Apparatus for displaying a three-dimensional image

Abstract

An apparatus for displaying a three-dimensional image comprises: a light field generating unit (110), which is configured to receive an incident light beam (112) and generate a three-dimensional light field; and an image revealing medium (120), which is arranged to receive the three-dimensional light field generated by the light field generating unit (110), wherein the image revealing medium (120) comprises a fluid with bubbles or particles suspended in the fluid, wherein the bubbles or particles have a size in the range of 40-500 nm.

Claims

1. An apparatus for displaying a three-dimensional image, said apparatus comprising: a light field generating unit, which is configured to receive an incident light beam and generate a three-dimensional light field; and an image revealing medium, which is arranged to receive the three-dimensional light field generated by the light field generating unit, wherein the image revealing medium comprises a fluid with bubbles or particles suspended in the fluid, wherein the bubbles or particles have a size in the range of 40-500 nm, wherein the bubbles or particles cause scattering of light based on at least one of Tyndall scattering or Rayleigh scattering for forming the three-dimensional image.

2. The apparatus according to claim 1, wherein the bubbles or particles have a size in the range of 40-200 nm.

3. The apparatus according to claim 1, wherein the bubbles or particles have a size in the range of 50-150 nm.

4. The apparatus according to claim 1, wherein the fluid is an aqueous liquid.

5. The apparatus according to claim 1, wherein the bubbles are filled with air or another gas comprising oxygen, nitrogen or carbon dioxide.

6. The apparatus according to claim 1, wherein a size of the bubbles or particles and a concentration of bubbles or particles in the fluid are selected for providing an optical attenuation constant in the range of 10-200 dB/m for a wavelength of light of the light beam.

7. The apparatus according to claim 1, wherein a size of the bubbles or particles and a concentration of bubbles or particles in the fluid are selected for providing an average distance between two adjacent bubbles or particles in the fluid below 200 μm.

8. The apparatus according to claim 1, wherein a concentration of bubbles in the fluid is larger than 2*10.sup.14 bubbles/m.sup.3.

9. The apparatus according to claim 1, further comprising a container in which the image revealing medium is arranged, wherein at least a portion of a wall of the container is transparent for output of light scattered by the bubbles or particles in the fluid.

10. The apparatus according to claim 9, further comprising at least one channel connected to the container for transporting the image revealing medium into and out of the container.

11. The apparatus according to claim 9, wherein the image revealing medium is arranged in the container to have an increasing concentration of bubbles or particles in a direction of propagation of light of the generated light field in the container.

12. The apparatus according to claim 1, further comprising at least one calibration sensor, which is configured to receive light being transmitted through the image revealing medium for detecting an intensity of received light as a measure of attenuation of a light beam propagating through the image revealing medium.

13. The apparatus according to claim 1, further comprising an optical system for transferring the light field generated by the light field generating unit into the image revealing medium.

14. The apparatus according to claim 1, further comprising a controller unit for controlling the light field generating unit for controlling distribution of light in the three-dimensional light field output by the light field generating unit.

15. The apparatus according to claim 1, further comprising at least one light source, which is configured to generate the light beam incident on the light field generating unit.

16. The apparatus according to claim 1, wherein the light field generating unit comprises a plurality of cells, wherein each cell is configured to interact with a portion of an incident light beam in order to provide interaction with the portion of the incident light beam for forming the three-dimensional light field.

17. An apparatus for displaying a three-dimensional image, said apparatus comprising: a light field generating unit, which is configured to receive an incident light beam and generate a three-dimensional light field; and an image revealing medium, which is arranged to receive the three-dimensional light field generated by the light field generating unit, wherein the image revealing medium comprises a fluid with bubbles suspended in the fluid, wherein the bubbles have a size in the range of 40-500 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above, as well as additional objects, features and advantages of the present inventive concept, will be better understood through the following illustrative and non-limiting detailed description, with reference to the appended drawings. In the drawings like reference numerals will be used for like elements unless stated otherwise.

(2) FIG. 1 is a schematic view of an apparatus according to an embodiment.

(3) FIGS. 2a-d are charts illustrating absolute and relative scattering cross sections for air bubbles in water and for polystyrene particles in water for different diameters of the bubbles and particles, respectively.

(4) FIGS. 3a-c are charts illustrating absorbance of an image revealing medium as function of wavelength for different sizes of particles of the image revealing medium.

(5) FIG. 4 is a chart illustrating absorbance of an image revealing medium as function of concentration of particles in the image revealing medium.

(6) FIG. 5 is a schematic view of control of the image revealing medium in an apparatus comprising a container with a plurality of compartments according to a first embodiment.

(7) FIG. 6 is a schematic view of control of the image revealing medium in an apparatus comprising a container with a plurality of compartments according to a second embodiment.

DETAILED DESCRIPTION

(8) Referring now to FIG. 1, an apparatus 100 for displaying a three-dimensional image will be generally described. The apparatus 100 comprises a light field generating unit 110, which is configured to generate a three-dimensional light field based on a light beam incident on the apparatus 100. The apparatus 100 further comprises an image revealing medium 120, which is configured to receive the three-dimensional light field. The image revealing medium 120 comprises scattering sources in form of bubbles or particles in the image revealing medium 120. The scattering sources will scatter light of the three-dimensional light field so as to form points of origin of light within the image revealing medium, as controlled by a distribution of the three-dimensional light field. The scattering sources may thus output light which may be seen by an observer, such that the scattering sources may reveal a three-dimensional image to the observer based on the three-dimensional light field in the image revealing medium 120.

(9) The apparatus 100 may further comprise a container 130 in which the image revealing medium 120 may be arranged. The container 130 may have walls 132 defining an interior hollow space, which may be filled by the image revealing medium 120.

(10) The container 130 may be arranged on a common substrate 102 of the apparatus 100. The light field generating unit 110 may also be formed on or in the substrate 102 or be mounted on or in the substrate 102. Thus, the substrate 102 may define a well-controlled relation between the container 130 and the light field generating unit 110, such that accurate control of the three-dimensional light field in the image revealing medium 120 may be provided.

(11) However, it should be realized that the container 130 need not necessarily be arranged on a common substrate with the light field generating unit 110. On the contrary, the container 130 may be arranged separately from the light field generating unit 110. The container 130 and the light field generating unit 110 may be mounted in a common housing so as to define a well-controlled relation between the container 130 and the light field generating unit 110. The container 130 may at least partly extend out of such a common housing in order for a view of the three-dimensional image by an observer not to be disturbed by external walls of the common housing.

(12) At least a portion of the walls 132 of the container 130 may be transparent, at least to light of the wavelength(s) being used for forming the three-dimensional image in the image revealing medium 120. The walls 132 may be transparent on the sides of the container 130 through which the three-dimensional image is to be observed. The walls 132 may thus comprise transparent windows through which the three-dimensional image is to be observed. Alternatively, the entire (or almost entire) walls 132 may be transparent.

(13) The container 130 may further comprise a surface or wall 134 through which light from the light field generating unit 110 is allowed to enter the container 130 and hence be projected into the image revealing medium 120 in the container 130. Light from the light field generating unit 110 may enter the container 130 through a bottom surface 134 of the container 130, which may be arranged on the common substrate 102.

(14) The wall 134 through which light enters the container 130 may be transparent to light, at least in the part of the wall 134 through which light is entered into the container 130. Alternatively, there may be an aperture or opening in the wall 134 through which light may be allowed to enter the container 130. An optical component, such as a lens, may be mounted in the aperture in the wall 134 for controlling the light being projected into the image revealing medium 120.

(15) The container 130 may comprise at least a first opening 136 and a second opening 138, which may allow flow of the image revealing medium 120 into and out of the container 130. This implies that the image revealing medium 120 may be replaced, such that if characteristics of the image revealing medium 120 deteriorate over time, the apparatus 100 may be provided with new image revealing medium 120 for maintaining a quality of the three-dimensional display.

(16) The first opening 136 and the second opening 138 may be formed in a bottom wall 134 of the container 130, such that the openings 136, 138 may not interfere with a view of the three-dimensional image. Thus, the openings 136, 138 may be arranged on respective sides of a part of the wall 134 through which light is allowed to enter the container 130.

(17) However, it should be realized that the openings 136, 138 may be arranged in any other manner allowing access to the interior space of the container 130 for replacing the image revealing medium 120.

(18) The apparatus 100 may comprise an inlet channel 104 associated with the first opening 136 and an outlet channel 106 associated with the second opening 138. The inlet channel 104 and the outlet channel 106 may be arranged in the common substrate 102 for forming a compact apparatus 100.

(19) The inlet channel 104 and the outlet channel 106 may each be associated with a valve for controlling whether flow of the image revealing medium 120 into or out of the container 130 is allowed. Also, the container 130 may alternatively comprise a single opening associated with a single channel, wherein a direction of flow through the channel may be controlled for using the channel both for transporting image revealing medium 120 into the container 130 and for transporting image revealing medium 120 out of the container 130.

(20) The light field generating unit 110 may be configured to generate a three-dimensional light field based on an incident light beam 112. The light field generating unit 110 may comprise portions which have different interactions with light so as to generate a non-homogeneous three-dimensional light field based on the incident light beam 112. The light field generating unit 110 may comprise a static arrangement for generating the three-dimensional light field, such that the apparatus 100 may be arranged to display a static three-dimensional image. However, according to an alternative embodiment, the light field generating unit 110 may comprise an array 114 of unit cells 116, wherein the unit cells 116 are individually addressable for controlling an optical property of the unit cell 116 and hence controlling an optical response of the array 114 of unit cells 116.

(21) Each unit cell 116 may be individually addressable. However, it should be realized that not necessarily each and every one of the unit cells 116 is individually addressable.

(22) By controlling the optical property of the unit cells 116, an effect on a light beam 112 incident on the array may be controlled. Thus, the unit cells 116 may in combination form a controllable effect on the incident light beam 112. Thus, the array 114 may be used for forming and controlling a distribution of a three-dimensional light field based on the incident light beam 112.

(23) The three-dimensional light field may be used for revealing a holographic image by the image revealing medium 120. Thanks to the unit cells 116 being controllable, a change in the holographic image formed may be provided. This implies that the apparatus 100 may be used for displaying a video of holographic images.

(24) The light field generating unit 110 may be set up for reflecting the incident light beam 112 or transmission of the incident light beam 112. The light beam 112 may be formed by a coherent light source, such as the light beam 112 being a laser beam, which provides a well-defined relation of the incident light field on the array 114 of unit cells 116 and, hence, is suitable for using as a basis for forming the desired distribution of the three-dimensional light field using the array 114 of unit cells 116.

(25) Each unit cell 116 may comprise a phase-change material (PCM), which may be switched between a first state and a second state, wherein switching of the PCM between the first state and the second state is configured to switch the optical property of the unit cell 116 between a first condition of the optical property and a second condition of the optical property.

(26) The PCM may be configured to switch between a crystalline state and an amorphous state. However, it should be realized that the first and second states may be other configurations of states of the PCM. For instance, the PCM may be configured to switch between two different crystalline states.

(27) The unit cell 116 may comprise a layer of PCM, which may be combined with other materials, e.g. in a stack of layers of materials, such that the combination of materials may define an optical property of the unit cell 116. The switching of a state of the PCM may then affect the optical property of, for instance, the stack of layers in the unit cell 116, such that a condition of the optical property of the unit cell 116 may be controlled by a state of the PCM.

(28) For instance, the switching of state of the PCM may affect refractive index and/or permittivity of the PCM, such that e.g. a PCM layer or a stack comprising a PCM layer may be switched from a highly reflective to a low reflective state for a given wavelength of incident light.

(29) The unit cell 116 may thus comprise a PCM layer for controlling a condition of the optical property of the unit cell 116. For instance, reflectivity or transmission of the unit cell 116 for a given wavelength may be configured to be highly dependent on the state of the PCM.

(30) According to an embodiment, the PCM is a compound of germanium, antimony and tellurium. For instance, the PCM may be formed by Ge.sub.2Sb.sub.2Te.sub.5 (GST). This is a material which may change between an amorphous state and a crystalline state and which may suitably be used for providing desired optical properties of the array 114 of unit cells 116.

(31) However, it should be realized that the PCM may be any material which provides a change in optical property based on the switching between two states. The PCM may for instance be any material which may undergo a phase change in relation to being exposed to a temperature (a thermochromic material) or in relation to being exposed to light (a photochromic material) or a combination of such materials. For example, a number of different forms of vanadium oxides, such as VO.sub.2 and V.sub.2O.sub.3, may be used. The PCM may include thermochromic materials formed from metal-oxide materials, such as vanadium oxide as mentioned above, polymers, such as azobenzene-containing polydiacetyelenes, or nanostructured polymers, such as diblock (poly[styrene-b-isoprene]) copolymers. The PCM may alternatively be an electro-optic material that changes a condition of an optical property based on an applied electric field, such as a birefringent material, or a magneto-optic material that changes a condition of an optical property based on an applied magnetic field, such as garnets and ferro-magnetic metals.

(32) In the specific case of using GST as the PCM, switching the material modifies the structure from a plasmonic (crystalline state) to a dielectric (amorphous state) antenna, which will exhibit very similar resonances but at different wavelengths, allowing to change a structure in the unit cell 116 from a highly reflective (transmissive) to a low reflective (transmissive) state for a given wavelength.

(33) Upon switching of the state of GST, the optical properties are significantly altered, resulting in large changes in both real and imaginary parts of refractive index and permittivity. It should be noted that in its crystalline state, GST has a negative real part of the permittivity, which implies that it shows metallic behavior and therefore supports plasmonic resonances. In its amorphous state, GST has a positive real part of the permittivity, accompanied with a large imaginary part, meaning that it acts as a highly lossy dielectric.

(34) The PCM in a unit cell 116 could be switched thermally (as for GST) but is not limited to that. It should be realized that in different embodiments, electro-optic materials, and magneto-optic materials may be used as alternative implementations.

(35) It should be realized that the controlling of the state of the PCM may be provided by individually addressing the unit cell 116, e.g. by sending a control signal to electrodes associated with the unit cell 116 for inducing the switching of the state of phase-change material locally in the unit cell 116, e.g. by local heating of the PCM, or by exerting the PCM to a local electric or magnetic field.

(36) The unit cells 116 may be configured so that the switching of conditions of an optical property of the unit cell 104 provide a strong effect of the unit cell 116 on a desired operational wavelength.

(37) The apparatus 100 may comprise control lines which may be integrated in the substrate 102 such that control signals may be provided to the unit cells 116. The control signal may induce a change in optical property of the unit cell 116, e.g. by the control signal providing heating of the unit cell 116 or forming an electric or magnetic field to cause a switching of the optical property of the unit cell 116.

(38) The apparatus 100 may comprise an individual control line for each of the unit cells 116 for controlling the unit cell 116. According to an alternative, the apparatus 100 may comprise a plurality of control lines arranged in columns and lines, such that an individual unit cell 116 may be controlled by cross-point addressing using the control lines.

(39) The light field generating unit 110 may further comprise an optical system 118 for transferring the three-dimensional light field generated by the array 114 of unit cells 116 into the image revealing medium 120. Thus, the optical system 118 may be arranged between the array 114 of unit cells 116 and the image revealing medium 120.

(40) The optical system 118 may guide the light from the array 114 of unit cells 116, e.g. by re-directing an optical path of the light. This may imply that requirements on a geometric relation between the array 114 of unit cells 116 and the image revealing medium 120 (in the container 130) may be relaxed.

(41) The optical system 118 may further comprise one or more lenses and/or one or more aperture stops, which may control that the three-dimensional light field is transferred into the image revealing medium 120 with desired dimensions.

(42) The optical system 118 may be arranged in or on the common substrate 102 between the array 114 of unit cells 116 and the container 130. According to one embodiment, a component of the optical system 118, such as a lens, may be arranged in a wall 134 of the container 130 for transferring light into the container 130 through the lens in the wall 134.

(43) The apparatus 100 may further comprise one or more light sources 140 for providing illumination light of the operational wavelength(s) of the apparatus 100. As mentioned above, the one or more light sources 140 may be configured to provide an incident light beam 112 on the array 114 of unit cells 116.

(44) The light field generating unit 110 may operate in a reflective arrangement or a transmissive arrangement, i.e. the array 114 of unit cells 116 being arranged to reflect or transmit the incident light beam 112. The one or more light sources 140 are mounted in relation to the light field generating unit 110 in dependence of whether a reflective or transmissive arrangement is used.

(45) In FIG. 1 a single light source 140 is shown mounted on or in the common substrate 102 above the array 114 of unit cells 116 for providing an incident light beam 112 which will be reflected by the array 114 of unit cells 116 towards the image revealing medium 120. Thus, a well-defined relation between the light source 140 and the array 114 of unit cells 116 may be provided.

(46) The light source 140 may be associated with an optical system 142, such as one or more lenses and/or one or more aperture stops for forming a desired shape of the incident light beam 112 on the array 114 of unit cells 116 and for illuminating the entire array 114.

(47) The light source 140 may be any type of laser source, such as a laser source with a well-defined operational wavelength or a laser source with a tunable operational wavelength.

(48) It should be realized that the apparatus 100 may comprise one or more light sources 140, which may be mounted in a pre-defined and accurate relationship to the array 114 of unit cells 116 in order to ensure that a desired incident light beam is provided on the array 114 of unit cells 116. It should also be realized that the one or more light sources 140 may be mounted in the apparatus 100 for providing an incident light beam 112 which is reflected or transmitted by the array 114 of unit cells 116.

(49) However, the apparatus 100 may alternatively be manufactured and delivered without including a light source. Thus, a user may be able to separately acquire light source(s) and the apparatus 100 in order to set up the apparatus 100 with the light source(s) for forming the three-dimensional display. This may provide flexibility to a user for designing an own system and e.g. choosing operational wavelength(s) of the light sources to be used with the apparatus 100.

(50) The apparatus 100 may further comprise a controller 150, which may control functions of the light field generating unit 110 and in particular the array 114 of unit cells 116. The controller 150 may be integrated in a unit on which the array 114 of unit cells 116 is formed and may provide control of when control signals are to be provided to unit cells 116 for switching a condition of an optical property of the unit cells 116.

(51) Thus, the array 114 of unit cells 116 with associated control circuitry and the controller 150 may be manufactured in one piece, such as in an integrated circuit package, providing pins for input and output of signals to the package. This may imply that the integrated circuit package may be mounted on the common substrate 102 providing integrated functionality for controlling the array 114 of unit cells 116 so as to control the display of the three-dimensional image.

(52) However, it should be realized that the controller 150 may be separately provided. Thus, the controller 150 may make use of a general-purpose processing unit of the apparatus 100 and may be implemented as a software being executed by the processing unit. This may be advantageous for enabling updating of functionalities of the controller 150. For simplicity, the controller 150 is illustrated in FIG. 1 as being arranged separately from array 114 of unit cells 116.

(53) The controller 150 may receive information of a desired holographic image to be displayed and may execute an algorithm for calculating of the condition of the optical property to be set for the respective unit cells 116 in order to form the desired distribution of the three-dimensional light field. Alternatively, the controller 150 may receive information of the conditions of the optical property to be set for the unit cells 116 from an external unit, which may execute the algorithm.

(54) The controller 150 may also control the light sources 140, e.g. for controlling when a light source 140 is to be activated or for controlling a property of the incident light beam 112, such as a polarization of the incident light beam 112.

(55) The controller 150 may also determine when the image revealing medium 120 is to be replaced and may be configured to output a signal when such a determination is made. The controller 150 may determine that the image revealing medium 120 needs to be replaced based on receiving a measurement result. However, according to an embodiment, the apparatus 100 may be set to replace the image revealing medium 120 at regular intervals and the controller 150 may be configured to keep track of when it is time to replace the image revealing medium 120.

(56) The image revealing medium 120 may have a long-term stability to maintain stable properties for a long time, even reaching up to several months. A frequency at which the image revealing medium 120 is replaced may be set in dependence on a desired quality of the three-dimensional image. For instance, the controller 150 may be set to control replacing of the image revealing medium once every hour, once every day or once every month.

(57) The controller 150 may output a signal which may trigger replacing the image revealing medium 120. Thus, the controller 150 may output a signal which may activate valves and/or pumps for controlling flow of the image revealing medium 120 into and out of the container 130. The replacing of the image revealing medium 120 may be automated, with the apparatus 100 being connected to a reservoir or supply of image revealing medium 120 or for enabling forming of new image revealing medium 120. Thus, the signal output by the controller 150 may trigger such automated control for replacing the image revealing medium 120. However, the controller 150 may alternatively output a signal to a user, e.g. in the form of presenting information on a display or activating a lamp or a speaker for alerting a user of a need of replacing the image revealing medium 120. The user may thus manually control replacing of the image revealing medium 120 such as connecting the apparatus 100 to a supply of new image revealing medium 120.

(58) It should be realized that the controller 150 may be implemented as one or more processing units, such as a central processing unit (CPU), which may execute the instructions of one or more computer programs in order to implement functionality of the apparatus 100.

(59) The controller 150 may alternatively be implemented as firmware arranged e.g. in an embedded system, or as a specifically designed processing unit, such as an Application-Specific Integrated Circuit (ASIC), a microcontroller unit (MCU) or a Field-Programmable Gate Array (FPGA).

(60) The apparatus 100 may further comprise at least one calibration sensor 160. The at least one calibration sensor 160 may acquire a measurement result which may be used by the controller 150. The measurement result of the calibration sensor 160 may be used for determining whether the image revealing medium 120 needs to be replaced.

(61) The calibration sensor 160 may be configured to receive light being transmitted through the image revealing medium 120 for detecting an intensity of received light as a measure of attenuation of a light beam propagating through the image revealing medium 120.

(62) The calibration sensor 160 may be mounted at an inner wall of the container 130 opposite to the wall 134 through which light enters the container 130. Thus, the calibration sensor 160 may receive light having propagated through the image revealing medium 120. However, as an alternative, a mirror or reflective surface may be mounted at an inner wall of the container 130 opposite to the wall 134 through which light enters the container 130. Then, the calibration sensor 160 may be arranged in proximity of the light field generating unit 110 and receive a back reflection from the mirror/reflective surface so as to detect light having propagated (twice) through the image revealing medium 120. In some embodiments, the apparatus 100 may comprise multiple calibration sensors 160 and/or mirrors for determining local density variations in the image revealing medium 120 (by determining propagation of light through different parts of the volume of the image revealing medium). This may give even improved feedback to the controller 150.

(63) The calibration sensor 160 may be used when the image revealing medium 120 in the apparatus 100 is replaced in order to determine attenuation of light of the new image revealing medium 120. The measurement result of the calibration sensor 160 may be used in setting an intensity of light to be received by the image revealing medium 120.

(64) When performing a measurement by the calibration sensor 160, a calibration pattern may be generated by the light field generating unit 110, wherein the calibration pattern focuses a light beam onto the calibration sensor 160 which allows measuring the attenuation of the propagating beam through the image revealing medium 120 for different colors.

(65) During replacing of the image revealing medium 120, measurement results of the calibration sensor 160 may also be used for determining whether a property of the image revealing medium 120 is to be adjusted. For instance, the calibration sensor 160 may be used for determining whether a concentration of bubbles or particles need to be increased, such that further bubbles or particles may be injected to the image revealing medium 120 in the container 130 (through the inlet channel 108 or through a separate inlet of bubbles or particles). Thus, the calibration sensor 160 may be used for controlling properties of the image revealing medium 120 when the image revealing medium 120 is being replaced.

(66) The controller 150 may also or alternatively use measurement results of the calibration sensor 160 in order to enable detecting if characteristics of the image revealing medium has changed (e.g. if concentration of bubbles has decreased or if a distribution of bubbles or particles in the image revealing medium 120 has changed) such that a need of replacing or adjusting the image revealing medium 120 may be identified.

(67) The controller 150 may be configured to generate the calibration pattern at regular intervals for performing of the measurements of the calibration sensor 160. A measurement of the calibration sensor 160 may alternatively or additionally be triggered upon start-up of the apparatus 100.

(68) As mentioned above, the apparatus 100 may be connected to a reservoir or supply of image revealing medium 120. The connection may be formed when a need of replacing the image revealing medium 120 occurs. The apparatus 100 may thus receive the image revealing medium 120 from an external supply or from a reservoir.

(69) The image revealing medium 120 may comprise bubbles or particles suspended in a fluid. Bubbles in the fluid may have a long-term stability in that the bubbles do not dissolve in the fluid and do not rise to the surface of the fluid to burst there. However, over a long period of time, some bubbles may disappear and the quality of the image revealing medium 120 may deteriorate.

(70) Particles in the fluid may not disappear in the same manner as for bubbles as explained above. However, quality of an image revealing medium 120 comprising particles may also deteriorate due to a distribution of the particles in the fluid changing.

(71) When the image revealing medium 120 needs to be replaced, the bubbles may be formed or particles may be introduced in the fluid of the new image revealing medium 120 to be introduced into the container 130. Thus, the image revealing medium 120 with particles or bubbles suspended in the fluid may be formed at a time when the image revealing medium 120 is to be replaced.

(72) The forming of the new image revealing medium 120 may involve manual operation for forming the image revealing medium 120 such as introducing particles into the fluid. However, the forming of the new image revealing medium 120 may alternatively be automated or semi-automated in that the fluid is mixed with particles or in that bubbles are injected into the image revealing medium 120.

(73) In one embodiment, the apparatus 100 may comprise a bubble generating device 170. The bubble generating device 170 may be configured to receive a fluid and may be configured to inject bubbles into the fluid such that bubbles are suspended in the fluid. The bubble generating device 170 may thus generate the image revealing medium 120 and may be connected to the inlet channel 104 for providing the image revealing medium 120 into the container 130.

(74) For instance, the bubble generating device 170 may comprise a gas inlet for receiving a gas from which the bubbles are to be formed. The bubble generating device 170 may further comprise a compressor for pressurizing the gas. The bubble generating device 170 may further comprise one or more nozzles for introducing the pressurized gas into the fluid and form bubbles therein. The pressure of the gas and the size of the nozzles may control a size of the bubbles formed in the fluid.

(75) By the apparatus 100 comprising a bubble generating device 170, the apparatus 100 may only need to be connected to a fluid inlet and a gas inlet in order to be able to form the image revealing medium 120. The fluid may be water and the gas may be air, which implies that the fluid and gas may be readily available. Also, the image revealing medium 120 may thus be formed from non-hazardous substances, which facilitates handling of the image revealing medium 120 when old image revealing medium 120 being replaced is to be disposed of.

(76) Now, the image revealing medium 120 will be described in further detail. The image revealing medium 120 may comprise a fluid, which should be transparent to the operational wavelength(s), and bubbles or particles suspended in the fluid, forming scattering points in the fluid.

(77) Below, factors for selecting a size of the particles or bubbles will be discussed. In this regard, size should be construed as a diameter of a sphere, but especially the particles may not necessarily be exactly spherical, so the size could also be construed as a largest cross-section of the particle or bubble or an equivalent spherical diameter of the particle or bubble.

(78) The following factors may be taken into account when selecting the size of the particles or bubbles: 1. The scattering medium should scatter with sufficient efficiency in order for the observer to be able to see a clear holographic image in terms of scattered intensity, translating into a desired resolution and contrast. 2. The scattering medium should not cause too high attenuation for the operational wavelengths, as high attenuation would limit the maximum dimensions of the volume of the image revealing medium 3. The scatterer density inside the medium should be large enough (i.e. bubble or particle concentration needs to be sufficiently high to always have at least one or several bubbles or particles inside the smallest discernible volume, voxel, of the image revealing medium 120) to efficiently scatter the light.

(79) The bubble or particle sizes being used in the present disclosure are in ranges to benefit from Tyndall scattering (scattering by particles/bubbles smaller than or similar in size to the operational wavelength) and Rayleigh scattering (scattering by particles much smaller than the operational wavelength). The Tyndall and Rayleigh scattering mechanisms are fairly similar in nature, especially in the sense that the scattering efficiency scales with the 4.sup.th power of the frequency. In sub-wavelength sized suspensions in a fluid with a constant background refractive index (e.g. particles in water suspension, bubbles in water suspension), the scattering effects result from the electrical polarizability of such particles/bubbles in which the charges oscillate along with the driving electric field, and as such the particle acts as a point dipole source whose radiation is observed as scattered light.

(80) In terms of size selection for ultrafine bubbles or particles, a trade-off has to be made in having high enough scattering efficiency in each voxel of the image revealing medium 120 and having low enough absorption/scattering for the image revealing medium 120 to still be sufficiently transparent as to not hamper the view and enable illuminating voxels over the entire volume of the image revealing medium 120.

(81) In a uniform medium that weakly absorbs and/or scatters light, the intensity decay is defined by Beer-Lambert's law which states that there is exponential decay of the light intensity with propagation distance. In its simplest form, Beer-Lambert's law can be written as

(82) $A={\log }_{10}\frac{I_t}{I_0}=-{\log }_{10}T,$

(83) wherein A is the absorbance of the medium, T is the transmittance of the medium, I.sub.t is the transmitted intensity of light and I.sub.0 is the intensity of the incident light. This can be rewritten as function of the optical depth τ as

(84) $T=\frac{I_t}{I_0}=e^{-\tau }=10^{-A}.$

(85) This implies that an absorption coefficient expressed in dB/m can be used to define the required concentration of bubbles or particles and sizes of bubbles or particles.

(86) For achieving sufficient propagation through the image revealing medium 120 and having reasonable brightness throughout the volume of the image revealing medium 120, the apparatus 100 may be designed such that between 10% and 1% of light is allowed to propagate through the entire volume of the image revealing medium 120. This corresponds to the power of light going down by 10 to 20 dB over the entire volume of the image revealing medium 120.

(87) The volume of the image revealing medium 120 may be defined by a size of a cube in a range of 0.1-1 m. This would imply that the apparatus 100 may provide a relatively large volume of the image revealing medium 120 so as to facilitate viewing of the three-dimensional image by an observer. However, it should be realized that even larger sizes of the volume of the image revealing medium 120 may be of interest, at least when technology of three-dimensional display evolves.

(88) Based on an assumption that power of light should go down by 10 to 20 dB over the entire volume and that a side of the volume of the image revealing medium being in the range of 0.1-1 m, the apparatus 100 may advantageously be designed to provide an attenuation constant between −10 to −200 dB/m.

(89) As the scattering intensity of small particles or bubbles scales with the 4.sup.th power of the frequency, larger illumination power will be required to reach similar scattering intensities for longer wavelengths compared with shorter wavelengths. This implies that intensity of light of incident light beams 112 should be lowest for blue light and larger for green light and red light, respectively, in order to provide similar scattering intensities of light in the image revealing medium 120.

(90) An air bubble in water may affect light in a similar manner as a particle in water, wherein the relation between the refractive index of the water and air is similar to the relation between the refractive index of the particle and water. In particular, water has a refractive index of approximately 1.33 whereas air has a refractive index of approximately 1.00. The optical behavior of air bubbles in water may then be compared with the optical behavior of plastic particles in water, wherein the plastic material may be selected, e.g. as polystyrene, having a refractive index of 1.60. Measurements have been made based on polystyrene particles suspended in water, and it may be assumed that corresponding behavior will be provided for air bubbles in water.

(91) Referring now to FIGS. 2a-d, simulations of scattering cross sections and relative scattering cross sections for air bubbles in water (FIGS. 2a and 2c) and polystyrene particles in water (FIGS. 2b and 2d) are shown.

(92) In particular for bubbles or particles having a diameter from 50 nm up to 200 nm, the relative scattering cross section is below 1, which implies that the image revealing medium 120 comprising bubbles or particles of such sizes will have fairly low scattering losses, such that the scattering medium will be relatively transparent. It is also clear that in order to have the same scattering intensity for blue, green and red light, respectively, the power of the incident light beam 112 should increase when going from blue light towards red light.

(93) Using polystyrene particles of different sizes, absorbance measurements were made, i.e. measurements to determine attenuation of an image revealing medium 120 comprising particles of different sizes and different concentrations.

(94) In FIGS. 3a-c, the absorbance for different sizes of the particles are shown as a function of wavelength. In FIG. 3a, the diameter size of particles is 65 nm. In FIG. 3b, the diameter size of particles is 120 nm. In FIG. 3c, the diameter size of particles is 250 nm. For each diameter size, absorbance is indicated for different concentrations in terms of weight percentage of polystyrene in the medium. The same weight percentage is illustrated by same dashing of the lines in FIGS. 3a-c. It should be realized that the same weight percentage does not correspond to the same number of particles per volume unit in the medium.

(95) It is clear from FIGS. 3a-c that with increasing particle size, the same weight percentage yields a larger absorbance of the medium.

(96) Based on these measurements, absorbance as a function of concentration of particles/m.sup.3 of the particles may be defined. In FIG. 4, the absorbance as a function of concentration is illustrated. The absorbance may advantageously be between 0.01-0.1 (corresponding to an attenuation constant of 10-100 dB/m), as indicated by the hatched area in FIG. 4. Such values would at least be interesting for a volume of the image revealing medium of 1 m.sup.3.

(97) In FIG. 4, appropriate concentrations of particles may be determined based on different sizes of the diameter of the particles. A higher concentration of particles may be needed for smaller sizes of the particles.

(98) Using a lower desired value of absorbance of 0.01 (attenuation factor of 10 dB/m) and an upper desired value of absorbance of 0.1 (attenuation factor of 100 dB/m), characteristics of the image revealing medium 120 for different sizes of particles may be defined according to the following table.

(99) TABLE-US-00001 Concentration Fill factor Distance Particles/mm.sup.3 Diameter Lower Upper Lower Upper Lower Upper Lower Upper  65 nm 10.sup.17 10.sup.19 10.sup.−5 10.sup.−3 1.0 0.1 10.sup.8 .sup. 10.sup.10 120 nm 10.sup.15 10.sup.17 10.sup.−6 10.sup.−4 10.0 1.0 10.sup.6 10.sup.8 250 nm 10.sup.13 10.sup.16 10.sup.−7 10.sup.−4 40.0 3.0 10.sup.4 10.sup.7

(100) In this table, the concentration is given as number of particles/m.sup.3, the fill factor is a volumetric fraction of the particles versus the fluid (water) in which the particles are suspended, the distance is a length in μm of a cubic volume in which a single particle is present (or in other words, the average distance between two adjacent particles in the fluid), and particles/mm.sup.3 illustrates the number of particles per volume of a cube having a side of 1 mm, which may give an indication of the number of particles per voxel.

(101) It is clear from this table that an image revealing medium 120 with particles in the tested range may be designed with a suitable attenuation factor for enabling propagation through a large volume of the image revealing medium 120 (so as to enable a large three-dimensional display) while allowing a large number of particles within a small volume of the image revealing medium 120 such that a high resolution of a three-dimensional image may be provided in that a smallest discernible volume may be set to be very small.

(102) Although test results have only been obtained for particles in a fluid, corresponding results may be expected based on bubbles in the fluid since the refractive index contrast between air and water is very similar to the index contrast between polystyrene and water and, therefore, in optical terms an image revealing medium 120 comprising bubbles is very similar to an image revealing medium 120 comprising particles.

(103) Ultrafine bubbles (bubbles with a diameter smaller than 500 nm and, in particular having a diameter smaller than 200 nm) may have a long-term stability, reaching up to several months.

(104) Instability of bubbles may be due to different mechanisms, depending on the size of the bubbles. So-called milli-bubbles (having a diameter in a range of 1 μm-1 mm) typically rise fairly fast in a fluid to then burst when reaching the surface. So-called microbubbles (having a diameter in a range of 10 μm-50 μm) tend to disappear under water and decrease in size. However, ultrafine bubbles (having a diameter below 500 nm and in particular below 200 nm) show long-term stability, reaching up to several months. Therefore, it is a realization that such bubble sizes are of particular interest in use in an image revealing medium 120, as regeneration or replacing of the medium would not need to happen frequently.

(105) It is believed that the long-term stability of bubbles can be attributed to the large surface to volume ratio for these small bubbles, making them highly reactive with their environment. Ultrafine bubbles in water have negative surface charge, as can be verified by Zeta-potential measurements. This negative surface charge enhances chemical interaction with oppositely charged molecules or small particles. In practical implementation, ultrafine bubbles are extremely long-lived when dispersed in electrolyte solutions, where the positive charges will arrange around the bubbles due to Coulomb interaction and will as such form a shield that avoids gas molecules to escape the bubble.

(106) Moreover, the surface charge will help to have a more uniform particle distribution within the medium due to the repelling Coulomb force between particles with the same charge. This effect will result on a fairly constant distribution of the bubbles in the fluid with a quite uniform average inter-particle distance that promotes uniformity of the scattering intensity within the image revealing medium 120.

(107) For the ultrafine bubbles of interest, the gas pressure inside is very large, in the range of about 10-30 atmospheres. This implies that, given the long-term stability of the bubbles, a surface tension of such bubbles is also very large. Due to their small sizes, ultrafine bubbles tend to randomly move around at high speed in solution, while continuously interacting with neighboring particles due to the Coulomb repulsion. This implies that there is little to no physical interaction between the bubbles, which probably contributes to the long-term stability of the bubbles as well. Therefore, such bubbles will further not have a tendency to rise to a surface of a medium or collapse, as is the case for micro- and milli-bubbles. Thus, the ultrafine bubbles may exhibit neutral buoyancy and a distribution of bubbles within the image revealing medium 120 may be maintained constant over long time.

(108) Ultrafine bubbles in water may have an anti-bacterial function when present in a sufficiently high concentration, meaning more than 2.10.sup.8/ml (2.10.sup.14/m.sup.3). An anti-bacterial function of the image revealing medium 120 may be advantageous as it may simplify handling of the image revealing medium 120.

(109) If the minimum concentration requirement for obtaining anti-bacterial functionality with the desired concentration in terms of optical scattering properties of FIG. 4, it may be seen that the minimum concentration may be provided at least for bubble sizes below 120 nm in diameter. Also, if a higher absorbance is allowed, bubble sizes of 250 nm in diameter may be used.

(110) Although it is described above that the image revealing medium 120 may be provided with bubbles or particles so that a distribution of the bubbles or particles in the image revealing medium 120 is maintained constant over a long period of time, it should be realized that it may still be of interest to provide active control of the image revealing medium 120.

(111) Thus, according to an embodiment, the controller 150 may be configured to control valves and/or pumps for actively controlling the image revealing medium 120 in the container 130. The controller 150 may dynamically control the image revealing medium 120, e.g. in dependence of the three-dimensional image to be displayed. Hence, the controller 150 may control adjustment of an optical attenuation constant by ensuring that characteristics of the image revealing medium 120 is changed when desired.

(112) Active control of the image revealing medium 120 may be provided in advance of display of a three-dimensional image requiring different optical attenuation constant by means of the controller 150 controlling the image revealing medium 120 to be replaced. Such control may be relatively slow and may not allow for changing optical attenuation constant within a sequence of images to be displayed in rapid sequence, such as in a video.

(113) According to another embodiment, the controller 150 may be configured to control a circulation of image revealing medium 120 for continuously pumping the image revealing medium 120 through the container 120. The apparatus 100 may then be provided with a flow control system for pumping the image revealing medium 120 through the container 130.

(114) The flow control system may provide a closed circuit wherein image revealing medium is continuously transported into the container 130 (e.g. through the inlet channel 104) and out of the container (e.g. through the outlet channel 106). The outlet channel 106 may then be connected to the inlet channel 104 for providing circulation of the image revealing medium 120.

(115) In a path of the flow control system outside the container 130, the flow control system may comprise a medium control unit 180 (illustrated in relation to embodiments of FIGS. 5-6 discussed in further detail below). The medium control unit 180 may be connected to the bubble generating device 170 for enabling bubbles to be introduced into the image revealing medium 120. Alternatively, the medium control unit 180 may be connected to a particle supply for enabling particles to be introduced into the image revealing medium 120. Also, the medium control unit 180 may comprise a filter for filtering bubbles or particles by size or density or a combination of size and density. The medium control unit 180 may then dynamically control a property of the image revealing medium 120 by controlling mixing of inlet of bubbles or particles into the image revealing medium 120 and/or controlling filtering of bubbles or particles in the image revealing medium 120.

(116) Filtering of bubbles or particles in the medium control unit 180 may be achieved by an external force. For instance, an external field, such as an acoustic (pressure) wave or an electromagnetic wave, may be controlled by the medium control unit 180 to exert a force on the bubbles or particles in the image revealing medium 120 so as to allow filtering of bubbles or particles. Filtering may also or alternatively be achieved using at least one semi-permeable membrane or porous membrane, optionally in combination with an applied pressure acting on the image revealing medium 120.

(117) Active control of the image revealing medium 120 may be used for advanced control of display of the three-dimensional image. The active control may be used for controlling brightness and contrast of the three-dimensional image to be observed.

(118) Also, the controller 150 may receive measurement results from the calibration sensor 160, which may be used for controlling the medium control unit 180. Such control based on the measurement results from the calibration sensor 160 may be used for ensuring that the optical attenuation constant of the image revealing medium 120 is maintained constant so as to ensure constant optical properties of the image revealing medium 120. Alternatively, the measurement results from the calibration sensor 160 may be used as input for a control of the image revealing medium 120, when the properties of the image revealing medium 120 are to be dynamically changed.

(119) In some embodiments, it would be useful to have local control over the scattering efficiency depending on a position within a volume of the image revealing medium 120. Since a constant bubble or particle concentration within the image revealing medium 120 causes light to decay exponentially as it propagates through the image revealing medium 120, larger intensities of light would be required for a voxel far away from the entrance of light into the image revealing medium 120 to scatter as intensely as for a voxel close to the entrance of light into the image revealing medium 120.

(120) Local control of the scattering efficiency in voxels of the image revealing medium 120 may be realized in multiple ways.

(121) According to an embodiment, the image revealing medium 120 may be divided in segments, where the concentration of bubbles in the image revealing medium 120 increases with a distance away from the entrance of light into the image revealing medium 120. Such implementation has the advantage that the intensity of the light field can be uniformly (or at least more evenly) distributed depending on the position of the voxels in the three-dimensional image. This may allow the algorithm for calculating the three-dimensional light field to be formed to take the decay of light through the image revealing medium 120 into account in a relatively simple manner.

(122) The segmentation of the image revealing medium 120 may be achieved by providing several compartments 130a-f within the container 130. Thus, each compartment 130a-f may be provided with a separate inlet/outlet and image revealing medium 120 having appropriate concentration of bubbles/particles may be arranged in each compartment 130a-f. The compartments 130a-f may be divided by transparent walls having a similar refractive index as the fluid so as to not influence propagation of light through the container 130.

(123) In a similar manner, instead of having a varying concentration of bubbles or particles in different segments of the volume of the image revealing medium 120, the image revealing medium 120 may be provided with particles or bubbles of different sizes in different segments. As discussed above, larger sizes of particles or bubbles may cause a larger decay of the propagation of light, such that the image revealing medium 120 may have an increasing size of bubbles or particles increases with an increased distance away from the entrance of light into the image revealing medium 120. Also, as mentioned above, the scattering efficiency scales with the 4.sup.th power of the frequency, so if multiple wavelengths are to be combined in the display of the three-dimensional image, a composition of the image revealing medium may be adjusted to intensity of different wavelengths for improving image quality.

(124) According to another embodiment, the algorithm for calculating the three-dimensional light field may correct for the exponential decay in the image revealing medium 120, when the three-dimensional light field to be formed is calculated. The algorithm may use exponential increase of the intensity for each voxel with increasing distance from the entrance of light into the image revealing medium 120. This may add to complexity of the algorithm for calculating the three-dimensional light field. However, there may not be any need of complex arrangements or control of the image revealing medium 120 in the container 130.

(125) As illustrated in FIG. 5, local control of the image revealing medium 120 may be provided by a medium control unit 180. The medium control unit 180 may be connected to the bubble generating device 170 (or alternatively to a particle supply).

(126) It should be realized that the embodiments illustrated in FIGS. 5-6 and discussed below may be combined, as understood by a person skilled in the art, with one or more of the features described above in relation to FIGS. 1-4 and are merely omitted here for brevity.

(127) Further, the container 130 is illustrated in FIGS. 5-6 with compartments 130a-f defined by opposing walls of partial envelopes of spheres such that a size of the compartment 130a-f increases with distance from position of entrance of light into the image revealing medium 120. It should be realized that such a shape of the container 130 may also be used with the embodiments described above in relation to FIGS. 1-4. It should also be realized that other shapes of the container 130 may be used in any of the embodiments, such as a container 130 having a cylindrical shape.

(128) The light field generating unit 110 (only generally indicated in FIGS. 5-6) may be configured to generate light that is projected into the container 130 through a bottom surface of a first compartment 130a so as to propagate through the first compartment 130a to reach a second compartment 130b and so forth through all the compartments 130a-f. The light field generating unit 110 may thus generate a three-dimensional light field in the compartments 130a-f, which may be used for revealing a holographic image by the image revealing medium in the compartments 130a-f.

(129) The medium control unit 180 may comprise a filter and mixing system for controlling concentration and/or size of bubbles or particles in the image revealing medium 120. The medium control unit 180 may thus control properties of the image revealing medium 120 output from the filter and mixing system. As illustrated in FIG. 5, a plurality of valves 182a-f may be used for controlling inlet of the image revealing medium 120 to the respective compartments 130a-f in the container 130. Also, as illustrated in FIG. 5, the compartments 130a-f may be provided with outlet channels, controlled by further valves, for enabling emptying of the respective compartments 130a-f. The image revealing medium 120 emptied from the compartments 130a-f may be transported to a waste.

(130) Optical properties of the image revealing medium 120 may be monitored in the medium control unit 180 to ensure that desired optical attenuation constant and scattering properties of the image revealing medium 120 for a target compartment 130a-f is achieved. Once target specifications are achieved, the appropriate valve 182a-f may be opened for providing the image revealing medium 120 into the selected compartment 130a-f.

(131) Each compartment 130a-f may also be provided with a separate calibration sensor 160 for monitoring the optical attenuation constant of the image revealing medium 120. The measurement result from the calibration sensor 160 may thus be provided to the medium control unit 180 during filling of the respective compartment 130a-f in order to ensure that the image revealing medium 120 in each compartment 130a-f meets target specification. The measurement result from the calibration sensors 160 in each compartment 130a-f may also or alternatively be used as input for the controller 150 for controlling three-dimensional light field to be formed and/or intensity of light to be received by the image revealing medium 120.

(132) As further illustrated in FIG. 6, the local control of the image revealing medium 120 in a plurality of compartments 130a-f may be combined with an active control of the image revealing medium 120.

(133) Thus, each compartment 130a-f may be associated with a flow control system for circulating the image revealing medium 120 through the respective compartments 130a-f. Further, the medium control unit 180 may then dynamically control a property of the image revealing medium 120 in each of the compartments 130a-f, such that properties may be differently changed in different compartments 130a-f.

(134) The medium control unit 180 may further receive measurement results from calibration sensors 160 in each of the compartments 130a-f, so as to enable control of the image revealing medium 120 in each compartment 130a-f to be based on measured properties of the image revealing medium 120 in the respective compartment 130a-f.

(135) This implies that accurate control of the image revealing medium 120 in the container 130 may be provided, with dynamic control of characteristics in different parts of the container 130. Thus, active and local control of the image revealing medium 120 may ensure that the apparatus 100 is able to continuously display three-dimensional images of high quality and may also enable to quickly adjust image revealing characteristics of the apparatus 100 to fit different three-dimensional images to be displayed.

(136) In the above the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.

(137) In the above description, the image revealing medium 120 has mainly been described in terms of comprising water and air bubbles or polystyrene particles suspended in water.

(138) It should be realized that the image revealing medium 120 may be formed based on other fluids and other gases in the bubbles or other particle materials. In particular, if similar refraction index contrasts are used, the image revealing medium 120 may be expected to provide similar characteristics.

(139) In particular, it should be realized that the fluid may be any aqueous liquid, such as water which may be provided with surfactants or electrolytic solutions, which may be helpful in providing long-term stability of bubbles in the fluid.

(140) Further, the fluid may be saturated with the gas used in generation of the bubbles in the image revealing medium 120. This may further improve long-term stability of the bubbles in the fluid.

(141) Further, the bubbles may be filled with any type of gas being relatively transparent. For example, the bubbles may be filled with a gas comprising oxygen, nitrogen or carbon dioxide or combinations thereof, as such gases may be easily available and may only involve non-hazardous substances.

(142) It should also be realized that particles may be formed in any other material having appropriate refractive index and which may be formed into small beads of the desired size. Thus, another plastic material may be used or even another material, such as silica. Silica particles would have a smaller refractive index (approximately 1.45) than the polystyrene particles discussed above, which implies that a higher concentration of particles would need to be used in the image revealing medium to obtain the same scattering properties.