Abstract:
An optical MEM device is encapsulated with a dampening gas to reduce oscillatory vibrations of movable parts during the operation of the device. Preferably, the dampening gas comprises one or more noble gases, such as neon and/or krypton with a partial pressure in a range of 50 to 100% of the total dampening gas pressure. In further embodiments, the dampening gas comprises a mixture of one or more noble gases and an inert carrier gas, such as nitrogen. Preferably, the optical MEM device is sealed within a die with a dampening gas pressure between 0.5 to 3.0 atmospheres at 20 degree Celsius. The current invention is particularly useful for reducing oscillatory vibrations of optical MEM devices having a plurality of movable ribbon structures configured to modulate light with one or more wavelengths in the near infrared (800 to 4000 nanometers) and which operate at high switching rates (4-40 Volts/nano second) and at high switching frequencies (1 kHz and greater).

Description:
FIELD OF THE INVENTION 
     The invention relates to an optical MEM device with movable ribbons for modulating light. More particularly, the present invention relates to an optical MEM device encapsulated within a dampening gas environment to reduce vibrations of the movable ribbons during operation. 
     BACKGROUND OF THE INVENTION 
     Optical MEM (micro-electro-mechanical) device have applications in display, print, optical and electrical technologies. One type of an optical MEM device is a grating light valve that is capable of modulating light by constructive and destructive interference of an incident light source. Exemplary grating light valves and methods for making grating light valves are disclosed in the U.S. Pat. Nos. 5,311,360, 5,841,579 and 5,808,797, issued to Bloom et al., the contents of which are hereby incorporated by reference. 
     SUMMARY OF THE INVENTION 
     Grating light valves of the instant invention generate the condition for constructive and destructive interference through a plurality of movable ribbons. The movable ribbons provide a first set of reflective surfaces that are movable relative to a second set of reflective surfaces. The second set of reflective surfaces are reflective surfaces on a substrate element or on a second set of ribbons. In operation, an incident light source having a wavelength λ impinges on the first set of reflective surfaces and the second set of reflective surfaces. The movable ribbons are displaced towards or away from the second set of reflective surfaces by λ/4, or a multiple thereof. The portion of light that is reflected from the first set of reflective surfaces and the portion of light that is reflected from the second set of reflective surfaces alternate between being in phase and being out of phase. Preferably, the first set of reflective surfaces and the second set of reflective surfaces are either in the same reflective plane or are separated λ/2 for generating the condition for constructive interference. 
     FIG. 1 a  illustrates a grating light valve with plurality of movable ribbons  100  that are formed in a spatial relationship over a substrate  102 . Both the ribbons  100  and the regions of the substrate between the ribbons have reflective surfaces  104  and  106 . The reflective surfaces  104  and  106  are provided by coating the ribbons  100  and the substrate with a reflective material, such as an aluminum or silver. The height difference  103  between the reflective surfaces  104  and  106  on the ribbons  100  and the substrate  102  is nλ/2 (where n is a whole number). When light having a wavelength λ impinges on the complement of reflective surfaces  104  and  106 , the portion of light reflected from the surfaces  104  of the ribbons  100  will be in phase with the portion of light reflected from the surfaces  106  of the substrate  102 . This is because the portion of light which strikes the surfaces  104  of the substrate  102  will travel a distance λ/2 farther than the portion of light striking the surfaces  104  of the ribbons  100 . Returning, the portion of light that is reflected from the surfaces  104  of the substrate  102  will travel an additional distance λ/2 farther than the portion of light striking the surface  106  of the ribbons  100 , thus allowing the complement of reflective surfaces  104  to act as a mirror. 
     Referring to FIG. 1 b,  in operation the ribbons  100  are displaced toward the substrate  102  by a distance  105  that is equal to λ/4 or λ/4 plus nλ/2 (where n is a whole number) in order to switch from the conditions for constructive interference to the conditions for destructive interference. When light having a wavelength λ impinges on the reflective surfaces  104 ′ and  106  with the ribbons  100 ′ in the down position, the portion of light reflected from the surfaces  104 ′ will be out of phase, or partially out of phase, with the portion of light reflected from the surfaces  106  and some or all of the light will be diffracted. By alternating the ribbon between the positions shown in FIG. 1 a  and FIG. 1 b,  the light is modulated. 
     An alternative construction for a grating light valve is illustrated in the FIGS. 2 a-b . Referring to FIG. 2 a,  the grating light valve has a plurality of ribbons  206  and  207  that are suspended by a distance  203  over a substrate element  202 . The ribbons  206  and  207  are provided with reflective surfaces  204  and  205 , respectively. The surface  208  of the substrate  202  may also be reflective. The first set of ribbons  206  and the second set of ribbons  207  are initially in the same reflective plane in the absence of an applied force. Preferably, the first set of ribbons  206  and the second set of ribbons  207  are suspended over the substrate by a distance  203  such that the distances  209  between the reflective surfaces  205  and  205  of the ribbons  206  and  207  and the reflective surface  208  of the substrate  202  corresponding to n λ/2. Accordingly, the portions of light reflected from the surfaces  204  and  205  of the ribbons  206  and  207  and the reflective surface  208  of the substrate  202  with a wavelength λ will all be in phase. The ribbons  206  and  207  are capable of being displaced relative to each other by a distance corresponding to a multiple of λ/4 and thus switching between the conditions for constructive and destructive interference with an incident light source having a wavelength λ. 
     In the FIG. 2 b,  the second set of ribbons  207  is displaced by a distance  203 , corresponding to a multiple of λ/4 of to the position  207 ′. The portion of the light reflected from the surfaces  205 ′ of the ribbons  207  will destructively interfere with the portion of the light reflected from the surfaces  204  of the ribbons  206 . While the FIG. 1 b  and FIG. 2 b  show ribbons touching the surface of the substrate, the instant invention is particularly useful in grating light valve designs where movable ribbons do not contact the substrate surface or where movable ribbons only partially contact the surface of the substrate. Accordingly, FIGS. 1 a-b  and FIGS. 2 a-b  are for illustrative purposes only and are not intended to limit the scope of the invention. Further, it is understood that the current invention is not limited to grating light valves and has applications for reducing vibrational oscillations in other micro machine devices with or without reflective surfaces. 
     FIG. 3 plots an idealized brightness response  107  of a grating light valve to an incident light source with a wavelength λ when voltage  108  is applied across a selected set ribbons (active ribbons) and the underlying substrate of the grating light valve to alternate between the conditions for constructive and destructive interference. From the discussion above, the brightness will be at a maximum  111  when the ribbons are in the same reflective plane or separated by λ/2, or a multiple of λ/2, and the brightness will be at a minimum  111  when the ribbons are separated by λ/4, or λ/4 plus (n)λ/2. Specifically, to operate the grating light valve, a voltage V 1  is applied across the active ribbons and the underlying substrate. At this point the active ribbons are in the constructive interference position and the maximum brightness  109  is observed. As the voltage is increased to V 2 , the active ribbons are moved to a destructive interference position and the minimum brightness  109  is observed. As the voltage is reduced, the active ribbons do return to their constructive interference position when V 1  is reached. 
     The rate (Volt/sec) at which voltage is applied to switch the ribbons of the grating light valve between the conditions for constructive and destructive interference is referred to as the switching rate, and is typically in the range of 4000 to 0.4 Volt/nano seconds. The frequency of at which the grating light valve is switched between the conditions for constructive and destructive interference is referred to as the switching frequency and is typically in the range of 100 KHz to 20 MHz. 
     Whether a grating light is constructed according to the principles illustrated in FIGS. 1 a-b,  FIGS. 2 a-b,  or any other construction including constructions where movable ribbons do not touch the surface of the substrate, there is the tendency for the ribbons to exhibit oscillatory vibrations when they are moved from one position to another. 
     In applications where the light source used has wavelengths λ corresponding to the near infrared (ca. 800-4000 nanometers), the distances that the ribbons are displaced to alternate between the constructive and destructive interference positions are between 200 to 1000 nanometers or greater. With greater displacement distances, the ribbons tend to exhibit oscillatory vibrations with greater amplitudes. These oscillatory vibrations reduce the ability of the device to effectively act as a light valve for light sources in the near infrared. 
     Many print applications utilize light sources that operate with wavelengths corresponding to the near infrared and the visible region of the spectrum. Because the oscillatory vibrations are a significant limitation for grating light valves operating at these wavelengths, grating light valves have had limited use in print applications. Further, the oscillatory vibrations reduce ability to operate under conditions of high switching voltages and high switching frequencies that are desirable for high speed print applications. 
     What is needed is a grating light valve that exhibits reduced oscillatory vibrations of reflective ribbons. Further, what is needed is a grating light valve that modulates light with minimized oscillatory vibrations at wavelengths in the near infrared for print applications. 
     According to the present invention the movable ribbons of a grating valves are sealed within a die structure along with a dampening gas environment. The damping gas preferably has a pressure of between 0.5 and 3.0 atmospheres within the die and more preferably between 0.5 and 1.5 atmospheres at 20 degrees Celsius. The dampening gas environment comprises an inert gas or noble group VIII gas including He, Ne, Ar, Kr, Xe, Rn or a mixture thereof. Preferably, the inert gas is 50 more molar percent of the total dampening gas. 
     The grating light valve comprises a plurality of spatially arranged elongated ribbons with a reflective surface and a substrate element with reflective regions between the ribbons. The grating light valve modulates light by constructive and destructive interference of the reflected light at an incident wavelength λ when the ribbons, or a portion thereof, are moved by a predetermined distance. The ribbons are moved to switch between a destructive interference position and a constructive interference position by applying the appropriate switching voltages across selected ribbons and the substrate. 
     The dampening gas environment attenuates oscillatory vibrations or “ringing” that results from the displacement of the ribbons while alternating between the destructive interference position and the constructive interference position. The current invention is particularly useful for grating light valves operating at wavelengths λ corresponding to the near infrared (800 to 4000 nm) and where the ribbons are required to move a distance equal to a multiple of λ/4. The current invention is also particularly useful for grating light valves that operate at high switching rates (4-40 Volts/nano second) and at high switching frequencies (1 kHz and greater). 
     In accordance with the instant invention, grating light valves are arranged in an array and configured to activate a print medium. The array has a plurality of independently operable grating light valves that are used to activate a pixel or spot in the print medium. The surface of the array is illuminated with a light source, such as a laser source or any other light source suitable for the application at hand. The print medium and the array are moved relative to each other and individual grating light valves or sets of grating light valves are actuated, thereby exposing the medium with the desired image or latent image. The array device and/or the imaging process may also utilize a suitable optical arrangement, including lenses and mirrors positioned between the light source and the array or between the array and the print medium in order to facilitate the imaging process. 
     The print medium is any print medium that is capable of being activated by the light source used. For example, the print medium is paper that is photo activated with a charge image, which subsequently collects a curable toner to produce an image. Alternatively, the system is configured to activate a developable latent image such as a silver halide-based medium. 
     The dampening gas environment is preferably sealed within the die structure by providing a metallized gasket on a sealing edge of the die structure. A glass cap is provided with a complementary metallized gasket. The glass cap is placed on the sealing edge of the die and the gaskets are aligned to overlap with a solder material between. The temperature of the die and the glass cap are adjusted and the pressure of the dampening gas environment within the cavity of the die is adjusted. The cap and the die structure are soldered together by virtue of the elevated temperature through the metallized gaskets and the solder material which form a hermetic seal and trap the damping gas environment within the die structure. The total pressure of the damping gas environment within the sealed die structure is preferably in the range of 0.5 to 3.0 atmospheres and most preferably in the range of 0.5 to 1.5 atmospheres. 
     Referring to FIGS. 9 and 10, a first metallized gasket  550  is formed around a portion of, or a lip portion of, the die structure  501  defining an inner sealing region. A second and complimentary metallized gasket  556  is formed on the lid  558 . A grating light valve  500 , which is either formed integral with the die  501 , or provided separately from the die  501 , is positioned within the inner sealing region on the die  501 . A solder material  560  is placed between the metallized gaskets  550  and  556  and the temperature is adjusted to a sufficient degree to cause the solder material  560 ′ to flow and seal the lid  558  to the die  501  through the metallized gaskets  550  and  556 . Alternatively, the lid  558  and the die  501  are sealed together using an epoxy material or other adhesive material. 
     According to a preferred embodiment, the metallized gaskets  556  and  550  are formed from first layers of chromium that are deposited on the lid  558  and the die structure  501  to a thickness of approximately 300 angstroms. A second layers of gold is then deposited on each of the first layer of chromium to a thickness of approximately 10,000 angstroms. It is also preferable, that the solder material  560  is an 80 Au/20 Sn solder that is approximately 50 microns thick. Other details and embodiments are described in U.S. patent application Ser. No. 09/124,710, the content of which is hereby incorporated by reference. The primary function of the metallized gaskets  550  and  556  in the instant invention is to provide a compatible interface between the lid  558  and the solder material  560  and between the die structure  501  and the solder material  560 , such that a hermetic seal is achieved. 
     The step of sealing the ribbon in the dampening gas environment is preferably performed in a isolation chamber where the temperature of the die structure and glass cap are adjusted to approximately 300 Celsius for a sufficient period of time to form the seal as described above. During the formation of the seal or prior to the formation of the seal, the pressure of the dampening gas environment is adjusted to between 1.0 to 6.0 atmospheres, such that the resultant grating light valve device after cooling has a encapsulated dampening gas environment with a pressure that is approximately between 0.5 to 3.0 atmospheres. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a cross-sectional view of a grating light valve with reflective ribbons in a constructive interference position. 
     FIG. 1 b  is a cross-sectional view of the grating light valve, shown in FIG. 1 a,  with the active ribbons displaced to a destructive interference position. 
     FIG. 2 a  is a cross sectional view of the grating light valve with set of active ribbons and a set of bias ribbons in the same reflective plane. 
     FIG. 2 b  is a cross sectional view of the grating light in FIG. 2 a  with the active ribbons displaced from the bias ribbons to a destructive interference position. 
     FIG. 3 is a plot of the brightness response versus bias voltage applied to a grating light valve. 
     FIG. 4 a  graphs the oscillatory vibrations of undamped ribbons in a grating light valve. 
     FIG. 4 b  shows the oscillatory vibrations of dampened ribbons sealed within a dampened gas environment, in accordance with the present invention. 
     FIG. 5 is cross-sectional view of a single ribbon supported on a substrate element. 
     FIG. 6 is a schematic representation of a print system utilizing and an array of grating light valves in accordance with the present invention. 
     FIG. 7 is a block diagram for the method of making a grating light valve with reflective ribbons sealed within a dampening gas environment in accordance with the present invention. 
     FIG. 8 is a schematic representation of a isolation chamber used in the preferred method of the instant invention. 
     FIGS. 9 a-b  show a schematic view of a grating light valve with a lid and a die structure with metallized gaskets and a solder material between for providing a hermetic seal, in accordance with a preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In general, the present invention is for a grating light valve capable of alternating between constructive and destructive interference conditions with an incident light source. The grating light valve switches between the conditions for constructive and destructive interference through at least one set of movable ribbons. In accordance with the invention, the oscillatory vibrations of the ribbons are reduced by an encapsulated dampening gas environment. Note that the present invention can also be used in conjunction with other types of micro-machines with movable parts that exhibit undesirable oscillations or vibrations during operation. 
     According to the preferred embodiment of the present invention, movable ribbons of a grating light valve device are encapsulated within a die structure having a substantial concentration of a noble gas. The noble gas provides an environment to dampen the oscillatory vibrations that result when the reflective ribbons move from a destructive interference position to a constructive interference position. The noble gas is He, Ne, Ar, Kr, Xe, Rn or a mixture thereof, that is preferably greater than 50 molar percent of the total dampening gas. The dampening gas is provided such that the total pressure inside the sealed die structure is in the range of 0.5 to 3.0 atmospheres and preferably in the range of 0.5 to 1.5 atmospheres at a temperature of 20±5.0 Celsius. However, higher or lower pressures are also considered to be within the scope of the invention. 
     FIG. 4 a  plots a response curve  120  for a grating light valve with movable ribbons, as described in detail above. The y-axis  122  is any response, such as the intensity or the brightness of light reflected from the ribbons and the x-axis  123  is time. The curve  120  reveals the effects of the oscillatory vibrations which typically occur in the ribbons of grating light valves when the ribbons are switched from the destructive interference position to the constructive interference position. While the vibrational energy is dissipated through the structure of the device over time, such oscillatory vibrations can persist for periods of time that are on the order of 10 microseconds and can significantly reduce the efficiency and the ability of the device to modulate light, especially at operating wavelengths corresponding to the infrared and the near infrared. 
     FIG. 4 b  shows a plot of a response curve  121  for a similar grating light valve with the ribbons and a Neon-based dampening gas environment encapsulated within a die structure. The partial pressure of the Neon in this example is approximately 1.0 atmosphere at 20 Celsius and the dampening gas is approximately 100% Neon. Again the y-axis  124  is any arbitrary response of the ribbons, such as the intensity or brightness of reflected light, and the x-axis  125  is time. It is clear from the plotted response curve  121  that the oscillatory vibrations of the ribbons are dampened by the presence of the Neon-based dampening gas environment. Further, in this example the oscillatory vibrations are barely apparent after about 2.0 microseconds. 
     FIG. 5 shows a cross-sectional view  125  of a movable ribbon element  120  supported on a substrate element  112 . As described previously the ribbon element  120  is provided with a reflective surface  114 . The ribbon element  120  is capable of moving up and down between the position shown and the position illustrated by the dotted line  111  when an appropriate bias voltage is applied across the ribbon element  120  and the substrate element  112 . Preferably, the spacing  115  is approximately equal to the distance that the ribbon  120  is displaced when alternating between a constructive interference position and a destructive interference position. Alternatively, the spacing  115  is greater than the distance that the ribbon  120  is displaced when alternating between a constructive interference position and a destructive interference position and the ribbon element does not contact the substrate element. 
     Referring to FIG. 5, both ends of the ribbon are supported by the substrate  112 . Therefore, only a central portion of the ribbon  120  will travel the entire distance  115 , leaving open spaces  119  near the attached ends of the ribbon  120 . However, it will be clear that the ribbon element  120  may be coupled to the substrate element  112  through any number of structural features including a single end of the ribbon  120 . Also, it is clear that a plurality of movable ribbons may be couple to a substrate structure with a single support element, such as taught in U.S. Pat. Nos. 5,311,360, 5,841,579 and 5,808,797, the contents of which are hereby incorporated by reference. 
     Still referring to FIG. 5, when a grating light valve is tailored to operated with light sources corresponding to the infrared and/or the near infrared, then the distance  115  that the ribbons are required to move in order to alternate between the conditions for constructive and destructive interference is greater than 200 nanometers. Ribbons that are deflected to the downward position, as illustrated by the dotted line  111 , and towards the substrate to generate the conditions for constructive or for destructive interference, are under considerable stress and tension. Thus the ribbons behave like stiff rubber bands and spring back into the upward position when the appropriate switching voltage is applied causing the oscillating vibrations observed. By providing the appropriate dampening gas, these oscillatory vibrations are considerably reduced even when the distance  115  that the ribbons are moved or displaced is 200 nanometers or greater. 
     FIG. 6 illustrates a system  150  configured to a print medium  129 . The print medium  129  is any suitable medium that is capable of being activated by light reflected from the array of grating light valves  122 . The array of grating light valves  122  has a plurality of grating light valves  125 ,  125 ′ and  125 ″ that are encapsulated within at least one die structure  121  along with a dampening gas environment. The grating light valves  125 ,  125 ′ and  125 ″ are sealed individually in separate die compartments or collectively in a single die compartment depending on the application and manufacturing process used. The grating light valves  125 ,  125 ′ and  125 ″ are arranged in a liner array  122  as shown, or alternatively are arranged in a two dimensional and/or three dimensional configuration (not shown). The system  150  is configured with suitable optics  128  between the medium  129  and the array  122 . Suitable optics  128  include a filter, a lens, a light activated screen, a photo multiplier screen or any combination thereof, depending on the application at hand. The system also has a light source  126  that provides light with a wavelength λ which the grating light valves  125 ,  125 ′ and  125 ″ are capable of modulating. The light source  126  is a monochromatic laser source, a broad band light source or multiple wavelength light source. In a further embodiment, there are suitable optics  127  positioned between the light source  126  and the array  122 . Suitable optics  127  include a filter, a lens, a light activated screen, a photo multiplier or a combination thereof, depending on the application at hand. 
     In operation, the light source  126  emits light with a wave length λ. The light is incident on the surface of the array  122 . The light is focused, filtered or intensified with the optics  127  to ensure that a portion of light with a wavelength λ strikes the surfaces of the grating light valves  125 ,  125 ′ and  125 ″. A controller  130  provides an actuating sequence of appropriate switching voltages to each of the individual grating light valves  125 ,  125 ′ and  125 ″ to alternate the grating light valves  125 ,  125 ′ and  125 ″ between the conditions for constructive and destructive interference. The portion of the incident light that is reflected by the array  122 , in accordance with the actuating sequence, passes through the optics  128  and is focused, filtered or intensified. The reflected light then strikes the print medium  129  to produce the desired image or latent image. Preferably, the system  150  is a scanning print system wherein the array  122  and medium  129  are moved relative to each other while the medium  129  is activated. 
     FIG. 7 shows a block diagram  300  for making a grating light valve or an array of grating light valves are encapsulated within a dampening gas environment. In the step  301 , a grating light valve is provided with a die structure. The grating light valve has a plurality of movable reflective ribbons as described previously. The die structure is monolithic with the ribbons formed during the production of the grating light valve or is provided separately. For descriptive purpose below, the die structure and the grating light valve are treated as separate entities. 
     In step  303 , moisture is removed from the surfaces of the grating light valve and the die structure. Preferably, moisture is removed form the surfaces by heating the grating light valve. After removing moisture in step  303 , in step  305  a dampening gas environment is provided. After the dampening gas environment is provided in step  305 , then in step  307  the die structure is sealed, thus encapsulating the ribbons and the dampening gas environment. 
     FIG. 8 illustrates an apparatus  400  configured for use in the preferred method of the instant invention. A grating light valve  413  is provided within a die structure  401  and both are placed within a isolation chamber  410 . Dampening gas is sealed within the die structure  401  by providing a suitable dampening gas environment  405 ′ in the isolation chamber  410  and sealing a portion of the dampening gas environment  405 ′ within the die structure  401 . The pressure and/or flow of the dampening gas environment  405 ′ within the isolation chamber  410  can be controlled with a vacuum source  407  coupled to the isolation chamber  410 . 
     Still referring to FIG. 8, the dampening gas environment  405 ′ is preferably encapsulated within the die structure  401  by providing a preformed metallized gasket  404  on the sealing edge  412  of the die structure  401 . A glass cap  402  is also provided. The glass cap  402  has a complementary preformed metallized gasket  406 . The glass cap  402  is placed on the die structure  401  with the gaskets  404  and  406  aligned and overlapping and with a solder material  408  between the gaskets  404  and  406 . 
     The gas source  425  and  415  comprise at least one noble group VIII gas  425  that preferably provides  50  molar percent or more of total dampening gas environment  405 ′. Dampening effects have been observed using a Neon and Krypton dampening gas environment that is approximately 100% Neon or Krypton with trace amounts of impurity gases. Alternatively, the dampening gas environment  405 ′ contains a second gas  415 , such as Nitrogen, Hydrogen, a second noble group VIII gas or any combination thereof. 
     Prior to sealing the die structure  401 , the pressure of the dampening gas environment  405 ′ is adjusted until the pressure meter  409  reads a value between 1.0 and 6.0 atmospheres. Then the temperature of the die structure  401  and the cap  402  are adjusted with a heating element  435  to a sealing temperature of approximately 300 degrees Celsius ±50, depending on the solder material materials  408  that are used. Preferably, the sealing temperature does not exceed 400 degrees Celsius, because higher temperatures can reduce the reflectivity of the ribbon surfaces. The sealing temperature is maintained for approximately 10 minutes or a sufficient time to thereby cause the solder material  405  to melt and thereby solder the glass cap  402  to the die structure  401  through the gaskets  404  and  406  and encapsulate a portion of the dampening gas environment  405 . The sealed die structure is then cooled and removed from the isolation chamber  410  to be installed and used in the desired device. 
     The present invention has been described relative to a preferred embodiment. Improvements or modifications that become apparent to persons of ordinary skill in the art only after reading this disclosure are deemed within the spirit and scope of the invention. Specifically, the operation of a grating light device has been described using a single set of movable ribbons which are alternating with respect to a set of stationary ribbons. However, it is understood that the conditions for constructive and destructive interference can be achieved by moving either set of ribbons or both sets of ribbons. Further, it is understood that practicing the instant invention is not dependent on a particular grating light valve construction or device. 
     The present invention has been described relative to a preferred embodiment. Improvements or modifications that become apparent to persons of ordinary skill in the art only after reading this disclosure are deemed within the spirit and scope of the application. Specifically, the operation of a grating light device has been described using a single set of movable ribbons which are alternating with respect to a set of stationary ribbons. However, it is understood that the conditions for constructive and destructive interference can be achieved by moving either set of ribbons or both sets of ribbons. Further, it is understood that practicing the instant invention is not dependent on a particular grating light valve construction or device.