AN APPARATUS FOR COLOUR CONTROL OF OBJECTS

Abstract: An apparatus for colour control of objects (11) has an approximately point-shaped light source in the form of a xenon flash lamp (16), which illuminates a specific, desired area through a diaphragm (17). Light reflected from the object is received by a detector unit (13) having three or more sensors (22) with their respective spectral sensitivities, which are positioned at such a great distance from the object that each sensor receives substantially the same amount of light from all parts of the illuminated area. The detector signals are amplified by amplifiers adapted to filter all signals exhibiting another timewise variation than the light source, so that disturbing influence form the surroundings is avoided. The apparatus is simple and fast and is versatile in use.

Description: An apparatus for colour control of objects The invention concerns an apparatus of the type defined in the introductory portion of claim 1. In industrial colour control or colour sorting it is often necessary to control objects which move with a great velocity, have different sizes, are hot or wet or are difficult to contact with colour control equipment for other reasons. It is therefore desirable that the colours of the objects can be controlled at a distance. It is known to perform colour control by illuminating an object with light from a pulsed source of light, with subsequent detection of the reflected light. The reflected light is imaged through a lens system on the centres of a detector unit. Since the light source is pulsed, light having another timewise variation may be filtered by suitable signal processing. The detected light is then compared with a reference signal from the pulsed light source, the colour of the light being determined on this background. The principle outlined here is known e.g. from the German Patent DE 3 244 286. It is a problem in colour measurement that the colour detectors have a finite light-sensitive area. The sensi- tivity in this area is constant for an ideal sensor, but in practice the sensitivity of the centres can vary very strongly even within small distances of the light-sensitive area. This gives problems when an object point is imaged in an image point. Thus, the detected light intensity will depend upon the position of the object, where even small changes in the position of the object will entail that different light intensities are measured. Since known colour measuring devices, where the measurement takes place without contact be¬ tween the object and the detector, mainly image the object on a plurality of sensors, it will i.a. be i - possible to obtain a constant colour measurement when an object, e.g. a shrimp, passes through the measure¬ ment area on a conveyor belt, or if the measurement 2 area is very large, e.g. 1 m . When imaging is performed on the sensors, a non-controllable, weighted averaging of the colour of the measurement area is effected. In addition, a lens system is to be trimmed to minimize the chromatic aberrations. To minimize these aberrations it is known to use an aperture diaphragm so that the used part of the lenses is limited, thus reducing the light intensity. For the measurement area to be imaged on e.g. three detectors, the system must be strictly symmetrical. Another problem of the prior art is that it is not possible to perform colour measurement where the object and the detector/light source have a great mutual dis¬ tance. This is a cleaning problem in the practical use with wet or dusty objects on a conveyor belt (fish, shell fish, chips, egg white, etc.). Finally, objects having different thickness give problems if their mutual thickness variation is not small with respect to the measurement distance. Such a variation causes problems in particular in a system where the measurement area is imaged on the sensors. For objects to be conveyed with an even speed it is necessary to perform simultaneous colour measurements. In the prior art it is often a problem that the useful wavelength area is small. This band limitation is caused by the fact that the known set-ups contain different sophisticated components whose losses are wavelength dependent, e.g. imaging components. Frequently, it may be advantageous to have a measurement area from 300 nm (ultra violet) to 2000 nm (infra red) at dis¬ posal. The object of the invention is to provide a colour measuring apparatus which measures the average value of the colour in the measurement area. Then, the result of the colour measurement will substantially not depend upon the position of the object in the measurement area when only the same part of the object is illuminated, Further, the measurement must be contact-free, the measurements being performed simultaneously in several frequency bands without using imaging components. Another object of the invention is to minimize the number of optical components used. Further, a great measurement distance is desirable, i.a. to be able to vary the dimensions of the measurement area from above 1 m 2 to below 1 mm2. A further object is to be able ,to use the apparatus for colour measurements far beyond the area with visible light. These objects are achieved in that the light source and in particular the detector are positioned at a great distance from the objects to be measured. No imaging components may be positioned between object and detector. The sensors will then receive substantially the same amount of light from all parts of the illumi¬ nated area. The lens and the diaphragm used in the system between light source and object are only used for varying the size of the illuminated area. The diaphragm is a visual field diaphragm in contrast to the aperture diaphragms which are used in the prior art to improve the image quality. The colour then recorded by the sensors will thus be indicative of the average colour in the entire ; illuminated area. Since no imaging lens systems or the like are incorporated between the object and the detector, the apperatus is not sensitive to changes in the position of the object within the measurement area. Since the number of optical boundary faces is limited, the losses in the system are likewise limited. These are now substantially propagation losses between object and detector, which makes it possible to measure with a great distance between object and measuring equipment. Further, the apparatus is suitable for measuring an average value for objects having greatly varying colour patterns.. When, as stated in the introductory portion of claim 2, the light source is a xenon flash lamp, which is approximately point-shaped, and a convext lens, the structure defined in the characterizing portion of this claim entails that the illumination can be limited to just the desired measurement area which may have an arbitrary, desirable form. Means for filtering signal contributions from the sur¬ roundings are stated in claim 3. The invention also concerns the use of the stated appa- ratus for measuring the dimension of an object in a specific direction, and the characteristic aspect of the use according to the invention is stated in the characterizing portion of claim 4. With this feature, the detected reflected light intensity is a measure of the dimension of the object in the beam direction, e.g. its width. When, as stated in claim 5, dimensional measurement is performed in a plurality of specific positions, the form of the object can be determined. The object may e.g. be positioned on a conveyor belt, and the individual measurements can be performed in predetermined positions of this belt. Means for eliminating light contributions from a specular background behind the objects whose colours are to be determined are stated in claim 6. This capitalizes the knowledge that polarized light remains polarized after specular reflection, but is spread as diffuse light by a non-specular surface. An embodiment of the apparatus of the invention is shown schematically in the drawing and will be described in more detail below with reference to this drawing. In the drawing, 10 is a conveyor belt on which objects 11 to be colour controlled are conveyed. When an object arrives at the control position at an illumination unit 12 and a detector unit 13, a control unit 14 is informed that measurement is to take place. This infor- mation is passed on to the detector unit 13 and via this to the illumination unit 12. The latter has a power supply unit 15 with a trigger to trigger a xenon flash lamp 16, which forms an approximately point-shaped light source and emits a strong light pulse with a duration of 2-6 ,us on reception of the signal from the control unit. This light passes through a visual field diaphragm 17 having a shape corresponding to the area of the object 11 whose colour is to be con¬ trolled and through a convex lens 18 which sends a pensil of substantially parallel rays 19 towards the object 11. When the object is non-specular, it spreads diffuse light, and part of this light impinges on the detector unit 13 which has a lens 21 which sends the light to¬ wards three sensors 22 having different spectral sensi¬ tivities. The detector unit is positioned at such a great distance from the object that each sensor receives substantially the same amount of light from the various parts of the illuminated area from the object. The lens 21 may be omitted since it only serves as an ampli¬ fying element and does not serve as an imaging element. The detector does not satisfy the imaging equation, but is positioned considerably more closely to the lens than to the image point. The sensors may optionally each have their own colour filter (not shown). The signals from the sensors are amplified by amplifiers 23, which are adapted to only allow signals having the same timewise variation as the light source to pass, and the amplified analog signals are converted to digital signals in an A/D converter and a computing unit 24. The computing unit computes the colour and intensity of the light on the basis of the strength of the three signals, and the measurement result is compared with stored values corresponding to approved/ rejected for the object in question. When sorting, the measurement results is compared with a plurality of stored limits, and the object is classified accord- ingly. The object may also be fluorescent and be illuminated with excitation light with a specific wavelength through a colour filter. The apparatus may be used for size determination by illumination of an area having at least the same size as the object. The measured intensity of light with the colour of the object is indicative of the size (area) of the object. If the object is placed on a background having a colour different from the object, the reliability of the size determination will be par- ticularly great, it being possible to determine the intensity of light both with the colour of the object and with the colour of the background. This procedure is of particular importance if the colour is not quite constant from object to object. When applying the system to width determination, the object is illuminated in a narrow streak transversely to the object. The measured intensity of light having the colour of the object is indicative of the width of the object. Shape determination may be performed by positioning the object on a conveyor belt running past a width determining set-up as described above. Comparison of measurements of the width of the object with simultaneous measurements of the position of the conveyor belt pro¬ vides an indication of the shape of the object. It may e.g. be determined in this way whether an ice-cream cone has its tip pointing forwardly or rearwardly on the conveyor belt, which is important in automatic packaging procedures. When using a set of intersecting polarization filters, placed at the illumination units and at the detector unit, respectively, the apparatus may be used for colour determination of objects present on a specular background, since specular light is completely suppressed by the polarization, while only 75% of diffuse light disappears. Similarly, undesirable reflections from wet glossy or otherwise specular surfaces are suppressed. This set-up may also be used for detecting whether an impurity is present on a specular surface and for determining the colour of the impurity. What is in mind here is particular detection whether an egg white present in a stainless steel container is polluted with yolk or something else. When using the colour controlling properties of the apparatus it may be deter¬ mined whether the impurity consists of yolk, blood or yolk string. The shown and described apparatus may be modified in several ways within the scope of the invention. E.g., a modulated lamp, which may be a xenon lamp, may be used as a light source instead of a flash lamp.

Claims: P a t e n t C l a i m s 1. An apparatus for colour control of objects (11) and containing a light source (12) for illuminating the object (11), a detector (13) for filtering and measuring the light reflected from the objects, c h a r a c t e r i z e d in that the light source (12) is adapted to evenly illuminate a measurement area comprising the part of the object whose colour is to be determined with another timewise variation than the background light and with great intensity, and that the detector (13) is positioned at such a great distance from the objects (11) that each of the sensors (22) of the detector (13) image-free receives substantially the same amount of light from all parts of the illuminated measurement areas. 2. An apparatus according to claim 1, in which the light source (12) is formed by a xenon flash lamp, c h a r a c t e r i z e d in that a diaphragm is inter¬ posed between the light source (12) and the objects (11), said diaphragm limiting the illumination of the objects (11) to the desired measurement area. 3. An apparatus according to claim 1 or 2, in which the detector (13) contains amplifiers (23) for ampli¬ fication of electric signals which each represent the light passed through a colour selective element, c h a r a c t e r i z e d in that the amplifiers (23) are adapted to filter the signal contributions originat¬ ing from light having another timewise variation than the illumination pulses. 4. Use of the apparatus according to claim 1 , 2 or 3 for measurement of the dimension of an object in a specific direction, c h a r a c t e r i z e d in that the object is illuminated in a streak in the direc¬ tion in question. 5. Use according to claim 4 for determination of the shape of an object, c h a r a c t e r i z e d by dimen- sional measurement in a plurality of specific positions of the object. 6. An apparatus according to claim 1 where the objects are positioned on a specular measurement area, c h a ¬ r a c t e r i z e d in that a first polarization filter having a first polarization direction is interposed between light source and object, and that a second polarization filter having a polarization direction orthogonal to the first polarization direction is inter¬ posed between object and detector.

Inventors: NIELSEN BJARNE CHRISTIAN, NIELSEN, BJARNE, CHRISTIAN
Applicants: NIELSEN BJARNE CHRISTIAN, NIELSEN, BJARNE, CHRISTIAN
Application year: 1988
Main classification: G01J3
IPC codes: B07C5, G01J1, G01J3, G01N21