Abstract:
A deformable mirror control device is provided with improved response characteristic by devising the mode of control even if the deformable mirror has a large time constant in comparison with the response speed required in applications such as the retinal camera. The deformable mirror device comprises a deformable mirror  10  having a reflective surface deformed with an applied voltage, and a voltage control circuit  20  for controlling the voltage applied to the deformable mirror  10.  Here, the voltage control circuit  20  produces a steady-state voltage at which the reflective surface of the deformable mirror  10  takes an intended shape in a steady state, and produces a transient voltage that causes the reflective surface of the deformable mirror  10  to deform toward the intended shape, and also produces a transient voltage that causes the shape of the reflective surface of the deformable mirror  10  to shift quickly toward the intended shape.

Description:
BACKGROUND OF THE INVENTION  
   1. Technical Field 
   The present invention relates to a deformable mirror device appropriate for use in retinal cameras, heads-up displays, astronomical telescopes, laser irradiation devices and so on. The present invention also relates to a device for observing the retina of an eye that casts light beam from a photographing light source to an eye to be examined and records the image of the light beam reflected from the retina of the eye as a retinal image, for diagnosing the retina of the eye. 
   2. Related Art 
   The device such as a camera for observing the retina of an eye is used by ophthalmologists and ophthalmic opticians to photograph the image of the retina of an eye for inspecting the state of the retina, hemorrhage on the retina of the eye, and so on. Incidentally, the human eyes optical system is composed of the cornea, the lens, the vitreous body and others with, unlike an ideal optical system used as a basis of the geometrical optics, some deformation. In particular in the clinical field of ophthalmology, the image of the retina of the eye is required to be clear and of little aberration because the extent of difference of the examined eye from a normal eye is used as diagnosis information. However, because the optical system for the human eyes constituting the photographing device is not ideal, in some cases sufficient resolution cannot be achieved. Therefore, to compensate for the deformation of the wavefront of the optical system for the human eyes, the deformable mirrors using the piezoelectric effect have been in use. 
   However, the conventional deformable mirrors using the piezoelectric element require a high voltage applied to the piezoelectric element and needs to use, as an electronic control circuit, a piezoelectric element with a high dielectric strength that is expensive. Therefore, commercially available retinal cameras employ deformable mirrors using electrostatic attraction that can be actuated with a lower voltage in comparison with the piezoelectric type. 
   However, the electrostatic type of deformable mirrors is low in natural frequency (for example, about 10 Hz) and the response speed is about 100 milliseconds in time constant. Thus, the electrostatic type of deformable mirrors, because of its slow response in deformation, has had a problem of difficulty in performing real time processing to the extent required in the ophthalmic examination (for example, a response speed of about 30 milliseconds in time constant). 
   Another problem with the deformable mirrors is that its shape is determined with the limitation of the specifications of the instruments to which it is incorporated, and its thickness, size, and material cannot be changed freely. Another problem with the deformable mirrors is that its dynamic range for the deformation amount is determined with the specifications of the instruments to which it is incorporated, and the distance between electrodes of the electrostatic type of deformable mirrors is also determined accordingly. Therefore, the response characteristic of the electrostatic type of deformable mirrors to the applied voltage is inevitably determined. Thus, it is still another problem that no measures can be taken in the structural design to shorten the response time of the deformable mirrors, such measures as employing hard materials for the deformable mirrors, increasing the thickness of deformable member, and reducing the size of the deformable member. 
   The present invention is to solve the above problems. An object of the present invention is to provide a deformable mirror device with which response characteristic is improved by devising the mode of control even if the time constant of the deformable mirror is large in comparison with the response speed required in applications such as images of the retina of an eye. Another object of the present invention is to provide a device for observing the retina of an eye with which a real time processing can be performed to the extent required in ophthalmic examinations. 
   SUMMARY OF THE INVENTION  
   A deformable mirror device of the present invention accomplishing the above object is a device, for example as shown in  FIG. 1 , comprising a deformable mirror  10  with its reflective surface deformed with the applied voltage and a voltage control circuit  20  for controlling the voltage applied to the deformable mirror  10 . Here, the voltage control circuit  20  produces a steady-state voltage at which the reflective surface of the deformable mirror  10  takes an intended shape in a steady state, and produces a transient voltage that causes the reflective surface of the deformable mirror  10  to deform toward the intended shape, and also produces the transient voltage that causes the shape of the reflective surface of the deformable mirror  10  to shift quickly toward the intended shape. 
   At the moment when the deformable mirror  10  is caused to deform (just after the voltage application) with the device constituted as described above, startup of deformation of the deformable mirror  10  is improved by applying the transient voltage with the voltage control circuit  20 . At the moment when the intended deformation is attained, the applied voltage to the deformable mirror  10  by the voltage control circuit  20  is changed from the transient voltage to the steady-state voltage, so as to improve the response characteristic of the entire deformable mirror device provided with the deformable mirror  10 . 
   It is preferable that, as shown in  FIG. 1  for example, the voltage control circuit  20  in the deformable mirror device of the present invention is a control circuit that controls the applied voltage with a DC voltage and is constituted that the transient voltage is in the direction of increasing the deformation amount of the reflective surface of the deformable mirror  10  toward the intended shape compared to the deformation amount with the applied voltage for producing the steady-state voltage. With the above constitution, because the voltage control circuit  20  can directly control the output DC voltage, the relationship between the applied voltage and the deformation of the deformable mirror  10  is easily known by intuition. 
   It is preferable that, as shown in  FIG. 5  for example, the voltage control circuit  20  in the deformable mirror device of the present invention is a control circuit that performs pulse width modulation (PWM) and is constituted that the transient voltage is produced with a duty ratio in the direction of increasing the deformation amount of the reflective surface of the deformable mirror  10  toward the intended shape compared to the deformation amount with the duty ratio for producing the steady-state voltage. With the above constitution, because the voltage control circuit  20  can control the output DC voltage with pulse width, it is possible to change average applied voltage without controlling the voltage level. 
   It is preferable that, as shown in  FIG. 8  for example, the voltage control circuit  20  in the deformable mirror device of the present invention is an electric circuit having a switching circuit  26  that outputs applied voltage toward a load with switching positive and negative polarities of the applied voltage, and is a control circuit that controls the positive or negative applied voltage, and is constituted that the transient voltage is in the direction of increasing the deformation amount of the reflective surface of the deformable mirror  10  toward the intended shape compared to the deformation amount with the applied voltage for producing the steady-state voltage. With the above constitution, as the polarities of the applied voltage to the deformable mirror  10  with the voltage control device  20  are always switched with the switching circuit  26 , the deformable mirror  10  does not happen to be charged in one polarity only, so that the deformed shape of the deformable mirror  10  is stabilized. 
   It is preferable that, as shown in  FIG. 10  for example, the voltage control circuit  20  in the deformable mirror device of the present invention is a reversing circuit  28  for energizing the deformable mirror with its polarity reversed and a control circuit that performs pulse width modulation, and is constituted that the transient voltage is produced with an on-time ratio in the direction of increasing the deformation amount of the reflective surface of the deformable mirror  10  toward the intended shape compared to the deformation amount with the on-time ratio for producing the steady-state voltage. With the above constitution, as the polarity of the voltage applied with the voltage control circuit  20  to the deformable mirror  10  changes without using two, positive and negative, kinds of high voltage power sources, the deformable mirror  10  does not happen to be charged in one polarity only, so that the deformed shape of the deformable mirror  10  is stabilized. Moreover, because the voltage control circuit  20  can control the output DC voltage with pulse width modulation, it is possible to change average applied voltage without controlling the voltage level. 
   In a preferable constitution of the deformable mirror device of the present invention, the time determined from the time constant of the reflective surface of the deformable mirror  10  is used as the time for applying the transient voltage in the direction of increasing the deformation amount of the reflection surface of the deformable mirror  10  toward the intended shape, to shift the shape of the reflection surface of the deformable mirror  10  near the status of the intended shape, followed by voltage control with the steady-state voltage, so that switching from the transient voltage to the steady-state voltage in the voltage control circuit  20  is carried out smoothly. 
   The device for observing the retina of an eye according to the present invention accomplishing the above objects is, as shown in  FIG. 13  for example, characterized by the use of a deformable mirror device of anyone of claims  1  to  6 . 
   The deformable mirror device of the present invention is constituted that the voltage control circuit produces a transient voltage in the direction of increasing the deformation amount of the reflective surface of the deformable mirror  10  toward the intended shape, and then produces a steady-state voltage with which the reflective surface of the deformable mirror  10  takes the intended shape in a steady-state. Therefore, it is possible to bring the device to an intended steady state quickly in comparison with the time constant determined from the size, material, and thickness of the deformable mirror. 
   With the device for observing the retina of an eye according to the present invention, it is possible to translate the measurement results of the examined eye into the mirror shape in real time, which can be utilized in automatic compensation of the retinal camera. 
   The basic Japanese Patent Application No. 2004-027128 filed on Feb. 3, 2004 is hereby incorporated in its entirety by reference into the present application. The Japanese Patent Application No. 2003-125279 is also hereby incorporated in its entirety by reference into the present application. 
   The present invention will become more fully understood from the detailed description given hereinbelow. The other applicable fields will become apparent with reference to the detailed description given hereinbelow. However, the detailed description and the specific embodiment are illustrated of desired embodiments of the present invention and are described only for the purpose of explanation. Various changes and modifications will be apparent to those ordinary skilled in the art on the basis of the detailed description. 
   The applicant has no intention to give to public any disclosed embodiments. Among the disclosed changes and modifications, those which may not literally fall within the scope of the present claims constitute, therefore, a part of the present invention in the sense of doctrine of equivalents. 
   The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1(   a ) and  FIG. 1(   b ) show a block diagram in cross section of an example of an electrostatic type of deformable mirror together with a voltage producing circuit and a voltage control circuit. 
       FIG. 2(   a ) and  FIG. 2(   b ) show waveforms representing response curves of the reflective membrane when voltage is applied stepwise to the deformable mirror.  FIG. 2(   a ) shows a waveform  1  corresponding to the applied voltage, and  FIG. 2(   b ) shows a waveform  2  corresponding to the deformation amount. 
       FIG. 3(   a ) and  FIG. 3(   b ) show waveforms representing response curves of the reflective membrane when transient voltage and steady-state voltage are applied in succession to the deformable mirror.  FIG. 3(   a ) shows a waveform  4  corresponding to applied voltage, and  FIG. 3(   b ) shows waveforms  3  and  5  corresponding to the deformation amount. 
       FIG. 4  is a plan view for explaining the electrode array of the deformable mirror. 
       FIG. 5  is a block diagram for explaining the second embodiment of the present invention. 
       FIG. 6(   a ) to  FIG. 6(   c ) are waveforms diagram for explaining the function of the device shown in  FIG. 5 . 
       FIG. 7(   a ) to  FIG. 7(   c ) show waveforms representing response curves of the reflective membrane when transient voltage and steady-state voltage are applied in succession to the deformable mirror of the second embodiment. 
       FIG. 8  is a block diagram of a voltage control circuit for explaining the third embodiment of the present invention. 
       FIG. 9(   a ) and  FIG. 9(   b ) show switching voltage waveforms for applying voltage stepwise to the deformable mirror of the third embodiment. 
       FIG. 10  is a block diagram of a voltage control circuit for explaining an exemplary modification of the third embodiment of the present invention. 
       FIG. 11(   a ) and  FIG. 11(   b ) show waveforms representing response curves of the reflective membrane when voltage is applied stepwise to the deformable mirror of the fourth embodiment. 
       FIG. 12(   a ) and  FIG. 12(   b ) show waveforms representing response curves of the reflective membrane when transient voltage and steady-state voltage are applied in succession to the deformable mirror of the fourth embodiment. 
       FIG. 13  is a block diagram for explaining the entire device for observing the retina of an eye. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
   [Principle] 
     FIG. 1  is a block diagram showing an example of an electrostatic type of deformable mirror. In the figure,  FIG. 1(   a ) is a plan view, and  FIG. 1(   b ) is a sectional view as seen along the line B-B of  FIG. 1(   a ) and shows a voltage control circuit also. As shown in the figures, the electrostatic type of deformable mirror  10  comprises: a glass substrate  11 , a silicon substrate  12 , a membrane  13 , spacers  14 , reflective membrane  15 , and electrodes  16 . The membrane  13  is produced by selective etching process of the silicon substrate  12 , which is with flexibility and of a thickness of about 4 μm for example. The reflective membrane  15  is produced by vapor deposition of a high reflectivity material to the membrane  13 , for example, by using a metallic material with a high reflectivity such as aluminum. The spacers  14  are used to keep the gap between the membrane  13  and the electrodes  16  at the predetermined value and are made of, for example, balls with a high rigidity. The electrodes  16  are provided in a specified number on the glass substrate  11 . The electrodes  16   a ,  16   b ,  16   c ,  16   d , and  16   e  are actuated individually with the voltage control circuit  20 . 
     FIG. 2  shows waveforms representing response curves of the reflective membrane when voltage is applied stepwise to the deformable mirror.  FIG. 2(   a ) shows a waveform  1  corresponding to the applied voltage.  FIG. 2(   b ) shows a waveform  2  corresponding to the deformation amount. In  FIG. 2(   a ), applied voltage Vi is applied stepwise as applied voltage X[V] at the time  0 .  FIG. 2(   b ) shows a measurement curve of change with time in the deformation amount Di of the membrane  13  as a response curve of a primary delay system of a time constant τ. Measurement of time t starts with the application of the stepwise voltage. With the response curve of a primary delay system, the deformation amount Di of the membrane  13  reaches about 10% of the total deformation amount D total  at t=τ/10, and the deformation amount Di of the membrane  13  reaches about 63% of the total deformation amount D total  at t=τ. Here, the total deformation amount D total  of the membrane  13  shows the total deformation amount A [μm] of the reflective membrane in a balanced state for the applied voltage when a sufficient time has elapsed (for example when a setting time t set  has elapsed) with reference to the time constant τ. 
   First Embodiment  
     FIG. 3  shows waveforms representing response curves of the reflective membrane when transient voltage and steady-state voltage are applied in succession to the deformable mirror.  FIG. 3(   a ) shows a waveform  4  corresponding to applied voltage.  FIG. 3(   b ) shows waveforms  3  and  5  corresponding to the deformation amount. As shown in  FIG. 3(   a ), the applied voltage Vi is a high voltage V high  for the period of time between 0 and t high , and is a steady-state voltage V stable  for the period of timeafter t high . In  FIG. 3(   b ), the waveform  3  in a dash-and-single-dotted line shows step response to the high voltage V high , and the waveform  5  in thin solid line shows the step response to the steady-state voltage V stable . The deformation amount Di of the membrane  13  is shown with: the waveform  3  for the period of time between 0 and t high , the curve interconnecting the waveforms  3  and  5  for the period of time between t high  and the setting time t set , and the waveform  5  after the setting time t set . 
   With the device of the above constitution, the voltage control circuit  20  applies a high voltage V high  as a transient voltage to the membrane  13  at the time  0  at which deformation of the membrane  13  is started. Thereupon, the deformation of the membrane  13  starts up more steeply than with the steady-state voltage V stable  and an intended deformation is attained with the lapse of the response time t high . Next, at the time t high , the voltage control circuit  20  sets the applied voltage again to the steady-state voltage V stable . This improves response characteristic of the deformable mirror  10  with a higher response speed. Here, the response time t high  is determined to be a time at which the response deformation amount Di of the membrane  13  with the high voltage V high  reaches the total deformation amount D total  of the membrane  13  for the steady-state voltage V stable . Incidentally, the response time t high  may be set for example to about 80% to 90% of theoretical response time to prevent the response deformation amount Di of the membrane  13  from overshooting its target, the total deformation amount D total . 
   Next, the relationship between the applied voltage Vi and the deformation amount Di of the membrane  13  is described. The relationship between the applied voltage Vi and the deformation amount Di is expressed with the equation (1).
 
 k·Di=ε   0   ·S·Vi   2 /2·( dg−Di ) 2    (1)
 
   where dg represents the gap length, k the spring constant, Di the deformation amount of the membrane  13 , S the surface area, Vi the applied voltage, and ε 0  the dielectric constant of vacuum. For example, in case that the gap length dg is 40 μm and the deformation amount Di is assumed to be increased from 5 μm to 10 μm as the total deformation amount D total  of the membrane  13  in steady state, the applied voltage V 10  for 10 μm relative to the applied voltage V 5  for 5 μm needs to satisfy the following relationship.
 
 V   10   /V   5 =1.21   (2)
 
   For example, a deformable mirror  10 , made of single crystal silicon, of a round shape of 15 mm in diameter and 4 μm in thickness is operated with the voltage control circuit  20  applying 50[V] as the applied voltage X[V]. Then, the total deformation amount D total  of the membrane  13  becomes 5 μm, the response waveform becomes as shown in  FIG. 2(   b ), and the setting time for reaching the total deformation amount D total  is about 200 milliseconds. Here, for doubling the total deformation amount D total  of the membrane  13  from 5 to 10 μm, 60.5[V] as the applied voltage X[V] is applied according to the equation (2) with the voltage control circuit  20 . The response time t high  becomes very short, for example about 30 milliseconds. 
     FIG. 4  is a plan view for explaining the electrode arrangement of the deformable mirror. Electrodes of the deformable mirror, 37 pieces including 1st to 37th, each in hexagonal shape, are arranged in a honeycomb shape, to which for example electrostatic voltage is applied to produce corresponding deformation to each electrode. 
   Second Embodiment  
   With the embodiment 1, it is explained that the response characteristic is improved by controlling the applied voltage. In this second embodiment, a linear voltage control of several hundred volts is performed to control the applied voltage level of the voltage control circuit  20 . Therefore, sophisticated voltage control technique is used. In order to enhance the shaping resolution of the deformable mirror  10 , the number of electrodes is increased. Voltage control is performed individually to each electrode (channel) shown in  FIG. 4 . 
     FIG. 5  is a block diagram for explaining the second embodiment of the present invention.  FIG. 6  is a waveform diagram for explaining the function of the device shown in  FIG. 5 . In  FIG. 6 , (a) shows saw-tooth-shape input signal in 1  and rectangular wave signal in 2 , (b) shows output signal out 1  of a comparator  22 , and (c) shows output signal out 2  of a high voltage buffer circuit  24 . The second embodiment employs a constitution in which the applied voltage level of the voltage control circuit  20  is not controlled directly, but is controlled by changing the average voltage through controlling the pulse width of a switching element where the pulse width modulation technique is applied. With such a constitution, circuit constitution is made simpler than in the first embodiment even when evenly performing voltage control for multiple channels. 
   A voltage control circuit  20   a  comprises a comparator  22 , and a high voltage buffer circuit  24   a . To the comparator  22  are inputted a saw-tooth-shape input signal in 1 , and a rectangular wave signal in 2  as a duty ratio control signal. Depending on the signal level of the rectangular wave signal in 2 , the comparator  22  slices the saw-tooth-shape input signal in 1  and outputs a high duty ratio output signal out 1  for the period in which the signal level of the rectangular wave signal in 2  is high (transient period) and a low duty ratio output signal out 1  for the period in which the signal level of the rectangular wave signal in 2  is low, to the high voltage buffer circuit  24   a . A high voltage HV is supplied from a high voltage power source (not shown) to the high voltage buffer circuit  24   a . There, the output signal out 1  is amplified as an output signal out 2  which is applied to the deformable mirror  10 . The logical voltage level is amplified by several tens times into the voltage for energizing, and for example, for the output signal out 1  of 5 V, the output signal out 2  is 300 V. 
     FIG. 7  shows waveforms representing response curves of the reflective membrane when transient voltage and steady-state voltage are applied in succession to the deformable mirror of the second embodiment. In  FIG. 7 , (a) shows a waveform  4 * corresponding to the applied voltage, (b) shows a waveform  5 * corresponding to the deformation amount, and (c) shows the waveforms  6 ,  6 * corresponding to the switching voltage. In  FIG. 7(   a ), the applied voltage Vi is the high voltage V high  for the period of time between 0 and t high  and becomes the steady-state voltage V stable  after the time t high . In  FIG. 7(   b ), the deformation amount Di of the membrane  13  is indicated with: a step response curve corresponding to the high voltage V high  for the period of time between 0 and t high , a straight line of the total deformation amount D total  for the period of time between t high  and t set , and a step response curve corresponding to the steady-state voltage V stable  after the setting time t set . In  FIG. 7(   c ), duty ratio of the switching signal indicated with the waveform  6  is high for the period of time between 0 and t high , and low after the time t high . In case the duty ratio for the steady-state voltage V stable  is, for example, 1:1, the duty ratio for the high voltage V high  becomes, for example, 1.21:1. The waveform  6 * indicates an output voltage signal produced by rectifying and smoothing the switching signal indicated with the waveform  6 , and corresponds to the waveform  4 *. 
   Third Embodiment  
     FIG. 8  is a block diagram of a voltage control circuit for explaining the third embodiment of the present invention. The voltage control circuit  20   b  comprises a switching circuit  26  for outputting applied voltage toward a load with switching positive and negative polarities of the applied voltage, and a high voltage buffer circuit  24   b . The high voltage buffer circuit  24   b  has a positive voltage DC source section and a negative voltage DC source section as a high voltage power source for supplying high voltage HV. The switching circuit  26  is a control circuit for controlling the polarity of the applied voltage. It is preferable to constitute the switching circuit  26  so that the transient voltage is in the direction of increasing the deformation amount of the reflective surface of the deformable mirror  10  toward the intended shape compared to the deformation amount with the applied voltage for producing the steady-state voltage. With this constitution, the polarities of the applied voltage with the voltage control circuit  20   b  to the deformable mirror  10  are always changed with the switching circuit  26 . Therefore, the deformable mirror  10  does not happen to be charged in one polarity only, so that the deformed shape of the deformable mirror  10  is stabilized. 
   In the above constitution, when voltage with its polarity switched in a sufficiently rapid cycle relative to the response time of the deformable mirror  10  is applied between the electrodes  16  and the membrane  13 , electrostatic attraction occurs between them without incurring charge-up and the membrane  13  deforms into a concave shape.  FIG. 9  shows switching voltage waveforms when voltage is applied stepwise to the deformable mirror. In  FIG. 9 , (a) shows waveforms  6 ,  6 * corresponding to the switching voltage of  FIG. 7(   c ), and (b) shows an applied DC voltage waveform  7  with the high voltage buffer circuit  24   b . The waveform  7  is shown with its period enlarged in comparison with the period of the waveform  6 . In other words, an operation is possible in which anti-charge-up measures for the deformable mirror  10  is realized by using the control of high pulse voltage with both polarities as indicated with the waveform  7 . 
   By the way, while the third embodiment of  FIG. 8  is described as an example having both the positive voltage DC source section and the negative voltage DC source section as the high voltage power source for the high voltage buffer circuit  24   b , the circuit constitution is simplified if the anti-charge-up measures for the deformable mirror  10  is realized with the positive voltage DC source section only. 
     FIG. 10  is a block diagram of a voltage control circuit for explaining an exemplary modification of the third embodiment of the present invention. As shown in the figure, the voltage control circuit  20  comprises: a PWM (pulse width modulation) circuit  22 , a high voltage buffer circuit  24   a , and a reversing circuit  28  for energizing the deformable mirror  10  as reversing the polarity. The reversing circuit  28  has four transistors Tr 1 , Tr 2 , Tr 3 , and Tr 4  so that polarity of voltage for energizing the deformable mirror  10  is reversed with timing signals supplied from outside. Here, the transistors Tr 1  and Tr 4  work as the positive side while Tr 2  and Tr 3  as the negative side. The PWM (pulse width modulation) circuit  22  for example uses a comparator as shown in  FIG. 5  to receive the saw-tooth-shape input signal in 1  and the rectangular wave signal in 2  as duty ratio control signal. 
   The above constitution makes it possible to realize measures against charge-up of the deformable mirror  10  using the reversing circuit  28  even if the high voltage source of the high voltage buffer circuit  24   a  includes the positive voltage DC source only. 
   Fourth Embodiment  
     FIG. 11  shows waveforms representing response curves of the reflective membrane when voltage is applied stepwise to the deformable mirror. In  FIG. 11 , (a) shows a waveform  8  corresponding to applied voltage, and (b) shows a waveform  9  corresponding to the deformation amount. In  FIG. 11(   a ), the applied voltage Vi is a high applied value X h [V] at first, and from the time  0 , it is stepped down to V stable .  FIG. 11(   b ) shows a measurement curve of change with time in the deformation amount Di of the membrane  13 , that is, a response curve of primary delay system of the time constant τ. Measurement of time t starts when the applied voltage is stepped down to the low voltage X 1 [V]. Here, the total deformation amount D total  appearing on the membrane  13  by the change in the applied voltage from the high X h [V] to the steady-state V stable  is shown as that of the reflective membrane in an equilibrium state relative to the applied voltage changed, appearing at the time when a sufficient period of time has elapsed (for example when the setting time t set  has elapsed) with reference to the time constant τ. 
     FIG. 12  shows waveforms representing response curves of the reflective membrane when the transient voltage and the steady-state voltage are applied in succession to the deformable mirror. In  FIG. 12 , (a) shows a waveform  11  corresponding to the applied voltage and (b) shows waveforms  10 ,  12  corresponding to the deformation amount. As shown in  FIG. 12(   a ), the applied voltage Vi is a high applied voltage X H [V] at first, followed by a low transient applied voltage X tr  for the period of time between 0 and τ, and after the time τ, a steady-state V stable  (X L ) which is higher than the transient applied voltage X tr . In  FIG. 12(   b ), the waveform  10  in a dash-and-double-dotted line shows the step response to the low applied voltage X tr , and the waveform  12  in solid line shows the step response to the steady-state voltage V stable  (X L ) The deformation amount Di of the membrane  13  is indicated with: the waveform  10  for the period of time between 0 and τ, the curve interconnecting the waveforms  10  and  12  for the period of time between τ and the setting time t set , and the waveform  12  after the setting time t set . 
   With the above constitution, the voltage control circuit  20  using a function of its transient applied voltage control, applies a low transient applied voltage X tr  to the membrane  13  at the time  0  when the deformation of the membrane  13  is started. Thereupon, the deformation rate of the membrane  13  is rapid in comparison with that produced with the steady-state voltage V stable  and the intended deformation is attained after the lapse of the response time τ. Next, the voltage control circuit  20  using a function of its steady-state voltage control, sets the applied voltage again to the steady-state voltage V Stable  at the time τ. Then, the deformation amount of the deformable mirror  10  quickly shifts into the amount of the reflective surface of the steady-state and is stabilized. In this way, response characteristic of the deformable mirror  10  is improved to be quick. Here, the response time t tr  is set, according to the low transient applied voltage X tr , to the time at which the response deformation amount Di of the membrane  13  reaches the total deformation amount D total  of the membrane  13  with the steady-state voltage V stable . Further, in order to prevent the response deformation amount Di of the membrane  13  from overshooting its target or the total deformation amount D total , the response time t tr  may be set to be slightly shorter, for example 70% to 100% of theoretical response time, preferably 80% to 90%. 
   Next, a device employing the above deformable mirror  10  for observing the retina of an eye is described.  FIG. 13  is a block diagram for explaining the entire device for observing the retina of an eye. As shown in  FIG. 13 , the device for observing the retina of an eye comprises: a wavefront compensation system  8 , a retinal illumination system  2 , a retinal observation system  3 , an alignment system  4 , a fixation system  5 , and a compensation optics  70 . The wavefront compensation system  8  has: a wavefront measuring system  80  including a point image projection optical system  81 , a point image reception optical system  82 , and a point image receiving section  83  (CCD); a computer  84 ; and a control section  85 . The computer  84  includes: an optical characteristic measuring section  841 , an image data forming section  842 , a compensation amount determining section  843 , a memory  844 , and a display section  845 . 
   The retinal illumination system  2  includes: a second light source section, a condenser lens, and a beam splitter, to cast the second light beam of the second light source section to a specified area on the retina of an examined eye. The retinal observation system  3  includes a retinal image forming optical system  36  and a retinal image receiving section  38  (CCD). The retinal image forming optical system  36  includes for example an afocal lens  88 , a compensation optics  70 , a condenser lens, and a beam splitter, to guide the light reflected from the retina  61  through the compensation optics  70  to the retinal image receiving section  38 . The compensation optics  70  has: the deformable mirror  10  for compensating aberration of measurement light, and movable prism and spherical lens that move in the optical axis direction to compensate spherical components of the aberration. The compensation optics  70  is placed in the point image projection optical system  81  and the retinal image forming optical system  36  to compensate aberration of the light beam reflected back for example from the examined eye  60 . 
   The alignment system  4  includes a condenser lens and an alignment light receiving section to guide the light beam emitted from the light source section and coming back as reflected from the cornea  62  of the examined eye  60  to the alignment light receiving section. The fixation system  5  includes a light path for casting a target for fixation and fogging of the examined eye  60  for example, and has a third light source section  51 , a fixation target  52 , and a relay lens. It is possible to cast the fixation target  52  with the light beam from the third light source  51  to the retina  61  so that the examined eye  60  observes the image. 
   The optical characteristic measuring section  841  determines optical characteristics including aberration of higher orders of the examined eye  60  according to the output from the point image receiving section  83 . The image data forming section  842  carries out simulation of perceived state of the target according to the optical characteristics, and calculates data of the examined eye such as MTF indicating the perceived state or simulation image data. The memory  844  stores a plurality of voltage change templates for adjusting the deformable mirror  10 . The compensation amount determining section  843  chooses from the voltage change templates stored in the memory  844  and, determines a compensation amount for the deformable mirror  10  according to the voltage change template chosen, and outputs the compensation amount to the control section  85 . The control section  85  deforms the deformable mirror  10  according to the output from the compensation amount determining section  843 . Further details of the device for observing the retina of an eye are described for example in the specification of a Japanese patent application No. 2003-125279 relating to the proposal of the applicant of the present invention. 
   In the above embodiment, the device employing the deformable mirror is described as the device for observing the retina of an eye. However, there are many devices employing the deformable mirror, including the head-up display, the astronomical telescope, the laser irradiation device, and so on. 
   DESCRIPTION OF REFERENCE NUMERALS  
   
       
         10 : Deformable mirror 
         20 : Voltage control circuit 
         22 : Comparator (PWM circuit) 
         24 : High voltage buffer circuit 
         26 : Switching circuit 
         28 : Reversing circuit