GB2188445A - Optical speckle-averaging system - Google Patents

Abstract

A system for speckle-averaging, especially for interferometric measurements and improving TV-based halographic images, either includes means, such as a glass plate or a mirror, or an acousto-optical modulator, in the path of the light 13 illuminating an object 11,12 and arranged to adjust the direction of illumination in steps, or includes an aperture 23 which may be rotated or otherwise displaced step by step or continuously, preferably by an electric motor 22, in the path of light from the object in order to adjust the direction of observation. Successive images are thus arranged to have completely or partly uncorrelated speckle-patterns. The objects 11,12 may be imaged onto a TV camera 15. <IMAGE>

Description

SPECIFICATION Speckle-averaging The present invention relates to speckle-averaging systems, particularly for interferometric measurements and for improving TV-based holographic images.

A speckle pattern is a granulation pattern observed in a laser beam. Such a pattern is chaotic and due to interference between several uncontrolled factors, which are often a result of reflections from the fine structure of an object surface. A speckle pattern forms in many techniques of measuring and imaging, and registration of interference between speckle-waves or between a speckle-wave and another wave is the basis of the technique.

Such is the case, for example, in holographic interferometry, speckle interferometry, and televison-based interferometry (ESPI).

However, even though the speckle-wave is an important part of such optical systems, the speckle-pattern per se may be regarded as noise in the final images, and in order to improve the image quality it is desirable to get rid of the final speckle-structure. This may be achieved by a technique known as SPECKLE AVERAGING.

The conditions for achieving a speckle-averaging effect without loss of band width are well established, and the present invention relates to techniques enabling speckleaveraging to be achieved in an efficient manner.

According to the invention there is provided a speckleaveraging system, particularly for interferometric measurements and for improving TV-based holographic images, in which an object is illuminated and speckle patterned images thereof are produced, and which includes means for adjusting either the direction of illumination of the object in steps or the direction of observation of the object either continuously or in steps.

As a result of the adjustment of either the direction of illumination or the direction of observation, successive images are arranged to have completely or partly uncorrelated speckle-patterns, thus enabling the images to be electronically processed to remove the speckle noise.

In TV-based holographic interferometry the video signal is electronically filtered in the case of each image, and the images are then summed so that the summed image has little or no speckle noise. The summing may be carried out in different ways, e.g. in a video memory, in a digital image store, by the aid of a computer, by presentation of the images directly on a TV monitor and utilizing the delaying properties of the eye and the human capability of singling out useful information, or by depicting the images on photographic film.

Adjustment of the direction of observation may be achieved by means of a movable aperture, preferably motor driven, which varies the image viewed by taking different sections from a larger aperture field.

Adjustment of the direction of illumination of an object field from one image to another may be achieved by means of a transmitting glass plate or a mirror which is provided in the path of the illuminating radiation at an inclined angle thereto and which is pivotable or rotatable in steps, preferably by a suitable motor. Alternatively the beam of illuminating light may be arranged to pass through an acousto-optical modulator which will change the direction of the beam by electronic frequency control of a signal supplied to the modulator.

When using a mirror or an acousto-optical modulator the beam of radiation is dispersed to several optical elements or to a composite optical element, and when using an acoustooptical modulator it is also necessary to ensure that all waves/radiation pass through the modulator.

The motors used may be electrically, pneumatically, or hydraulically driven.

The invention will now be described in more detail with reference to the accompanying drawings which illustrate embodiments of the invention and in which: Figure 1 is a diagrammatic view of one example of a speckleaveraging system of the invention; and Figure 2 is a diagrammatic view of part of another embodiment of the invention.

Figure 1 shows diagrammatically the optical system of a TV based holographic interferometer utilizing a multi-aperture member, and with both interfering waves being speckle fields. Figure 1 also shows how a rotatable or displaceable aperture may be placed to have different portions of the multi-aperture element utilized at different times as a result of the aperture having different positions at different times. In each position the aperture effectively defines a different direction of observation in relation to the object that is imaged.

More particularly, Figure 1 shows two objects 11 and 12 illuminated by a source of laser light 13 via a microscope lens 14. The objects are imaged onto a TV camera 15 by means of a lens 16, via three mirrors 17, 18 and 19, and a multi-aperture member 20. Due to the construction of the aperture member 20 the light from the object 11 will have different angles of incidence on the TV camera 15 from those of the light from the object 12.

In this manner the cross interference between light from the objects 11 and 12 will yield contributions of intensity on other space frequencies than would the objects 11 and 12 per se, and the first contribution may be removed electronically from the remaining contributions with the aid of a band pass filter. This is important when the arrangement is used for vibration analysis.

An electric motor 22, which may be con nected directly to an aperture 23, or may be connected via a transmission unit 26, is uti lized to rotate (or otherwise displace) the aperture to make different portions of the multi-aperture member 20 operative at differ ent times. The unit 26 may comprise a sto rage and friction clutch, or a gear, chain or belt transmission. As a result of the aperture 23 rotating, the different portions of the multi aperture member 20 will yield uncorrelated speckle-patterns, and by detecting completely or partly uncorrelated speckle-patterns on different TV images with the aid of the TV cam era 15, the video signals for each of the images may be passed through an electronic filter and rectifier circuit 24 before the images are summed by a summing unit 25, or are presented directly on a TV monitor included in the unit 25.The summing unit 25 may be a digital image store, a TV monitor having a long delay time, or a slow exposure (greater than 1/25 second) photographic film. The different images will have uncorrelated specklepatterns if the displaceable/rotatable aperture 23 moves through its own width during the exposure time (generally 1/25 second).

Figure 2 shows an example of how the invention may be applied to a TV-holographic interferometer by providing it with means for achieving an efficient change of the direction of illumination. The drawing illustrates only the direction changing arrangement, and the remainder of the TV-holographic interferometer may be of conventional construction. The electronic processing of signals and image summing occurs in the same manner as described in connection with Figure 1.

In Figure 2 the direction of illumination represented by wave 34 relative to an object 33 is changed by step-wise rotation (alternatively tilting) of a glass plate 32, and keeping it motionless between the movement steps.

Ideally, rotation of the glass plate should take a minimum of time, and it should be kept motionless for 1/25 second or more before the next step of rotation occurs. The glass plate 32 may be moved manually or by means of a suitable motor, such as an electric motor.

This is indicated diagrammatically in Figure 2 where 31 represents a suspension means for the glass plate 32 and a transmission in the form of a friction clutch or a gear, chain, or belt transmission which is connected to the electric motor 30. The optical axis is indicated by the broken line 35. Speckle-averaging is achieved either by step-wise rotation of the glass plate 32 about the optical axis, or by step-wise rotation (or tilting) of the glass plate 32 about another axis that is not parallel to the surface normal to the plate.

The electric motor 30 may be a step-motor, and the glass plate 32 may be attached directly to the motor shaft. Additional mirrors or other stationary optical components may be provided, if desired, in front of or behind the glass plate 32, but are not shown in the Fig ure. Such components will of course need to be placed in dependence on the arrangement of other components of the interferometer.

In an alternative arrangement the glass plate is replaced by a mirror that may be firmly connected to the motor shaft. The mirror reflects the illuminating beam towards the object, and the normal to the mirror makes an acute angle with the rotational axis of the motor. When the motor shaft rotates step-wise the direction of illumination is changed stepwise. Otherwise the system is as described in connection with the glass plate.

Claims (8)

1. A speckle-averaging system, particularly for interferometric measurements and for improving TV-based holographic images, in which an object is illuminated and speckle patterned images thereof are produced, and which includes means for adjusting either the direction of illumination of the object in steps or the direction of observation of the object either continuously or in steps.

2. A system according to claim 1, in which a glass transmission plate or a mirror is mounted in the path of illumination at an inclined angle to the path and is rotatable in steps to adjust the direction of illumination of the object.

3. A system according to claim 2, including an electric, pneumatic, or hydraulic motor for rotating the glass plate or mirror in steps.

4. A system according to claim 1, in which an acoustooptical modulator is positioned in the path of illumination and is arranged to adjust the direction of illumination of the object in steps in accordance with changes in frequency of a signal received by the modulator.

5. A system according to claim 1, in which a movable aperture is provided for varying the image which is viewed and hence adjusting the direction of observation of the object.

6. A system according to claim 5, in which the aperture is rotatable either continuously or in steps by an electric, pneumatic, or hydraulic motor.

7. A system according to claim 1, substantially as described with reference to Figure 1 or Figure 2 of the accompanying drawings.

8. A device for speckle-averaging, especially for interferometric measurements and to improve TV-based holographic images, characterized in that a glass plate or a mirror is inclined in the path of light and is rotated step-wise by hand or by the aid of a motor, or in that an acousto-optic modulator is placed in the path(s) of light and changes the direction of radiation step by step with changed frequency of signals received by said modulator, or in that an aperture, provided in front of or behind diffusion of a light beam (i.

e. formation of a speckle band), is rotated continuously or step by step by a motor.