DE19513233A1 - Determination of phases and phase differences of light reflecting from specular or diffusely reflecting, or transparent surface of object - Google Patents

Abstract

The method uses light or infrared radiation with rays reflected in a shifting or distorted condition, either from an object with a diffusely reflecting surface, from a transparent object or from the surface of a mirror. The object (10) is irradiated by a coherent beam (11) of known frequency. The reflected rays (12) pass through an imager (13) to a sensor (15). The sensor is composed of many regularly arranged pixels (16). A superimposed reference ray (14) of equal frequency and defined phase produces an speckle interference pattern (17). The intensity values of the pattern are processed to give the calculated phase difference.

Description

The invention relates to a method for determining phases and phase differences of radiation, especially of light Radiation and infrared radiation diffused by an object reflected, transmitted and smoothly reflected, as well a device for performing such a method.

The determination of phases and phase differences is the basis the methods used in optical measurement technology, such as holo graphic interferometry, interferometry, speckle measurement ren, shearography, fringe projection method, Moire method ren and the like. The phase relationships are in shape represented by stripe patterns, which either derive from Ver linkage of images taken in two states or that directly through the superposition of two wave fronts arise.

For the automatic evaluation of the resulting images the so-called phase shift process is particularly efficient proven that emanates from the stripes and these Strip images manipulated in a way that a point by point mathematical analysis allowed by a computer. To do this the at any point of a captured image measuring intensity described by the following equation:

I = a (x) (1 + m (x) cos ϕ)

where I is the light intensity in the relevant state of the Object, a the background brightness, m the contrast and ϕ the Phase position in the relevant state means. For calculating the parameter of interest, the phase ϕ, is in the phase shift procedure introduced a known additional phase θ:

I = a (x) (1 + m (x) cos (ϕ + θ))

If you take this additional phase θ and A determination of the intensity I before results in a System of equations with 3 unknowns, from which the searched phase ϕ can be calculated.

This phase shift method for the formation of is disclosed Line images of all kinds, for example in DE-OS 37 23 555. The procedure described here corresponds to the classic one Development of the interferometric measuring technique, in which initially the phase relationship between two light beams by the above Equation is described, and the holographic interferome trie, which also produce interference fringes directly, which are charged analogously. The disadvantage is consider that there is a need to stood to take at least three phase-shifted images, to calculate the phase and then by Subtract the change of the object between two states to determine. This procedure shapes the procedure quite well time consuming. Add to that the pictures in every condition be read one after the other, which presupposes that the Object is at rest during this reading time. In addition, must also the measurement setup for the duration of the time-consuming image take and evaluate in peace. The phase shift process can therefore only be used for slow processes and extremely susceptible to interference due to influences from the surrounding area exercise.  

Another method of performing the phase shifting process rens is disclosed in EP 0 419 936 B1. For this, the Ob project in at least two states with coherent or partially coherent irradiated radiation of a predetermined frequency, and in each of at least two changed states is the reflec radiated or traversed radiation from an imaging optics in depicted an image plane in which a sensor with a A large number of preferably regularly arranged sensor elements ten or pixels. On this sensor is a reference radiation with a certain preferably the same frequency superimposed with a defined phase position, and from the intensity The phase difference becomes signals from the sensor elements or pixels the radiation from the object or through the object between the determined in both states. This will help determine the phase also three or more neighboring ones according to the above equations Sensor elements of an image-recording sensor summarized to per condition only with one picture and to also be able to examine moving processes. At the Untersu This is how the speckles become optically rough surfaces magnified that they each have at least three sensor elements cover.

As a disadvantage, however, is to be seen that through the together three sensor elements to reduce the spatial resolution to a third compared to the temporal phase shift. But since the image resolution in full-field measurement technology is always the is limited size, the practicality is this method known from EP 0 419 936 is very limited. There continue to make the speckles so big when shooting must cover at least three sensor elements, and this serves to reduce the aperture, also occurs a significant loss of light. In addition, that this known method for each state of the object Equation system with at least three equations and  must be solved, which leads to a not inconsiderable calculation wall and storage requirements.

The invention is based, task and a method To continue device of the type mentioned in the beginning that a high image per object state Image resolution is achieved.

According to the invention, this task is carried out on the process side by the Features mentioned claim and device side by solved the features mentioned in claim 12. Preferred Features that further favor the method or the device form, are ge in the respective subordinate claims called.

According to the invention, for example, an object that is diffuse has reflective surface, with coherent or partially coherent renter radiation irradiated in any polarization direction, and the reflected light is converted into an optical system Image plane depicted in which there is a light-sensitive sensor or a multi-chip camera with a variety of sensor elements or pixel. With an analog sensor, such as Röhrenenkame ra, diode, corresponds to a sensor element of the resolution. Soon the sensor is timed with further light, preferably the same Frequency and defined phase position, illuminated in such a way that a Interference pattern on the sensor arises. Object and reference beam are adjusted so that they have an interference mu ster with preferably constant spatial carrier frequency testify, the imaging optics being designed and inserted in this way is that when speckles develop, the image of a only two speckles generated by the radiation in the image plane Sensor elements or pixels covered, and which for each Condition recorded respective intensity values of each only two sensor elements or pixels for the phase difference mutually or crosswise.  

In addition to the phase difference, at least two of these can Recordings also the phase, the contrast and the background hel be calculated.

If the examined object is not a body deals with a diffusely reflecting surface, but a transparent medium or a reflective surface, do not occur Speckles on. In these cases the interference field will be that of Object coming and the reference radiation also only through the use of two sensor elements or pixels in two states that determines.

In contrast to the phase shift method discussed above are not in the proposed method according to the invention The phases are determined directly for each state, but the phases intensity values taken alternately or crosswise com trimmed or offset. This advantageously makes it possible to get by with two pieces of information in each state Solvable system of equations consisting of four equations with a total of 4 To solve unknowns (a, m, ϕ, Δϕ).

According to the invention, a completely new path is thus followed that of conventional thinking developed from interferometry approach to having to analyze sinusoidal stripes has been. To analyze a sinus strip, at least three support points needed. In the invention, however, is based on the strip analysis is omitted, and only the information available in the image directly for determining the Phase and, if necessary, other parameters.

According to a preferred development of the invention, the Representation of the object in the different states at the same time tig on different image levels, with Sensorele elements or pixels on two different sensors or multi-chip cameras are used. It is provided that in each case  corresponding sensor elements or pixels in two or more Image planes can be used for phase mapping or in each Image plane is registered in a wave-selective manner. In the second Al ternative can simultaneously use multiple lighting beams and several corresponding reference beams of different Frequencies are used, with the possibility of using different illuminating beams and reference beams different measuring directions, from which the more dimensional phase difference can be determined.

According to the invention, it is advantageous over the prior art image resolution improved by a third, the image memory requirements reduced by a third, the computational effort ver reduces and thus increases the computing speed. Come in addition, that the optical system can be set much easier can, and that about a factor of 3 increased light output is achieved.

The provided on only about two sensor elements or pixels Illustration of a speckle can be made according to a preferred embodiment device of the invention also in different areas of a single Sensor or a single multi-chip camera can be made.

Furthermore, the reference radiation from the object radiation according to the shearing procedure or by double simultaneous Illumination of the object with coherent and partially coherent light be generated. Alternatively, it can also be provided that more re reference radiation with the same or different Pha layer overlay.

It is also advantageous if in the imaging beam path an intermediate image is generated to differentiate advantageously Use lenses (interchangeable lenses and zoom lenses) to be able to.  

The sensor elements are preferably along parallel lines aligned. Alternatively, a multi-chip camera can also be used the phase position between the two camera chips is defined, where then the corresponding pixels for the Phase difference determination can be used.

The reference light is advantageously made by an optical waveguide ter, an optical grating, a beam splitter or the like directed to the sensor or the multi-chip camera. On the other hand the reference radiation can also, as already mentioned, from the light reflected from the object can be obtained, for example wise by arranging a wedge, a prism, a tilt mirror, a Michelson interferometer and the like, wel are arranged in, in front of or behind the imaging optics.

When illuminating the object with two light beams and the Illustration of the resulting interference pattern in the Image plane, serves as a variant that lighting together as Reference radiation for the other and vice versa.

According to a preferred further embodiment of the invention is the spatial carrier frequency by an optically effective Influencing the object and / or reference radiation defined, for example by tilting, introducing an optical Keils, an optically active medium with variable refraction number, a holographic optical element or the like.

The device according to the invention can also be used for simultaneous detection development of the phase differences in different spatial directions be used by the object with two or three radiation is illuminated against different frequencies two or three image planes are mapped simultaneously and two or three reference beams also of different waves lengths, preferably according to the illumination wavelengths, are mapped in the three image planes. The registration  is then carried out advantageously wavelength-selective such that because corresponding reference radiation and object radiation gen overlap in an image plane, whereby as registration medium preferably used a multi-chip color camera can be. The lighting arrangement is preferred chosen such that a two- or three-dimensional detection the phase differences is made possible.

The invention is described below with reference to the beige added drawings explained in more detail. Show it:

Figure 1 push a flow chart to explain the sequence in the inventive method with spatial phases.

Fig. 2 is a schematic representation of the method;

Fig. 3 is a schematic representation of the sensor elements of a sensor;

Fig. 4 is a schematic representation of the method with education from two different image planes;

FIG. 5 shows a schematic representation of the method, the mapping being carried out on two different areas of a single sensor; FIG.

FIG. 6 is a schematic illustration showing the extraction of the reference radiation from the object radiation by the shearing method;

Fig. 7 is a schematic representation illustrating the generation of the reference radiation by double simultaneous illumination of the object with coherent or partially coherent light;

Is a schematic representation of the method, are superimposed in which several reference radiations with the same or below schiedlicher phase position. 8;

Fig. 9 is a schematic representation of the imaging Strah lengangs with an intermediate image;

Fig. 10 is a schematic representation of the method according to the invention, wherein the spatial carrier frequency is generated by influencing a second illumination beam by introducing an optical wedge; and

Fig. Is a schematic representation of the method according to the invention, wherein the reference radiation from the object radiation after the shearing method is obtained, wherein the spatial carrier frequency by a take is generated an optical wedge in the reference beam 11.

In Fig. 1, the sequence of the method according to the invention is shown roughly schematically in the form of a flow chart. Then, in a first state of the object, an image is taken or a single image is read in, the structure is changed or the object is loaded, moved or deformed, and then another image is taken or read in the second state that is then reached.

Next, a decision can be made as to whether the images should be evaluated or not. If not, either the process is terminated or the The process begins with reading a new image for one possibly new, second state of the object. If others On the one hand, an evaluation is to be carried out in any condition recorded respective intensity values of the sensor elements or pixels for the phase difference determination alternately or  processed crosswise in order to then calculate the phase difference and then the result for illustration and / or control purposes to process further.

In FIG. 2, a simple construction is shown schematically for imple out the method for determining phase differences and radiation for the case of an object 10 with diffuse reflec render surface shown. The object 10 is irradiated in at least two different states (deformation, displacement or the same) with coherent or partially coherent radiation 11 of a predetermined frequency. In each state of the object 10 , reflected radiation 12 is imaged by imaging optics 13 into an image plane in which there is a sensor 15 which, according to FIG. 3, has a plurality of regularly arranged sensor elements 16 . On the sensor 15 , a differential radiation 14 with a specific, preferably constant frequency with a defined phase position, which is at an angle to the radiation 12 reflected by the object, is superimposed, whereby speckles occur. The image of a speckle 17 generated by the radiation is adjusted according to the invention by the imaging optics 13 such that two sensor elements 16 are covered in the image plane. To determine the phase difference, it is then only necessary to take into account the respective intensity values of the sensor elements 16 taken from an image acquisition for each state alternately or crosswise, for which purpose a computer is used, not shown.

Fig. 4 shows schematically a partial modification of a device according to the invention, in which the object radiation 12 passing through the imaging optics 13 is performed by means of a double prism 18 in two orthogonal image planes 19 and 20 .

In Fig. 5, another imaging variant is shown schematically, in which the imaging on different areas of a single sensor 21 via an optical element 22 downstream of the imaging optics, z. B. grid, prism or the like, is made.

Fig. 6 shows the extraction of the reference radiation from the object radiation 12 according to the known shearing method as an alternative to the representation according to FIG. 2, while Fig. 7 shows the generation of the reference radiation by double simultaneous illumination of the object 10 with coherent or partially coherent light 11 , 11 'illustrated.

The superimposition of several reference radiations 14 and 14 'as a variant of Fig. 2 is shown in Fig. 8.

In some cases, the generation of an intermediate image in the imaging beam path 12 can be favorable in order to be able to use different lenses. Fig. 9 shows schematically the course of the imaging beam path from an object 10 'through two modified imaging lenses 13 ' to the imaging plane in which the sensor 15 is arranged. An intermediate image plane 23 is located between the lenses 13 'of the modified imaging optics.

In Fig. 10 is schematically shown how in the generation of the reference radiation already explained according to FIG. 7 by double simultaneous illumination of the object 10 with coherent tem or partially coherent light 11 , 11 'bring a defined carrier frequency by influencing the reference beam 11 ' by a an optical wedge 23 is generated.

Fig. 11 illustrates the generation schematically a defi ned carrier frequency by controlling the reference beam 11 by means of introducing an optical wedge 23 for the in Fig. Shearing method already shown. 6

Claims (18)

1. A method for determining phases and phase differences of radiation, in particular light radiation and infrared radiation, which is either reflected by an object with a diffusely reflecting surface in at least two displaced or deformed states, or a transparent object in at least two changed states stands or is reflected from a reflecting surface of an object in at least two displaced or deformed states, consisting of the following process steps:

• - The object is irradiated in at least two states with coherent or partially coherent radiation of a predetermined frequency;
• - In each state, the reflected or transmitted radiation is imaged by imaging optics in an image plane in which a sensor with a plurality of preferably regularly arranged sensor elements or pixels is located;
• - On the sensor, a reference radiation with a certain, preferably the same frequency with defined phase position is superimposed; and
• the phase difference of the radiation from the object or through the object between the two states is determined from the intensity signals of the sensor elements or pixels,

the object and reference beam being set such that they generate an interference pattern with a preferably constant spatial carrier frequency,
wherein the imaging optics are designed and set in such a way that when speckles are formed, the image of a speckle generated by the radiation in the image plane only covers about two sensor elements or pixels, and
wherein the intensity values recorded for each state of only approx. two sensor elements or pixels are taken into account mutually or crosswise for the phase difference determination. 2. The method according to claim 1, characterized in that only one sensor element or pixel per speckle on two or more different sensors are used, each one corresponding sensor elements or pixels in two images planes are used for phase formation. 3. The method according to claim 1, characterized in that the Image simultaneously on different image planes is taken, and that in each image plane wavelength selec tiv is registered. 4. The method according to claim 3, characterized in that several lighting beams and several cor responding reference beams of different frequencies zen can be used. 5. The method according to claim 4, characterized in that with different illuminating beams and reference beams different measuring directions are spanned from which the multidimensional phase difference can be determined. 6. The method according to claim 1, characterized in that the Mapping to different areas of a single sensor is made.   7. The method according to claim 1, characterized in that the Reference radiation from object radiation according to the Shea ring process is won. 8. The method according to claim 1, characterized in that the Reference radiation through double simultaneous lighting of the object with coherent or partially coherent light is fathered. 9. The method according to any one of the preceding claims, characterized characterized in that the spatial carrier frequency by a optically effective influencing of the object and / or ref limiting radiation is defined. 10. The method according to claim 1, characterized in that several reference radiations with the same or different phase position are superimposed. 11. The method according to any one of the preceding claims, characterized characterized in that in the imaging beam path Intermediate image is generated. 12. The apparatus for performing the method according to claim 1, consisting of
at least one radiation source ( 11 ), in particular a light source, for emitting coherent or partially coherent radiation of a predetermined frequency onto an object ( 10 ) with a diffusely or smoothly reflecting or transmitting surface,
at least one adjustable optical system ( 13 ) for imaging the radiation ( 12 ) reflected by the object ( 10 ) or passing through the object ( 10 ) in an assigned image plane;
a sensor ( 15 ) arranged in each image plane with a plurality of preferably regularly arranged sensor elements ( 16 ) or pixels;
at least one reference radiation source for superimposing the sensor or sensors ( 15 ) with reference radiation ( 14 ) with frequencies which are preferably the same as the respective illuminating radiation ( 11 ) and defined phase positions such that the respective superimposition preferably results in constant spatial carrier frequencies;
the imaging optics ( 13 ) being adjustable so that the image ( 17 ) of a speckle formed by the radiation onto the object ( 10 ) covers only about two sensor elements ( 16 ) or pixels in the respective image plane, and off
a computer by means of which the phase difference of the radiation from or through the object can be determined from the respective intensity signals of the two sensor elements or pixels alternately or crosswise in at least two states of the object. 13. The apparatus according to claim 12, characterized in that the sensor elements ( 16 ) or pixels of each sensor are aligned along parallel lines. 14. The apparatus according to claim 12 or 13, characterized in that an optical waveguide, an optical grating ter or a beam splitter is provided for directing the reference radiation ( 14 ) onto the sensor ( 15 ). 15. The apparatus according to claim 12 or 13, characterized in that the reference radiation from the object ( 10 ) reflected or the object passing light by arranging a wedge, a prism, a tilted mirror or a Michelson interferometer in, before or behind the imaging optics ( 13 ) can be obtained. 16. The apparatus according to claim 12, characterized in that the reference radiation can be generated by a second illumination of the object ( 10 ) with coherent or partially coherent light ( 11 '). 17. The device according to one of claims 12 to 16, characterized characterized in that for a wavelength-selective regi a multi-chip color camera is provided.