Color detection device

Color detection apparatus wherein only light that has been diffuse-reflected, and has not been mirror-reflected, from the object whose color is to be detected is processed. Light from a source passes through a first polarizer before the light is incident on the object whose color is to be detected. Light reflected from this target object passes through a second polarizer which blocks all mirror-reflected light while permitting a portion of the diffuse-reflected light to pass. After passing through the second polarizer a portion of the light is directed onto a color filter after which the intensity of the transmitted light is detected. The intensity of the portion of light not applied to the color filter is also detected. The ratio of the output of the two detectors is a function of the color of the reflected light. The principal axis of the light incident on the target object is coincident with the principal axis of the reflected light that is to be processed, thus eliminating the need to accurately position the target object.

The invention relates to a color detection device. 
Devices are known for detecting color and are applied in, among others, 
fruit sorting machines. In this device the light strikes the object so 
that a substantial angle exists between the principal axis of the incident 
light and the axis normal to the surface of the object. Also, the light to 
be detected leaves the object at a substantial angle with respect to the 
normal axis. This is necessary in order to detect light resulting solely 
from diffuse-reflection, because this reflected light contains much more 
color information, than light resulting from mirror-reflection, that 
contains hardly any color information. The angle of incidence of the light 
from the source and the angle of leaving of the light to be detected must 
for this reason, be different. Because the angles between the axis normal 
to the surface of the object and both the axis of the incident light beam 
and the axis of the reflected light, to be detected are substantial, the 
point of intersection of the latter two axes is well defined. Thus the 
positioning of the surface of the object must be accurate. When this point 
of intersection, little or no light at all will reach the detector. The 
drawback of these severe demands with reference to the accuracy of the 
positioning of the surface in practice appears to be very large, 
particularly when fruit or other asymmetrical objects, are being sorted 
according to color. The device according to the invention lacks these 
drawbacks. In the present invention an object is at least partially 
illuminated by means of a light-source. A portion of the light 
diffuse-reflected from the object passes through a lens system to be 
divided into various beams. After having passed at least one color filter 
the beams are detected by various colored light intensity detectors, which 
emit electric currents proportional to the received colored light 
intensity, which can be further processed. 
In order to detect only diffuse-reflected light, light from the source 
passes through a first polarization filter with a first polarization 
direction, before striking the object whose color is to be detected. A 
portion of the reflected light passes through a second polarization filter 
with a second polarization direction. 
The first polarization direction of the first polarization filter is such, 
that the mirror reflected light has principally a polarization direction 
that differs preferably by 90.degree., from the second polarization 
direction of the second polarization filter. 
In one embodiment of the present invention the axis, along which light 
principally propagates before striking the object, coincides with the axis 
along which the light to be detected principally propagates after 
reflection by the object.

In FIG. 1, white light along axis 1 from a light source is incident upon an 
object 2. Of the incident light, a portion reflects as a result of 
mirror-reflection back along axis 1, which in this case is normal to the 
surface of about2, and object 2, part scatters as a result of 
diffuse-reflection. Of the last mentioned part, the part that reflects in 
the direction of axis 3 is detected by a device installed along axis 3. 
In FIG. 2, white light from a light source travels substantially parallel 
to axis 1, which axis makes an angle .alpha. axis normal to 4, the surface 
of object 2. A portion of the light, as a result of diffuse-reflection, is 
directed back to a detector device along axis 3 which makes an angle 
.beta. with normal axis 4, wherein angle .beta. must be unequal to angle 
.alpha. in order to avoid incidence on the detector device of 
mirror-reflected light. 
FIGS. 3 and 4 indicate the importance in the case of FIGS. 1 and 2 of the 
placement of object 2 is with respect to the point of intersection of axes 
1 and 3. If the object plane lies under the point of intersection of axes 
1 and 3, as illustrated in FIG. 3, or if the object plane lies above the 
point of intersection of axes 1 and 3, as illustrated in FIG. 4, a 
detection device on axis 3 will not receive light reflected from the 
surface of object 2. In both cases the detection device will receive too 
little or no light at all. 
Because the light beam, incident upon object 2, in fact will be a somewhat 
diverging beam, the angle between axis 1, the principle axis of light 
propagation, and axis 3, the principle axis along which light is detected, 
must not be too small in order to eliminate the detection of 
mirror-reflected light. As this angle increases the point of intersection 
of axis 1 and 3 becomes better defined, and, therefore it becomes more 
difficult, in practice, to accurately adjust the location of the plane of 
reflection on object 2 to include the point of intersection. Moreover, 
when object 2 has a bent surface, part of the light received by the 
detection device will be a result of mirror-reflection. 
FIG. 5 is an illustration of a device according to the present invention. 
White light from light source 11 is transmitted via lens systems 12 and 13 
through polarization filter 14. The beam then reflects off of a plane of 
prism 15, and, as a somewhat diverging beam, is incident on a plane of 
object 16, the color of which is to be detected. The light reflected off 
of object 16, both as a consequence of mirror-reflection and as a 
consequence of diffuse reflection, propagates along an axis coincident 
with the before-mentioned incident beam axis, and is incident upon lens 
17. Behind lens 17 is a second polarization filter 18. At an angle of 
45.degree. to polarization filter 18 is dividing mirror 19 mounted behind 
polarization filter 18 so as to transmit a part of the incident light 
which is detected via optical color filter 20 by detector 21, and to 
reflect the remaining light toward detector 22. 
The polarization direction of the light having passed through polarization 
filter 14, is such that the part thereof that, returns to lens 17 as a 
result of mirror-reflection and therefore, retains its polarization 
direction, is arrested by second polarization filter 18, because the 
polarization direction thereof differs by 90.degree. from the polarization 
direction of the mirror-reflection light reflected off an object 16. Only 
the part of the light that returns in the direction of the lens 17 as a 
consequence of diffuse-reflection, is able to pass through second 
polarization filter 18, since as a consequence of the diffuse reflection, 
this light is no longer polarized in the original direction. The part 
transmitted through dividing mirror 19 contains the desired color 
information. Since the axis of light incident on object 16 coincides with 
the axis of light reflected toward detector 21 the location of the plane 
of impact of the light reflected from prism 15 object 16 is not critical 
so that object 16 can even move between wide limits. In the shape of 
object 16 is not critical, considerably enlarging the number of objects 
capable of undergoing color detection. 
By use of dividing mirror 19 and optical color filter 20 before detectors 
21 and 22, the frequency of light eliciting maximum sensitivity for 
detector 21 may be made different from the frequency of maximum 
sensitivity for detector 22. Detectors 21 and 22, transform incident 
radiation into electric currents proportional to the intensity of the 
radiation. The voltages resulting therefrom are divided one by another, 
from which an electric voltage results, having a magnitude dependent on 
the color of object 16. By determination of the magnitude of the ratio of 
the output voltages of detectors 21 and 22, the result is independent of 
the total quantity of light reaching the detector so that the result 
solely reflects the spectral composition of the light flux resulting in a 
detection device with a large dynamic range. The electric voltage 
representative of the ratio of detector outputs can be supplied to an 
electronic signal conversion unit, containing, for example threshold 
devices, in which the supplied voltage is compared with a number of 
adjustable voltages generated in the signal conversion unit. This a broad 
spectrum ca be can in a number of smaller spectral fields, the frequency 
limits of which are adjustable in accordance with the adjustable voltages 
generated in the signal conversion unit. When a voltage, generated by the 
detection devices, is outside of these values, a relay on the signal 
conversion unit is excited, which can for example control solenoids, which 
operate mechanical switching systems. 
The color detection device according to the invention can find application 
for example in color sorting devices for a large number of objects, such 
as fruits, dye-stuffs, plastics and many others.