the method according to the invention and the arrangements based on the method rely on basic constructions of two - channel polarimeters , known per se , using linearly polarized light for analyzing phase shifts ( birefringence effects ). known arrangements of this kind comprising , successively , a light source , a linear polarizer , the measurement sample , a polarizing splitter , one of whose two orthogonal polarization directions is oriented at 90 ° to the linear input polarization , and two light receivers measuring incident light intensities in two orthogonal polarization channels , are expanded , according to the invention , by an optical unit which produces an elliptical polarization of light . the elliptical polarization has the particular feature that the polarization ellipse has a very large ratio of the large principal axis to the small principal axis and , in two measurement phases a and b , a first state of the direction of rotation of the elliptical polarization can be switched to a second state with the opposite direction of rotation . this step can be achieved on the one hand by changing the ratio of the principal axes and on the other hand by changing the direction of the incident linear polarization . an optical unit , known per se , by means of which these requirements can be implemented is a phase - retarding device ( known to the person skilled in the art as a retarder ). for the novel purpose according to the invention , a retarder of this kind can be constructed so as to be controllable in different ways in principle : a ) alternating angle orientation of the retarder ( mechanically controlled retarder ); b ) alternating phase retardation of the retarder ( electro - optical or magneto - optical phase modulator with alternating control voltage or control current ). persons with skill in the field of polarization optics will be familiar with additional techniques . in case ( a ), due to the comparatively slow movement speeds of the necessary mechanical adjusting elements , the difference is given by sequential individual measurement and subsequent numerical subtraction of the measured values . in case ( b ), on the other hand , due to the high settling speeds of electro - optical components , the change in the direction of rotation of the elliptical polarization is carried out in the form of a modulation technique , so that the difference signal can be formed by means of the known technique of lock - in detection with the modulation frequency as reference clock , which allows a considerable increase in the measurement speed up to real - time capability . in the arrangement shown schematically in fig1 , with reference to which the method according to the invention will be described initially , a beam of linearly polarized light is generated by means of a laser source 1 and a polarizer 2 . it passes the optical phase shifter 3 which generates a path difference that is comparatively small relative to the wavelength of the laser light ( typically λ / 10 . . . λ / 1000 . the light exiting the optical phase shifter 3 is accordingly elliptically polarized with a comparatively large ratio of the large ellipse axis to the small ellipse axis . the optical phase shifter 3 has the characteristic that the direction of rotation of the elliptical polarization and the ratio of the axes can be controlled — as will be described more fully later on in the specific embodiment forms — in order to divide the measurement of the optical stress state into the two measurement phases a and b . the measurement phases a and b differ in that the direction of rotation of the elliptical polarization is reversed in the transition into the other respective phase , as is made clear in fig1 by the different arrow directions at the polarization ellipse for measurement phases a and b . after the elliptical polarization has been impressed on the beam impinging on the measurement sample 4 , the beam suffers disturbance of its polarization state characteristic of the ( local ) stress state in the measurement sample 4 when penetrating the measurement sample 4 . a polarizing splitter 5 divides a light bundle exiting the measurement sample 4 into two linearly polarized components whose intensities i ⊥ and i ∥ are measured by the light receivers 6 ( channel of perpendicular polarization ) and 7 ( channel of parallel polarization ). the associated photocurrents s ⊥ and s ∥ obtained from the light receivers 6 and 7 are registered in the processing unit 8 and processed as a function of the control state of the optical phase shifter 3 . fig2 and 3 show implementations of the method according to the invention in a transmission arrangement , wherein the controllable phase shifter 3 shown in fig1 is realized by a birefringent phase retardation plate 9 with adjustable orientation . the position of the phase retardation plate 9 is shown in phase a ( fig2 ) and in phase b ( fig3 ) of a measurement for determining the stress state . the measurement phase a shown in fig2 is characterized in that the phase retardation plate 9 is rotated by angle φ a relative to the polarization direction of the entering linearly polarized light ( shown by the double arrow ). the phase shift of the phase retardation plate 9 is in the range of λ / 1 − . . . λ / 1000 , angle φ a is in the range of 0 . . . 45 °. the resulting direction of rotation of the polarization ellipse is marked by an arrow at the ellipse . the measurement phase 3 in fig3 shows that the phase retardation plate 9 is rotated by angle φ b relative to the polarization direction of the entering linearly polarized light , where φ b = φ a + 90 °. the resulting direction of rotation of the polarisation ellipse is again identified by an arrow at the ellipse . the measurement device 8 receives and amplifies the photocurrents obtained from the light receivers 6 and 7 in the same manner as in fig2 and 3 . when the light of the laser source 1 is intensity - modulated for improved suppression of extraneous light influences , which can be carried out by means of any chopper arrangement or by direct modulation of the laser source 1 , the measurement device 8 advisably comprises two lock - in amplifiers ( also known as lock - in detectors ). the measurement device 8 processes the photocurrents of the light receivers 6 and 7 in the following manner : 1 . the photocurrents of the light receivers 6 and 7 are measured during the rotation of the phase retardation plate 9 by angle φ a . the measurement values s ⊥ a and s ∥ a are measured ; 2 . the photocurrents of the light receivers 6 and 7 are measured during the rotation of the phase retardation plate 9 by angle φ b . the measurement values s ⊥ b and s ∥ b are measured ; 3 . the measurement values s ⊥ a , s ∥ a and s ⊥ b , s ∥ b of the light receivers 6 and 7 are normalized with respect to the intensities i ⊥ and i ∥ such that in the arrangement according to fig2 and 3 , the zero balancing is carried out in that , with the measurement sample 4 removed from the measuring arrangement , one of the two angles φ a or φ b is changed slightly relative to the condition φ b = φ a + 90 °, which applies to an ideal system , until δi ⊥ is sufficiently close to zero . in fig4 , the method according to the invention is realized in a transmission arrangement in which the controllable phase shifter 3 shown in fig1 is implemented by means of an electro - optical phase modulator 10 . the light source 1 is controlled by a modulator 15 which is clocked by an intensity clock generator 14 with frequency f 1 . the electro - optical phase shifter 10 is controlled by a control voltage generator 11 which is clocked by a phase shifter clock generator 12 with frequency f 2 . the electro - optical phase modulator 10 is oriented at ρ = 45 ° in the beam path , so that the reversal of its control voltage leads to a sign reversal of its phase shift λ . the phase shift λ generated by the control of the phase modulator 10 is typically in the range of λ / 10 . . . λ / 1000 . the signals proceed from the light receivers 6 and 7 ( measurement values s ⊥ a and s ⊥ b ; s ∥ a and s ∥ b ) to a two - channel lock - in detector 16 which detects the signal sum from s ∥ a , s ⊥ a , s ∥ b and s ⊥ b required for normalizing . the lock - in detector 16 receives its reference signal with frequency f 1 from the intensity clock generator 14 . further , the signals proceed from light receiver 6 ( s ⊥ a and s ⊥ b ) to another lock - in amplifier 13 which obtains its reference signal with frequency f 2 from the phase shifter clock generator 12 . a particularly advantageous implementation results by selecting f 1 = 2f 2 . for this case , fig5 shows time charts of some signals of the measurement arrangement . fig5 shows , from top to bottom : the output signal of the intensity clock generator 14 with frequency f 1 ; the light intensity of the light source 1 ; the output signal of the phase shifter clock generator 12 with frequency f 2 ; the control voltage generated by control voltage generator 11 and applied to the phase modulator 10 , wherein the control voltages applied to the phase modulator 10 in measurement phases a and b are designated by v a and v b , respectively ; the polarization state of the light which exits from the phase modulator 10 and enters the measurement sample 4 ; the intensity i ⊥ measured at the light receiver 6 when a sample is located in the beam path ( solid line ) and when no sample is located in the beam path ( dashed line ); and the intensity i ∥ measured at the light receiver 7 . when selecting the time functions for the control frequencies f 1 and f 2 , it is particularly advantageous to set the switching moment for the control voltage for the phase modulator in the off state of the light source , since in that case the always finite changeover time period , i . e . the finite switching velocity , causes minimal disturbance of the measurement results . as can be seen from the time charts , the lock - in detection of the output signals of the light receivers 6 and 7 with reference frequency f 1 , according to the invention , supplies the arithmetic averages of the measurement signals s ⊥ and s ∥ from the time segments a and b which are required for normalizing the intensity values . the detection of the output signal of the light receiver 6 with reference frequency f 2 supplies the value δi ⊥ · n , where n is the normalization constant . in the arrangement according to fig4 , the zero balancing is carried out in that , with the measurement sample removed from the measuring arrangement , one of the two control voltages v a or v b is changed slightly relative to the condition v b =− v a applicable for an ideal system until the output signal of the lock - in amplifier 13 supplies an average δi ⊥ which is sufficiently close to zero . in the embodiment form shown in fig4 , the calibration of the measurement sensitivity is particularly simple . with the measurement sample 4 removed from the beam path ( γ = 0 ), the quantity k ′= i ⊥ /( i ⊥ + i ∥ ) is formed from the output signals of the lock - in detector 16 , where k ′= sin 2 λ sin 2 2ρ because γ = 0 . in the embodiment form according to fig4 , ρ = 45 °, so that the quantity k = sin 2 λ sin 2ρ = sin 2λ introduced above can easily be determined from k ′= sin 2 λ . fig6 illustrates the possibility of implementing the inventive solution mentioned above as a reflection arrangement , for which purpose an additional beam splitter 19 is introduced in the form of a partially transparent mirror 19 . the latter deflects the elliptically polarized light onto the measurement sample 4 . the portion of intensity reflected on the latter then passes through the partially transparent mirror 19 again into the polarizing splitter 5 acting as analyzer . the optical phase modulation is carried out and the signal is obtained in a manner analogous to fig4 . while the foregoing description and drawings represent the present invention , it will be obvious to those skilled in the art that various changes may be made therein without departing from the true spirit and scope of the present invention .