Method for determining recording exposure for holographic recording medium and method for recording thereon

An optimal recording exposure is determined by varying in the first to nth stages the recording exposure with a write laser beam to record a bright pattern image and a dark pattern image in each stage as the first to nth bright pattern images and the first to nth dark pattern images, respectively; irradiating the respective pattern images with a read beam to detect the intensity of a diffracted beam from the central portion of each image of the bright and dark patterns; calculating Sa1/Sb1=SNR1, . . . , and San/Sbn=SNRn, where Sa1 to San are the intensity of a diffracted beam from the first to nth bright pattern images and Sb1 to Sbn, are the intensity of s diffracted beam from the first to nth dark pattern images; and determining a recording exposure which is given at the SNRmax being the maximum value of the resulting SNR1 to SNRn.

TECHNICAL FIELD

The present invention relates to a method for determining a recording exposure to record a plurality of dot-shaped elemental holograms when forming the elemental holograms on a surface of a holographic recording medium to create a hard copy of a three-dimensional image. The invention also relates to a method for recording a stereoscopic image with the resulting recording exposure.

BACKGROUND ART

A holographic stereoscopic hard copy which can be used as a hard copy of a three-dimensional object, and a method and an apparatus for creating the same are disclosed in Patent Literature 1. The holographic stereoscopic hard copy is constructed by forming a plurality of dot-shaped elemental holograms on a surface of a medium.

In the aforementioned recording method, an original image pattern associated with each point on a holographic recording medium is displayed from three-dimensional image data on display means, and then a dot-shaped elemental hologram is formed on the aforementioned holographic recording medium corresponding to the displayed original image pattern.

As can be seen from this example, to form a three-dimensional image using an elemental hologram, it is necessary to find an optimal recording condition for recording the elemental hologram.

More specifically, it was required to determine an optimal recording condition for a certain holographic recording medium by varying the intensity of an object beam and a reference beam, the intensity ratio between the object beam and the reference beam, and the exposure time.

In contrast to this, for example, a method and a system for evaluating a holographic optical element is disclosed in Patent Literature 2, but cannot be used to quantitatively determine recording conditions. Accordingly, sample images with elemental holograms recorded under a variety of conditions have to be visually observed under white light in order to determine the optimal recording condition. This raises the problem that an enormous number of samples for determining the optimal recording condition and an enormous amount of time for visually observing the samples are required.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

The present invention has an object of providing a method for determining a recording exposure on a holographic recording medium and a recording method using the same, the method enabling the optimal recording condition for recording a stereoscopic image with elemental holograms to be qualitatively determined in a short time.

In summary, the above-described objectives are achieved by the following embodiments of the present invention.

(1) A method for determining a recording exposure to record a plurality of elemental holograms when forming the elemental holograms on a holographic recording medium to create a three-dimensional image, the method comprising: a bright pattern image recording step including a first bright pattern image recording substep of continuously recording a plurality of elemental holograms with a first stage recording exposure to record a first bright pattern image which is brighter at a central portion than at a peripheral portion, and second to nth bright pattern image recording substeps of recording second to nth bright pattern images having the same pattern as that of the first bright pattern image with a recording exposure varied in second to nth stages; a dark pattern image recording step including a first dark pattern image recording substep of continuously recording a plurality of elemental holograms with the same recording exposure as that for the first stage in the first bright pattern image recording substep to record a first dark pattern image, the first dark pattern image having a central portion darker than a peripheral portion and having reversed bright and dark portions with respect to the first bright pattern image, and second to nth dark pattern image recording substeps of recording second to nth dark pattern images with the same recording exposure as that for the second to nth stages, the second to nth dark pattern images having the same pattern as that of the first dark pattern image; a step of irradiating the first to nth bright pattern images and the first to nth dark pattern images with a read beam to detect an intensity of a diffracted beam occurring from each image; an SNR calculating step of calculating Sa1/Sb1=SNR1, Sa2/Sb2=SNR2, . . . , and San/Sbn=SNRn, where Sa1to Sanare an intensity of a diffracted beam from the first to nth bright pattern images and Sb1to Sbnare an intensity of a diffracted beam from the first to nth dark pattern images; an SNRmaxdetermining step of determining an SNRmaxbeing the maximum of the resulting SNR1, SNR2, . . . , and SNRn; and a step of determining a recording exposure at the SNRmaxas an optimal recording exposure among the recording exposures in the first to nth stages.

(2) The method for determining a recording exposure for a holographic recording medium according to (1), wherein an image area in the central portion in the first to nth bright pattern images and the first to nth dark pattern images is 0.01 to 0.5 times an entire pattern area.

(3) The method for determining a recording exposure for a holographic recording medium according to (1), wherein a trial writing area is set on the holographic recording medium in addition to an image area for recording images so as to record the first to nth bright pattern images and the first to nth dark pattern images on the trial writing area.

(4) The method for determining a recording exposure for a holographic recording medium according to (1), wherein the holographic recording medium is a three-color image recording medium, the colors including red, green, and blue, and the bright pattern image recording step and the dark pattern image recording step include a substep of recording a red bright pattern image and a substep of recording a red dark pattern image using a red write laser beam; a substep of recording a green color bright pattern image and a substep of recording a green color dark pattern image using a green write laser beam; and a substep of recording a blue dark pattern image and a substep of recording a blue bright pattern image using a blue write laser beam, and a recording exposure is determined for each of the colors, i.e., red, green, and blue.

(5) The method for determining a recording exposure for a holographic recording medium according to (1), wherein a dark portion and a bright portion of the bright pattern image and the dark pattern image are formed from an entirely dark pixel and an entirely bright pixel of a spatial light modulator, respectively.

(6) A method for recording on a holographic recording medium wherein a write laser beam with an optimal recording exposure is used to record a stereoscopic image on a holographic recording medium, the optimal recording exposure being determined according to either one of (1) to (5).

Advantageous Effects of Invention

The present invention has an effect of enabling a condition for recording good images at a high SNR to be determined qualitatively in a short time.

DESCRIPTION OF EMBODIMENTS

The method for determining a recording exposure for a holographic recording medium according to an exemplary embodiment of the present invention employs a recording exposure determination optical system10, shown inFIG. 1, for determining the amount of exposure for recording on the holographic recording medium. In the system10, as shown inFIG. 2, a plurality of dot-shaped elemental holograms14are continuously recorded on a holographic recording medium12as follows. The amount of exposure for recording with a write laser beam is varied in first to nth stages to record a bright pattern image25and a dark pattern image27shown inFIG. 3as first to nth bright pattern images and first to nth dark pattern images in each stage, respectively. These images are irradiated with a read beam to detect the intensity of a diffracted beam from the central portion of each bright pattern image and each dark pattern image. Then, Sa1/Sb1=SNR1, . . . , and San/Sbn=SNRnare calculated, where Sa1to Sanare the intensity of a diffracted beam from the first to nth bright pattern images and Sb1to Sbnare the intensity of a diffracted beam from the first to nth dark pattern images. A recording exposure at SNRmaxor the maximum of the resulting SNR1to SNRnis employed as an optimal amount of exposure for recording.

As shown inFIG. 1, the recording exposure determination optical system10is installed in an apparatus for performing a method for determining a recording exposure for a holographic recording medium according to a first exemplary embodiment of the present invention. The system10is configured to include: a light source optical system20which includes a laser16to a beam splitter18; a reference beam optical system30for directing a transmitted beam from the beam splitter18as a reference beam to the holographic recording medium12; an object beam optical system40for directing a reflected beam from the beam splitter18as an object beam to the holographic recording medium12in the direction opposite to the reference beam; and a detection optical system50which is disposed so as to be opposed to the object beam optical system40with the holographic recording medium12therebetween.

Furthermore, the system10has a reading optical system60for supplying a read beam for reading a stereoscopic image formed on the holographic recording medium12.

The light source optical system20is configured to include a shutter21, a collimator lens22, a half-wave plate24, and a polarizing filter26in that order from the laser16side.

The reference beam optical system30is configured to include an ND filter32, a rotatable mirror34, and an aperture36.

As shown inFIG. 1, the rotatable mirror34is configured to be rockable between a position at which a reference beam having passed through the beam splitter18and the ND filter32is reflected towards the holographic recording medium12and a position at which the reference beam is blocked and the read beam from the reading optical system60is allowed to be incident upon the holographic recording medium12.

The object beam optical system40is configured to include an ND filter41, a mirror42, a beam expander43, a spatial light modulator (SLM)44, and an objective lens45in this order between the beam splitter18and the holographic recording medium12.

The detection optical system50is configured to include a detection-side objective lens52disposed to be opposed to the objective lens45with the holographic recording medium12therebetween and a light-receiving device54for receiving a diffracted beam which has been collimated through the detection-side objective lens52.

The reading optical system60is configured to include an LED62for emitting a white read beam and a collimator lens64.

Here, the holographic recording medium12is mounted on an XY stage13to be movable in the XY-plane, and the light-receiving device54of the detection optical system50is a CCD.

In this exemplary embodiment, using the SLM44of the recording exposure determination optical system10shown inFIG. 1, the elemental holograms14of the bright pattern image25and the dark pattern image27as shown inFIG. 3are continuously formed and thereby stored in each area of the holographic recording medium12.

After recording, the power of the laser16is shut down or the laser16is covered with a shield plate immediately after the emission of the laser beam. Then, the rotatable mirror34of the recording exposure determination optical system10is rotated so as to allow the read beam from the LED62to pass therethrough toward the holographic recording medium12. The LED light serving as the read beam is directed to irradiate therewith the recording area of the bright pattern image25and the recording area of the dark pattern image27recorded on the holographic recording medium12to produce diffracted beams.

Now, a description will be made to a control system70shown inFIG. 4of the recording exposure determination optical system10.

The control system70is configured to include a system controller72; an optical unit74which includes the shutter21, the light-receiving device54, the SLM44, the half-wave plate24, and the XY stage13; a timing controller76for controlling the shutter21, the light-receiving device54, the half-wave plate24, and the XY stage13; and a laser unit78for controlling the laser16.

The system controller72allows control software72A to read from an image file72B an image to be recorded and then output the image data of the resulting image from a signal processor72C to the SLM44.

Furthermore, the timing controller76is configured to control, on the basis of a command from the control software72A, the timing at which the light-receiving device54receives a diffracted beam, the amount of driving the half-wave plate24and the timing at which the same is driven, and the timing at which the shutter21is opened or closed. The timing controller76also controls the XY stage13to provide control to the point of forming (the point of recording) the elemental hologram14on the holographic recording medium12on the basis of an instruction from the control software72A.

Now, referring toFIG. 5, a description will be made to the step of recording the bright pattern image25and the dark pattern image27on the holographic recording medium12by the recording exposure determination optical system10.

Note that in this exemplary embodiment, for reference purposes, an entirely bright pattern image26and an entirely dark pattern image28shown inFIG. 6are also recorded on the holographic recording medium12.

In step101ofFIG. 5, the holographic recording medium12is loaded into the recording exposure determination optical system10. In the next step102, the process determines whether a light shielding film is adhered to the holographic recording medium12. If YES, then the light shielding film is removed in the next step103; if NO, the process skips step103to move to step104.

In step104, the process acquires information on the recording condition for recording on the holographic recording medium12having no light shielding film. In the next step105, the process determines whether the holographic recording medium12requires a precure. If YES, then the medium is subjected to a precure exposure in step106; if NO, then the process skips step106to move to step107.

In step107, the holographic recording medium12is driven by the XY stage13to move the trial writing area (not shown) on the holographic recording medium12to the intersection between the object beam and the reference beam.

In step109, in the recording exposure determination optical system10, the reference beam and the object beam are used to record the bright pattern image25and the dark pattern image27with the elemental hologram14. This step will be discussed in greater detail later.

At this time, the process provides control to one of or both the write laser power by the half-wave plate24and the recording time by the shutter21, so that each pattern image is recorded with the respective recording exposure in the first to nth stages. In the next step110, the rotatable mirror34shown inFIG. 1is rocked from the status ofFIG. 1in the clockwise direction to be located so that a white read beam from the LED62reaches the holographic recording medium12without being hindered by the rotatable mirror34. Under this condition, the read beam is used to irradiate each of the first to seventh bright pattern images, the first to seventh dark pattern images, the first to seventh entirely bright pattern images, and the first to seventh entirely dark pattern images, which have been recorded on the holographic recording medium12with the respective recording exposure in the first to seventh stages. The resulting diffracted beams are acquired from the light-receiving device54in step111. Examples of the results are as shown inFIGS. 7 and 8.

Furthermore, in the next step112, the data on the diffracted beam intensities Sai and Sbi which have been detected for each recording exposure is supplied from the light-receiving device54through the timing controller76to the system controller72, where SNR=Sai/Sbiis calculated for each recording exposure. As shown inFIGS. 7 and 9, it can be seen that the relation between the recording exposure and the SNR provides the highest SNR of 14 at a recording exposure of 80 mJ/cm2. In steps113, the amount of recording exposure, that is, the write laser power and the exposure time at an SNR of 14 is determined as the optimal amount of recording exposure.

In step114, the aforementioned amount of recording exposure, i.e., the recording conditions of the write laser power and the exposure time are read into the system controller72.

In steps115, the XY stage13is used to drive the holographic recording medium12and move the image area thereof to a position to be Irradiated with the write laser beam so as to start recording images in step116.

After the recording of images is ended in step117, the holographic recording medium12is postcured in step118; the image area is cut therefrom in step119; the resulting image area is covered with a black sheet in step120; and then in step121, the holographic recording medium12is ejected from the apparatus.

Now, the steps of trial writing and reading with white light in steps107to110ofFIG. 5will be described in detail below.

An emitted beam of light from the laser16is collimated through the collimator lens22, while the write laser power thereof is controlled through the half-wave plate24and the polarizing filter26, and thereafter allowed into the beam splitter18, where the beam is split into a reference beam to be transmitted and an object beam to be reflected.

The object beam is adjusted in intensity by the ND filter41in the object beam optical system40; reflected on the mirror42and expanded in beam diameter by the beam expander43; amplitude modulated through the SLM44into an object beam; and then condensed through the objective lens45onto the holographic recording medium12. The SLM44performs modulation so that the bright pattern image25, the dark pattern image27, the entirely bright pattern image26, and the entirely dark pattern image28, shown inFIGS. 3 and 6, are formed when the elemental holograms14are continuously formed.

The reference beam is adjusted in intensity through the ND filter32; then adjusted through the aperture36in the size and shape of the elemental hologram being recorded; reflected on the rotatable mirror34; and allowed to enter the focal point of the object beam on the holographic recording medium12, where the resulting reference beam is interfered with a signal beam condensed by the objective lens45to form an elemental hologram.

Each area of the bright pattern image25, the dark pattern image27, the entirely bright pattern image26, and the entirely dark pattern image28recorded on the holographic recording medium12is irradiated with the read beam (white light) from the LED62in the reading optical system60. The read beam is collimated by the collimator lens64and incident to generally overlap the reference beam, thereby causing the elemental hologram14to produce diffracted beams.

The diffracted beam is generally collimated by the detection-side objective lens52to be incident upon the light-receiving device54or a detector, where the intensity and the intensity distribution of the diffracted beams are detected.

In the aforementioned exemplary embodiment, as shown inFIGS. 4 and 7, the ratio of the area of the central portion of the bright pattern image25and the dark pattern image27to the entire image is 0.20, so that the optimal amount of recording exposure of 80 mJ/cm2corresponds to this area ratio of 0.20. Thus, a different area ratio results in a variation in the optimal exposure.

Table 1 shows the optimal recording exposures which have been obtained by varying the area ratio of the central portion of the bright pattern image and the dark pattern image from 0.005 to 1 in 18 stages and by repeating steps109to113shown inFIG. 5for each area ratio.

Furthermore, Table 1 also shows the results which are obtained by visually determining the image contrast when images are recorded at the resulting optimal recording exposure for each area ratio. From the determined results, an adequate image contrast was obtained within the range of area ratios from 0.01 to 0.5.

In the above exemplary embodiment, both the aforementioned bright and dark pattern images have the circular central portions25aand27a. Various exemplary embodiments of the invention also applicable to the bright and dark pattern images that is the pattern images with other shapes as long as the following conditions are met. That is, for the bright pattern image, the central portion should be brighter than the peripheral portion, whereas for the dark pattern image, the central portion should be brighter than the peripheral portion and contains the pattern center. Furthermore, the peripheral portion should be configured such that the four corners of the pattern are reversely bright or dark with respect to the central portion. For example, as shown inFIG. 10, the central portion may be quadrangular, rhombic, triangular, vertically elongated, horizontally elongated, or crisscross in shape.

The optical system shown inFIG. 1is of a transmitting type in which an object beam and a reference beam are incident upon the holographic recording medium in the opposite directions. However, various exemplary embodiments of the invention are also applicable to the optical system that is a recording exposure determination optical system80for a reflection type holographic recording medium as shown inFIG. 11. In this case, since the optical system80is different from the optical system ofFIG. 1only in the position of the reading optical system and the rotatable mirror, like components to those ofFIG. 1are denoted with like symbols without additional explanations.

Furthermore, the SLM44may be a transmission-type liquid crystal panel or a reflective device such as a digital micro mirror device (DMD) or LCOS. The light-receiving device54may be required only to detect the spatial optical intensity distribution such as of CCDs and CMOSs or alternatively an array of multiple simple photodetectors (PD).

Furthermore, the reading LED62is preferably of a white light type as in the exemplary embodiment. The white light type may be capable of reading either color images or monochrome images which are recorded on the holographic recording medium12.

Furthermore, in the aforementioned exemplary embodiment, the trial writing on the holographic recording medium12is to record monochrome images. However, various exemplary embodiments of the present invention are applicable to the trial writing such that color images may also be recorded with three color laser beams, i.e., red, green, and blue. In this case, the flowchart will be such as shown inFIG. 12.

To record color images, in steps109R,109G, and109B ofFIG. 12, the holographic recording medium12is moved by the XY stage13for each color to record the bright pattern image25, the dark pattern image27, the entirely bright pattern image26, and the entirely dark pattern image28, which are shown inFIGS. 3 and 6, each with a red write laser beam, a green write laser beam, and a blue write laser beam. For each color, i.e., red, green, and blue, performed are the step110of reading with white light, the step111of acquiring the output from the detector, the step112of calculating SNRs, the step113of determining an optimal recording exposure, and the step114of reading the recording conditions. Note that the amount of recording exposure has to be determined by taking into accounts the color balance in terms of the intensity ratio between red, green, and blue. To keep the color balance, color information on the medium is preferably measured using a gonio spectral colorimetry system or the like. Measurement results may be recorded as data such as an XYZ chromaticity diagram, an L*a*b* chromaticity diagram, or an L*u*v* chromaticity diagram on part of the holographic recording medium12or a memory provided on the cartridge thereof. When images are recorded, this makes it possible to read the recorded data to calculate the optimal intensity at each recording wavelength, thereby determining the optimal recording conditions.

Furthermore, in the aforementioned exemplary embodiment, the optimal amount of recording exposure is determined. This may be done by fixing the recording power of the write laser beam and varying the recording time, or conversely, by fixing the recording time and varying the recording power of the write laser beam, or may also be done by varying both.