GB2106243A - Optical measuring system employing coaxial illumination - Google Patents

Abstract

An optical measuring system employs coaxial illumination and is characterised in that it comprises a light source S coaxially illuminating an object (2) situated in front of a lens (3) in order to determine the dimensions or position of the object (2), a photodetector (4) being disposed on the image plane of the lens (3) at the location of the image of the object (2) under measurement. Reflecting surfaces may be provided at 5 or on the object itself. The detector which may be a photodiode, or a photodiode, photoresistor or CCD array, may be situated perpendicularly to the line of sight and cooperate with beam separator 9 and light trap 10. A light slit 8 and detector 4 may be at equal distances from lens 3. The main application of the invention is in the optical measuring industry. <IMAGE>

Description

SPECIFICATION Optical measuring system employing coaxial illumination This invention relates to an optical measuring system employing coaxial illumination. Optical measuring systems are already known in which contrast is improved by disposing a light source behind the object. This source is, however, always too bulky and requires complex adjustments.

The object of this invention is to obviate these disadvantages and proposes a universal contactless measuring system whereby it is possible to determine and measure the position of an object, its dimensions, or rapid phenomena such as the very rapid movement of an object.

Another object of the invention is to measure the dimensions or speeds of moving objects in the case of rapid movements or non-solid materials.

Another object of the invention is to provide an optical measuring system using photodetectors with high adaptability and good contrast so that the measurement is more reliable and insensitive to the ambient illumination, the measurements being made with a low power of illumination.

Finally, another object of the invention is to provide a lightweight and compact system incorporating the illumination and sighting means, the system being adapted to most industrial scientific conditions.

To this end, the invention relates to an optical measuring system employing coaxial illumination, characterised in that it comprises a light source coaxially illuminating an object and disposed in front of a lens in order to determine the dimensions or position of the object, a photodetector being disposed on the image plane of the lens at the location of the image of the object being measured.

According to another characteristic, the light source applies a beam of light on to rearwardly reflecting surfaces, the beam being returned to the photodetector, which is associated with an electronic system for processing the signal.

The use of rearwardly reflecting surfaces eliminates the need for, or simplifies, any installations situated behind the object requiring to be measured. Thus it is only necessary, for example, to mark the object with one or more sighting marks, which constitute the rearwardly reflecting surfaces, or to dispose a rearwardly reflecting surface behind the object.

According to another characteristic of the invention, the system comprises a condenser for the beam of light, in front of which there is disposed a screen formed with a slit, the beam of light passing through the slit to define a slit of light, and then passing through the lens in order to illuminate coaxially the object situated in front of a rearwardly reflecting surface.

According to another characteristic of the invention, the photodetector is disposed perpenducularly to the sighting axis and cooperates with a beam separator disposed between the photodetector and a light trap adapted to trap the unwanted beam.

Accordinq to another feature of the invention the light slit is larger than or equal to the photodetector in respect of its size.

According to another characteristic of the invention, the light slit and the photodetector are situated at equal distances from the lens.

Finally, according to another characteristic of the invention, the rearwardly reflecting surfaces are disposed either behind the illuminated object, or on the object itself.

The invention will be more readily understood from one embodiment of an optical measuring system according to the invention, which is shown diagrammatically by way of example without any limiting force in the accompanying drawings wherein: Fig. lisa side elevation of the system and Fig. 2 is a diagram showing the path of the rays of light.

Referring to Fig. 1, the optical measuring system 1 employing coaxial illumination comprises a light source S coaxially illuminating an object 2 disposed in front of a lens 3. A.

photodetector 4 is disposed on the image plane of the lens 3 at the location of the image of the object 2 under measurement.

The system 1 also comprises reflecting surfaces 5 disposed behind the object although they may be disposed on the object itself in the form of sighting marks. The function of the reflecting surface is to return the beam of light emitted by the source S to the photodetector 4 with only low illumination power.

The beam of light emitted by the light source S first passes through a condenser 6 where it is applied to a screen 7 formed with a light slit 8. The dimensions of the slit 8 are greater than or at least equal to the dimensions of the photodetector 4.

Slit 8 and photodetector 4 are also disposed at equal distances from the lens 3. Photodetector 4 is also disposed perpendicularly to the sighting axis and co-operates with a beam separator 9 situated on the line of sight X between the photodetector 4 and a light trap 10 intended, to trap the unwanted beam of light. Thus the beam of light emitted by the light source S is reflected by the rearwardly reflecting surface 5 to the beam separator 9 which applies some of this beam art a right angle to the photodetector 4, the other part, which is deflected perpendicularly upwards by the beam separator 9, being trapped by the light trap 10. Photodetector 4 is connected to an electronic signal processing system not shown in the drawing.

The system 1 thus makes it possible to determine the size or position of an object disposed in front of the lens 3 by forming the image of this object on a photodetector 4. The latter may, for example, be a photodiode, a CCD photodiode, photo resistor or similar array. In the case of a CCD photodiode array connected to an electronic signal processing system, the video signal is logicalized in a comparator which discriminates the illuminated diodes and the dark diodes. A counter defines either the dimensions of the measured zone or its position in relation to the photo-detector origin.

The system 1 coaxially illuminates the object 2 by means of the taking lens 3, the beam of light being condensed and then passing through a light slit 8 before passing through a beam separator 9.

The latter may comprise a semi-transparent plate, a Lumer cube, or a Fresnel prism or the like. The reflected beam of light is separated by the semitransparent plate 9, for example downwardly to the photodetector 4 and upwardly to the light trap for the unwanted beam. The light trap 10 also reduces interference effects.

The rearwardly reflecting surfaces 5 improve the sighting contrast, being disposed either on the background behind the object, which then appears dark on a light background, or being disposed on the sighted object itself, e.g. in the form of sighting marks, so that the position or the limits of this object can be defined. The object then appears light on a dark background.

The rearwardly reflecting surface 5 may be in the form of an easily fitted adhesive plastic or paint. It may also be in the form of catadioptric plastic blocks adapted to be secured by adhesive or screws. The use of these rearwardly reflecting surfaces requires a minimum of work. When they are disposed behind the object, they become larger than the image spot of the light source S.

These reflecting surfaces are also relatively insensitive to the angle of incidence of the light over a range depending upon the technique used for their production.

Referring to Fig. 2 the system 1 illustrates the path of the beam of light E. This is emitted by the light source S and then passes to the condenser 6 before passing through the light slit 8 and the semi-transparent plate 9 and lens 3 to illuminate the object and impinge on the rearwardly reflecting surface 5, from which a beam E is returned. The reflected beam is returned to the beam separator 9, consisting of a semitransparent plate where the unwanted beam is applied at right angles to the line of sight X to the light trap 10 where this part of the beam is trapped. The light trap 10 is disposed above the semi-transparent plate 9 in relation to the line of sight X.

The wanted beam is fed to the photodetector 4 situated below the line of sight X. Photodetector 4 is connected to an electronic signal processing system 11 which allows either the dimensions of the object 2 or its position or movement to be measured.

System 1 enables the properties of the rearwardly reflecting surfaces to be used to the optimum of their efficiency, i.e. for zero divergence between the incident and reflected beams. An impression of the photodetector is thus obtained with a good efficiency, using only low and very localized illumination power. The use of the rearwardly reflecting surfaces provides high constrast, which allows accurate measurements with the different types of detectors, the ambient illumination having little influence.

The system 1 is also adaptable to all focusing distances and distances of the measured object.

Thus if the lens is moved for a different focus, the light source S remains in focus as soon as the image of the object 2 is distinct on the photo detector 4. This greatly simplifies adjustment operations.

Claims (10)

1. An optical measuirng system employing coaxial illumination, characterised in that it comprises a light source coaxially illuminating an object disposed in front of a lens in order to determine the dimensions or position of the object, a photodetector being disposed on the image plane of the lens at the location of the image of the object being measured.

2. A measuring system according to claim 1, characterised in that the light source passes a beam of light on to rearwardly reflecting surfaces, the beam being returned to the photodetector, which is associated with an electronic signal processing system.

3. A measuring system according to claims 1 and 2, characterised in that it comprises a condenser for the beam of light, in front of which there is disposed a screen formed with a slit, the beam of light passing through the slit to define a slit of light, and then passing through the lens in order coaxially to illuminate the object situated in front of a rearwardly reflecting surface.

4. A measuring system according to any one of claims 1 to 3, characterised in that the photodetector is disposed perpendicularly to the line of sight and co-operates with a beam separator disposed between the photodetector and a light trap intended to trap the unwanted beam.

5. A measuring system according to any one of claims 1 to 4, characterised in that the light slit is of dimensions greater than or equal to those of the photodetector.

6. A measuring system according to any one of claims 1 to 5, characterised in that the light slit and the photodetector are situated at equal distances from the lens.

7. A measuring system according to any one of claims 1 to 6, characterised in that the beam separator is a semi-transparent plate, a Lumer cube, a Fresnel prism or the like.

8. A measuring system according to any one of claims 1 to 7, characterised in that the photodetector comprises a photodiode or a photodiode, photo-resistor or similar CCD array.

9. A measuring system according to any one of claims 1 to 8, characterised in that the rearwardly reflecting surfaces are disposed either behind the illuminated object or on the object itself.

10. An optical measuring system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.