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    • 专利摘要:
    Calibration procedure of spatial light modulators. A calibration procedure of spatial light modulators is described. In particular, the present invention relates to a method for characterizing a spatial light modulator, of the tn-lcos type commonly used, for example, in imaging systems, which has the ability to completely predict its modulation properties such as complex optical modulation (irradiance, amplitude and phase) or modulation of the polarization state of light. To this end, microscopic optical models based on the mathematical treatment of jones matrices are used, the liquid crystal is modeled as layers of microscopic optical phase shifting material (or retarder) whose light retarding properties depend on the applied voltage. (Machine-translation by Google Translate, not legally binding)
    公开号:ES2604684A1
    申请号:ES201531282
    申请日:2015-09-08
    公开日:2017-03-08
    发明作者:Ignacio MORENO SORIANO；José Luis MARTÍNEZ FUENTES
    申请人:Universidad Miguel Hernandez de Elche；
    IPC主号:
    专利说明:

    OBJECT OF THE INVENTION
    The present invention relates to a method of calibration of spatial light modulators. Specifically, the present invention relates to a method for the characterization of a spatial light modulator, of the TN-LCOS (Twisted Nematic Liquid Crystal on Silicon) type commonly used, for example, in imaging systems, which It has the ability to completely predict its modulation properties such as complex optical modulation (intensity, amplitude and phase) or modulation of the polarization state of light. BACKGROUND OF THE INVENTION
    Spatial Light Modulators or SLM (English Spatial Light Modulators) are opto-electronic devices commonly used in imaging systems, where they act as modulators of the intensity or brightness of light that can be controlled spatially by changing the voltage of each of the cells or pixels of the device. Also, in other systems, especially in research and new technological developments, these devices are also used to modify other light parameters such as their polarization state, or their phase.
    The use of SLM technology is common, for example, in video projection systems and screens, such as liquid crystal or LCD screens (English, Liquid Crystal Display). Of these screens, subtypes TN-LCD (English, Twisted Nematic-LCD), PAL-LCD (English, Parallel Aligned-LCD), or FLCD (English, Ferroelectric-LCD) are known.
    Regardless of the type of screen used, it is of great importance in order to use these devices efficiently to develop a precise physical model that allows to evaluate and predict the properties of optical modulation, as well as the development of techniques of characterization of the optical response by a procedure of


    calibration. This calibration is necessary since the optical modulation properties depend decisively on the orientation of the polarization elements (polarizing filters and / or phase shifters) that are always located at the entrance and exit of the screen.
    In addition, there is an additional problem in certain SLM devices derived from the simplification of electronic circuitry for the generation of the pixel control electrical signals. This problem has been registered, for example, in LCOS devices (Liquid Crystal On Silicon). These represent an LCD technology that allows to reduce the size of the pixels with respect to other types of screen and, therefore, produce high resolution screens. In these types of screens, the different voltage levels are preferably achieved by a pulse width modulation or PWM modulation (in English, Pulse Width Modulation). This technique is very suitable when the final detector is the human eye, which has a very slow response time, as is the case in imaging systems. However, these voltage levels are not stable over time and their fluctuations have a decisive impact on the characteristics of optical modulation in applications that employ phase modulation or polarization status, typically for laser beam control. In addition, these temporal fluctuations make the calibration and optimization of the optical response highly difficult, so it is necessary to measure signals over time and apply the optimization procedures for each temporal sample of the characterization.
    Various characterization methods and procedures are known for different technologies of liquid crystal modulators. Specifically, document US2005 / 0225859 discloses a characterization procedure focused on the technology of DMD modulators (English, Digital Micro-Mirror Display). DMD technology (also called DLP) consists of arrays of pixels formed by micro mirrors whose inclination is adjustable by voltage. The objective of this procedure is to form an interferential image of the pixels of the device on a CCD camera (English Charge-Coupled Device) by which it is possible to deduce the inclination of the micro-mirrors according to the voltage applied to them. In order to form said interferential image, it is appealed to the use of various systems that allow applying a technique called displacement interferometry (or, Shearing Interferometry). By


    For example, by placing a suitable diffraction network prior to the CCD camera, it is possible to form the interference of two or more displaced versions of the original image that would exist if the network was omitted. This interference allows the evaluation of the optical path difference properties (which can be translated into a lag, or an inclination) of the observed object. When measuring images of the surface of the modulator, the procedure of this patent has the advantage of being able to also calibrate the aberrations of the device (differences in inclination of the micro-mirrors in the entire surface of the device for the same voltage). However, this technique cannot be applied directly to LCD devices. Likewise, the specific treatment to be followed is not contemplated in case of possible temporary fluctuations in the response of the device.
    On the other hand, US7580559 discloses a variation of the prior art for calibrating DMD modulators in which interferometry techniques are replaced by the use of a "apodized" pupil (semi-blocking object) in the path of the system of imaging This pupil allows to obtain images in the plane of the CCD that are highly sensitive to the inclination of the DMD micro-mirrors. By processing these images it is possible to deduce the same information as in the previous case, with the same advantages and disadvantages described. Again, it is not a technique applicable to LCD-type SLMs, nor can it be applied to devices with temporary fluctuations.
    Finally, in the articles, A. Lizana, I. Moreno, C. Iemmi, A. Márquez, J. Campos, M.
    J. Yzuel, “Time-resolved Mueller matrix analysis of a liquid crystal on silicon display”, Appl. Opt. 47, 4267-4274 (2008) and A. Lizana, I. Moreno, A. Márquez, C. Iemmi, E. Fernández, J. Campos, MJ Yzuel, “Time fluctuations of the phase modulation in a liquid crystal on silicon display : characterization and effects in diffractive optics ”, Opt. Express 21, 16711-16722 (2008), characterization and study methods of LCOS modulators that contemplate the presence of fluctuations are described. However, the methods described in these articles have certain limitations, namely:
    • Mueller's matrix only allows you to evaluate the modulation of the light intensity, or that of the polarization state of the light. But it does not allow to determine the phase modulation, which is essential for the


    laser beam control; Y
    • These characterizations do not consider a microscopic model of the birefringence properties of the TN-LCOS modulator. Therefore, it is only possible to obtain useful information at those wavelengths for which calibration is performed. Since it is an expensive procedure and requires a significant number of measurements, it is not convenient if you want to obtain information in many wavelengths.
    In short, none of the known methods employs a procedure capable of completely predicting modulation properties, understanding this complete prediction as the ability to predict the modulation of the state of polarization of light and the complex optical modulation, amplitude and phase, which wins the light depending on the following parameters:
     Depending on the applied voltage signal; In most cases, this voltage is not directly controlled, but rather it is through the gray level that is sent from the graphics card of the computer that controls it, which usually has values between 0 (null voltage) and 255 (maximum voltage);
     depending on the wavelength of the light radiation (usually visible light with wavelength values between 380 nm and 700 nm, although light is also usually used in the near infra-red range, with wavelengths between 700 and 1600 nm, without understanding an exclusive limitation to these ranges); Y
     depending on the orientation of the polarizing filters and / or phase shifters located at the input and output of the TN-LCOS modulator. DESCRIPTION OF THE INVENTION
    Accordingly, the present invention discloses a method of calibration of spatial light modulator, this being a modulator of the TN-LCOS type, comprising two calibration phases: a first spectral calibration phase (with several lengths


    wave) of the device turned off; and a second phase of monochromatic calibration (with a single wavelength) with the device turned on. This phase is performed based on the level of gray applied to the TN-LCOS modulator also referred to throughout this memory as a modulator.
    For monochromatic calibration, the use of the optical scheme that incorporates a microscopic physical model is contemplated, which allows extrapolating the offset properties (or delay) of the TN-LCOS modulator to wavelengths that have not been used in the calibration. In this way, with a reduced set of measurements, the optical response can be obtained at any wavelength within a previously calibrated range. Finally, being a time-resolved technique, it allows predicting the temporal evolution of the complete optical response (polarization, spectral, intensity, irradiance and phase) of the light beam modulated by the device. This information can be processed to also obtain the average irradiance modulation reflected by the TN-LCOS modulator.
    Specifically, the present invention discloses a calibration procedure for an optical modulator, this being a liquid crystal modulator, as already indicated of the TN-LCOS type and which throughout this document is referred to interchangeably as. liquid crystal modulator, TN-LCOS space light modulator, TN-LCOS modulator, LCOS modulator, optical modulator, or simply modulator; which has a director (molecular orientation of the liquid crystal) with an angle of rotation (D) in the
    entrance face and an angle of rotation () from the entrance face to the rear.
    To carry out the calibration procedure of the modulator, an emitter is emitted that emits a beam of light towards the modulator, a radiometer that receives the light reflected by the modulator, a first polarizer arranged between the emitter and the modulator with a first rotation angle; and a second polarizer arranged between the modulator and the radiometer with a second angle of rotation.
    The calibration procedure of the TN-LCOS modulator comprises the steps of:
    a) Calibration off.
    b) Calibration on.


    In the off-calibration stage, a series of offset (or delay) in-off values is determined for at least three wavelengths of beams emitted by the emitter, and in the on-calibration stage, a series of values of values are determined. Offset on for at least one wavelength of beams emitted by the emitter depending on the level of gray applied to the modulator, and characterized in that in step a) the series of offset values is calculated taking into account microscopic parameters of the device which include at least the angle of rotation (D) of the liquid crystal director at the inlet face and the angle of rotation () of the
    Director to the back face.
    To relate these different terms to the mathematical formulas that will be presented below, it must be interpreted that: the angle of rotation of the first polarizer is called θ1; the angle of rotation of the second polarizer is called θ2; the angle of rotation of the director of the liquid crystal on the input face of the modulator is called D; Offset offset values are called βOFF; the offset values on ignition are called β; the offset values on the walls of the modulator are called δ; the angle of twist of the director is called α; the values measured by the radiometer are irradiations referred to as I; and the wavelength of the beam emitted by the emitter is called λ, with λR being a wavelength chosen as a reference equally emitted by the emitter;
    Preferably, an interpolation for various wavelengths is defined from the series of off-phase offset values. This is achieved, for example, by a Cauchy adjustment although other equally acceptable methods are performed in other embodiments of the present invention.
    In a particular embodiment, step a) comprises the steps of:
    a1) emit at least three beams of three different wavelengths of
    reference (1, 2y 3) towards the modulator;
    a2) using the radiometer, make irradiance measurements I i , i = 1,2,3
    for these three reference wavelengths, for a series of
    rotation angles θ1 of the first polarizer and a series of angles of


    θ2 rotation of the second polarizer;
    a3) find, for these reference wavelengths, and by means of the measurements made in step a2), the off-set values apagado  i , i = 1,2,3, which best fit the experimental data
    OFF
    5 to the following mathematical expressions:
    I X2 Y 2  Z22cos22 1 4Y 2X cos2 1 2D  Z sen2 1 2D  2;
    X  cosOFF OFF sen0; OFF
    And  cosOFF OFF senOFF; OFF

    10 Z  senOFF; yOFF
    22
     OFF
    OFF 
    a4) by means of numerical adjustment the values of the off-phase offset are interpolated for other wavelengths different from those of reference by means of a Cauchy adjustment given by
    AB C  35
    15 OFF    ...  
    being A, B, C,… numerical constants of adjustment.
    In a particular embodiment, step b) comprises the steps of: b1) applying a voltage (V) to the modulator; normally the control parameter
    20 is the gray level (g) applied from the graphics card of the computer that controls the device; b2) emit at least one beam of a reference wavelength R towards
    the modulator; b3) using the radiometer, make irradiance measurements for a series of 25 rotation angles θ1 of the first polarizer and a series of angles
    of rotation θ2 of the second polarizer, and the range of gray levels (g) applied to the modulator; b4) find, for said reference wavelength, and by means of the 8


    measurements made in step b3), the values of offset on  (g) and offset on the walls of the modulator  (g) for the reference wavelength, and depending on the level of gray (g) applied, that best fit the measured experimental data to the following
    5 equations:
    I X 2 Y 2  Z22cos22 1 4Y 2X cos2 1 2D  Z sen2 1 2D  2; 
    X  coscos2 sensen2;


    And  cossen2 sencos2;
     
    10 Z  sen; Y

    22
      
    In order to reduce the computational cost of operations to calculate the
    15 values of g and g, in the second stage, the present invention contemplates that, preferably, the series of rotation angles of the first and second polarizer are selected as angles dependent on the angle of rotation of the director ( D) obtained in the first stage. In this way considerable simplifications can be made to
    the equations of stage b4). As an example the series of rotation angles of the
    First and second polarizers are selected from: D + 90º, D + 45º, D + 135º, D + 135º, D + 22.5º, D-22.5º, D + 112.5º, D + 157.5º or D + 67.5º. In addition, in step b4) for the ignition offset parameter (β) two restrictions must be added, the inequality: 0OFF for the offset of the central layer, and the inequality
    0OFF  for the offset of the lateral layers.
    25 Since the values of OFF, α and D were already known from the calibration stage in off (stage a), it is possible to determine the values of g
    and g, at least, for the reference wavelength.9


    In a particularly preferred embodiment, the process of the present invention comprises a step c) in which, from the series of off-phase offset values obtained in step a) and from the series of off-set phase values and the offset in the walls of the modulator obtained for the reference wavelength in step b4), extrapolation to other wavelengths is performed by normalization using the equations:
    OFF 
    g,  g, R ; Y
    OFF R  OFF 
    g,  g, 
    R  R 
    OFF DESCRIPTION OF THE DRAWINGS
    To complement the description that is being made and in order to help a better understanding of the characteristics of the invention, according to a preferred example of practical implementation thereof, a set of drawings is attached as an integral part of said description. where, for illustrative and non-limiting purposes, the following has been represented:
    Figure 1. Shows, schematically, a microscopic representation of the orientation of the molecular director of the liquid crystal inside a spatial light modulator of the TN-LCOS type; Where there is no voltage. The arrow represents the electric field and its transformation into the modulator.
    Figure 2. Schematically shows a microscopic representation of the orientation of the molecular director of the liquid crystal inside a spatial light modulator of the TN-LCOS type; where voltage has been applied. The arrow represents the electric field and its transformation into the modulator.


    Figure 3. Shows a schematic view of the components necessary for the calibration of a spatial light modulator.
    Figure 4. Shows a graph in which the different off-phase offset values are represented in a calibration procedure according to the present invention as well as the values extrapolated by Cauchy adjustments.
    Figure 5. Shows a scheme of an especially preferred embodiment for the calibration of spatial light modulators in the presence of temporal fluctuations. PREFERRED EMBODIMENT OF THE INVENTION
    Microscopic optical models are based on the mathematical treatment of Jones matrices. Specifically, the liquid crystal is modeled as layers of microscopic optical phase shifter (or retarder), whose delay properties on light depend on the applied voltage.
    As can be seen in Figures 1 and 2 where a representation of a molecular director is shown inside a TN-LCOS cell acting as an optical modulator (3) type TN-LCOS working in reflection, for two voltage situations; in said figure 1 it can be seen in the absence of voltage while in figure 2 you have the same device but with voltage application you have to complete optical system composed of a linear polarizer (2), the modulator (3) type LCOS or TN-LCOS with a front face (31) and a rear face (32), the orientation of the liquid crystal molecules (molecular director) (4), which describe the rotation of angle  from the front face (31) to the rear face (32) in the absence of voltage (figure 1), but that disappears when applying voltage (figure 2), causing optical modulation.
    When the modulator (3) is turned off, as shown in Figure 1 the molecular director (4) follows a turn, said turn is defined by a turning angle (α), from the front face (31) of input to the internal part (32) of the modulator (3) TN-LCOS. The orientation of the molecular director on the input face (31) is called the angle of rotation of the director (D). Figure 2 shows the case for a maximum voltage, a situation in which the molecular director is preferably aligned in the


    longitudinal direction There are areas next to the walls of the modulator (3) TN-LCOS that do not change their orientation with respect to the situation in which the voltage is zero. Thus, the parts of the TN-LCOS modulator with the inclined molecular director and with the molecular director anchored to the walls are modeled by two angular parameters, which represent the global lags (delays) associated with the central zone and the cell walls, respectively. In the present description, these angular parameters will be referred to as offset (which can be offset in off (β0FF), with the modulator (3) with no voltage applied, or offset in ignition (β), with the modulator (3) with a voltage applied through the gray level g, in the usual case of modulators controlled from a computer) and offset (δ) in the walls of the modulator (3), which is only considered under the ignition situation.
    Thus, by controlling the voltage applied to the modulator (3) of the TN-LCOS type, the polarization of a light beam is gradually modulated, and by placing a polarizer at the output, its irradiance. On the other hand, this control voltage translates into a digital control signal, as is the case in modern LCOS devices, which is time dependent. This temporal dependence is what produces the temporal fluctuation of the optical response of the device. Although this dependence does not affect applications where the final receptor is the human eye, it must be properly characterized in other applications.
    Applying the ideas presented, it is possible to demonstrate that Jones' complex matrix that models the device working in reflection, under normal incidence or small incidence angles, is as follows:
    i24 X 2 Y 2  Z2 i2XY i2YZ 
    MLCD  e RD  RD ; i2YZ X2 Y 2  Z2  i2XY
    
    x
    where e represents the exponential function, i 
    1 is the imaginary unit, RD 
    represents the rotation matrix


     cos  sin 
    RD DD;
    sinD  cosD 
    and the parameters X, Y and Z are auxiliary variables that depend on the microscopic parameters of the modulator as indicated in the following expressions:

    X  cos cos2 sensen2;


    5 And  cossen2 sencos2; Y


    Z  sen;

    with the parameter  defined as:
    22
    
     
    When the modulator is off, the offset of the central zone is considered 10 OFF and that the offset associated with the walls of the modulator is  0; This allows simplifying the above expressions by replacing these values when you want to characterize the device in power off. Calibration procedure of a spatial light modulator.
    15 In general, space light modulators are calibrated using, first, a measurement without the application of voltage to the modulator (3) -calibration in off-and a modulation with a voltage applied to the modulator (3) -calibration in ignition- . The voltage is typically controlled by a gray level applied to the device from
    20 a graphics card from the computer that controls the device. From these, a series of calibration parameters are obtained, among which, in order to have a more adequate calibration, the microscopic aspects of the operation of the modulators must be taken into account.
    A first embodiment of the calibration procedure will be described with reference to Figures 3 and 4.


    Calibration Off Procedure
    In this part of the procedure, the offset offset parameters (β0FF), the angle of rotation of the director (D) on the input face (31), and the angle of rotation (twist) (α) up to the rear face (32) of the cell acting as a modulator (3).
    To determine these parameters, a light beam emitter (6) is used, preferably a laser or any collimated beam of light, which allows sequential lighting with different wavelengths (preferably at least three). The light is passed through a first polarizer (7), is reflected in the modulator (3), forming a small angle with its surface, and the reflected beam passes through a second polarizer (8). The light finally affects a photodetector or radiometer (9), in order to be able to measure the intensity transmitted as a function of the orientation (rotation) of the first and second polarizers (7, 8).
    Since the transmission axis of the first polarizer (7) is arranged with a rotation angle of the first polarizer (θ1) and the second polarizer (8) is disposed at a rotation angle of the second polarizer (θ2), the normalized irradiance ( I) detected by the radiometer (9) responds to the expression:
    22222 2 2
    I1, 2X Y Z  cos214Y Xcos212DZsen212 D
    In which the values of the polarization rotation angles (1 and 2) are known (they are configured by the user) and various irradiance measurements are taken (for different rotation angles and for the three wavelengths of calibration i, i = 1,2,3) in order to determine the values of the birefringence off parameters (β0 (i)), the angle of rotation of the director (D) and the angle of rotation ( α). Consequently, at least three measurements of irradiance (I), one for each calibration wavelength i, i = 1,2,3 should be made.
    At the end of this calibration, the appropriate values of the


    Microscopic angular parameters of the liquid crystal director of the modulator (3), as well as the offset offset value (β0FF) for several wavelengths, at least two.
    In this embodiment of the present invention, it is especially convenient to perform an extrapolation of these measured values in order to, by means of a few measurements, have an estimated value of the offset off for all possible wavelengths. This can be achieved, for example, by performing a Cauchy adjustment of the off-phase offset (β0FF) and, with said adjustment, obtaining the necessary information to predict the spectral behavior of the device (at different wavelengths).
    Figure 4 shows the evolution of the off-phase offset (β0), in which, on the one hand, the experimental measurements (10) of the off-phase offset (βOFF) and their corresponding theoretical extrapolation (11) are shown by means of a Cauchy adjustment for a given device calibrated according to the described method. For this same device, the rotation values of the director and the angle of rotation of the estimated molecules were D = 16.3º and α = -88.5º respectively.
    Ignition Calibration Procedure
    Once the twist angle (α) and the director rotation (D) of the modulator cell (3) have been determined, it is possible to determine the offset parameters of the device, depending on the level of gray (or with the voltage, if applicable). This section will ignore the treatment of temporary fluctuations of the device parameters, which will be explained later.
    For this work, the same system is used to perform the calibration in off but slightly modified in case there are temporary fluctuations. A series of polarizer configurations (7.8) can be established, that is to say the angle of rotation of the first polarizer (θ1) and the angle of rotation of the second polarizer (θ2) of its transmission axes, at the same time as measures the irradiance transmitted by the system for different gray levels applied to the


    modulator (3). To perform this calibration it is sufficient to use a single wavelength.
    The following table specifies the placements of these angles for each
    5 measurement configuration as well as the corresponding expression of the value of theexpected irradiance transmissions for each of them. The use ofconfigurations in the following table allow a reduced computational costalthough it is not essential to limit yourself to these rotation angle settingsof the first polarizer (θ1) and the rotation angle of the second polarizer (θ2).
    SETTING θ1θ2I (θ1, θ2) 222224
    one DD ZYXY X  22
    2 D + 90ºD4Y Z 222224
    3 D + 45ºD + 45º ZYXY Z  22
    4 D + 45ºD + 135º4 XY 2
    5 D + 22.5ºD + 112.5º 22 ZXY  2
    6 D + 22.5ºD + 22.5º   22222 ZYXZXY  2
    7 D -22.5ºD + 157.5º   22222 ZYXZXY  2
    8 D -22.5ºD + 67.5º 22 ZXY 
    From the expressions in the table, it would be possible to determine the values of X, Y, and Z by operating the measures taken together. However, there would be a quadruple indetermination in the values obtained, both of their absolute value and the sign of
    15 these.
    To avoid this problem, it is advisable to perform a search of the offset values (β and ) instead, so that β is bounded between zero and the value of the offset offset of the modulator (3), that is, it is added as a restriction the
    20 inequality:


    0 OFF
    And  is restricted to inequality
    0 OFF 
    These values must minimize the quadratic error between the measurements of the irradiance transmitted by the system in Figure 3 and those provided by the expressions of
    10 the table above.
    Taking these aspects into account, as a result of the process of calibration and minimization of the mean square error between the experimental measurements and the theoretical measurements, the data of phase shift on (β), phase shift in the
    15 walls of the director (δ), and with them all the necessary information to predict the response of the device for each level of gray and each wavelength.
    Calibration procedure in the presence of temporary fluctuations.
    In a possible more preferred embodiment of the object of the invention, the calibration procedure is carried out in the presence of temporary fluctuations. In this case in which the modulator (3) TN-LCOS has temporary fluctuations, the ignition procedure described above cannot be applied directly since the irradiance values are not constant but have a
    25 variation over time.
    In order to carry out the procedure, a beam splitter is added to obtain a second beam of laser light, which is passed through a first additional polarizer (13), as shown in Figure 5.
    30 The beam of light emitted by the laser or light source (6), which passes through the first polarizer


    additional (13) is reflected in a zone of the modulator (3) called constant zone that remains with a constant voltage (or gray level) value applied throughout the ignition procedure and its reflection is passed through a second polarizer additional (14) and is detected in an additional radiometer (15).
    As can be seen in Figure 5, the light beam coming from the first polarizer (7) is passed through the modulator (3) in another zone of the modulator (3) called the variable zone where the voltage level is varied ( or gray level applied) and is reflected; its reflection is passed through the second polarizer (8) and is detected in the radiometer (9); the radiometer (9) and the additional radiometer (15) being connected to an oscilloscope (16) having at least two channels.
    The measurement of both radiometers is captured by an oscilloscope or a data acquisition system that allows temporary measurements of sufficient temporal resolution (of the order of one millisecond) to appreciate fluctuations.
    For each voltage level (or gray level) applied in the variable zone of the modulator (3), a calibration is performed consisting of taking the measurement in the eight possible configurations of the first and second polarizers (7,8) indicated in the Table of the procedure in previous ignition, obtaining eight temporary measures of
    In t  n 1,2, ..., 8
    irradiance with. Simultaneously the corresponding ones are measured
    I
    reference measurements REF _n t , with n 1,2, ..., 8. These measurements correspond to a zone of the modulator that has a constant gray level during the whole process, and with the additional polarizers (13,14) oriented at arbitrary angles, but which also remain constant throughout the process. That way you get that
    In t 
    REF _
    the reference measures are always the same, simply temporarily displaced.
    I _ t
    The temporal irradiance function REF n corresponding to n = 1 is taken as a temporary reference signal.
    IREF _nt
    The other reference temporary irradiance functions, n1, are versions of the same function but temporarily displaced, that is:


    IREF_nt IREF_n1t tn  n  2,3 ..., 8
    , where tn, with, are the temporary delays. The
    IREF _nt IREF _n1
    Comparison of functions with allows to determine tn delays. Once these tn delays are known. functions are calculated numerically
    I 'n (t)  In t  tn  I' n (t)
    . Temporary irradiance functions are versions
    5 sampled over time, and temporarily synchronized, of the eight measures ofirradiance of the ignition calibration procedure described above.Therefore, for each applied voltage level (or gray level g), the
    g, t  g, t 
    Determination of temporary offset functions and.
    10 Once the time lag functions have been determined for each applied voltage level (or gray level g), the Jones matrix that describes the TN-LCOS modulator (3) as a function of time is:
    2 2 2
    i   X   Y  t  Z   i XtYt  2  
    2 t 4 t  tt 2   i YtZt
    MLCD and  D  R
    t  R D
    2 22   X   Yt  Z   i2XtYt
    i2YtZt t  t   
     
    where t
    Xt cos 2 t  sen 2 t 
       cos t    sen  t    
    t
    ; t
    Yt sen 2 t  cos 2 t 
       cos t    sen  t    
    t
    ; Y

        
    Zt sen t   
    t
    ;   t 
    22
    t
    And the temporal irradiance for any pair 1, 2 of rotation angles of the
    polarizers P1 and P2, can be calculated as:
    2 22 2
    ()  t  2
    It X   Y  t  Zt  cos 2 1  22
     And tXt  sen 1 D 
    25 4    cos2   2D  Zt 2   2;


    The irradiance detected by a radiometer without temporal resolution can be calculated as the average of this function over time:
    I 
    Item)
    . The calibration procedure then consists in determining the functions of
    g, t  g, t 
    5 time lags that best fit the irradiance curves
    I 'n (t)
    synchronized .
    In this way it is possible to obtain, for each gray level applied to the modulator (3)
    g, t  g, t 
    TN-LCOS, the temporal variation of the lags and, and to be able to predict at 10 through the two previous equations, the temporal and average irradiance for any pair of angles 1, 2 of the input and output polarizers.
    In addition, using the Cauchy ratio derived from the off calibration, it is also possible to extrapolate the results to other wavelengths other than the calibration.

    权利要求:
    Claims (8)
    [1]
     R E I V I N D I C A C I O N E S
    1. Calibration procedure of a spatial modulator (3) of liquid crystal type TN-LCOS light (4) with a director that defines the molecular orientation of the liquid crystal (4), with a rotation angle of the director (D ) on a front face (31) of entry, and a turning angle () to a rear face (32); procedure that makes use of:  a transmitter (6), which emits a beam of light towards the modulator (3);  a radiometer (9), which receives the light reflected by the modulator (3);  a first polarizer (7) disposed between the emitter (6) and the modulator (3) with a rotation angle of the first polarizer (θ1); and  a second polarizer (8) disposed between the modulator (3) and the radiometer (9) with a rotation angle of the second polarizer (θ2), the calibration procedure being characterized in that it comprises: a) a calibration stage in off; which in turn includes calculating: -a rotation angle of the director vector (D) on the input face, -a twist angle (α) of the director vector, and -a series of offset off values (βOFF) for at least two wavelengths of the beam emitted by the emitter (6), and b) an ignition calibration step which in turn comprises - supplying voltage to the modulator (3) such that an optical response occurs at said voltage, -determine a series of offset values on (β) for the aforementioned at least two wavelengths of the beam emitted by the emitter based on the aforementioned optical response, where the offset values off (βOFF) They are calculated by the following steps:
    i. emit at least one beam of a reference wavelength (λR) towards the modulator (3),
    ii. perform, using the radiometer (9), intensity measurements (I) for a series of rotation angles of the first polarizer (θ1) and a series of rotation angles of the second polarizer (θ2),
    iii. find, for said reference wavelength (λR), and by means of the measurements made in the previous stage, on-off phase values (β) and of

    offset in the walls of the modulator (δ) for the reference wavelength (λR) that satisfy the equations:
    22222 2 2
    I X  Y  Z  cos   4Y X cos  2  Z sen  2;
    21 21 D 21 D

    X  cos cos2 sen sen2;


    5 And  cos sen2 sen cos2;


    Z  sen; Y

    
    2 2
    [2]
    2. TN-LCOS light space modulator (3) calibration procedure, according to
    10 claim 1, characterized in that it further comprises defining, from the series of off-phase offset values (βOFF), an interpolation for various wavelengths.
    [3]
    3. TN-LCOS light spatial modulator (3) calibration procedure, according to
    Claim 2, characterized in that the interpolation is performed by means of a Cauchy adjustment.
    [4]
    4. TN-LCOS light spatial modulator (3) calibration method, according to claim 1 characterized in that the rotation angles (θ1, θ2) of the first
    20 polarizer (7) and second polarizer (8) respectively are selected as angles dependent on the angle of rotation of the director (D).
    [5]
    5. Spatial modulator (3) calibration procedure according to claim 1,
    characterized the rotation angles (θ1, θ2) of the first polarizer (7) and second polarizer (8) respectively are selected from the group consisting of: D + 90 °,
    D + 45º, D + 135º, D + 135º, D + 22.5º, D-22.5º, D + 112.5º, D + 157.5º and D + 67.5º.
    [6]
    6. TN-LCOS light space modulator (3) calibration procedure, according to
    claim 1, characterized in that the ignition offset parameter (β) 30 meets the inequality:

    [7]
    7. TN-LCOS light spatial modulator (3) calibration procedure, according to
    claim 1, characterized in that it further comprises performing extrapolation to other wavelengths (λ) by means of the equations:
      

    g,   g,  off; Y
    R off R 
      

    g,  g,  off
    R off R 
    from the series of off-phase offset values (βOFF) and on-phase offset values (β) and offset on the modulator walls (δ) obtained for a
    10 reference wavelength (λR).
    [8]
    8. The TN-LCOS light spatial modulator (3) calibration method according to claim 1, wherein there are temporary fluctuations, a method characterized in that it additionally comprises:
    15 add:
    or a beam splitter (17) to obtain a second beam of laser light,
    or a first additional polarizer (13) between the beam splitter (17) and the modulator (3), and
    or a second additional polarizer (14) between the modulator (3) and an additional radiometer (15),
    20 additional polarizers (13,14) through which the second beam of laser light will be passed which will be measured by an additional radiometer (15) after passing through said additional polarizers (13,14), and
     capture the measurement of the radiometer (9) and the additional radiometer (15).

     DRAWINGS 

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