WO2009004108A1

WO2009004108A1 - High-speed optical profilometer

Info

Publication number: WO2009004108A1
Application number: PCT/FI2007/000185
Authority: WO
Inventors: Alexei Kamchiline
Original assignee: Oy Optoinspection Ltd
Priority date: 2007-07-03
Filing date: 2007-07-03
Publication date: 2009-01-08

Abstract

The invention provides apparatus and methods for fast non-contact measuring distance between an optical head and an optically rough surface with improved accuracy. The technique involves spatial filtering of a speckle pattern formed when the surface is scanned by a focused laser beam with the predefined speed. In one embodiment, the apparatus includes several independent photo-receivers for receiving properly selected portions of the spatially filtered light and a signal processor for measuring the temporal frequency of electrical signals from these photo-receivers and further calculation of z- distance according to the predefined equations. In another embodiment, the apparatus provides multiple scanning of the surface by the focused laser beam with simultaneous displacement of the scan by the predefined distance.

Description

High-speed optical profilometer

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 measurement of distance to an optically rough surface by using dynamic speckle pattern scattered from this surface.

Background of the Invention

Distance measurements are of great importance in modern industry. With the increasing speed of industrial production lines, there is a need for fast distance sensors enabled for online control of as-deposited coating thickness, monitoring surface profile (and its dynamic variations and vibrations), and online thickness measurements of paper, metallic, or plastic sheets. Such measurements are preferably to be accomplished in non-contact manner to exclude the influence of the contact sensor on the measuring surface. Moreover, accurate measurements should be frequently carried out at elevated temperature or hazardous environment. An optical profilometer satisfies abovementioned requirements.

Various types of optical profilometers have been disclosed. The most widely used optical profilometers 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 captured 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 an optically 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

³ITlJTE 8H- 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 optically 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.

Recently, modified method of z-distance measurements by using dynamic speckles was disclosed (Semenov, D., Nippolainen, E., and Kamshilin, A., "Accuracy and resolution of a dynamic-speckle profilometer," Applied Optics, 45, 411-418, 2006). In this method, the dynamic speckle pattern is created when the laser beam scans an optically rough surface to which the distance is to be measured. Since the speed of laser scanning can be very high and precisely controlled, the modified technique enables distance measurements independently on the object velocity. However, the accuracy of distance measurements is not very high because of stochastic nature of the laser speckles.

It is the object of the present invention to provide a method and apparatus for fast measuring of the surface profile with improved accuracy of distance measurements.

It is another object of the invention to make the fabrication of apparatus cost- effective and robust.

Summary of the Invention

The present invention provides a method and apparatus for fast non-contact measuring of the surface profile with improved accuracy. 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 dynamic speckle pattern created when an optically rough surface is scanned by a laser beam, the method of the present invention involves either selecting of several statistically independent portions of light scattered from the rough surface or providing multiple scanning of different areas of the rough surface with further measuring and 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; scanning the said surface by the said focused beam with predefined speed; selecting at least two portions of the light scattered by said surface so that the angle between their axes satisfies predefined condition; spatially filtering said selected portions by either a single spatial filter or different spatial filters; delivering said filtered portions into different photo-receivers; transforming the optical power received by said photo- receivers into electrical signals; measuring the mean temporal frequency of said electrical signals from said photo-receivers; and calculating the signal being characteristic of the current distance between the said surface and optical measuring head according to the predefined equations.

According to another aspect of the invention, there is a method including additional step of displacing the scanning beam on the said surface so that the distance of this displacement satisfies the predefined conditions with further calculating the signal being characteristic of the instant distance between the said surface and optical measuring head using another set of the predefined equations.

According to yet another aspect of the invention, there is a method including additional step of changing the focus position of the said coherent beam so that this change satisfies the predetermined conditions with further calculating the signal being characteristic of the instant distance between the said surface and optical measuring head using yet another set of the predefined equations.

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 coherent beam out of the said surface; an optical deflector providing scanning of the said surface by the said focused beam; either one or several spatial filters for filtering light scattered from said surface; several optical collecting means for collecting and delivering said light filtered by spatial filters to the photo-receivers; several 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 of said electrical signals and for calculation of z-distance between the said surface and said optical measuring head by using the set of the predetermined equations.

According to another advantageous embodiment of the invention, the optical deflector can provide multiple scanning of the focused beam over the said surface while the optical measuring head additionally comprises a displacing means for changing the position of the scanning beam in respect to the said surface.

According to yet another advantageous embodiment of the invention, the optical measuring head can further comprise an optical assembly for variation of the focus position of the scanning beam.

According to the invention the spatial filters and optical collecting means may by fabricated as a single diffractive-optical element.

According to yet another advantageous embodiment of the invention, the optical deflector can be fabricated in the form of the rotating polygonal mirror with light-reflecting facets tilted by a predefined angle in respect to the axis of rotation to provide multiple scanning over different areas of the said surface.

The foregoing as well as other objects, features, and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the appended drawings.

Brief description of the drawings

The figures depict one or more embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the method and apparatus illustrated herein may be employed without departing from the principles of the invention. FIG. 1 shows the perspective view of the typical optical measuring head (prior art) for measurement the distance to an optically rough surface by using dynamic speckles generated during scanning the surface by the focused laser beam. FIG. 2 shows a schematic perspective view of an exemplary arrangement of the optical measuring head according to the first advantageous embodiment of the invention, which is also useful for understanding the principle of an advantageous method of the invention. FIG. 3 shows a side view of the exemplary arrangement of the optical measuring head according to the first advantageous embodiment of the invention. FIG. 4 shows a flow diagram of the signal processing means of embodiments of the invention. FIG. 5 shows a side view of the exemplary arrangement of the optical measuring head according to another advantageous embodiment of the invention.

Detailed description of the preferred embodiments

Preferred embodiments of the present invention are now described with reference to the figures where like reference numbers indicate identical or functionally similar elements. Also in the figures, the left most digit(s) of each reference number correspond(s) to the figure in which the reference number is first used. The optical measuring head 10, which is essential part of the method and apparatus of this invention, is shown in FIG. 1. Referring to the FIG. 1, there is an illumination means 11 for generating a coherent light beam of certain wavelength and polarization. There is also focusing 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 outside an optically rough surface 13, the distance z between which and the optical measuring head is to be measured. The light beam is deflected by optical deflector 14 (such as acousto-optic deflector, or rotating polygonal mirror, or piezoelectric deflector, or any other type of known optical deflectors) so as to provide scanning of the rough surface 13 by the focused laser beam.

When a coherent light is directed onto an optically rough surface, there is produced irregular distribution of the scattered light intensity referred to as a speckle pattern. If the laser beam is moving in respect to the surface 13, the speckle pattern becomes dynamic and in many cases it moves as a whole with certain velocity. The velocity of speckles movement, Vsp in the translation mode is defined by 3 parameters: (i) scanning speed, V_BS, of the laser beam displacement in respect to the said rough surface; (ii) radius of the light-beam wavefront, Rw, of the illuminating beam when it crosses the surface 13; (iii) distance, D, between the rough surface in the point of illumination and the point of observation:

It is worth noting that the situation when the object surface is moving with the speed of V_OM in respect to the laser beam, which in its turn is fixed in respect to the observer, is usually considered in the literature (see, for example, Giglio, M., Musazzi, S., and Perini, U., "Distance measurement from a moving object based on speckle velocity detection," Applied Optics, 20, 721-722, 1981). For such a situation, an equation for the speckles-movement velocity V_SP = F₀^(I + D/R_W) is applied.

However, when the observer is fixed in respect to the object surface but the laser beam scans the said surface, the speckles velocity is given by the abovementioned

Eq.1. In more general case, when the speed of the laser beam is V_BS in respect to the observer but the object surface is moving with the speed of V_0M relative to the observer, the velocity of speckles is

V_SP = V_OM +{V_OM + V_BS)^- (2)

Installation in the point of observation of the spatial filter 15, which is a series of stopping and transmitting stripes oriented orthogonal to the direction of the speckle motion and having the spatial period Λ, leads to the power modulation of the transmitted light at the temporal frequency of

where for the sake of simplicity we consider the case when the scanning speed is much higher than the speed of the surface movement: V_BS » V_OM- All the other cases of the surface movement can be considered in a similar way. Optical collecting means 16 (such as a lens or objective or concave mirror or diffractive optical element) serves for collecting the light filtered by the spatial filter 15 and delivering it into a photo-receiver 17, which transforms the received light power into an electrical signal.

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 TEM₀₀ mode. For such a beam the wavefront radius Rw is equal to the distance between the beam waist (also called as a focus position) and the illuminated surface 13 when this distance is larger than the Rayleigh range Z_R = πwl/λ, where W₀ is the beam waist and λ is the light wavelength. Referring to the FIG.l, it is seen that when the distance z is changing, both D and Rw varies resulting in changing of fsp. Therefore, after measuring the temporal frequency fsp of the electrical signal one can calculate the momentary distance z to the surface by using Eq.3 and assuming that the position of the spatial filter 15 in respect to the focus position of the focusing optical assembly 12 is preliminary known. This is the essence of the previously disclosed method (prior art) for distance measurements using the speckle effect.

However, owing to stochastic nature of laser speckles, the accuracy of z- measurements provided by this technique is not very high. The Authors of the present invention have found from the one hand that when the parameters Rw, D, and V_BS are accurately defined and kept stable, the modulation frequency ^p is changing if different areas of the surface 13 are scanned by the focused laser beam, which results in failure of reproducibility of the measured distance z. From the other hand, it was found that the shape of the electrical signal from the photo-receiver 17 is always the same if the laser beam is repeatedly scanning one and the same area of the surface 13 that does not allow applying an averaging method for increasing the accuracy of z-measurements. Experimental study carried out by the Authors has revealed that accuracy of z-measurements can be improved by delivering of at least one more portion of the scattered light (properly separated in space from the first portion) into another photo-receiver and averaging the electrical signals from the photo-receivers by using equations described therewith.

FIG. 2 shows a schematic perspective view of a high-speed non-contact optical profilometer being a first advantageous embodiment of this invention. Illumination means 11, optical deflector 14, and focusing optical assembly 12 are the same as in the prior-art method and apparatus. There are also spatial filter 15, optical collecting means 16, and photo-receiver 17 used in the prior-art technique. A major contribution of the invention is addition of the spatial filter 25 together with optical collecting means 26, which serves for delivering the filtered light into additional photo-receiver 27. It is also possible to add more spatial filters with respective optical collecting means and photo-receivers thus forming multiple measuring channels. One skilled in the art may appreciate that it is also possible to use one spatial filter of larger aperture and collect light from different parts of the filter into several photo-receivers by means of respective optical collecting means. Each optical collecting means collects its own portion of the light scattered from the rough surface 13. This portion of the scattered light can be specified by an optical axis connecting the center of the light beam at the rough surface 13 with the central point of the scattered light collected by lens 16 (26) at the plane of the spatial filter 15 (25). All the spatial filters and respective optical collecting means should be separated in space so as to generate statistically independent electrical signals from all the photo- receivers. As it was found by the Authors, this condition is achieved when the angle between the optical axes of any of two portions of the scattered light is equal or larger than 4λ/πW , where λ is the wavelength of the scanning light beam and W is the linear size of the focused beam on the surface 13. Note that the footprint of the illuminating beam at the surface 13 may have non-circular shape. In this case the linear beam size W is measured along the intersection of the surface 13 with the plane containing the optical axes of the respective portions of the scattered light.

FIG. 3 shows a side view of the optical profilometer according to the first advantageous embodiment of the invention in which the relative positions of the optical elements are shown in details. Let the spatial filter 15 be situated at the distance Df from the surface 13 and at the distance Df from the focus position of the scanning beam, while the distance from the spatial filter 25 to the surface 13 and to the focus position is D₂ and D₂ , respectively. Spatial period of the filter 15 and filter 25 is A₁ and Λ₂, respectively. Then in accordance with Eq.3, the light power of the first selected portion of the scattered light after the filter 15 is modulated at the frequency of f_PDl = V_BS [Df + z_F JJA₁Zp assuming that the angle θ_\ shown in the FIG. 3 is small. In a similar manner, the light power of the second selected portion after the filter 25 is modulated at the frequency of f_PD2 = V_BS\Dζ + Z_F)/A₂Z_F , where Z_F is the distance between the focus position and the surface 13. If N portions of the scattered light are selected, the light power of the spatially filtered portion number / is modulated at the frequency of f_PDl =

+ z_F)IK_tz_F where Df is the distance from the focus position to the spatial filter number /, and Λ, is its spatial period. It is worth noting that abovementioned assumption of smallness of the angle θi, which defines the geometrical position of the spatial filter of the number /, does not imply any restrictions for implementation of the invention. One skilled in the art could easily express the distance Df (which is the distance between the spatial filter of the number / and the surface 13) through Df and zp for arbitrary #,.

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. 2, the scattered light after filtering by spatial filter 15 (25) is delivered to the photo-receiver 17 (27) by using optical collecting means 16 (26) such as a lens or objective or another optical device capable to collect light (for example, diffractive optical element). The photo- receivers 17 and 27 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-receivers 1, 2,..., N will be temporally modulated at the mean frequency of fpm, fpoi,---, /PD_N, respectively. These frequencies may be different but all of them correspond to the same distance Z_F, which is expressed as following:

Referring to FIG. 3 it is seen that A₁ is a calibration coefficient, which is defined by the geometrical position of the central point of the "z" portion of the scattered light at the respective spatial filter. In the simplest case of small angle θ,, this coefficient is expressed as A₁ = Df , and it can be preliminary determined during the calibration. Parameters Λ, and V_BS can be also preliminary determined. Therefore, the distance, Z_F, between the surface 13 and the focus position can be calculated by using Eq.4 after measuring the modulation frequency of the electrical signal in any measuring channel.

Since the electrical signals from different photo-receivers are statistically independent, averaging of zp computed from Eq.4 in different measuring channels will result in improved accuracy of the measurements. It is convenient to introduce a characteristic time, T_AV, of the collection of the electrical signals from N measuring channels as

As seen from FIG. 3, the sought distance z between the optical head 10 and the rough surface 13 is expressed as z = z_o + τ_AVV_BS , (6) where z₀ is the distance between the optical head 10 and the focus position.

Different electronic circuits may be employed to measure the frequencies

An advantageous configuration of the signal processing means 40 for the frequency measurements and for further calculation of the instant distance to the rough surface is shown in FIG. 4. For clarity of explanation only two measuring channels are described in FIG. 4 however, the number of channels may be easily increased because all the processing steps are the same for all the channels. Both electrical signals from the photo-receivers 17 and 27 are separately amplified in the processing step 41 up to the necessary level. For convenience, typical example of the amplified electrical signal is shown in the graph 47 representing one of the channels. Thereafter, in the step 42, the DC-level is subtracted from each amplified signal using a high pass filter. The example of the signal after the processing step 42 is shown in the graph 48. In the step 43 both signals are separately limited so as to form a sequence of binary pulses of the type shown in the graph 49. A measurement of the time, T_MP_\ and T_NPI, at which Np binary pulses occur, is performed in the step 44 for electrical signals from the photo-receiver 17 and 27, respectively. In the same step 44, the frequencies f_PD1 = T_NPl/N_p and f_PD2 = T_NP2/N_P are calculated. Then the characteristic time T_AV of both electrical signals is calculated in the step 45 by using Eq.5. In the final step 46, the instant 2-distance between the rough surface 13 and the optical measuring head 10 is calculated by using Eq.6.

It should be noted that the frequency-measuring procedure of Fig.4 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, the mean frequency of the electrical signals after the step 42 may be estimated by using fast Fourier transfer of the electrical signals.

In accordance with the present invention, there is provided a non-contact method for measuring a distance between an optical measuring head 10 and an optically rough surface 13, the method involves steps of illuminating an optically rough surface with a focused coherent beam; scanning of the said surface by said focused beam with a predefined speed of V_BS,' selecting at least two portions of the light scattered by said surface so that the angle between the axis connecting the central point of one of the said portions on the spatial filter with the center of the said focused beam on the said rough surface and the axis connecting the central point of another portion on the spatial filter with the said center of the said focused beam on the said surface is equal or larger than 4λ/πW; spatially filtering said selected portions by either a single spatial filter or different spatial filters; delivering said filtered portions into different photo-receivers; transforming the optical power received by said photo-receivers into electrical signals; measuring the mean temporal frequency of said electrical signals from said photo-receivers; calculating the characteristic time T_AV of the said electrical signals by using Eq.5; and calculating with Eq.6 the signal being characteristic of the instant distance z between the surface 13 and optical measuring head 10.

A first advantageous apparatus for non-contact distance measurements according to the invention generally includes the optical measuring head 10 and signal processing means 40. The optical measuring head 10 further comprises the illumination means 11 for generation coherent light beam with a predetermined wavelength; a focusing optical assembly 12 for focusing said laser beam out of the rough surface 13; an optical deflector 14 providing scanning of the said surface by the said focused beam; at least two spatial filters, 15 and 25, for filtering light scattered from the said surface; at least two optical collecting means, 16 and 26, for collecting and delivering said light filtered by spatial filters, 15 and 25, into two photo-receivers, 17 and 27, respectively; two photo-receivers, 17 and 27, for receiving said collected light and converting it into electrical signals dependent on the light power of said collected light. The signal processing means 40 further comprises electronic components arranged for evaluation of the temporal frequency, fp_Di and fpm, of said electrical signals, respectively, and calculation z-distance between the rough surface 13 and the optical measuring head 10 using predetermined Eq.5 and Eq.6.

FIG. 5 shows a side view of a second advantageous embodiment of the invention, which includes a displacing means 51 to produce additional displacement of the scanning beam so that different areas of the surface 13 are covered by sequential scans. It is assumed that optical deflector 14 provides multiple scanning of the focused beam over the surface 13. In FIG. 5 the direction of the beam scanning is perpendicular to the plane of the drawing while the displacement of the scanning beam provided by the displacing means 51 is in the plane of the drawing. However, this displacement may be in any direction relative to the direction of the scan. To be specific, the displacing means 51 is shown in FIG. 5 as a tilting mirror. Nevertheless, any other type of device or system capable to change position of the scanning beam in respect to the surface 13 (such as electro-mechanical or piezoelectric actuators, which moves/tilt either a mirror or focusing means 15, or acousto-optical or electro- optical deflectors, or even a device/system producing movement of the surface 13 itself, mentioned a few) may be used without departing from the spirit and scope of the invention.

The Authors of the present invention have found that each subsequent scan of the surface 13 will produce an electrical signal from the photo-receiver 17, which is uncorrelated with that of the previous scan, only when the subsequent scan is displaced from the previous scan on the distance Δ > W /2 , where W is the size of the focused beam measured at the surface 13 along the direction of the displacement. Suppose that a scan of the surface 13 results in an electrical signal from the photo- receiver 17 modulated at the mean frequency offsci and the subsequent scan (shifted by the distance Δ) gives an electrical signal with the mean frequency o£fsc2- If the surface is scanned repeatedly M times, the scan number "Λ" gives an electrical signal with the mean modulation frequency offsck- After measuring all the frequencies fscu fsc2,---, fsc_M, we calculate the characteristic time, TA_V, of the whple sequence of multiple scans as

Here Λ is the spatial period of the spatial filter 15 and Ak is a calibration coefficient, which is defined by the geometrical position of the spatially filtered light collected by the optical means 16 in a similar way as it is described above for the first embodiment of the invention and shown in FIG. 3.

In the final step, the distance z between the optical head 10 and the rough surface 13 is calculated by using Eq.6. Since each scan produces the electrical signal from the photo-receiver 17 statistically independent from that of any other scan, the accuracy of z-distance calculated by using Eq.7 and Eq.6 increases with the number M of the multiple scans.

In accordance with the present invention, there is provided a second non- contact method for measuring a distance between an optical measuring head 10 and an optically rough surface 13, the method involves steps of illuminating an optically rough surface with a focused coherent beam; providing multiple scanning of the said surface by said focused beam with a predefined speed of Vsc in such a way that any scan is displaced from each other scan by the distance equal or larger than a half of the size of said focused beam on the said surface; spatially filtering the light scattered from the said surface by spatial filter 15; delivering said filtered light into photo- receiver 17 and transforming the optical power received by said photo-receiver into electrical signal; measuring the mean temporal frequency of said electrical signal after each of multiple scans; calculating the characteristic time TA_V of the said electrical signals by using Eq.7; and calculating with Eq.6 the signal being characteristic of the instant distance z between the rough surface 13 and optical measuring head 10.

A second advantageous apparatus for non-contact distance measurements according to the invention generally includes the optical measuring head 10 and signal processing means 40. The optical measuring head 10 further comprises the illumination means 11 for generation coherent light beam with a predetermined wavelength; a focusing optical assembly 12 for focusing said laser beam out of the surface 13; an optical deflector 14 providing multiple scanning of the said surface by the said focused beam; a displacing means 51 providing displacement of the said focused beam relatively with the said surface; a spatial filters 15 for filtering light scattered from the said surface; an optical collecting means 16 for collecting and delivering said light filtered by the spatial filter 15 into a photo-receivers 17; the photo-receiver 17 for receiving said collected light and converting it into electrical signals dependent on the light power of said collected light. The signal processing means 40 further comprises electronic components arranged for evaluation of the temporal frequenciesj/sci,./sc2₅---_>./sc_Mθf said electrical signals after each of multiple scans, and calculation z-distance between the rough surface 13 and the optical measuring head 10 using predetermined Eq.7 and Eq.6.

Improvement of the accuracy of z-measurements in the first advantageous embodiment of the invention is achieved through calculation of the characteristic time TAV of the electrical signals received simultaneously from several photo- receivers (parallel averaging), while in the second advantageous embodiment, the characteristic time T_AV is calculated for electrical signals received from single photo- receiver in a sequence (sequential averaging). It is believed simply sufficient notice that one skilled in the art would have no difficulty in combining these two embodiments: the characteristic time TA_V calculated after single scan by parallel averaging may be simply averaged with other TAV calculated during multiple scans with final calculation of z-distance by using the same Eg.6. According to another advantageous embodiment of the invention, the optical measuring head 10 comprises the focusing optical assembly 12 capable of changing its focus position by a variable distance, the magnitude of which is measurable and characterized by certain indicative signal. This focusing optical assembly 12 may be for example, a zoom-objective frequently used in photo/video cameras. After calculating the characteristic time TA_V in the first or second advantageous embodiment of the invention, an additional parameter/^ is calculated as

J AV = T > W

where Λ,- is the spatial period of anyone of the spatial filters comprising the optical head 10 and A_t is the calibration coefficient defined by the geometrical position of the central point of the portion of scattered light filtered by this filter in respect to the focus position. If the calculated parameter /jjr is different from the previously chosen value of f_AV then the focusing optical assembly 12 is requested to change its focus position (which results in change of the modulation frequencies fp£n_?fpD2,»;fpDN or fschfsc2,--;fscM, and consequently, in change of TAV andfAv) until the parameter /AV becomes equal to f_AY . Let the equality f_AV = f_AV be achieved when the focus position is shifted from its initial position by the distance of δ, then the z-distance between the rough surface 13 and the optical measuring head 10 is calculated by using following equation:

* = Z_o +/l + ^y + V_BSτ_Λr , (9)

where z₀ is the distance between the optical measuring head 10 and the initial focus position of the focusing optical assembly 12; V_BS is the scanning speed of the coherent light beam over the rough surface 13; τ_AV is the characteristic time of the electric signals calculated for the initial position of the focusing optical assembly 12 during the calibration process; and A_f is the same calibration coefficient, which was used for calculating the parameter/) γ using Eq.8. Since the coefficients τ_A°_V , A_t, and

ZQ are defined during the calibration process, one can simplify Eg.9 by replacing them with the new coefficients B and C both of which are to be defined during the calibration:

∑ ^ B + Cδ . (10) According to yet another advantageous embodiment of the invention there is provided a single diffractive optical element, which combines the spatial filters 15, 25, and others with respective optical collecting means 16, 26, and others. Such a combination could be made for example, by evaporation of the equidistant metallic stripes on the diffractive element containing a series of lenses. It simplifies adjustment of the advantageous apparatus and diminishes its cost.

According to yet another advantageous embodiment of the invention, there is provided optical deflector 14 in the form of the rotating polygonal mirror fabricated so that each light-reflecting facet of the polygonal mirror is tilted in respect to the axis of rotation by an angle, which is different from the tilting angle of any other facet so that any respective scan of the said focused beam over the said surface is shifted from each other scan by the distance equal or larger than W/2. Here the linear size W of the focused beam is measured at the surface 13 along the direction of the shift between scans. The magnitude of the tiltϊng-aπgle difference depends on the angle of incidence of the laser beam on the polygonal mirror and on the focal length of the focusing optical assembly 12. Anyway, it can be defined for any particular configuration of optical elements in the optical head 10. One should notice that use of the polygonal mirror with the tilted facets allows combining of the optical deflector 14 with the displacing means 51 into one optical element.

Although there has been hereinabove described methods and apparatus for measuring the profile of an optically 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 an optically rough surface by illuminating said surface with a focused laser coherent beam in such a way that the said beam is scattered in any extent by said surface, providing scanning of the said surface by said focused beam with a predefined speed of V_BS, spatially filtering said scattered light by spatial filter, delivering the light transmitted through the said spatial filter into photo-receiver, transforming the optical power received by said photo-receiver into electrical signal, and measuring the mean temporal frequency of said electrical signal, characterized by selecting at least two parts of the light scattered by said surface so that the angle between the axis connecting the central point of one of the said parts on the spatial filter with the center of the said focused beam on the said surface and the axis connecting the central point of another part on the spatial filter with the said center of the said focused beam is equal or larger than 4λ/πW , where λ is the wavelength of the said laser beam and W is the linear size of the said focused beam on the said surface measured along the intersection of the said surface with the plane containing the said axes; spatially filtering said selected parts by either a single spatial filter or different spatial filters; delivering said filtered parts into different photo-receivers and transforming the optical power received by said photo-receivers into electrical signals; measuring the mean temporal frequency of said electrical signals from said photo-receivers,

fpm,-- •, /_PDN, where N is the number of said selected parts of the scattered light transmitted through the said spatial filters; calculating the characteristic time χ_Aγ of the said electrical signals by using

I ^N A the equation τ_ΛV = — 2] , where Λ, is the spatial period of the spatial

filter, which is filtering the part number i of the scattered light, and A₁ is the calibration coefficient defined by the relative position of the said central point of the said part number i on the respective spatial filter and the said focus position; and calculating the distance z between the said optical measuring head and the said surface by using the equation z - z_o + τ_AVV_BS , where z₀ is the distance between the said optical measuring head and the said focus position.

2. A non-contact method for measuring a distance between an optical measuring head and an optically rough surface by illuminating said surface with a focused laser coherent beam in such a way that the said beam is scattered in any extent by said surface, providing scanning of the said focused beam in respect to the said surface with speed of V_BS, spatially filtering said scattered light by spatial filter, delivering the light transmitted through the said spatial filter into photo-receiver, transforming the optical power received by said photo-receiver into electrical signal, and measuring the mean temporal frequency of said electrical signal, characterized by providing multiple scanning with at least two times of the said focused beam over the said surface in such a way that any scan is displaced from each other scan by the distance equal or larger than a half of the size of said focused beam on the said surface while the size of said focused beam is measured along the direction of the displacement; calculating the characteristic time T_AV of the said electrical signals by using

A_n ^M 1 the equation τ_AV =—Υ] , where Mis the number of said scans, Λ is the

M _k^ Af_PDk -V_sc effective spatial period of the said spatial filter, AQ is the calibration coefficient defined by the relative position of the said spatial filter and the said focus position, and f PD_k is the mean frequency of the said electric signal measured during the scan number t, calculating the distance z between the said optical measuring head and the said surface by using equation z = z_o + t_AVV_sc .

3. A method according to claims 1 or 2 characterized in that the said method additionally comprises the steps of: calculating the parameter^ by using equation f_AV = — — -^v -^ , where TAV

Λ_tτ_Λr is the said characteristic time, A_\ is the said calibration coefficient, and Λ,- is the effective spatial period of the spatial filter, which is filtering the z-part of the said scattered light; displacing the said focus position until the said parameter./^ becomes equal to the previously chosen magnitude and measuring the value of this displacement δ; calculating the distance z between the said optical measuring head and the said rough surface by using equation z = B + Cδ , where B and C are the calibration coefficients defined by the relative position of the said spatial filters and the said focus position.

4. An apparatus for measuring a distance between an optically rough surface and an optical measuring head, said optical measuring head comprising illumination means for producing a beam of the coherent light, focusing optical assembly for focusing said light beam out of the said surface, optical deflector for scanning the said focused beam over said surface, spatial filter for filtering light scattered from the said surface, optical collecting means for collecting and delivering filtered light onto a photo-receiver, and the photo-receiver for receiving collected light and converting it into electrical signal dependent on the light power of the collected light characterized in that said optical measuring head further comprises at least one more photo-receiver and one more respective optical collecting means for collecting light filtered by either the same or different spatial filters into said photo-receivers, said collecting means installed so that the angle between the optical axis connecting the central point of a portion of light collected by one of said optical collecting means with the center of the said focused beam on the said surface and the optical axis connecting the central point of a portion of light collected by another optical collecting means with the said center of the said focused beam on the said surface is equal or larger than Aλ/πW , where λ is the wavelength of the said coherent light and W is the linear size of the said focused beam on the said surface measured along the intersection of the said surface with the plane containing the said optical axes, 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 the said surface and said optical measuring head according with the predefined equations of claim 1.

5. An apparatus for measuring a distance between an optically rough surface and an optical measuring head, said optical measuring head comprising illumination means for producing a beam of the coherent light, focusing optical assembly for focusing said light beam out of the said surface, optical deflector for scanning the said focused beam over the said surface, spatial filter for filtering light scattered from the said surface, optical collecting means for collecting and delivering filtered light onto a photo-receiver, and the photo-receiver for receiving collected light and converting it into electrical signal dependent on the light power of the collected light characterized in that said optical measuring head further comprises displacing means for changing the position of said scanning beam in respect to the said surface so that any scan is shifted from each other scan by the distance equal or larger than a half of the size of said focused beam on the said surface while the size of said focused beam is measured along the direction of the said shift, 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 the said surface and said optical measuring head according with the predefined equations of claim 2.

6. An apparatus according to any of claims 4 or 5 characterized in that said optical measuring head further comprising focusing optical assembly for variation of the focus position of the said light beam, and a signal processing means arranged for evaluation of the temporal frequency of said electrical signals and calculation of the distance between the said surface and said optical measuring head according with the predefined equations of claim 3.

7. An apparatus according to any of claims 4 to 6 characterized in that the said spatial filters and the said optical collecting means are fabricated as a single diffractive-optical element.

8. An apparatus according to claim 5 characterized in that the said optical deflector is a rotating polygonal mirror fabricated so that each light-reflecting facet of the said polygonal mirror is tilted in respect to the axis of rotation by an angle, which is different from the tilting angle of any other mirror-facet so that any respective scan of the said focused beam over the said surface is shifted from each other scan by the distance equal or larger than a half of the size of said focused beam on the said surface while the size of said focused beam is measured along the direction of the said shift.