FR2733827A1 - Spectrometer with Dammann type network for recognition of objects by their colour. - Google Patents

Abstract

The spectrometer includes the following elements arranged along an axis(X-X): - Means(1) defining an input slit(2); - A Dammann type network(4) for decomposing the light emanating from the object in pre- determined ranges of wave length(R,V,B); - Focusing means(5) for focusing at least one of the wave length ranges in an image plane(6); - measuring means placed in the image plane(7a,7b,7c) so as to measure the light corresponding to at least one of the wave length ranges(R,V,B). The measuring system(7a,7b,7c) includes at least a photo-sensitive detector for transforming the light form at least one of the wave length ranges into an electrical signal.

Description

 The present invention relates, in general, to optical spectrometers comprising a diffraction grating for the decomposition of light. More particularly, the invention relates to the application of such a spectrometer to the recognition of objects from one or more color information.

 A particular type of diffraction grating was first described by H. Dammann in an article dated August 1, 1978 in the review "Applied Physics", Vol.17, N 15, which has since been called by the name of the author of this article.

 As described in detail in the aforementioned article, the Dammann network comprises a transparent plate in which are formed parallel streaks in the form of steps having steps which may be two or more in number. Such a plate has the particularity of decomposing the visible visible light into so-called fundamental spectral bands which correspond more or less to the color bands to which the retina of the human eye is accorded. For more details on the spectral distribution of light from a Dammann grating, we can refer to this article.

 Much more recently, Michael Farn et al. took up the study of Dammann networks and described their conclusions in "Conference on Binary Optics", NASA Conference Publication n 3227, p 409,1993.

 The authors of this article describe in particular an experimental device for analyzing the spectrum from the Dammann grating using a light source whose light, after passing through the grating, is analyzed in a spectrometer equipped with a photomultiplier, a slit of entrance being placed in front of Dammann's network.

 This experimental setup makes it possible to identify, either separately or individually, the three ranges of wavelengths in which the light from the source is decomposed.

 To the knowledge of the Applicant of this patent application, Dammann networks have so far found no practical application and the invention therefore essentially aims to provide a spectrometric device to Dammann network leading to an application usable industrially .

The subject of the invention is therefore a network spectrometer of the Dammann type, in particular for recognizing objects from the perception of their colors, this spectrometer being characterized in that it comprises the following elements placed on the same optical axis :
- means defining an entry slot;
- a Dammann type network for decomposing the light coming from said object into predetermined ranges of wavelengths;
- focusing means for focusing at least one of said wavelength ranges in an image plane; and
- measuring means placed in said image plane for measuring the light corresponding to at least one of said wavelength ranges.

 Thus, the spectrometer according to the invention allows a perception of colors of an object, analogous to that of the human eye. It can therefore generate signals which translate the reactions that a human would have towards the observed object.

This can be particularly useful for guiding robots, for example.

 The spectrometer can also be used advantageously in industry in many applications where the color or colors of an object are important. A typical example in this regard may be the sorting of objects having different colors.

 According to advantageous features of the invention, the spectrometer can be designed to perceive its environment through an entrance slit of essentially rectangular shape making it possible to observe a strip of an object which is presented in front of it. However, the slit can also be circular and allow the spectrometer to have a panoramic field of vision extending over 360.

Other characteristics and advantages of the invention will become apparent during the detailed description which follows, given solely by way of example and made with reference to the appended drawings in which:
- Figure 1 is an optical diagram of a spectrometric device according to the invention allowing in particular the recognition of objects by their colors;
- Figure 2 is a schematic perspective representation of the same spectrometric device;
- Figure 3 is a schematic perspective representation of a particular application of the spectrometric device according to the invention;
- Figure 4 shows in an axial sectional view, a concrete embodiment of an assembly usable as a spectrometric device according to the invention;
- Figure 5 schematically shows a variant of the device according to the invention.

 Referring to Figures 1 and 2, it can be seen that the spectrometric device according to the invention comprises a plate 1 defining an inlet slot 2 of essentially rectangular shape. This slot is oriented perpendicular to the optical axis X-X of the device. The plate 1 is placed so as to receive light from an object 0 symbolized here by an arrow.

A collimating lens 3 is placed on the optical axis
XX behind slot 2. It is followed by a network 4 of
Dammann which in practice is formed by a plate in a transparent material, quartz for example on which is engraved the network of Dammann. This can be carried out in accordance with the description given in the first aforementioned article.

 A focusing lens 5 is placed behind the Dammann grating 4. This lens focuses the light from the grating 4 in an image plane 6 in which is placed at least one photosensitive detector. In the example shown, three detectors 7a, 7b and 7c have been provided.

 Each of the three detectors 7a, 7b and 7c is sensitive to the wavelengths of one of the respective ranges diffracted by the Dammann grating 4. However, if it is desired to detect light only from one or two ranges of the incident light, the number of detectors will be planned accordingly.

 In the example of FIGS. 1 and 2, each of the detectors has a sensitivity area which corresponds to that of the slot 2. This area has a rectangular shape and is oriented in the same direction as the entry slot 2.

 The detectors 7a, 7b and 7c can be connected to an operating circuit 8 which is designed so as to be able to analyze the signals supplied to it by the detectors.

Thus, the device according to the invention allows rapid recognition of the colors of the light coming from the object O.

 It can thus be seen that the detectors 7a, 7b and 7c are arranged in two perpendicular directions R and S defining a plane perpendicular to the optical axis XX and corresponding respectively to a spatial direction (long side of each rectangular detection zone) and to a spectral direction (short side of each of these areas). The spectrometric device according to the invention therefore has a perception of the object which it "sees" according to a relatively limited region of this object which corresponds to the orientation and the shape of the entry slit. We will see that this limitation to the power of perception is not a problem in many applications in which it is only a question of noting what is the color of an object.

 For example, if of a set of colored objects, it is a question of separating the objects of red color, it suffices to scroll the objects in front of a device according to the invention and to exploit the electrical signal supplied by the detector 7c which is placed so as to receive the red component of the light coming from the object. In this case, it is obviously possible to provide only one detector placed so as to receive the red component of the Dammann network.

 The electrical signal thus obtained can then be used to control a switch, known per se in the handling technique, and placed on the path of the objects which pass in front of the device according to the invention. This switch will be ordered each time the electrical signal appears to keep red objects out of the way. Specialists will understand that the same can be done for green and / or blue objects.

 The example which has just been described constitutes only one of the very numerous possibilities offered by the application of a spectrometric device according to the invention. Thus, the device according to the invention can be used (without this being in any way limiting) for the inspection of moving food on a support forming part of a production line, for controlling the quality of surfaces such as those of paper, metals and plastics, for medical examinations by skin color for example, for analysis by color of flow of liquid, in spectrofluorimetry, for flame monitoring, in emission spectroscopy atomic, in 74 thermometry etc.

 Each photosensitive detection zone can also be constituted by a row of photosensitive cells so that the color information can be discriminated according to the spatial direction R. By interconnecting the photosensitive elements for example using an electronic neural network, the rows of photosensitive cells can be likened to an artificial retina.

 It is also possible to design the Dammann grating so that its diffraction spectrum presents at least some of the wavelength ranges located outside the visible light range between the ultraviolet band and the near infrared (between 250 nm and 2m, for example), the choice of the diffracted bands depending on the dimensions and the number of rungs of the network, as described in the aforementioned articles.

 FIG. 3 shows another embodiment of the spectrometric device according to the invention. The entry slit is here irradiated by a bundle of optical fibers 9 which open side by side in this slit. At the output, the photosensitive devices comprise rows of cells, each of which can be associated with an optical fiber in the input beam 9. Thus, the optical fibers can collect several pieces of information at various locations, for example on a production line of food products, this information being converted into information of light passing through the optical fibers (pressure, temperature, color, humidity, vibration etc.).

This information will then be discriminated along the spatial axis R of the device, while the spectral information is given along the direction of the spectral axis S.

 FIG. 4 shows an exemplary embodiment of a device according to the invention in the form of a box 10 of compact construction. To fix ideas, this case which is preferably cylindrical in shape, has a length along the optical axis X-X of approximately 52 mm and a diameter of approximately 35 mm.

 We find in this housing all the elements described above with reference to Figures 1 and 2, these elements being housed in a cylindrical body made in two parts 11 and 12 nested coaxially one inside the other. The rear part 13 of the device can comprise, in the form of a single integrated circuit, both the network of photosensitive detectors 7a, 7b and 7c, as well as the use circuit 8.

FIG. 5 represents a variant of the spectrometric device according to the invention in which the network of
Dammann 4 is flanked on either side by microlens arrays 14a and 14b. At the output of the downstream matrix, there then appear for each microlens, three different colored information items (red, blue and green, for example). At the level of the photosensitive detectors, it will be necessary in this case to provide a matrix of photosensitive cells whose cells are sensitive (by forming groups of one to three cells per microlens respectively) to the color information provided by the microlenses.

Claims (6)

 1. Dammann type network spectrometer, in particular for recognizing objects from the perception of their colors, this spectrometer being characterized in that it comprises the following elements placed on the same optical axis (x-X):  - means (1; 15) defining an entry slot (2; 16);  - a Dammann type network (4; 19) for decomposing the light coming from said object into predetermined ranges of wavelengths (R, G, B);  - focusing means (5; 20) for focusing at least one of said wavelength ranges in an image plane (6; 21); and  - measuring means placed in said image plane (7a, 7b, 7c; 22a, 22b, 22c) for measuring the light corresponding to at least one of said wavelength ranges (R, V, B).  2. Spectrometer according to claim 1, characterized in that said measuring means (7a, 7b, 7c; 22a, 22b, 22c) comprise at least one photosensitive detector for transforming the light coming from at least one of said ranges of wavelengths in an electrical signal.  3. Spectrometer according to claim 2, characterized in that said photosensitive detector (7a, 7b, 7c; 22a, 22b, 22c) is composed of a matrix of photosensitive cells.  4. Spectrometer according to claim 3, characterized in that it comprises a beam of light guides (9) for bringing light to be observed in said entry slit (2; 16).  5. Spectrometer according to any one of the preceding claims, characterized in that said Dammann grating (4) comprises steps parallel to each other, in that said entry slit (2) is of essentially rectangular shape with its long side arranged parallel to said rungs, and in that said measuring means (7a, 7b, 7c) comprise at least one photosensitive detector whose sensitivity extends over an essentially rectangular area with the same orientation as said entry slit (2 ).  6. Spectrometer according to any one of the preceding claims, characterized in that it comprises at least one matrix of microlenses 14a, 14b) placed against said Dammann grating (4, 19) on said optical axis.