BE1022447B1 - DEVICE FOR FOCUSING A LASER BEAM BY CAMERA (II) - Google Patents

Abstract

Device for focusing a laser beam on the surface (3) of a liquid, viscous or pulverulent product, the level of which is variable comprising: - an image processing system coupled to the imaging device (2) for measure the distance between two points or two lines; - means for calculating the distance between the laser source (1) and the surface (3) as a function of the aforementioned distance, on the basis of a preliminary calibration; - Means for modifying the focusing of the laser beam, as a function of the distance calculated between the laser source (1) and the surface (3).

Description

DEVICE FOR FOCUSING A LASER BEAM BY CAMERA (II)

Object of the Invention [0001] The present invention relates to a distance measuring device using a camera and intended for focussing a laser beam, mainly but not exclusively on a liquid surface at high temperature, that is, that is greater than at least 600 ° C. The invention also relates to the method implemented by the device.

BACKGROUND AND PRIOR ART [0002] When it is necessary to focus a laser beam on a surface located at a variable distance from the source, it is important to know this distance with good accuracy.

For this purpose, there is a whole series of devices available commercially. These are based on different principles and can be used in widely varying ranges of distances, with an accuracy that depends both on the measuring principle and the conditions under which it takes place.

When the surface considered is that of a product at high temperature, most existing devices are inappropriate. In general, contactless measurement methods based on different optical principles are used. The two best known are the methods of triangulation and flight time.

The method of triangulation is to send on the surface a light beam whose direction forms a certain angle, usually quite small, with the perpendicular to this surface. The reflected beam is collected and measured, at the surface of the same sensor, the half distance between it and the beam emitted. The knowledge of the angle of the incident beam and this distance is sufficient to define a right triangle whose long side of the right angle represents the distance to be measured (the half-distance on the sensor between the emitted beam and the reflected beam being the small side of the right angle of this right triangle).

This method can be very accurate. Precision increases with the angle of incidence but at the expense of the size of the device. However, the method has some disadvantages: - if the temperature of the target surface is very high, the clean radiation of the latter may disturb the measurement; - if the surface is very little reflective or very rough (scattering), the presence of this radiation can even make the measurement impossible; - the presence of dust or smoke between the sensor and the surface to be measured may also affect the measurement.

[0007] The flight time method analyzes the time taken by a very short pulse of light to travel the return path of the measuring device to the surface. This method requires a very fast electronics to allow the measurement of short distances and suffers from disadvantages similar to those of the method of triangulation, even if the fact of using a short pulse of light of high intensity makes it possible to better get rid of the radiation clean of the surface.

[0008] EP 1 584 893 A1 discloses a method for controlling or correcting the orientation of a laser light beam with respect to a hollow body, in which the laser beam is directed, in particular for a spectroscopy application. laser-induced plasma (LIBS). In order to detect and correct, in a simple and reliable manner, deviations of the laser beam from the desired orientation in the hollow body, it is proposed to take a coaxial image to the laser beam of a light entry opening. and / or a light outlet opening of the hollow body, respectively of a substance completely included in the hollow body and to acquire, where appropriate correct, respectively adjust, the orientation of the laser beam in the hollow body to from the position of the light entry aperture and / or the light exit aperture, respectively from the surface of the substance, in the image.

OBJECTS OF THE INVENTION [0009] The object of the present invention is to allow a measurement of the distance to the surface of a liquid, mainly a liquid carried at a high temperature, while guaranteeing good accuracy as well as better immunity. dust and clean radiation from the surface.

In particular, this distance measurement must be able to be combined with a focusing system of a power laser beam in order to focus the laser on said surface. An application of this device is the measurement by the Laser Induced Breakdown Spectroscopy (LIBS) method of the chemical composition of the liquid.

Finally this invention is also an application to the level measurement of liquid products, viscous or powdery.

Main features of the invention [0012] A first aspect of the present invention relates to a device for focussing a laser beam on the surface of a liquid, viscous or powdery product whose level, that is, that is to say the vertical position of the surface, is variable over time, comprising: a source emitting a laser beam that can be focused on said surface; an optical element associated with the source capable of generating, from the laser beam emitted by said source, a plurality of points or lines or a given geometric pattern at said surface; - A digital imaging device, fixed relative to the source for capturing the image of the plurality of points or lines or the geometric pattern; an image processing system coupled to the imaging device for measuring the distance between two points or two lines or a characteristic size of the geometric pattern; means for calculating the distance between the laser source and the surface as a function of the distance measured between two points or two lines or the measured size of the geometric pattern, on the basis of a prior calibration; means for modifying the focusing of the laser beam, as a function of the distance calculated between the laser source and the surface.

According to examples of embodiments of the invention, the device is further limited by at least one, or an appropriate combination, of the following characteristics: the optics associated with the source is a beam splitter at lateral displacement; the optics associated with the source is an assembly comprising a splitter cube or a splitter plate and a mirror or a right-angle prism capable of generating two parallel output beams of adjustable spacing; the mirror or the right-angle prism is pivotally mounted to allow the two output beams to be slightly convergent; the optics associated with the source is a DOE-type shape-generating optics; the laser source is a laser diode emitting at a wavelength of less than 500 nm; the digital imaging device is a color digital camera with a low opening optics to increase the depth of field, optionally provided with a narrow bandwidth filter; - the digital camera is a CCD or CMOS camera with at least 256 levels or 8 bits.

A second aspect of the present invention relates to a method for focusing a laser beam on the surface of a liquid product carried at high temperature, the level of which is variable over time, by means of the device. as described above, characterized by the following steps: - the exposure time of the digital imaging device is set so that the image of the surface of the liquid has a luminous level below a given threshold; - Generating on the surface of the liquid carried at high temperature, with a laser source emitting in the blue and its associated optics, at least two points or two lines or a geometric pattern; the intensity of the laser source is adjusted so that the image of the points, the lines or the geometrical pattern is saturated or has a light level higher than the aforementioned threshold; the points, the lines or the geometrical pattern are isolated in the image by applying a thresholding algorithm at the level of the image processing; the number of pixels of the digital imaging device is measured as the distance between the points or the lines or a characteristic size of the geometric pattern; thanks to a preliminary calibration, this gap or size is computationally associated with a distance value between the laser source and the aforementioned surface; the focus of the laser beam is modified as a function of the calculated distance between the laser source and the surface.

BRIEF DESCRIPTION OF THE FIGURES [0015] FIG. 1 schematically represents an exemplary assembly illustrating a form of use of the technique according to the present invention.

FIG. 2 schematically illustrates an exemplary beam splitter configuration according to the invention, based on the use of a splitter cube and a 90 ° prism.

FIG. 3 shows examples of images obtained at two distance values, before and after treatment according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION [0018] The principle of the method according to the present invention is based on the fact that an object of given size will generate on a detector such as a camera, an image which will appear all the smaller as this object will be placed at a greater distance.

The object of the invention is then to materialize a geometrical figure on a plane identical to that of the surface of the liquid, viscous or powdery material which it is desired to measure the distance to the detector and to use the dimensional characteristics of the image of this geometrical figure on the detector to calculate the distance of said material to the detector, by means of image analysis and appropriate calibration.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION According to a preferred embodiment of the invention, a laser diode emitting a blue beam at 445 nm is used. The advantage of using a light in the blue is that the high temperature surfaces (600 to 3000 ° C) emit only a small part of their thermal radiation in this range of wavelengths. As a result, it is relatively easy, by image processing, to separate the light from the laser from that emitted by the high temperature material.

One can optionally use a color camera and extract the channel "blue" to increase the contrast and reject the longer wavelengths (green and red). If necessary, the use of a narrow bandwidth optical filter centered on the wavelength of the laser further enhances the contrast.

The configuration used, shown schematically in Figure 1, comprises, in addition to a laser diode 1, a color camera 2 and a halogen lamp 3 serving as a target and covered with a tracing paper serving as a diffuser for homogenizing the light. This halogen lamp 3 simulates the material at high temperature. Indeed, the color temperature of such a lamp may be higher than 3000 ° C.

Thus, a light point or a small vertical line is generated by adjusting the output optics of the laser diode 1. This point or line is divided into 2 by placing, at the output of the laser diode, a separator of Lateral beam beams 4 (lateral displacement beamsplitter).

The latter is known to those skilled in the art as a separator producing two parallel output beams. It consists of one or two rhomboid prisms cemented at a right angle prism. The displacement value is determined by the size of the rhomboid prism (s).

In this way, we project on the surface of which we want to measure the distance, a set of two points or two parallel lines separated by a constant distance as a function of distance. This gap is 20 mm in this case.

Alternatively, one can replace the lateral displacement beam separator 4 by a cube (or blade) separator (separator) 5 and a mirror or a right-angle prism 6 (see Figure 2). The advantages of this last configuration are as follows: the distance between the two output beams is adjustable and can be optimized according to the application and the measurement conditions (total distance, size, etc.); - The two beams can be rendered slightly converging by turning the prism 6 around a vertical axis. In this case, the apparent spacing between the two projected points on the surface will decrease even more with the increase in the measurement distance, which will increase the precision by the same amount.

The camera will ideally have a small aperture (large number of aperture f / number) so as to increase the depth of field, which improves the accuracy by reducing the fuzzy areas around points or projected lines.

To measure, simply adjust the exposure time of the camera so that the entire image has a light level below a given threshold.

For example, settings can be made so that the blue level is less than 128 on the 256 levels accepted by current cameras.

If the laser diode has a sufficient intensity, it is possible to obtain an image of the points or lines that is saturated (level = 255) at these locations, or at least whose brightness is greater than the maximum level of the rest of the picture.

By applying a thresholding algorithm to the complete image, it is thus possible to isolate the points or the lines. Image thresholding replaces the pixels of an image one by one using a fixed threshold value. Thus, if a pixel has a value greater than the threshold, it will take for example the value 255 (white) and if its value is less than the breast, it will then take the value 0 (black).

It is then sufficient to measure the spacing, in number of pixels, between the centroids of the points or lines. By proper calibration, this gap can be directly related to the distance to the surface to be measured.

FIG. 3 shows images obtained before and after thresholding processing. The difference in spacing between the two spots, even if it is not really visible to the eye in the figures is 10 pixels, which correspond to a distance difference of 50 mm (distances 1300 and 1250 mm respectively). We have a precision of 5 mm per pixel.

Finally we can formulate the following remarks: - the accuracy is doubled if the spacing between the spots is doubled. So, if this spacing, from 20 mm in the example above, is increased up to 40 mm, a precision of 2.5 mm per pixel is obtained. As mentioned above, this can be further improved by making the two beams slightly convergent; - if a part of the image is saturated despite the settings, it is still possible to use the method using lines whose length will be greater than the saturated zone. In this case, it will remain possible to identify the original lines with the help of an image processing based on the recognition of objects; - rather than using dots or lines, any geometric shape may be suitable if it is easy to identify by image processing depending on the measurement conditions (camera sensitivity, temperature, size, ambient light, etc.). Complex shapes can be generated from a laser and a suitable optics such as a diffractive optical element (DOE), that is to say a pattern with a thin microstructure that alters the phase of the light passing through it. by interference and diffraction, to form arbitrary light distributions.

List of reference symbols 1. Laser diode 2. Camera 3. Measured distance surface 4. Lateral beam splitter 5. Splitter cube 6. Right angle prism 7. Optical points associated with source

Claims (9)

1. Device for focusing a laser beam on the surface (3) of a viscous or powdery liquid product whose level, ie the vertical position of the surface, is variable during the time, comprising: - a source (1) emitting a laser beam that can be focused on said surface (3); an optics associated with the source (4), capable of generating, from the laser beam emitted by said source (1), a plurality of points or lines or a given geometric pattern (7) at said surface ( 3); - A digital imaging device (2) fixed with respect to the source (1) for capturing the image of the plurality of points or lines or the geometric pattern (7); an image processing system coupled to the imaging device (2) for measuring the distance between two points or two lines or a characteristic size of the geometric pattern; means for calculating the distance between the laser source (1) and the surface (3) as a function of the distance measured between two points or two lines or the measured size of the geometric pattern, on the basis of a prior calibration; means for modifying the focusing of the laser beam, as a function of the distance calculated between the laser source (1) and the surface (3). 2. Device according to claim 1, characterized in that the optical associated with the source (4) is a lateral displacement beam separator. 3. Device according to claim 1, characterized in that the optical associated with the source (4) is an assembly comprising a separator cube or a splitter blade (5) and a mirror or a right-angle prism (6) capable to generate two parallel output beams of adjustable spacing. 4. Device according to claim 3, characterized in that the mirror or the right-angle prism (6) is pivotally mounted to allow the two output beams to be slightly convergent. 5. Device according to claim 1, characterized in that the optics associated with the source (4) is a shape-generating optics DOE type. 6. Device according to claim 1, characterized in that the laser source (1) is a laser diode emitting at a wavelength of less than 500 nm. 7. Device according to claim 1, characterized in that the digital imaging device (2) is a color digital camera with a low opening optics to increase the depth of field, optionally provided with a narrow bandwidth filter. 8. Device according to claim 1, characterized in that the digital camera is a CCD or CMOS camera with at least 256 levels or 8 bits. 9. A method for focusing a laser beam on the surface (3) of a liquid product carried at a high temperature, the level of which is variable over time, by means of the device according to any one of claims 1 to 8, characterized by the following steps: - the exposure time of the digital imaging device (2) is adjusted so that the image of the surface of the liquid has a light level below a given threshold; - Generating on the surface of the liquid carried at high temperature, with a laser source (1) emitting in the blue and its associated optics (4), at least two points or two lines or a geometric pattern; the intensity of the laser source (1) is adjusted so that the image of the points, the lines or the geometrical pattern is saturated or has a luminous level higher than the aforementioned threshold; the points, the lines or the geometrical pattern are isolated in the image by applying a thresholding algorithm at the level of the image processing; the number of pixels of the digital imaging device (2) is measured as the distance between the points or the lines or a characteristic size of the geometric pattern; thanks to a preliminary calibration, this gap or this size is associated by calculation with a distance value between the laser source (1) and the aforementioned surface (3); the focus of the laser beam is modified as a function of the calculated distance between the laser source (1) and the surface (3).