WO2005090908A1

WO2005090908A1 - High-speed optical profiler

Info

Publication number: WO2005090908A1
Application number: PCT/FI2004/050032
Authority: WO
Inventors: Alexei Kamchiline
Original assignee: Oy Optoinspection Ltd.
Priority date: 2004-03-23
Filing date: 2004-03-23
Publication date: 2005-09-29

Abstract

An apparatus and method for fast non-contact measuring z-distance between an optical head and a rough surface independently from their relative velocity having two spatial filters for filtering a speckle pattern formed when the rough surface is illuminated by a focused coherent beam, two independent photo-receivers, and a signal processor for simultaneous measuring the temporal frequency of signals from the photo-receivers and further calculation of z-distance.

Description

HIGH-SPEED OPTICAL PROFILER

Field of the Invention

The present invention relates to an apparatus and method for fast non-contact measuring surface profiles with height variations. More particularly, the apparatus and method relate the frequency content of the dynamic speckle pattern to the distance from the rough surface.

Background of the Invention

With the increasing speed of the papermaking or like industrial machines, there is a need for quality assurance equipment to monitor surface profile (and its dynamic variations or vibrations) of a rough surface (like rollers, belts, paper web, etc) moving with high speed (30 m/s or higher). Such equipment must be preferably of the non-contact type to exclude the influence of the contact sensor on the measuring surface. Optical profilers provide a convenient means for non-contact measuring the surface profile.

Furthermore, dimensional metrology, or the measurement of size and shape of the object (3D-vision), is very important in today's manufacturing environment in which machines perform much of the fabrication and assembly of complex objects composed of many subassemblies. The shape and size of each component in a complex assembly must be held in close tolerances to ensure that that the components fit together properly. Such measurements are preferably to be accomplished in a non-contact manner to save time in making measurements. Many non-contact measurement methods make use of available machine vision systems. The measurement of surface contour information is an especially difficult problem in machine vision systems since depth information is often lost or difficult to interpret. An optical profiler is useful instrument for 3D-vision, however such a profiler must be very fast to provide on-line measurements of size and shape of the object.

Various types of optical profilers have been disclosed. The most widely used optical profilers are based on the geometrical approach as classified by Strand (T. C. Strand, "Optical three-dimension sensing for machine vision," Optical Engineering, 24, 33-40, 1985). In this approach, a light pattern of the predetermined shape (spot, line, grid, etc.) is projected onto the surface under study and then image is raptured and analyzed providing the information about the surface profile. Image capturing and analysis is time-consuming procedures thus limiting applications of these methods to slowly moving surfaces.

Spatially irregular intensity distribution of the coherent electromagnetic wave (which is referred to as a speckle pattern), while it is scattered from a rough surface, is a consequence of the fundamental physical property of diffraction. Since the variations of a speckle pattern is determined by the scattering surface, optical methods involving analysis of speckle pattern are considered as very convenient and cost-effective among the non-contact sensing apparatus. The size of speckles is easily scaled therefore it can be used over a broad range of distances in contrast with the interferometric techniques. However, according to available review devoted to these methods (I. Yamaguchi, "Theory and application of speckle displacement and decorrelation," in: Speckle Metrology, ed. by R. S. Sirohi, Marcel Dekker, New York 1993, Ch.l), the most of them are applied for measurements of in-plane displacements and velocity of rough surfaces.

Nevertheless, in the US patent No 4,210,399, a method for measuring distance between an observation point and rough moving surface is provided that uses dynamic speckle pattern formed in free space, when a coherent light beam is reflected from a rough surface. According to this method, a coherent beam through a predetermined aperture D illuminates the moving surface under study, and the intensity of the reflected wave Is then registered and analyzed to find a characteristic size of irregular speckle pattern. This characteristic size is representative of the distance between the photo-receiver and the surface. However, signal registering and calculation of the characteristic size of dynamic speckle pattern via correlation analysis is a time-consuming procedure, which again limits application of this method to slowly moving objects.

Thereafter, the method of z-distance measurement using the laser-speckle effect was disclosed (Giglio, M., Musazzi, S., and Perini, U., "Distance measurement from a moving object based on speckle velocity detection," Applied Optics, 20, 721-722, 1981) that benefits from spatial filtering of dynamic irregular speckle pattern and measuring temporal frequency of the light power modulation after spatial filtering that represents instant distance to the surface. Owing to optical data processing via spatial filtering, this method offers high speed of distance measurements and can be applied to fast moving objects. However, it requires precise knowledge of the object velocity, which in many cases is either unknown or unstable.

It is the object of the present invention to provide a method and apparatus for fast measuring of the surface profile, which are independent on the object's velocity.

It is another object of the invention to make the production of apparatus cost- effective and robust, ϊt Is stfj! another object of the invention to provide the apparatus for, surface profile measurements of non-moving object and fast formation of 3D-images.

Summary of the Invention

The present invention provides a method and apparatus for fast non-contact measuring of the surface profile independently on the relative velocity between the surface and the measuring optical beam.

The method according to the invention is based on the analysis of dynamic speckle pattern by means of spatial filtering. Unlike previously known method of z-distance measurement using spatial filtering of the speckle pattern, the light scattered from rough surface in the present invention is filtered by at least two spatial filters with further measuring and joint processing of light-power-modulation frequency. According to one aspect of the Invention, the method Involves steps of illuminating an optically rough surface with a focused coherent beam; providing movement of said focused beam relatively to said rough surface; spatially filtering the light scattered from said rough surface by two spatial filters; delivering said filtered light respectively to two different photo-receivers; transforming light power received by said photo-receivers into two electrical signals; measuring the temporal frequency of said electrical signals; and calculating the signal being characteristic of the current distance between said rough surface and optical measuring head.

The present invention also provides an apparatus for non-contact distance measurements from a rough surface, which generally includes an optical measuring head and signal processing means. The optical measuring head further comprises the illumination means for generating a coherent light beam with a predetermined wavelength; an optical assembly for focusing said laser beam nearby said rough surface; two spatial filters for filtering light scattered from said rough surface; two optical collecting means for collecting and delivering said light filtered by each spatial filters onto two photo-receivers; two photo-receivers for receiving said collected light and converting it into electrical signals dependent on the light power of said collected light. The signal processing means further comprises electronic components arranged for evaluation of the temporal frequency, f n. and f_m, of said electrical signals, respectively, and calculation z-distance between said rough surface and said optical measuring head by using the predetermined equation ^;'(z_>/_/,_i,₁,/_røj) = 0, where F(z,f_pm,f_P[yι ) is a functional defined by the geometry of the focused beam and by positions of the said spatial filters.

According to another advantageous embodiment of the Invention, spatial filters of the apparatus can be designed to provide separation of said scattered light in two parts so that the light power of said parts is temporally modulated in the counter phase in respect to each other, and optical measuring head further comprises additional optical collecting means and additional photo-receivers.

According to yet another advantageous embodiment of the invention, an apparatus can further comprise an optical scanner for providing movement of the said light beam over said rough surface.

The invention has many potential applications. Particularly, it can be used for online non-contact profile monitoring of fast-rotating rolls and reels in papermaking machine or alike. The invention is also useful for fast formation of three-dimensional images of real objects comprised from different components during assembling of complex systems. With the help of the present Invention, the shape and vibrations of different parts of modern machines can be monitored during their exploitation. Using of two apparatus according to the invention allows on-line measurements of the thickness of paper, steel, plastic, or other materials, on-line during their production with high speed. The invention can also be used for on-line monitoring the thickness of evaporated thin layers.

Brief description of the drawings

Further features and advantages of the invention will be better understood from the following description of preferred embodiments as illustrated by way of examples in the accompanying drawings in which:

FIG. 1 illustrates an exemplary arrangement of the optical head, a key-element of an advantageous apparatus according the invention, which is also useful for understanding the principle of an advantageous method of the invention.

FIG. 2 shows a flow diagram of the signal processing means of embodiments of the invention.

FIG. 3a illustrates a dividing refractive spatial filter.

FIG. 3b illustrates a dividing reflective spatial filter.

FIG. 4a shows an example of a simple spatial filter.

FIG. 4bshows an example of a spatial filter with non-uniform spatial frequency.

FIG. 4c shows another example of a spatial filter with non-uniform spatial frequency.

One should notice that the drawings are only exemplary to illustrate the embodiments of the invention, not for limiting the invention. One should notice also that the scale or dimensions in the drawings are only illustrative and therefore may depart considerably from those measures for corresponding objects in reality. Detailed description of the preferred embodiments

The optical measuring head 10, which is essential part of the method and apparatus of this invention, is shown in Fig.l. Referring to the Fig.l, there is an illumination means 11 for generating a coherent light beam of certain wavelength and polarization. There is also optical assembly 12 (such as a lens or objective or concave mirror or diffractive optical element) for focusing light beam so that the focus plane is situated nearby a rough surface 13, which profile is to be measured.

When a coherent light is directed onto an optically rough surface, there is produced irregular distribution of the scattered light intensity called as a speckle pattern. If the surface is moving relatively to the illumination beam, a speckle pattern varies with the surface motion. Dynamic variations of the speckle pattern may be categorized roughly in two groups: (a) when the speckle pattern is displaced In the space as a whole in accordance with the surface motion referred to as translating motion, and (b) when the individual speckles vary their shape and size chaotically referred to as boiling motion. It is known that when the object is illuminated by divergent coherent beam, the dynamic motion of speckles has preferably the translating type (Yoshimura, T., "Statistical properties of dynamic speckles," J. Opt. Soc. Am. A, 3, 1032-1054, 1986). This type of motion is the most advantageous for distance measurements and it certainly ensured, when the illuminating beam is focused nearby the surface to be studied. The velocity of speckles movement, V_& In the translation mode is defined by 3 parameters: (i) relative velocity, _>_» of the illuminating spot displacement in respect to the said rough surface; (ii) radius of the light wave front, R_w, of the Illuminating beam when it crosses said rough surface; (iii) distance, D, between the said surface In the point of illumination and the point of observation:

Installation In the point of observation of the spatial filter 14, which is a series of stopping and transmitting stripes oriented orthogonal to the direction of the speckle motion and having the spatial period Λ as shown in Rg.4a, leads to the power modulation of the transmitted light at the temporal frequency of

It is seen that during the surface motion, both and V_Λ varies resulting in changing of f$p. Therefore, one can calculate the momentary distance to the surface, if the frequency, f_Α is measured. This is the essence of the previously disclosed method for distance measurements using the speckle effect. However, one can see from Eq.2 that the speed, κ» of the surface movement must be known while calculating the distance from &.

The Author of the present invention has found that installation of the second spatial filter 15 allows to measure the instant distance to the rough surface independently on its velocity.^' While! many of different type of coherent illuminating beam can be used in the method and apparatus of the present invention, it will be described, for the sake of simplicity, a method and apparatus using an illuminating beam of

Gaussian TEM_M mode. The wave front radius of such a beam is known to be varied with the propagation (2) coordinate as

Z²

R_w = z + -S-, (3) z

where Z_R = πw /λ is the Rayleigh range of the Gaussian beam, w& is the beam waist, λ is the light wavelength, and zis measured from the waist position, which Is situated at the focus position. Power of the light scattered from the rough surface 13 will be temporally modulated after being transmitted through each spatial filter 14 and 15. According to Eq.2, the frequency of this power modulation depends on the distance to the spatial filter position and on the wave front radius. Let the frequency of the power modulation after the filter 14 is f_m and that after the filter 15 is fpm. Then the distance between the illuminated spot at the rough surface and the focus position can be calculated as a root of the quadratic equation: a, a, 2r² /«Λ cos² ^- -/_rø2Λ- cos^: + *( /K>A - «H^Λ2)^{+ Z}i!( «>Λ - o.Λ₂) = 0, (4)

where Λ₍ is the effective spatial period of the first spatial filter, Λ₂ Is the effective spatial period of the second spatial filter, is the distance between the spatial filter 14 and the focus position, is the distance between the spatial filter 15 and the focus position, α. is the angle between the axis of the Gaussian beam and the line connecting the geometrical center of the spatial filter 14 and the center of the Gaussian beam in its focal plane, α. is the angle between the axis of the Gaussian beam and the line connecting the geometrical center of the spatial filter 15 and the center of the Gaussian beam in its focal plane, as it is shown in Fig.l. It is seen from Eq.4 that after the frequencies, fm and fm, have been measured, one can calculate the distance -r independently on the velocity of the rough surface, *

Equation 4 can be further simplified assuming that the spatial fitters 15 and 16 have the same spatial period, Λ, = Λ-, and the angles a and as are small:

(2^Ϊ2 +ZlXf_m -/«a)+*fø/»ι -A/«a)- (5)

When the illuminating coherent beam Is not of the Gaussian TEMM mode, a calibration function F(z,f_PD f_pm) may be introduced by estimating experimentally or theoretically the dependence of the effective wave front radius, R_m on the z- coordinate, and then introducing the obtained dependence R_w(z) into Eq.2.

Measuring the temporal frequency of the light-power modulation is the key process of the present invention. It can be implemented by using any available technique. In the advantageous embodiment shown in Fig.l, the scattered light after filtering by spatial filter 14 is delivered to the photo-receiver 16 by using optical collecting means 17 such as a lens or objective or another optical device capable to collect light (for example, diffractive optical element). Similarly, the light transmitted through the spatial filter 15 is delivered to another photo-receiver 18 by using optical collecting means 19. The photo-receivers 16 and 18 may be any known optical-electrical device capable to transfer the received light power into an electrical signal. Particularly, a photo- diode or photo-multiplier can be exploited as a photo-receiver. After this light-to- electric transformation, the electrical signal from the photo-receiver 16 will be temporally modulated at the frequency f_m, while the temporal frequency of the electrical signal from the photo-receiver 18 is f_m.

Different electronic circuits may be employed to measure the frequencies f_m and fpoi. An advantageous configuration of the signal processing means 20 for the frequency measurements and for further calculation of the instant distance to the rough surface is shown in Fig.2. Both electrical signals from the photo-receivers 16 and 18 are separately amplified in the processing step 21 up to the necessary level. Typical example of the amplified signal is shown in the graph 26 representing one of the channels. Then, in the step 22, the DC-level is subtracted from each amplified signal using a high pass filter. The example of the signal after the processing step 22 is shown in the graph 27. In the step 23 both signals are separately limited so as to form a sequence of binary pulses of the type shown in the graph 28. A measurement of the time, T_m and Tta, at which N binary pulses occur, is performed in the step 24 for electrical signals from the photo-receiver 16 and photo-receiver 18, respectively. In the same step 24, the frequencies f_PDl = T_m/N and fp_D2 = ^T _NII^N are calculated. In the final step 25, the instant ^-distance to the rough surface is calculated by using either Eq.4 or equation (z,/_røl, _PD2)⁼ ° ■

It is worth noting that the frequency-measuring procedure of Fig.2 is shown just as an example. A person skilled in the art of electronics may use any of known method of the frequency measurements. For example, using the dual-channel frequency counter, model #8940 HK of Hioki, with further data transfer into a personal computer, is also possible.

In accordance with the present invention, there is provided a non-contact method for measuring a distance between an optical measuring head 10 and a rough surface 13, the method involves steps of illuminating an optically rough surface with a focused coherent beam; providing movement of said focused beam relatively to said rough surface; spatially filtering the light scattered from said rough surface by two spatial filters 14 and 15; delivering said filtered light respectively to two different photo-receivers 16 and 17; transforming light power received by said photo-receivers into two electrical signals; measuring the temporal frequency of said electrical signals; and calculating with Eq.4 the signal being characteristic of the instant distance between the rough surface 13 and optical measuring head 10.

An advantageous apparatus for non-contact distance measurements according to the invention generally includes the optical measuring head 10 and signal processing means 20. The optical measuring head 10 further comprises the illumination means 11 for generation coherent light beam with a predetermined wavelength; an optical assembly 12 for focusing said laser beam nearby said rough surface; two spatial filters, 14 and 15, for filtering light scattered from said rough surface; two optical collecting means, 17 and 19, for collecting and delivering said light filtered by spatial filters, 14 and 15, into two photo-receivers, 16 and 18, respectively; two photo-receivers, 16 and 18, for receiving said collected light and converting it into electrical signals dependent on the light power of said collected light. The signal processing means 20 further comprises electronic components arranged for evaluation of the temporal frequency, fm and f_m, of said electrical signals, respectively, and calculation z-distance between the rough surface 13 and the optical measuring head 10 using either predetermined equation *^'(*,/„₁,/_J∞)= 0 or .-q.4 or 5.

According to another advantageous embodiment of the invention, the optical measuring head 10 comprises at least one spatial filter having the form of a sequence of refractive prisms 34 with the spatial period Λ as shown in Fig.3a. After the scattered light passes through such a filter 34, it will be collected into two spatially separated spots by using similar optical collecting means 17. Since a pair of spatial filter 34 and optical collecting means 17 provides division of the scattered light Into two spots, the filter 34 may be called as a dividing filter in contrast with the simple transmitting filter 14. The photo-receiver 16 is situated so as to receive the light collected into one of the spots, while an additional photo-receiver 36 receives the light collected into another spot. It was found that, when the dynamic speckle pattern moves in respect to the spatial filter 34, the light power collected into one spot is modulated In time in counter phase with that light power collected into another spot Photo-receivers 16 and 36 make transformation of the received light power into a pair of electrical signals. Since these electrical signals are in the counter phase, their amplification by a differential amplifier results In rejecting of DC-component. Therefore, the processing steps 22 and 24 In the signal processing means 20 are joined in one step. Such differential amplification results in increasing of variety of object surfaces to be measured in the sense of their light reflectivity. Anyone skilled in the art of optical design appreciates the numerous variations of implementation of the dividing spatial filter 34 and optical collecting means 17. Particularly, these two elements may be joined in one diffractive optical element. The main function of this element is division of the scattered light into two spatially separated spots in which the light power is temporally modulated in the counter phase.

The dividing spatial .filter 34 of the refractive type shown in Fig.3a may be replaced by a dividing spatial filter of the reflective type 34a, as shown in Fig.3b. The filter 34a is advantageously a series of reflective stripes (mirrors) almost equidistantiy evaporated on a transparent substrate with the spatial period Λ. In this case an additional optical collecting means 37 is used to deliver the filtered light into the photo-receiver 36. The reflecting filter 34a provides the same function as the refractive filter 34, namely, it separates the scattered light into two parts in which the light power is temporally modulated in the counter phase in respect to each other.

One should notice that replacement of both simple transmitting filters (14, 15) by two dividing filters of the type either 34 or 34a inσeases the variety of measurable surfaces even better.

According to yet another advantageous embodiment of the invention, there is provided a spatial filter 14 having a sequence of stopping, or reflecting, or refracting strips positioned in the plane of the filter so that their spatial period varies with both coordinates, the variation of the spatial period is calculated so as to compensate possible difference in the speed of the speckle-pattem movement in different parts of the spatial filter aiming to get uniform frequency fs> of the intensity modulation after the filter. This compensation is especially useful when the plane of the filter is tilted in respect to the axis connecting the geometrical center of the filter and the center of the illuminating spot on the rough surface 13. For example, such tilting occurs in the case of dividing reflection filter 34a shown in Fig.3b. Examples of the spatial filters with variable spatial frequency are shown in Fig.4b and 4c. The spatial frequency Λ of the spatial filter shown in Rg.4b depends on the coordinates x, /as

Λ(x, y) = C, + C₂ JD² +x² +y² -2D_lxcos& , (6)

where Q. and C₂ are constants defining by the geometry of the illuminating beam, A is the distance between the geometrical center of the spatial filter and the center of said focused beam in its focal plane, θ\s the angle of spatial filter tilting (θ = 45° in the particular case shown in Fig.4b). It was assumed calculating the filter shown in Rg.4b that the direction of the speckle-pattern movement is orthogonal to the plane of the tilting. Alternatively, the filter shown in Rg.4c was calculated suggesting that the direction of the speckle pattern movement is parallel to the plane of tilting (θ = 45°). For the filter shown in Rg.4b, the spatial frequency varies as

Λ(x,y)= C_l + C₂ JD² + x² +y² ± 2D_lycos3 . (7)

Spatial filters with variable spatial frequency (Rg.4b,c) provide higher squeezing of the temporal frequency peak near /spthan the simple filter shown in Rg.4a does, thus resulting in higher resolution in z-distance measurement. Even in the case when the filter is not tilted (θ = 0), variable spatial frequency improves the measurement accuracy. It should be noted that the spatial filters with variable spatial frequency are shown in Rg.4b and 4c just for illustrative purpose. One can calculate a compensated filter for any particular position and tilting of the spatial filter.

It Is also worth noting that any of advantageous embodiments according to the invention may be used for measurement the distance between the optical measuring head and a rough surface even in the case when the surface is not moving in respect to the optical measuring head. To achieve this, one can scan the Illuminating beam over the rough surface with the predetermined direction of scanning. This scanning may be implemented with help of any known optical deflector, such as acusto-optical deflector, electro-mechanical (galvanometer), piezo-electric driver, and others. Due to independency of the advantageous embodiment on the speed of scanning, usually hard requirements to linearity and stability of the scanning system can be significantly softened. Since the embodiments of the invention are capable to operate at very high velocity of the relative movement of the optical beam over a rough surface, exploitation of a highspeed optical scanner is very beneficial for profile measurements of objects that are moving at relatively low velocity in respect to the optical head with unknown direction of motion (for example, in a production line). This benefit is achieved owing to well-defined direction of the speckle-pattern movement along the direction of predefined scanning.

Although there has been hereinabove described a method and apparatus for measuring the profile of a rough surface, for the purpose of illustrating the manner in which the invention may be used to advantage, it will be appreciated that the invention is not limited thereto. Accordingly, all modifications, variations, or equivalent arrangements which may occur to those skilled in the art, should be considered to be within the scope of the invention as defined in the appended claims.

Claims

Claims:

1. A non-contact method for measuring a distance between an optical measuring head and a rough surface by illuminating said rough surface with a focused laser coherent beam in such a way that the said beam is scattered In any extend by said rough surface and providing movement of the said focused beam in respect to the said rough surface characterized in that said method comprises steps of: spatially filtering said scattered light by at least two spatial filters, which geometrical centers are positioned either at different distances from the focus position of the said focused beam or at different angles In respect to the axis of the said focused beam; delivering at least part of the light transmitted through each spatial filter into two separate photo-receivers; transforming the optical power received by said photo-receivers into electrical signals, J_m and J_m measuring the temporal frequency, f_m and _/ of said electrical signals, J_m and

calculating the distance between said optical processing means and said rough surface by using equation F(z,f_PDl,f_PD2)= 0, where F(z,f_pm,f_PD2) is a functional defined by the geometry of the focused beam and by positions and the spatial frequency of the said spatial filters.

2. A method according to claim 1 exploiting such a focused laser beam that its light power is distributed in space according to the Gaussian TEM_M mode, characterized in that said functional l z) of the method step (v) is expressed as

= 2z J_røΛ, cos² ^— J_rø2Λ₂ cos² ^> + ^z( Λ_oΛ - /_m2Λ₂)+Z (/^" _rølΛ₁ -J_rø2Λ₂)

where A is the distance between the said first spatial filter and said focus position, A is the distance between the said second spatial filter and said focus position, αi is the angle between the optical axis of the said focused beam and the line connecting the geometrical center of the first spatial filter and the center of said focused beam in its focal plane, - is the angle between the optical axis of the said focused beam and the line connecting the geometrical center of the second spatial filter and the center of said focused beam in its focal plane, Λi is the effective spatial period of the first spatial filter, Λ₂ is the effective spatial period of the second spatial filter, and Z_R is the Rayleigh range of the said focused beam.

3. A method according to daim 2 exploiting spatial filters with the same spatial period and situated at small angles in respect to the optical axis of the said focused beam characterized in that said functional f{ of the method step (v) is expressed as

F(z,f_Pm,f_PD2) = {2z² +Z X _røl -f_m)+z(p₂f_m - /_«β)

4. Ah apparatus for measuring a distance between a rough surface and an optical measuring head, said optical measuring head comprising illumination means for producing a beam of the coherent light and optical assembly for focusing said light beam nearby said rough surface characterized in that said optical measuring head further comprises two spatial filters for filtering light scattered from said rough surface, two optical collecting means for collecting and delivering said light filtered by each spatial filters onto two photo-receivers, two photo-receivers for receiving said collected light and converting it into electrical signals dependent on the light power of said collected light, and said apparatus also comprises a signal processing means arranged for evaluation of the temporal frequency of said electrical signals and calculation of the distance between said rough surface and said optical measuring head according with the predefined equation of any of claim 1 to 3.

5. An apparatus according to claim 4, characterized in that at least one said spatial filter separates the light scattered from said rough surface into two parts so that the light power of said parts is temporally modulated in the counter phase in respect to each other, and said optical measuring head further comprises additional optical collecting means for collecting said separated part of said scattered light onto an additional photo-receiver, an additional photo-receiver for receiving said separated light part and converting it into an electrical signal dependent on the light power of said separated part, a differential amplifier for amplification the difference between electrical signals of photo-detectors receiving said light parts separated by said spatial filter.

6. An apparatus according any of claims 4 or 5, characterized in that said spatial filter is a sequence of stopping, or reflecting, or refracting strips positioned in the plane of the filter so that their spatial period varies with both coordinates, the variation of the spatial period is calculated so as to compensate possible difference in the speed of the speckle-pattern movement in different parts of the spatial filter aiming to get uniform frequency &»of the intensity modulation after the filter.

7. An apparatus according any of claims 4 to 6, characterized in that said optical measuring head further comprises means for providing movement of the said light beam over said rough surface.

8. An apparatus according to claim 7, characterized in that the means for providing movement of said light beam comprises an optical scanner.

9. An apparatus according to claim 7, characterized in that the means for providing movement of said light beam comprises a mechanical means for moving the measuring head in respect to the surface under measurement.

10. An optical measuring head for measuring a distance between a rough surface and the optical measuring head, said optical measuring head comprising illumination means for producing a beam of coherent light and an optical assembly for focusing said light beam nearby said rough surface, characterized in that the said optical measuring head further comprises:

(a) two spatial filters for filtering light scattered from said rough surface, and said filters geometrical centers are positioned either at different distances from the focus position of the said focused beam or at different angles in respect to the axis of the said focused beam; (b) two optical collecting means for collecting and delivering said light filtered by each spatial filters onto two photo-receivers,

(c) two photo-receivers for receiving said collected light and converting it into electrical signals dependent on the light power of said collected light.