Apparatus for the illumination of a region of a bottle or the like to be inspected such that bottle acts as waveguide and secondary light source

The apparatus is used for testing the surface structure of hollow, cylindrical regions of transparent, hollow bodies (e.g. bottles) moved along a track. It comprises an illuminating device (10), a conveyor (14) by means of which the hollow bodies (34) are moved through an inspection region, and an imaging device (12). The illuminating device (10) has several light sources (26) which are arranged so that, in every position within the inspection region (9), the hollow bodies (34) receive light at an angle of incidence such that the hollow bodies (34) act as wave guides and appear as secondary light sources with respect to the imaging device (12), so that the imaging device (12) yields a high-contrast image of the surface structures of the hollow bodies. The device is principally used for the testing of recycled bottles.

BACKGROUND OF THE INVENTION 
The invention relates to an apparatus for inspecting a substantially hollow 
cylindrical region of transparent hollow bodies travelling along a track, 
whereby the bodies pass through an inspection zone on a section of the 
track, with an imaging device located on one side of the track including 
the inspection region, and a light source containing an illumination 
device located on the opposite side of the track and illuminating the 
inspection region. 
A known device of this type (U.S. Pat. No. 4,691,231) is used for 
illuminating the sidewalls of bottles which are located on a continuously 
moving conveyor and are inspected by transmitted light, in order to detect 
defects in the walls of the bottles. For this purpose, the known device 
employs six video cameras for the side walls, three more video cameras for 
a lower side wall region and three video cameras for a side wall region 
above this. The video cameras may either be arranged in the shape of an 
arc around the bottle or in a line along the bottle conveyor. The cameras 
are arranged to provide each time data relating to regions of different 
height, over the whole circumference of the bottles. To achieve this, 
diametrically opposed bottle wall regions (front and back sides) are 
illuminated simultaneously and superimposed on each other. For each pair 
of cameras arranged one above the other a light source is provided at the 
side of the bottle opposite the camera; for a total of six cameras there 
are therefore three light sources. A bottle to be inspected is moved 
between the first pair of cameras and its corresponding light source and 
is thus recorded, then between the second pair of cameras and their 
corresponding light source, and finally between the third pair of cameras 
and their corresponding light source. A diffusing, transparent plate is 
disposed between the cameras and the light sources, so that the bottle is 
more or less equally illuminated for each pair of cameras. 
For this known apparatus the illumination comprises illumination purely by 
transmitted light, for which the rear side and the front side (with 
respect to the camera) have the same effect on the path of the light beam. 
Such illumination is not suitable for examining one of the special regions 
of the bottle, for example the mouth region which is usually provided with 
a thread, when this is facing the imaging device, since this type of 
illumination does not give sufficient or uniform contrast to make faults 
or geometric structures in the glass, e.g. the thread visible. Such a 
region requires intensive high-contrast and uniform illumination to form 
the image necessary for inspection, in order that both internal and 
external defects, such as deposits or changes in the geometry of the 
glass, can be detected in this region, where the glass surface is very 
uneven due to the thread. Illumination purely by transmitted light is also 
unsuitable for inspection if the bottle is filled up to the mouth region 
with a substantially opaque liquid, or the bottle glass itself is only 
slightly transparent. The known apparatus therefore only provides for the 
inspection of bottles made of clear or transparent glass, which also have 
no thread in the region of the mouth, and which furthermore must not be 
filled with opaque liquids in this region. Moreover, internal defects in 
the glass are scarcely illuminated with the known apparatus, because much 
of the light which travels in the direction of a normal passing through 
the longitudinal axis of the bottle on to the surface of the bottle is 
lost by reflection at the surface of the bottle or at water which is 
present thereon or by refraction. The incident light falls in the normal 
direction in the known apparatus because the light source and associated 
camera are always diametrically opposed on opposite sides of the bottle. 
SUMMARY OF THE INVENTION 
The object of the invention is to develop an apparatus of the type 
specified at the outset which is capable of providing an image of a 
region, substantially in the form of a hollow cylinder, of a transparent 
body with high contrast, in order to make geometric structures or defects 
in the internal structure of the hollow body more easily visible on 
inspection. 
This object is achieved in apparatus according to the invention in that the 
light sources are distributed around the inspection region so that the 
hollow bodies in each position on the track section receive light at such 
an angle of incidence that the hollow bodies act as wave-guides and appear 
as secondary light sources in relation to the imaging device. 
The apparatus according to the invention makes use of the wave-guide 
effect, and therefore of an effect which was previously mainly used in the 
field of fiber optics. In this respect that part of the light emitted by 
the light sources is used which impinges on the wall of the hollow 
cylindrical region of the hollow body and enters it at such an angle that 
it is totally reflected when it next impinges on an interface between the 
material of the hollow body and the air. There is then a high probability 
of it being totally reflected again, so that the hollow body acts like a 
wave-guide. On the other hand part of this light which is reflected to and 
fro inside the hollow body escapes from the latter if it impinges on the 
interface at an angle less than the critical angle for total reflection. 
In this way the region of the hollow body to be inspected becomes a 
secondary light source. Depending on the angle of the surfaces of the 
hollow body with respect to the cylindrical axis of the hollow cylindrical 
region, the surfaces of the hollow body which has become a secondary light 
source radiate at different intensities, so that a high-contrast image of 
the surface of the cylindrical region can be obtained. 
This is a highly significant advantage compared with the state of the art, 
since a particular object of the device according to the invention is not 
to illuminate a region of the hollow body in which the latter is 
completely cylindrical in every respect, but to illuminate a region such 
as the mouth, where the surface of the hollow body exhibits externally 
projecting structures due to threads, for example, and which according to 
the invention are displayed and depicted by the imaging device by being 
illuminated at high contrast so that the hollow body acts as a secondary 
light source. Likewise cracks, bubbles, etc. in the interior of the 
material act as interfaces on which light can impinge, so that internal 
structural defects of this type can also be identified. 
In one advantageous embodiment of the invention conveyors, especially 
circular conveyors, are provided for moving the hollow body through the 
inspection region. A device may be provided on the conveyors which enables 
the hollow bodies to be rotated on their path through the inspection 
region. 
The light sources are preferably adjusted amongst themselves with respect 
to their arrangement and/or their light output so that they illuminate the 
inspection region homogeneously. The light sources may be adjustable with 
respect to the distance between them or with respect to their light 
output. These possibilities of adjustment enable homogeneous illumination 
of the inspection region to be maintained, for example, in the event of 
the failure of a light source at any time. 
The light sources may be arranged in an arc around the inspection region. 
Particularly advantageous is apparatus for which light sources of the same 
light output are arranged equidistant from each other in an arc in the 
form of a circle around the centre of the inspection region with a radius 
significantly larger than half the distance between the outermost points 
of the inspection region, or for which light sources of equal light output 
are arranged in an arc in the form of a part of an ellipse, the focal 
points of which are the outermost points of the inspection region, with 
the distances between the light sources on the elliptical arc inversely 
proportional to the distance to the nearest focus. The use of light 
sources of equal output in such arrangements ensures that the inspection 
region is irradiated with light of approximately homogeneous intensity 
over the whole of its extent. However it is possible to employ other 
arrangements of the light sources in order to irradiate the inspection 
region with light of approximately homogeneous intensity. 
Moreover, it is advantageous to arrange a wall of a material which is 
opaque to light between the light sources and the inspection region, where 
the wall has an aperture for light to pass through in the shape of an 
elongated slit extending at perpendicular to the longitudinal axes of the 
hollow bodies, the vertical dimension of which is at the same height as 
the height of the regions of the hollow bodies to be inspected and which 
is larger than the vertical dimension of these regions. These walls may be 
plane or curved. 
Light sources may be used which comprise rod-shaped high-power lamps, 
parallel to the longitudinal axis of the hollow body, for which the middle 
light source is opposite to the line which bisects the length of the slit, 
and their length is at least equal to double the slit height. However 
light sources may also be used which comprise elongated tubular lamps, 
each with the shape of a circular or elliptical arc and perpendicular to 
the longitudinal axis of the bottle. 
A row of high-power lamps can be specified for use in the device. 
By providing an arc-shaped reflector parallel to an arc-shaped light source 
arrangement in one embodiment of the invention, additional light energy 
can be made available in the inspection region. This measure may be 
further reinforced by a reflecting coating on the wall with the slit on 
its internal surface which faces the light sources. 
Further arc-shaped reflectors ensure that light which would otherwise 
escape to the outside at the ends of the slit is reflected back into the 
slit and thereby to the inspection region. 
Acting as a secondary light source the hollow, cylindrical region of the 
hollow body emits light which depicts the surface structure with high 
contrast. On meeting the imaging device, the light which passes through 
the hollow cylindrical region of the hollow body without reflection acts 
as an interfering background for the high-contrast display of the surface 
structure of the hollow cylindrical part by the secondary light source. 
This interference is reduced if the imaging device is aligned at the 
centre of the inspection region, or obliquely with respect to the 
inspection region, so that it sees the region of the hollow body to be 
inspected over the whole of the inspection region, but as little as 
possible of the light sources directly.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the drawings the hollow bodies are shown as bottles 34 (FIG. 2), for 
which the threads in the region of the mouth, and therefore the necks 1 
(FIG. 3), are to be examined. The term "bottles" will therefore be used in 
part below instead of hollow bodies, where the same numeral 34 is used for 
both. These bottles 34 are examined inside an inspection region 9, which 
is shown in FIG. 4. 
FIG. 1 is an overall view of a bottle examination machine which has an 
illuminating device 10 for the regions of the mounths of the bottles 34 to 
be examined. 
Instead of being used for the illumination of the mouth regions of bottles 
which are provided with threads, the device 10 could also be used to 
illuminate other regions of bottles 34 or other more or less transparent 
bodies of all types such as hollow glassware, hollow bodies made of PET, 
etc. The machine has two circular conveyors 14 which deliver the bottles 
34 and which move them on two circular tracks, so that various 
examinations can be made of the bottles 34. Longitudinal conveyors may 
also be used in such bottle examining machines instead of the circular 
conveyors 14. The illuminating device 10 is assigned to the circular track 
14 on the right of FIG. 1, and has a slit 20 in its front wall 18 which is 
covered by a glass plate 22. A more detailed drawing of an arrangement 
according to the invention is shown in FIGS. 2 and 4. 
One of the imaging devices 12 assigned to the illuminating device 10, which 
is not visible in FIG. 1, is shown in FIG. 2. FIG. 2 also shows in 
simplified form a circular conveyor 14 which supports the bottles 34. 
The illuminating device 10 according to FIG. 2 has a housing 16. This has a 
front wall 18 made of material which is opaque to light, which is disposed 
between the light sources 26 provided in the illuminating device 10 and 
the bottles 34. In the wall 18 there is an elongated slit 20, 
perpendicular to the longitudinal axis of the bottles 34, the vertical 
location of which is at the same height as the height of the regions of 
the hollow bodies 34 to be inspected and the vertical dimension of which 
is larger than the vertical dimension of these regions. The wall 18 may be 
plane or curved. The slit 20 is normally open. However under certain 
conditions it may be covered by a heat-resistant glass plate. A holder 24 
is provided in housing 16 for light sources 26. FIG. 2 shows a type of 
construction for this suitable for rod-shaped high-power lamps, e.g. xenon 
lamps, parallel to the longitudinal axis of the bottles. The middle one of 
these light sources 26 is located opposite the line which bisects the slit 
20 longitudinally and its length is preferably approximately equal to 
twice the slit dimension. The light sources 26 are arranged on the holder 
24 in the shape of an arc, which is shown as an elliptical arc here. 
However it could also be a circular arc. The upper and lower ends of the 
light sources 26 are all arranged at the same height. 
The light sources 26 are linked to a power supply, which is not shown, to 
which they are connected in parallel. The circuit arrangement or the power 
source is designed so that the light output is adjustable for each light 
source 26 individually or for all light sources 26 jointly. 
The circular conveyor 14 rotates in the direction of the arrow 30 about a 
central axis 32 and thus moves the bottles 34, which are individually 
vertically mounted on rotating plates 35 and are rotated clockwise about 
their longitudinal axes past the illuminating device 10. 
In principle, arrangements are also possible in which the light sources 26 
are arbitrarily arranged on one side of the track section 15 and of the 
axes of the hollow bodies 34. However for each arrangement of the light 
sources 26 it is advantageous to position them so that the light intensity 
is homogeneous in the inspection region. 
FIG. 3 shows a bottle neck 1 in cross section; this has an outer surface 2 
and an inner surface 3. Light of all angles from a half-space impinges on 
the bottle neck 1, where the planar limit of the half-space is shown by 
the line 8. This is considered as the same light which is incident on a 
point 4 on the outer surface of the bottle neck. Two solid angle regions 
a, b arise, which are shown in the plane of FIG. 3. Incident light from 
angular region a contributes to the formation of the secondary light 
source in the bottle neck; in contrast, incident light from angular region 
b mainly contributes to the light which passes straight through bottle 
neck 1. The limiting rays 5,5' separate these angular regions from each 
other. The limiting rays 5,5' are the same light rays which just attain 
the critical angle for total reflection at the first interface between the 
bottle material and the air. For bottle glass with an index of refraction 
of about n=1.5, the critical angle is about 41.degree.. 
A denotes a light ray coming from angular region a which enters the bottle 
neck 1 at point 4 and impinges on the next interface between the bottle 
material and the air at an angle greater than the critical angle for total 
reflection. There is a high probability of it undergoing further total 
reflections in the bottle neck 1 until it again escapes from this at some 
point, and thus it contributes to the secondary light source in the bottle 
neck. 
B denotes a light ray coming from angle region b which enters the bottle 
neck 1 at point 4. It impinges on the interface between the bottle 
material and the air at an angle less than the critical angle for total 
reflection, and is transmitted with only a small reflection loss. It thus 
makes a contribution to the light transmitted through the bottle neck 1. 
In the spatial representation, which is not shown here, a right cone of 
limiting radiation arises, the axis 6 of which goes through point 4 and 
bottle axis 7. The light sources 26, which are not shown, illuminate the 
half-space above the bottle neck 1 (shown in FIG. 3), the line 8 thus 
forms the limit of the illuminated region. If it is considered that the 
light sources 26, which are preferably arranged in an arc around the 
inspection region 9 (FIG. 4), preferably illuminate the bottle neck 
homogeneously, it may be seen that appreciably more light enters the 
secondary "bottle neck" radiation source and is also emitted by the latter 
in a high-contrast form than is the case for the transmitted radiation, 
which interferes with the contrast formation. 
To estimate the applicable light intensity over 11 points 4 of the outer 
surface in the secondary light source and the interfering transmitted 
intensity, the reflection losses must be taken into account and then 
integrated over all points 4 to obtain both intensities. 
A schematic diagram of a device is shown in FIG. 4 as an example. Track 
section 15 is provided with an inspection region 9, which is traversed by 
the bottles 34 (FIG. 2), of which only the cross section of the bottle 
neck 1 is shown. The bottle neck 1 is located at the right-hand end of 
inspection region 9, after having passed through the positions 
corresponding to the bottle necks 1' and 1", which are shown in dashed 
lines. These positions are now empty. The illuminating device 10 comprises 
a holder 24, light sources 26 arranged in an elliptical arc, an elliptical 
reflector 39 which is provided at a location in the illuminating device 10 
opposite the inspection region 9, and two further, arc-shaped reflectors 
50 and 52 provided at both ends of the elliptical arc. These reflectors 
increase the optical efficiency of the illuminating device 10. The focal 
points F of the ellipse formed by the light sources 26 are the limits of 
the inspection region 9. The reflectors may also be provided on the outer 
or inner walls of the light sources 26. 
The imaging device 12 is provided on the opposite side of the illuminating 
device 10 with respect to the inspection region 9. In this example it is 
located at the centre of the inspection region 9 and adjusted so that it 
just takes in the whole inspection region 9. At the instant corresponding 
to the drawing, the imaging device 12 only sees the bottle neck 1 located 
at the position on the right, since the positions of the bottle necks 1' 
and 1" are already empty. Because of its alignment the imaging device 12 
only receives very little transmitted light, but only sees the bottle neck 
1 essentially as a secondary source which is illuminating the structure of 
its surface with high contrast. 
The same considerations also apply to the positions 1' and 1" and for all 
the intermediate positions in inspection region 9. It is therefore evident 
that, at every position in the inspection region 9, light is received at 
an angle of incidence such that the hollow body 34 acts as a secondary 
light source with respect to the imaging device 12. 
High-power lamps, such as xenon lamps, halogen lamps or sodium vapour 
lamps, may be used as light sources 26. In addition, high-pressure or 
low-pressure lamps may be used. It is important that the lamps provide a 
high light intensity. 
Moreover, the arc on which the light sources 26 are arranged may also 
comprise a semicircle around the mid-point of the inspection region 9, 
with a radius significantly larger than half the distance between the 
outermost points of the inspection region. 
FIGS. 5 and 6 comprise schematic representations in front view and in plan 
view of an advantageous type of design of elongated light sources 26', 
which are each shaped as a semi-ellipse and comprise tubular lamps 
arranged perpendicular to the longitudinal axis of the bottles. The light 
sources 26' are also arranged with respect to each other at such distances 
apart that they provide a homogeneous light intensity in the inspection 
region 9 (FIG. 4). 
In order that a single imaging device 12 may cover the total perimeter of 
the bottle neck 1, the bottle 34 is advantageously rotated during its 
progress through the inspection region 9. 
The device according to the invention produces a secondary light source in 
the substantially hollow and cylindrical regions of hollow bodies 34 which 
are to be inspected, for example in bottle necks 1, and suppresses the 
interfering transmitted radiation. This makes it possible to display the 
surface structures and the like of the regions to be inspected, at high 
contrast, by means of the imaging device 12.