Apparatus for detecting impurities in translucent bodies

An apparatus including a source for producing a concentrated light beam, such as a laser, a background element spaced from the source against which the light beam is directed, and a transport arrangement for moving translucent bodies, such as French cut potatoes, through the light beam. When light from the source impinges upon a pure translucent body, the light is scattered or diffused within the body around the point of impingement so as to illuminate a surface area of the body larger than the cross-sectional area of the light beam, but light from the source impinging upon an impurity in the translucent body is scattered or diffused to a lesser extent. The background element is formed of a material which causes light from the source which impinges on it to be diffused within the material in a manner similar to the diffusion of the light in the translucent bodies. A receiver having a field of view larger than the cross-sectional area of the light beam receives light reflected from the background and from translucent bodies moving through the light beam. The receiver is insensitive to light in the part of its field of view which is operatively aligned with the point of impingement of the light beam on the translucent bodies or on the background element. The receiver produces an output signal which changes when an impurity enters the concentrated light beam, and this changed signal is used to operate a device which removes the impurity from the remainder of the translucent bodies.

This invention relates to detecting impurities in translucent bodies, and 
removing from a mass of such bodies those in which impurities are 
detected. 
The invention finds utility in detecting impurities, i.e., relatively 
opaque discolorations, in French cut potatoes, i.e., raw potatoes cut into 
strips which after being deep fried become French fried potatoes. 
Therefore, although the invention has other applications as well, it will 
be described with reference to French cut potatoes. 
It is an object of the present invention to provide an apparatus for 
detecting impurities in translucent bodies, e.g., French cut potatoes, 
which permits high speed scanning of the individual potato strips, is 
extremely accurate, enabling detection of even small impurities, and does 
not require a high level of ambient light. 
The invention is predicated on a realization that when a concentrated light 
beam impinges upon a French cut potato strip, the light diffuses into the 
translucent potato so that the surface area of the potato which is 
illuminated is considerably larger than the diameter of the incident light 
beam. For example, a 2 mm helium-neon laser beam directed at a potato 
strip illuminates a surface area of the strip ten to thirty times larger 
than the diameter of the laser beam; the potato strip has the appearance 
of a miniature fluorescent lamp. However, when the concentrated light beam 
impinges upon a defect or impurity in the potato strip, which is opaque, 
or much less translucent than the pure potato material, there is very 
little or no diffusion of the light into the potato. As a result, the 
illuminated area of the potato is about equal to the diameter of the light 
beam, i.e., the illuminated area is much smaller than when the light beam 
strikes a pure region of the potato strip. 
With this difference in illuminated area in mind, it was realized that if a 
sensing means is arranged to receive light reflected from an area of the 
potato strip considerably larger than the diameter of the incident light 
beam, the sensing means can detect a drop in the reflected light received 
when an impurity in the potato enters the light beam. This decrease in 
received light can be used to signal the presence of an impurity which 
should be removed. Moreover, the sensitivity of the sensing means, to 
detection of the drop in reflected light, is increased by making the 
sensing means "blind" at its center, i.e., insensitive to light in the 
part of its field of view operatively aligned with the point of 
impingement of the light beam on the translucent bodies or on the 
background element. 
The invention is based upon an understanding that advantageous use can be 
made of the fact that impact of a concentrated light beam, such as a laser 
beam, upon some materials, including various fibrous and cellulose 
substances, will cause a light scattering effect. The impact of the 
concentrated light beam causes an intense light spot, having a size equal 
to the cross-sectional area of the beam, caused by diffuse reflection on 
the material's surface at the point where the beam strikes, and also 
around the intense light spot, in a circular zone, a less intense 
illumination, the intensity of which gradually decreases from the center 
outwards. If the beam, however, touches an impurity or irregularity in the 
fibrous structure, e.g., a black spot, this scattering phenomenon 
disappears completely. 
This invention thus is characterized by the fact that the detection of 
impurities is obtained by measuring the intensity of only that part of the 
reflected light which is a result of the scattering, around the point of 
light beam impact. The photosensitive receiver is made blind in the part 
of its field of view corresponding to the beam impact spot itself. 
By making the photosensitive detector blind at the center of the reflected 
image, the influence of the diffuse reflected light from the concentrated 
light beam at the point of incidence is eliminated. Thus, although the 
intense light spot at the point of incidence of the concentrated light 
beam is indeed projected toward the center of the detector, such 
projection has no influence on the output signal of the photosensitive 
detector, e.g., a photomultiplier tube. 
The clearly illuminated zone around this spot, however, results in a 
certain intensity of light and therefore in a certain output signal of the 
light sensitive detector. The signal fully disappears when the scanning 
beam touches a dark spot on or in the product being inspected. 
It is another object of the present invention to provide an apparatus for 
detecting impurities in translucent bodies wherein the intensity of 
reflected light from a body changes appreciably when an impurity enters 
the concentrated light beam, but does not change, or changes only 
minimally, when the edges of the body enter and leave the light beam. 
In this connection, use is made of a background in the sorting zone made 
from a material that also spreads the incident light beam by internal 
scattering underneath its surface. By doing this, transitional signals 
that may occur when the scanning beam strikes the edges of the objects to 
be inspected are almost completely eliminated. 
The term "scattering" is used herein in the sense defined in Webster's 
Third New International Dictionary: "to diffuse or disperse (a beam of 
radiation) in a random manner as a result of collision of the particles, 
photons, or waves with particles of the medium traversed". 
The apparatus according to this invention can also successfully be used for 
detection of irregularities among conveyable objects being conveyed, e.g., 
for detection of white stones among blanched navy beans (white beans). 
Blanched beans do present the above-described scatter properties, while 
little stones do not. This permits sorting out foreign objects from among 
navy beans at high speed and very efficiently. 
In one preferred embodiment of the invention, a second photosensitive 
detector is used to receive that part of the reflected light which was 
directed to the insensitive zone of the first detector. This combination 
provides a simple means for choosing scatter reflection or diffuse 
reflection in carrying out specific sorting requirements. The second 
photosensitive detector receives the diffuse reflected light from the 
scanning beam on the point of incidence and the first photosensitive 
detector receives, via a mirror, the reflected scatter-light. Selection of 
the detection mode may be made simply by switching the selected signal 
processing circuit to the detector. 
One preferred solution for separating the two sorts of reflected light is 
to obtain a picture of the illuminated product by using a focusing means 
which focuses the reflected image upon a mirror having a small hole at the 
center, behind which hole the second photosensitive detector is installed 
while the first photosensitive detector only receives the light reflected 
by this mirror less its hole. 
Additional objects and advantages of the present invention will be apparent 
from the following description in which reference is made to the 
accompanying drawings.

The apparatus chosen to illustrate the present invention, and illustrated 
in FIG. 1, includes a conveyor 2, for transporting French cut potato 
strips 6 toward an inspection zone 4, and another conveyor 3 for 
delivering the potato strips which reach it for further processing, such 
as freezing and packaging. The conveyors and other components of the 
apparatus are supported by a framework 1. Between the 2 conveyors is the 
sorting area 4 and underneath is a chute 5 for evacuating the rejected 
product and/or rejected impurities. 
The objects to be sorted, e.g., French cut potatoes, cross the sorting zone 
4 in free flight because conveyor 2 moves at high enough speed so that, as 
the potato strips leave the forward end of the conveyor, they are tossed 
through the air along trajectories indicated by broken line 7 and onto 
conveyor 3. 
Underneath the trajectories 7 followed by the potato strips 6 in the 
sorting zone 4, a background element 8 is mounted. The background element 
is made of a substance having the property of dispersing, by scattering, a 
small incident concentrated light beam in such a way that around the 
highly illuminated spot, where the incident beam strikes the surface, 
another well illuminated area is created, the so called scatter effect. 
When the objects to be sorted follow trajectory 7, they are illuminated 
while in the trajectory by a very fast scanning, highly concentrated 
lightbeam. The scanning lightbeam is produced by a stationary laser unit 
10, mounted on the frame 1, the beam 11, emerging from the laser, striking 
one of the mirrors 13a of the multifaceted polygon shaped mirror 13 after 
passing through an opening in a mirror 12. The polygon mirror is mounted 
on the shaft of an electric motor 14 which rotates the mirror at high 
speed. Therefore the beam reflected by a mirror 13, along the path 14', 
"scans" the full width of sorting zone 4 forming a detection plane above 
the background 8. All product tossed from infeed conveyor 2 towards output 
conveyor 3 will cross this detection plane. 
As described below, this illumination results in a return beam 15 which 
also strikes the multifaceted mirror 13 and then is reflected in a 
direction as indicated by arrowhead 16 on to the mirror 12 which reflects 
the beam in the direction indicated by arrowhead 17. Via a focusing lens 
18 the returning light then reaches the photosensitive detector 20, e.g., 
a photomultiplier, possibly via polarising filter 19, which results in a 
certain output signal of the detector, a signal which is a function of the 
quantity of returning light. 
Above the trajectory 7 of the objects to be sorted, a plurality 22 of 
individual air jets 23 is mounted, each one being connected with an air 
valve 24 and with an air pressure unit 25. The air valves 24 are 
individually operated from a central processing unit 26 which will be 
described later on. 
A basic principle of the present invention is illustrated in FIG. 3. As 
mentioned before, there are materials such as various synthetics 
(polyamides) and also natural products such as potatoes, pears, carrots, 
apricots, and blanched beans, which under a concentrated lightbeam, will 
disperse light underneath their surface in all directions so that around 
the place where the beam touches the surface an enlarged circular light 
zone occurs. 
FIG. 3 shows a part of a background element 8 made of, e.g., polyamide, 
polyethylene, and polyacetate. An incident lightbeam striking the 
background will result in a highly intensive light spot 30 surrounded by a 
less intensive circular light spot 31. When the scanning lightbeam 32 
which produces the intensive light spot reaches a scattering object, e.g., 
a potato strip 33, then on the surface of the potato strip a high 
intensity light spot 34 appears surrounded by a circular light zone 35. 
Via the optics described above, this combination of the central light spot 
and the surrounding circular light zone is projected on to the surface of 
a photosensitive detector 20, as shown in FIG. 4, as a circular lightzone 
36. According to the invention, the optical centerpoint of the 
photosensitive detector has been made blind by means of a small black spot 
37 so that the light spot 30 or 34 caused by diffuse reflection, has no 
influence on the output signal of the photosensitive detector. This effect 
can naturally also be obtained in a different, e.g., an optical way. 
When the scanning beam strikes a pure part of the French cut potato, the 
output signal of the photosensitive detector will not or only slightly 
change. When, however, the scanning beam, along line 32a in FIG. 3, passes 
on to an impurity 39 or a defect in a French cut potato strip 33, this 
will cause a substantial reduction of the light intensity of illuminated 
zone 31, resulting in a large change in the output signal of detector 20. 
The output signal of detector 20 is sent along line 40 (FIG. 1), via a 
pulse shaping network 41 and a threshold circuit 42, to the central 
processing unit 26 which will produce an output signal indicating when, in 
the product being inspected, an irregularity has been detected. The 
rotating mirror 13 is combined with a position detector 43 passing 
information along line 44 to the central processing unit 27 concerning the 
exact position at any moment in time of the mirror facet 13a which is 
illuminated at that moment. This information is directly representative of 
the momentary position of the scanbeam in the detection zone. A suitable 
process circuitry commands the operation of the appropriate air jet 23 
located at the exact place where the detected impurity will pass by. 
Let us assume that in the inspection station 4 a French cut potato 
containing a defect traverses the plane defined by moving beam 14'. The 
process control unit, at the moment it receives the detection signal, 
"gathers" (from the instant of time being referred to the angular position 
of mirror 13a) the exact position of the detected impurity and energizes, 
via the appropriate air valve, the air jet unit 22 located just above the 
impurity's trajectory. The French fry is then deviated from its normal 
flying curve 7 and blown down into the reject chute 5. 
This invention has been shown and described in preferred form only, and by 
way of example, and many variations are possible within the invention 
which will still be comprised within its spirit. For example, FIG. 5 
illustrates a variant showing the reflected beam 17 being focused by lens 
50 on to mirror 51, a front view of which is shown in FIG. 6. As shown in 
FIG. 6, mirror 51 comprises a circular mirroring or reflective part 52 
surrounded by a non-reflective part 53 and further comprises a narrow 
central opening 54 adjusted to coincide with the image of the point of 
incidence of the scanning beam. 
A part 17a of the returning light beam which reaches mirror 51 will pass 
through the central opening 54 and reach a photosensitive detector 55. The 
other part, beam 17b, will reach a second photosensitive detector 56. 
Diffuse reflection on a surface results only in a single light spot which 
will be reflected only on to photosensitive detector 55. Scatter 
reflection, however, results in an illuminated zone which will be 
reflected on to photosensitive detector 56. The respective outputs of both 
detectors are connected to selector switch 57, the output of the latter 
being line 40. This allows easy switching of the apparatus from sorting 
according to diffuse reflection to sorting by using the scattering 
principle. For certain sorting requirements, both detection principles may 
have to be used simultaneously.