application processes for example. It is of course also 
possible for the CD to be partially unprinted, so that the metal layer is 
visible through the UV lacquer from the top. Furthermore a CD is often 
neither printed nor provided with a metal layer on the inner and outer 
edge region and is therefore transparent in those regions. 
Generally speaking an optical process is used for testing the printed 
surface of the CD, in which the surface of the CD is photographed by a 
light-sensitive receiver, generally an electronic CCD camera, in the top 
view from above. In a data processing unit the actual image taken is 
compared with a previously calibrated desired image of a perfect surface 
with certain test criteria. Any deviation is then detected as a fault and 
the CD is graded according to the size and nature of the deviation. 
In principle it should be noted that differently printed or unprinted 
regions of a CD appear different if the CD is illuminated differently. 
With illumination from above, the light reflected by the surface is 
received by the camera. Metal or reflective surfaces appear light, for 
example, whereas the colours remain substantially dark with this kind of 
illumination. When the surface of the CD is illuminated at a sharp angle 
from the side, the light scattered by the surface is received by the 
camera and the chrominance and the colour saturation can be detected. 
It is evident that when a CD is tested with illumination from above only or 
with illumination from the side only, many imprints and hence potential 
deviations cannot be detected. With lateral illumination metallic surfaces 
appear black in the top view, and black imprints cannot be detected on 
them as they are only identifiable in the upper light. With illumination 
from above, however, colours usually appear dark in the top view, so that 
the colour saturation or the chrominance cannot be tested. It should also 
be noted that with no imprint in the data region of the CD, the pit 
structure, which can also be seen on the metal surface, behaves like an 
optical reflective grid so that a colour splitting of the lower light is 
caused. This means that any colour imprints of the same colour as the 
generally accidental colour splitting can no longer be reliably detected. 
For this reason, in a known process the camera is exposed once during the 
test period whereas the CD is simultaneously illuminated both from above 
with the so-called upper light and from the side at a sharp angle with 
respect to the surface with the so-called lower light, and this has 
produced acceptable results. Simultaneous illumination with upper and 
lower light cannot, however, always detect some types of and faults in the 
print or the surface of the CD. Furthermore there is the risk that the 
faults which might be readily identifiable in the one light are reduced by 
the simultaneous illumination with the other light as far as their 
identifiability is concerned, so that small or low-contrast faults can 
barely be detected for example. 
A metallic or metallized surface appears black in the lower light in the 
top view, for example, whereas in the upper light it appears light. This 
causes the actual image to take on a grey shade there, because the camera 
is exposed by both the one and the other light. A grey imprint with the 
same grey shade as this generated grey shade cannot therefore be detected 
and tested. Nor can the effects of the colour splitting because of the 
reflective grid formed by the pit structure be reliably prevented. 
The consequence is that either a fault cannot be detected or a so-called 
pseudo-fault is indicated. This generates an unnecessary reject, however. 
To compensate at least partially for the mutual disturbing effect of the 
lighting it is also known that in order to produce the desired image, 
before the actual test process the printed surface of the CD is 
alternately illuminated by the upper light only and by the lower light 
only, in order to detect the metallic surfaces, the reflective surfaces or 
the transparent edge regions in advance, for example. The comparison 
between actual image and desired image can then be carried out in these 
regions with different criteria. 
All faults cannot, however, be reliably detected in this case either. 
Scratches on the surface can, for example, weaken the reflected upper 
light whereas they intensify the scatter of the lower light in this 
region. This causes the scratch to appear dark in the upper light and 
light in the lower light, so that the scratch cannot be seen on the 
resulting camera picture, or can only be seen with very low contrast, and 
does not lead to a rejection. Nor can different types of surface roughness 
be detected, because a rougher surface reduces the proportion of reflected 
light and simultaneously increases the proportion of scattered light, so 
that scarcely any difference can be perceived in the picture taken by the 
camera. 
Furthermore, with simultaneous illumination, which is often also used 
because of the short test time that is available, faults in the UV lacquer 
cannot be reliably identified. Overprinted lacquer faults in particular 
cannot be detected because ultimately they only cause a profile change in 
the CD. A profile change or a missing lacquer layer also appear light in 
the one light but dark in the other, however, so that here also 
simultaneous illumination by means of the upper light and the lower light 
causes a weakening of the contrast and it is no longer possible to detect 
the fault. 
SUMMARY OF THE INVENTION 
The object of the invention is therefore to provide a process and a device 
for the optical testing of a surface of an object with which the 
above-mentioned disadvantages can be avoided. A further object of the 
invention is to enable the process to be incorporated into the continuous 
production process of the object in question without problems. 
According to the invention the object is achieved in that during the test 
period the surface is recorded at least twice by the light-sensitive 
receiver and during the recording times in question is illuminated by 
means of at least intermittent illumination from above and/or from the 
side in a different manner, in order to obtain at least two actual images 
with different illumination of the surface. The images obtained can then 
be compared with corresponding desired images which have also been 
obtained, for example, by means of corresponding illumination by the upper 
light and/or lower light. This has the advantage that a weakening of the 
contrast of certain fault phenomena or an effect on the colour 
reproduction due to a mixing of the reflected and the scattered light with 
simultaneous illumination with the upper light and the lower light can be 
avoided. 
In particular it is possible to select the illumination in such a way that 
in each recording period, a certain region, a certain feature, a certain 
potential fault phenomenon or the like of the surface can be recorded 
particularly clearly and compared with a corresponding desired image. In 
particular, therefore, faults in the UV lacquer layer, which may even be 
overprinted, can also be detected because in a recording period the 
illumination can be selected in such a way that a weakening of the 
pictorial representation which is only changed by the deviating profile no 
longer occurs. 
It can be appropriate if the illumination of the surface takes place only 
by the upper light or the lower light in each case in the recording 
periods in question. This has the advantage that the light-sensitive 
receiver is exposed only by the upper light in one recording period and 
only by the lower light in the other recording period, so that mutual 
influence is prevented. 
It has, however, been shown that it is not possible frequently to switch 
the lamps of the corresponding light sources for the upper light and/or 
lower light on and off in the generally short test time available, which 
is only approx. 100 milliseconds in a test of a CD, without problems. This 
causes problems with halogen lamps or halide lamps in particular, because 
although they generate a wave spectrum that is desired for the lighting 
they cannot be switched on and/or off in this short time. 
An embodiment of the invention can therefore provide that during the test 
period the surface is illuminated substantially continuously by the upper 
light whereas the surface is additionally illuminated by the lower light 
solely during at least one portion of time in at least one recording 
period. Alternatively provision can be made that during the test period 
the surface is illuminated substantially continuously by the lower light 
whereas the surface is additionally illuminated by the upper light solely 
during at least one portion of time in at least one recording period. Even 
if the surface of the CD has been illuminated by both the lower light and 
the upper light in one of the recording periods, the influence of the 
mutual effects can be compensated for in the data processing unit by the 
presence of the other actual image which had been taken with the upper 
light only or with the lower light only. 
An advantageous embodiment of the invention provides that the upper light 
and the lower light bring about an exposure of the light-sensitive 
receiver with different intensity in each case and that the exposure times 
are correspondingly matched for an approximately maximum recording level 
of the light-sensitive receiver in the recording times in question. This 
has the advantage that when the CD is continuously illuminated by the 
lower light, for example, at a luminous intensity which causes a weaker 
exposure of the light-sensitive receiver, when the upper light is switched 
on in the second recording period, for example, in which a shorter 
exposure time of the light-sensitive receiver than the first exposure time 
is set because of the stronger upper light, the light-sensitive receiver 
is predominantly exposed by the light generated by the upper light. The 
effects due to the lower light can thus be reduced. 
A particularly advantageous embodiment of the invention provides that the 
intensities of the illuminations in question are selected in such a way 
that when the surface is simultaneously illuminated by the upper light and 
the lower light in an exposure time of the light-sensitive receiver the 
one light causes an at least approximate maximum recording level of the 
light-sensitive receiver whereas in the same exposure time the other light 
causes only a fraction of the maximum recording level of the 
light-sensitive receiver. This has the advantage that an influence on the 
exposure during the exposure time in which both the upper light and the 
lower light illuminate the CD can be most extensively prevented by the 
weaker light because in the short exposure time the weaker light is 
scarcely able to cause an exposure of the light-sensitive receiver. It can 
be appropriate for the intensities of the illuminations in question to be 
dimensioned in such a way that the ratio of the desired exposure times by 
the one or other light for the maximum recording level of the 
light-sensitive receiver in the recording times in question is at least 
1:10 to 1:200. Basically it should be borne in mind that the smaller the 
exposure ratio, the greater the influence from the weaker light. 
Basically provision can be made that the particular illumination of the 
surface from above or from the side is achieved by corresponding switching 
on and off of corresponding light sources. It is also possible to achieve 
the illumination of the surface from above or from the side by covering 
corresponding light sources with corresponding cover means, such as 
mechanical elements. 
It can be appropriate for flashlight to be used at least as upper light. 
This has the advantage on the one hand that the corresponding light can be 
produced in the desired time interval. On the other hand it is an 
advantage that flashlight in the form of a discharge lamp also produces a 
wave spectrum desired for illuminating the CD. 
It is also appropriate that a discharge lamp, such as a halide lamp, is 
used at least as lower light. This is particularly advantageous when the 
light in question is used as constant light. The surface of the CD can be 
illuminated with the desired wave spectrum by means of a discharge lamp. 
Basically it is advantageous when the surface is illuminated from above 
and/or from the side with diffuse light. This has the advantage that 
potential effects or erroneous assessments as a result of directly 
incident light are prevented. 
It should be noted that the device for implementing the process according 
to the invention is generally incorporated into an automated device for 
producing and/or printing the object, such as the CD. It is therefore 
necessary for the CD to be moved to the mounting device of the test device 
and away from the mounting device by transport means. Generally speaking 
this means that the lower light cannot be screened with respect to the 
ambient area without problems, particularly not without obstructing the 
transport means. The lower light reflected at the CD thus shines freely 
into the ambient area. For this reason, a use of flashlight as lower light 
is generally dispensed with because this would be a considerable 
inconvenience to the operator because of the high light intensity and the 
high pulse frequency. Otherwise the lower light can also take the form of 
flashlight. 
A preferred embodiment provides that the surface of the CD is continuously 
illuminated by means of the lower light and additionally at least once by 
means of an upper light in the form of flashlight in one of the recording 
periods, wherein the exposure times of the light-sensitive receiver are 
correspondingly set. An appropriate embodiment provides that at least the 
beam path of the upper light is at least partially screened with respect 
to the ambient area in order to prevent dazzle. The beam path of the upper 
light can be screened with respect to the ambient area more easily because 
the upper light is not arranged in the immediate vicinity of the CD. 
Furthermore the reflected light of the upper light does not shine freely 
into the ambient area but is collected in the objective of the 
light-sensitive receiver. In the ambient area the upper light in the form 
of flashlight is only perceptible because of scatter on the surface of the 
CD and does not therefore generally intrude. 
In automatic optical test processes, electronic CCD cameras which convert 
the light beams received directly into an electrical signal are generally 
used. With these types of cameras the exposure time is set by means of 
corresponding control in the so-called shutter mode. This means that the 
camera is only light-sensitive for fractions of seconds which correspond 
to the exposure time. In this way, illuminations with high intensity, such 
as by flashlight for example, also produce correctly exposed photographs. 
The conventionally used video cameras operate at 50 Hz frequency to produce 
a field line by line. This means that a field is present after 20 
milliseconds and a frame after 40 milliseconds irrespective of the actual 
exposure time. To be able to produce a frame with flashlight it is 
necessary for the light-sensitive receiver to be exposed in both the first 
and in the second period to produce a field. A further embodiment of the 
invention therefore provides that the surface is exposed by flashlight 
twice during at least one recording period. This is perfectly possible 
within the given time frame. The distance between the flashlights is 
selected in such a way that in the case of a light-sensitive receiver, 
such as a CCD camera, which produces a frame in two successive periods for 
producing a field, the first flashlight falls in the first period for 
producing a field and the second flashlight in the second. 
Producing two successive images in two recording periods thus takes 
approximately 80 milliseconds, whilst a test period of approx. 100 
milliseconds is available. Particularly when a constant light is used, in 
addition to which at least one flashlight is switched on in the at least 
one recording period, incorporation in a continuous production process is 
therefore possible even when the available test period is very short. 
The invention also relates to a device for the optical testing of a 
surface, particularly a compact disc (CD), in which the surface can be 
illuminated by at least one upper light substantially from above and by at 
least one lower light from the side at a sharp angle with respect to the 
surface and at least one light-sensitive receiver is provided in order to 
receive the light reflected and/or scattered by the surface, and in which 
a device is provided in order to produce at least one actual image in a 
test period and compare with at least one desired image. In order to be 
able to implement the above-mentioned process according to the invention 
in an advantageous manner it is provided that a first control means is 
provided in order to trigger the light-sensitive receiver at least twice 
in the test period and that a second control means is provided that is 
connected to the first control means and to the upper light and the lower 
light and thus cooperates with the first control means, that during the 
recording times in question the surface can be illuminated by means of an 
at least intermittent illumination with the upper light and/or the lower 
light in a different manner, in order to obtain at least two actual images 
with different illumination of the surface. With this device it is 
possible to set the desired optimum illumination during the recording 
periods in question by means of an alternating and/or by means of a 
combined illumination of the surface with the lower light and/or the upper 
light. 
Provision can be made for the upper light or the lower light to be an 
intermittent light. Provision can also be made for at least the lower 
light to be a constant light. By this means, different illumination 
conditions can be produced during the recording times in question in a 
simple manner. 
An advantageous embodiment provides that the upper light and the lower 
light illuminate the light-sensitive receiver with different intensity and 
the exposure times of the light-sensitive receiver in the recording 
periods are adapted according to the illumination prevailing at the time. 
In the different recording periods different illuminations can therefore 
be set in such a way that influence from the weaker light on the image 
taken in the exposure time of the light-sensitive receiver in which the 
stronger light illuminates the CD as well as the weaker light can be 
avoided. 
The device according to the invention can provide that at least the upper 
light has a flashlight as the light source. Provision can also be made for 
at least the lower light to have a discharge lamp such as a halide lamp as 
the light source. The required pulse time of the intermittent light can be 
maintained by using flashlight. Furthermore the use of flashlight and also 
a discharge lamp has the advantage that the CD can be illuminated with the 
desired wave spectrum which at least approximately corresponds to that of 
daylight. 
An appropriate embodiment of the invention provides that at least the beam 
path of the upper light is screened at least partially with respect to the 
ambient area by means of at least one cover element in order to prevent 
dazzle. This has the advantage that even when flashlight is used as the 
upper light, inconvenience to the operator of the production machines is 
extensively prevented.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The device 10 for the optical testing of a surface of a compact disc (CD) 
which is shown in FIG. 1 has a generally columnar vertical structure to 
which the individual functional elements are secured. For reasons of 
clarity the retaining devices required to secure the individual functional 
elements are not shown in the drawing. 
In its lower section the device 10 is provided with a mounting device for a 
CD, which is also not shown, with which the CD 11 under test is received 
for the test and centred. In the upper region of the device in the 
direction of the longitudinal axis 12 of the device 10, which 
substantially corresponds to the axis of rotation of the CD 11, a 
light-sensitive receiver 13, such as an electronic CCD camera, is located 
in order to be able to photograph the surface of the CD 11 in the top 
view. 
For the photographs by the light-sensitive receiver 13 the CD 11 can be 
illuminated from above by means of an upper light 14 and/or from the side 
at a sharp angle with respect to the surface of the CD 11 by means of a 
lower light 15. More specifically the arrangement in the case of the upper 
light 14 is such that a semi-reflecting mirror 16 is arranged between the 
CD 11 and the light-sensitive receiver 13 approximately in the upper half 
of the device 10 in such a way that the light emitted by a lateral light 
source 17 is reflected at the mirror 16 in the direction of the CD 11 and 
the CD 11 is illuminated from above. The light reflected by the CD 11 
passes through the semi-reflecting mirror 16 in the direction of the 
light-sensitive receiver 13 and exposes it. To achieve a uniform 
illumination of the CD 11, a diffusor 18 is provided between the light 
source 17 and the semi-reflecting mirror 16. This spatial arrangement of 
the device has been selected in such a way that a gripping means for 
transporting the CD is moved to and fro between the upper light 14 and the 
lower mounting for the CD 11 in the continuous production process. The 
distance between the CD 11 and the lower edge of the unit receiving the 
upper light and the camera depends on the local circumstances. 
In the case of the lower light 15 the arrangement is such that a 
substantially annular light source 19, which can also comprise several 
individual lamps arranged at a distance from each other in the 
circumferential direction, is provided laterally above in the immediate 
vicinity of the CD 11. This means that the CD 11 is illuminated with a 
light impinging at a sharp angle. The lower light 15 is aligned in such a 
way that only light scattered from the surface of the CD 11 can reach the 
light-sensitive receiver 13. The colour saturation and chrominance of the 
print are well accentuated by this means in particular. For uniform 
illumination of the CD 11, a diffusor can also be provided between the 
light source and the surface of the CD 11 in this case. 
Corresponding cover elements 20 are provided both for the light source 17 
of the upper light 14 and for the light source 19 of the lower light in 
order to prevent the light-sensitive receiver 13 from being directly 
illuminated by the light sources in question. To prevent the operator from 
being dazzled the upper light 14 is screened with respect to the ambient 
area by means of a sleeve-like cover element 21. This cover means 21 
surrounds the light source 17 of the upper light 14 and the mirror 16 and 
can extend as far as the light-sensitive receiver 13 to prevent ambient 
light from influencing the image that is taken. 
The output of the light-sensitive receiver 13 is connected to a data 
processing unit 22 in which the recorded images of the surface of the CD 
11 are compared with at least one previously calibrated desired image. 
Depending on the test result, for example, the data processing unit 22 
supplies a corresponding signal to the handling device for the CD in order 
to grade out a faulty CD or pass on to the rest of the production process. 
The data processing unit 22 is also connected to a first control means 23 
and a second control means 24, to control the different illuminations by 
the upper light 14 and/or the lower light 15 during the recording periods 
in question and to set the light-sensitive receiver 13 correspondingly. 
More specifically the arrangement is such that the first control means 23 
is connected to the trigger of the light-sensitive receiver 13, to set its 
exposure times. On the one hand the second control means 24 is connected 
to the light source 17 of the upper light 14 and the light source 19 of 
the lower light 15, to switch them on and off according to the specified 
modes of illumination. On the other hand the control means 24 is connected 
to the first control means 23 to enable the illumination to be 
synchronized with the triggering of the light-sensitive receiver 13. It is 
of course also possible for the tasks of the control means 23 and 24 to be 
performed directly by the data processing unit 22. 
FIG. 2 shows a further embodiment of a device 30 for the optical testing of 
a surface of a CD. The mode of operation of device 30 according to FIG. 2 
is the same as that of device 10 according to FIG. 1, and identical 
elements are provided with identical reference numerals. 
The substantial difference between the two devices lies in the fact that 
the arrangement of the light-sensitive receiver 13 and the CD 11 under 
test is in the form of a telecentric structure. For this purpose a lens 
arrangement 31, an achromatic lens for example, is provided, by means of 
which the CD 11 is illuminated by the upper light with substantially 
parallel light rays. The reflected light rays of the CD 11 are 
concentrated in this lens arrangement 31 and projected onto the objective 
32 of the light-sensitive receiver 13. By means of this structure it is 
possible for the width of the device 30 to be substantially reduced 
compared with the width of a traditional test device. In this case the 
width is substantially determined by the diameter of the lens arrangement 
31 which only needs to be slightly larger than the diameter of a CD or the 
largest width of the surface under test. 
The lower light 15 can also be annular in form. It is appropriate, however, 
to provide the lower light on only one side of the CD 11. In this case the 
lower light is provided with a lens arrangement which enables the CD to be 
illuminated uniformly. In the embodiment shown in FIG. 2 the lower light 
15 has a light source 33, a convergent lens 34, a cylindrical lens 35 and 
a mirror 36. A concave mirror 37 can also be provided behind the light 
source 33 so as better to utilize the radiated light. More specifically 
the arrangement is such that the light of the light source 33 is initially 
concentrated through the spherical convergent lens 34 and is then 
concentrated again through the cylindrical lens 35 in a plane into 
substantially parallel rays so that a pencil of rays is produced which 
illuminates the surface of the CD 11 with slightly convergent rays viewed 
from above and with substantially parallel rays viewed from the side. The 
order of the lenses can also be changed. 
For uniform illumination provision is also made for the cylindrical lens 35 
to be tiltable. By this means the constantly present lens errors can be 
utilized by means of a corresponding tilting with the effect that the 
surface of the CD 11 is illuminated with substantially identical light 
intensity, starting from the edge that is nearest to the lower light, as 
far as the edge facing away from the lower light. 
Access to the handling means of the production device is substantially 
facilitated in that the lower light is now arranged on one side of the CD 
only. Furthermore the width of the test device 30 is reduced in the lower 
region because the illumination is no longer provided by an annular lower 
light which surrounds the CD laterally along its circumference and thus 
requires greater radial space. 
By means of these measures, viz. in particular the telecentric arrangement 
and optionally also the use of a lower light which irradiates the CD 11 
from one side only it is possible for a test device of this kind to be 
used in a so-called twin unit for producing CDs. In a twin unit the CDs 
are very close together, with an axial spacing of 135 mm for example (the 
diameter of a CD is 120 mm), so that when traditional units which are 
substantially wider in construction were used in the past, compromises 
always had to be accepted as regards illumination and test sharpness. It 
is now possible to incorporate two test devices which operate with full 
test sharpness and consistently faultless lighting alongside each other in 
the production process. 
To reduce the structural height, the convergent light rays behind the lens 
arrangement 31 are deflected at least once before they are projected into 
the objective 32 of the light-sensitive receiver 13. In the embodiment 
shown in FIG. 2, at a mirror 38 the light rays are initially reflected 
downwards onto a reflecting element and then reflected upwards again 
towards the objective 32 of the light-sensitive receiver 13. The height of 
the device can be reduced by means of this double beam deflection. 
The light for the upper light can be supplied, for example, by a light 
source which is located in the focal point of the lens arrangement 31, via 
a semi-reflecting mirror which is arranged in the beam path between the 
light-sensitive receiver 13 and the lens arrangement 31. This should be 
understood to mean that the focal point can of course also be displaced by 
corresponding beam deflections. In the embodiment shown in FIG. 2 a prism 
40 whose one side 41 represents the reflecting element for the rays 
reflected by the CD 11 and has the form of a semi-reflecting mirror is 
provided to supply the light of a light source 42 for the upper light. 
More specifically the prism 40 is formed in such a way that a further side 
39 of the prism 40 behind the semi-reflecting mirror side 41 runs at a 
certain angle a to the mirror side 41 depending on the material of the 
prism 40. The angle a should be selected in such a way that the light rays 
emanating from a light source 42 are deflected onto the mirror 38 by means 
of a refraction at the side 39 and at the side 41 in such a way that the 
rays are aligned substantially parallel onto the CD 11 through the lens 
arrangement 31. This means that the rays converging from the lens 
arrangement 31 are reflected in an imaginary reverse path through the 
mirror 38 and are refracted at the one side 41 and the other side 39 of 
the prism 40 in such a way that the focal point of the lens arrangement 31 
at least approximately impinges on the light source 42. Furthermore the 
angle is selected in such a way that the light rays emanating from the 
light source are refracted in such a way that they do not impinge on the 
objective 32 of the light-sensitive receiver 13 directly and on the other 
hand the light-sensitive receiver 13 cannot "see" into the light source. 
This means that imaginary light rays emanating from the light-sensitive 
receiver 13 undergo a total reflection at the inner boundary surface 44 of 
the side 39 of the prism 40 after the refraction at the side 41. The third 
side 45 of the prism 40 on which the imaginary light rays from the 
light-sensitive receiver are totally reflected by the boundary surface 44 
can be blackened in order to prevent influence from diffuse light. 
By means of this arrangement it is possible to reduce the dimensions of the 
device in depth also, because the light source 42 for the upper light 14 
can be arranged in or close to the optical axis of the light-sensitive 
receiver 13 without influencing it. A lateral arrangement of the light 
source, such as that of the light source 17 in FIG. 1, for example, is no 
longer required. For uniform illumination, corresponding diffusers for the 
upper and/or lower light can also be provided but these are not shown for 
reasons of clarity. 
In the embodiment shown in FIG. 2 the mirror 38 is additionally designed to 
be movable. This has the advantage that when calibrating the desired 
images for the test process, for example, the mirror 38 and hence the 
(light rays) reflected at the CD 11 can be displaced by a fraction, half 
for example, of a pixel of the light-sensitive receiver in at least one 
coordinate axis of the light-sensitive receiver. By this means an 
improvement in detection can take place in that exposure measurements of 
adjacent pixels can be compared with each other and/or combined in order 
to determine the type of surface present in that region. Grid prints, 
metallic prints or regions with a sharp light/dark contrast, for example, 
can therefore be unequivocally detected when reading-in the desired image 
so that a correspondingly matched comparison between the desired images 
and the actual images can be carried out. This can increase the test 
accuracy and reduce pseudo-rejects. 
Basically the mirror 38 can be pivoted by any means. A piezoactuator 43 is 
provided in the embodiment shown in FIG. 2 because of the possible short 
paths and uncomplicated operation by means of an electronic control. This 
actuator can, for example, be provided with three rams acting on the 
mirror back, to move the mirror in such a way that the light rays are 
displaced on the light-sensitive receiver 13 approximately by a fraction 
of a pixel in the x-direction, y-direction and x,y direction in each case. 
In the rest position the mirror is pressed by a spring element against the 
stop face of the piezoactuator 43 so as to have a fixed position. Four 
images each in different viewing directions can therefore be produced in a 
simple manner for reading-in the desired image. The continuous test 
process can be carried out with the mirror stationary for example. If 
there is sufficient time in a continuous test process it is of course also 
possible to move the mirror in at least one direction in the test period. 
FIGS. 3a to 3d show examples of possible modes of illumination with which 
the surface of the CD 11 can be illuminated differently by means of the 
upper light OL and/or the lower light UL within a test period TP for a CD 
11 in the course of the particular recording times TA1 and TA2. The light 
intensity I is shown on the ordinate and the time t on the abscissa. In a 
graph, two abscissae for the upper light OL and the lower light UL in each 
case are shown over each other. 
In FIG. 3a the surface of the CD is illuminated either by the upper light 
OL only or by the lower light UL only in the recording times TA1 and TA2. 
The images of the surface obtained in this way are free from any influence 
from the other light. 
In FIG. 3b the surface of the CD is continuously illuminated by the upper 
light OL whereas the lower light UL is switched on with a higher intensity 
in the second recording period TA2. 
The exposure time TB of the light-sensitive receiver 13 has therefore been 
made correspondingly shorter in the second recording period TA2. The 
higher intensity of the lower light means that the effects on the recorded 
image because of the simultaneous illumination of the surface by the 
weaker upper light OL are reduced in the course of this exposure time TB. 
In FIG. 3c the surface is continuously illuminated by the lower light UL 
whereas in the second recording period TA2 the upper light OL is switched 
on with a substantially higher intensity I, in the form of flashlight for 
example. This means that the required exposure time TB of the 
light-sensitive receiver is reduced on the one hand and the effect of the 
lower light UL on the recorded image is reduced on the other hand, as the 
lower light brings about scarcely any additional exposure of the 
light-sensitive receiver 13 during the short exposure time. 
An electronic CCD camera which converts the light rays received directly 
into an electrical signal can be used as the light-sensitive receiver. 
With this type of camera the exposure time is set by corresponding 
operation in the so-called shutter mode. This means that the camera is 
light-sensitive for only fractions of seconds which correspond to the 
exposure time. 
The conventionally used video cameras operate with a 50 Hz frequency to 
produce a field line by line. This means that a field is present after 20 
milliseconds and a frame after 40 milliseconds irrespective of the actual 
exposure time. In order to be able to produce a frame with flashlight it 
is necessary for the light-sensitive receiver to be exposed in both the 
first and the second period P1, P2 for producing a field. 
As shown in FIG. 3d, provision is therefore made for the surface to be 
exposed by flashlight twice during the second recording period. The 
interval between the flashlights is selected in such a way that the one 
flashlight falls in the first period and the second flashlight in the 
second period for producing a field P1, P2. 
Producing the first image, which is generated by means of continuous 
exposure with the lower light, for example, takes approx. 40 milliseconds. 
The camera can also be light-sensitive for this entire time. In the second 
exposure time the camera is switched into the shutter mode and initially 
exposed by a flashlight, wherein the actual exposure time is 1/2000 of a 
second for example. After the period for producing the first field has 
passed after 20 milliseconds the light-sensitive receiver is exposed by 
the flashlight again under identical conditions. The second field is ready 
after a further 20 milliseconds so that the frame of the second recording 
period is also produced after approx. 40 milliseconds. Only approx. 80 
milliseconds are therefore required to produce two successive images 
whereas a test period of approx. 100 milliseconds is available. 
In the modes of operation shown in FIGS. 3a to 3d it was assumed that the 
flashlight is fired in the second exposure time for example. It is of 
course also possible for this to take place in the first recording. 
Furthermore, the start of the exposure time TB for the light-sensitive 
receiver and the start of the particular recording periods TA1 and TA2 
coincide in FIGS. 3a-d. It is of course also possible for the exposure 
times to be basically shorter than the recording times on the one hand 
and/or to start at a different time from the recording times on the other 
hand. Furthermore, so-called dead times are provided bewteen the recording 
periods and also between the test periods. These dead times substantially 
depend on the performance characteristics of the light-sensitive receiver. 
It is of course also possible for the recording times and also the test 
times to follow each other immediately. 
It is evident that an improved test of a surface can take place by means of 
the process and with the device according to the invention. In particular, 
two images of the surface under test can be produced, one of which was 
taken substantially with the light reflected by the surface only and the 
other with the light scattered by the surface only. Upper light and lower 
light are correspondingly aligned so that the light-sensitive receiver 
substantially receives the light of the upper light reflected at the 
surface and/or the light of the lower light scattered by the surface. 
Even if an image has been produced with illumination by both the upper 
light and the lower light, however, with the aid of the other image the 
corresponding portion of light can be separated and compensated for in the 
data processing unit for evaluation purposes. If illumination takes place 
by upper and lower light, one of which brings about a substantially more 
intensive exposure of the light-sensitive receiver, this compensation is 
not absolutely necessary. 
List of Reference Numerals 
10 Device 
11 CD 
12 Axis 
13 Light-sensitive receiver 
14 Upper light 
15 Lower light 
16 Semi-reflecting mirror 
17 Light source 
18 Diffusor 
19 Light source 
20 Cover element 
21 Cover element 
22 Data processing unit 
23 First control means 
24 Second control means 
30 Device 
31 Lens arrangement 
32 Objective 
33 Light source 
34 Lens 
35 Cylindrical lens 
36 Mirror 
37 Concave mirror 
38 Mirror 
39 Side 
40 Prism 
41 Mirror side 
42 Light source 
43 Piezoactuator 
44 Boundary surface