HK1080952B - Method for using a deep image hologram as a security device and a deep image hologram - Google Patents

Description

Method for using a deep image hologram as a security device and deep image hologram

Technical Field

The present invention relates to methods of using Holographic Optical Elements (HOE) containing deep image holograms. The element is particularly useful as a security device.

Background

Holography is a form of optical information storage. The general principle is described in many documents, such as e.n. leith and j.uptnieks at SCIENTIFIC AMERICAN, 212, No.6, 24-35, "laser-generated holograms" (month 6 1995). Briefly, an object to be photographed or displayed is illuminated with collimated light from, for example, a laser, and a photosensitive recording medium (e.g., a photographic plate) is positioned to receive the light reflected from the object. Each point on the object reflects light onto the entire recording medium, and each point on the medium receives light from the entire object. This reflected beam is called the object beam. At the same time, a portion of the collimated light is transmitted by the mirror through the object directly onto the medium. This beam is called the reference beam. Recorded on the recording medium is an interference image produced by the interaction of the reference beam and the object beam impinging on the medium. When the treated recording medium is subsequently illuminated and viewed in a suitable manner, light from the illumination source is diffracted by the hologram to recreate the wavefront originally arriving at the medium from the object, so that the hologram resembles a window through which a virtual image of the object can be viewed in substantially three dimensions (including parallax).

The hologram formed by causing the reference beam and the object beam to enter the recording medium from the same side is called a transmission hologram. The interaction of the object and reference beams in the recording medium forms material interference fringes with different refractive indices that are orthogonal or nearly orthogonal to the plane of the recording medium. When the hologram is viewed in transmitted light for playback, these fringes decompose the light, producing a virtual image that is viewed. Such transmission holograms may be produced by methods well known in the art, such as those described in U.S. Pat. nos. 3,506,327; 3,838,903 and 3,894,787, both of which are incorporated herein by reference.

Holograms formed by bringing the reference beam and the object beam into the recording medium from opposite sides (so that they move in substantially opposite directions) are called reflection holograms. The interaction of the object beam and the reference beam in the recording medium forms material interference fringes with different refractive indices, which are substantially parallel to the plane of the recording medium. When the hologram is played back, these interference fringes act as a mirror image, refracting the incident light back to the viewer. Thus, the hologram is viewed in reflection rather than transmission. Since the wavelength sensitivity of such holograms is very high, it can be reproduced with white light. The use of off-axis methods for generating reflection holograms is disclosed in U.S. Pat. No. 3,532,406, which is incorporated herein by reference.

More and more of the holograms described above are used as enhanced security devices (security devices) in connection with commercial products such as digital optical disks, compact disks, batteries for electronic products, and other products that are susceptible to counterfeiting. It is known to use holograms to authenticate such products. In most prior art documents, such holograms are surface relief holograms formed by stamping. The method can be incorporated into the production of products. It is well documented that holograms (volume phase holograms) are formed and then applied to products by means of labels. Although the use of holograms is advantageous for security devices, this approach has a significant disadvantage, as holograms can be counterfeited and applied to counterfeit products. Therefore, such hologram postmarks or markers have limited value as security devices.

Accordingly, there is a strong need for a security device that provides a higher level of security than that described above. The present invention provides a solution to this important need.

Disclosure of Invention

The present invention relates to a method of viewing a reconstructed image of a deep image hologram within a holographic optical element having a surface, said method comprising providing a deep image hologram at a substantial distance from the surface of the holographic optical element, wherein the deep image hologram is not recognisable to the human eye under a diffuse light source; the image is illuminated with a light source having a center wavelength (center wavelength) suitable for the depth of the image, a spectral bandwidth, and an illumination angle, wherein the image can be resolved.

The invention also relates to a method of determining the authenticity of an article containing a holographic optical element comprising a deep image hologram having a surface, said method comprising the steps of: (a) providing a holographic optical element on an article to be authenticated, said holographic optical element comprising a deep image hologram having a surface, wherein the deep image hologram is at a substantial distance from the surface of the holographic optical element, wherein the deep image hologram is not recognizable by the human eye under a diffuse light source; (b) illuminating the image with a collimated or at least partially collimated light source having a center wavelength in the visible region of the electromagnetic spectrum and a spectral bandwidth, wherein the spectral bandwidth of the collimated or at least partially collimated light source at least partially overlaps the spectral bandwidth of the deep image hologram; and (c) determining that the article carrying the holographic optical element is authentic only if the deep image hologram is observable when placed at least one angle θ with respect to the normal to the surface of the holographic optical element using a suitably collimated light source of step (b).

The invention also relates to a deep image hologram contained within a holographic optical element having a surface, wherein the image comprises a hologram at a distance from the surface of the holographic optical element, wherein the deep image hologram is not recognizable by the human eye under a diffuse light source.

Brief Description of Drawings

FIG. 1 depicts an example of generating a back-beam (back-beam) hologram.

Fig. 2A and 2B illustrate cross-sectional enlarged views of the photographic plate, depicting an example of the arrangement of interference fringes in the emulsion of a front-beam (front-beam) hologram and a back-beam hologram.

FIG. 3 depicts the reconstruction of an image from a back beam hologram.

FIG. 4 depicts an arrangement for constructing a front beam transmission or surface relief hologram.

Fig. 5 depicts an arrangement structure for reconstructing a true undistorted image of an object from a transmission hologram.

FIG. 6 depicts a physical model.

Fig. 7 and 8 depict the imaging steps.

Fig. 9 depicts a light source spectrum.

FIG. 10 is a graph depicting spectral curves for a reflection hologram.

FIG. 11 is a graph depicting the light source curve overlaid with the hologram curve.

Fig. 12 is a diagram describing a physical model.

Fig. 13 is a diagram describing an irradiation angle.

Detailed description of the preferred embodiments

One embodiment of the present invention utilizes holograms produced by the volume reflection method used in the holographic industry to produce holograms and reconstruct three-dimensional images from such holograms, which method comprises forming a pattern of interference fringes on a photographic plate in which an object-bearing beam (information) and a reference beam impinge on the back of the plate, then illuminating the hologram with an at least partially collimated light source to reconstruct the image, and viewing the reconstructed image. A partially collimated light source is a light source that produces at least some light in the form of parallel rays of radiation. The reproduced image can be used as a security device for confirming authenticity. The light source may be the light of a laser or other monochromatic collimated light source. The backward beam hologram acts as a selective reflective filter, reproducing an image in a monochromatic display in a narrow band of wavelengths. The particular band visible in the reconstruction depends largely on the geometry of the construct. The reproduced color tends to shift to shorter wavelengths due to distortion or shrinkage of the emulsion that changes the spacing of the interference fringe pattern. However, by adjusting process variables during development, the amount of spectral shift can be controlled. Additionally, multiple images may be stored within the hologram as well as images using radiation having multiple wavelengths. By reflecting in white light and observing, a multicolor image can be reconstructed from the back beam hologram, each color being selectively reflected from the hologram and combined in the image, producing a colored image in true three dimensions.

In addition, referring to FIG. 1, the light beam 11 from the collimated coherent light source 13 is split into a reference beam 17 and an incident beam 19 by suitable means, such as a beam splitter 15. The incident beam 19 impinges on an object 21. The reflected light from the object 21 or the object-carrying beam 23 passes to the photographic plate 25. The reference light beam 17 passes by suitable means, such as a mirror 27, to the photographic plate 25, but impinges on the side of the plate 25 opposite to that illuminated by the object-carrying beam 23. This produces an interference pattern that is recorded in the photographic plate 25. The path lengths of the reference beam 17 and the object-carrying beams (19 and 23) from the beam splitter 15 are preferably approximately equal, but this is not necessary if the light is suitably coherent. Two or more sets of waves having an equiphase relationship are present in the electromagnetic radiation generated by the coherent light source. Generally, a coherent light source is only coherent over a certain distance.

Of course, considerable variations can be made to the arrangement that allows the two beams (the object-carrying beam and the reference beam) to pass to opposite sides of the recording device. Even two separate light sources may be used, as long as their "phases are locked" (i.e., they interfere with each other); of course, the optics used to direct each beam may also be conveniently selected.

Fig. 2a and 2B are comparisons of examples of interference fringe patterns (20A, 20B) produced in the emulsions of two photographic plates. Fig. 2a is an example of a forward beam hologram and fig. 2b is an example of a backward beam hologram. These holograms were generated and then sectioned to determine the difference in interference patterns of the two methods. It is known that interference patterns are generated by maxima and minima of the waveform when two beams meet. In fig. 2a, an emulsion 31 is located on a transparent substrate 33 (e.g. glass). After exposure with the forward beam technique, a cross section of the developed plate was taken and examined under a microscope. Dark silver particles or interference fringes 35 in the emulsion 31 represent the interference maxima between the object-carrying beam and the reference beam, i.e. the antinodes of the standing wave. These interference fringes 35 are tilted at 30-40 degrees from the normal to the surface of the photographic plate, which depends largely on the angle between the two beams and the angle at which they strike the plane of the photographic plate. The angle is substantially parallel to a line bisecting the angle between the object-carrying beam and the reference beam. The maximum angle allowed by the forward beam technique is limited by the refractive index of the emulsion 31 and thus by the critical angle of total internal reflection, which for silver halide emulsions is about 40 degrees. In fig. 2B, the photographic plate is used to record a back beam hologram, and the interference fringes 36 are a few degrees from being parallel to the outer surface of the photographic plate and are substantially parallel to a line bisecting the angle formed between the object-carrying beam 23 and the reference beam 17. Both holograms of fig. 2a and 2b may be referred to as special diffraction gratings, but it is clear that their diffraction characteristics are very different. Thus, the backward beam hologram can be reconstructed in reflected incoherent light, a property not found in the forward beam hologram.

FIG. 3 shows the reconstruction of an image from a back beam hologram 46. The hologram 46 is illuminated by reflection of incoherent light 41 (daylight or incandescent light), and while the observer 43 sees the reflected image, he sees a three-dimensional image 45 of the object 41 through the "hologram window" as if the object were behind the hologram 46. If the emulsion does not shrink during plate processing, the image has the color of the light used to form the hologram. This process is further described in U.S. Pat. No. 3,532,406, which is incorporated herein by reference.

Another solution is the surface relief method (surface relief method) conventionally used in the holography industry. The general protocol is described below. Figure 4 depicts an arrangement that forms a conventional surface relief hologram. A beam splitter 14 splits collimated coherent light 12 from laser 10 into two components 12A and 12B. Component 12A passes through a telescopic arrangement having lenses 16 and 18 to increase the cross-section of beam 12A. It illuminates the object 20. Object 20 reflects and scatters light from illuminating beam 12A. Wavefront 22 is a portion of the light reflected and scattered by object 20. The formation of the wavefront 22 is functionally related to the object 20. It is incident on a photosensitive material 24, such as a photographic plate. The wavefront 22 contains optical information about the object 20. With all the information needed to see a three-dimensional perspective view of object 20.

The component 12B of the collimated coherent light 12 passes through a telescope having lenses 26 and 28 to increase its cross-section and provide a preselected shape to the waveform. A beam 12B having a preselected waveform is used as a reference. The waveform of beam 12B should be reproducible. For this purpose, it is made to converge at point P. The reference beam 12B is also incident on the photosensitive material surface 24. The light of the reference beam 12B and the wavefront 22 interfere on the photosensitive surface 24 to form a complex diffraction pattern uniquely associated with the object 22. The pattern is a hologram of the object 20. Holograms or diffraction patterns are often complex and thus difficult to view directly in the manner of viewing conventional photographs. Special illumination techniques are required to view the image of the object.

Figure 5 shows a method suitable for illuminating the previously generated hologram 54 and forming a true undistorted image 58. The laser 50 generates coherent light 52. The lens 53 directs the light 52 through a focal point P where a point source of light is formed and then gives it a predetermined waveform 55. The direction and waveform 55 of illumination beam 52 is chosen so that it represents the time reversal of reference beam 12B used to form the hologram of FIG. 4. A time-reversed beam is a beam that has a direction and waveform associated with the holographic image, and therefore appears to emanate from the point where the original reference beam having the same waveform converges. Specifically, if the reference beam 12B converges to a point "P" remote from the photosensitive surface 24, the temporal reversal of the reference beam 12B is a beam that, in relation to the hologram 54, appears to emanate from P having the same waveform as the reference beam 12B. The hologram 54 is obtained using the method described in figure 4. When time-reversed illumination light 55 is incident on the hologram 54, the holographic image of the hologram 54 diffracts a portion of the light 55 into a wavefront 56. During diffraction, the wavefront 56 forms the same waveform as the wavefront 22 from the holographic image, but the wavefront 56 propagates in the opposite direction. The wavefront 56 forms a true image 58. The real image 58 is pseudoscopic, i.e. looking up and down in reverse. It can be recorded by placing photographic material in the space where the image is located. The image 58 has all of the optical information in the wavefront 22 that the object 20 originally imparted and incident upon the photosensitive surface 24.

The hologram image elements described in the embodiments of the present invention are in positions that are generally to be avoided because those skilled in the art believe that such positions produce defective, blurred, and unusable images. These image elements produce a deep image hologram. In general, much effort has been put into making conventional commercial graphic works or holographic security marks by bringing all elements of the image very close to the surface of the holographic recording material. This ensures maximum clarity under non-ideal lighting conditions. Typically, any portion of the image at a large distance from the film surface (typically more than 0.318 cm, including all values found therein) will be blurred, depending on the particular diffuse light conditions. Scattered light is incident light that is redirected or scattered over a range of angles from the surface on which it is incident. The ideal viewing condition for commercial holographic reconstruction images is diffuse illumination, such as fluorescent or incandescent light.

The present invention utilizes the above-described conventional method, except for the position of the picture element. The image element is placed at a large distance, typically more than 0.318 cm, and all values therein are far from the film surface. This location selection effectively blurs or hides the image, which takes advantage of the properties of volume reflection holograms to blur deep images under diffuse lighting conditions. Blurred deep images are considered "defects" by those skilled in the art of hologram technology. Image reconstruction of deep image holograms uses an illumination source similar to that used in making holograms. The light source (132) is positioned at an angle theta in the range of +55 degrees to-55 degrees (including all angles and partial angles) with respect to the normal to the surface of the holographic optical element (130) (see fig. 13).

Suitable light sources for viewing the deep image holograms of the present invention are those that provide collimated or at least partially collimated light in the visible region of the electromagnetic spectrum. Typical examples of suitable light sources include, but are not limited to, lasers, including laser pointers and various portable LED lights that provide collimated or near-collimated light output. In one aspect, a collimated light source may be employed, including, for example, a laser. In another approach, quasi-monochromatic or near-collimated point sources of light may be used, such as portable LED lights, or diffused or expanded laser pointers (laser pointers). These light sources are relatively inexpensive and quite common. These light sources are used to allow the observer to recognize the reproduced image. Both horizontal and vertical movements of the laser pointer will cause a corresponding movement of the hidden image.

"defects" in the scattered light appear as source size defocusing and color blurring. Holograms reproduce the original model with maximum fidelity when the image is reproduced with exactly the same wavefront and wavelength as used at the time of production. Generally, spherical waves from a point produce maximum fidelity.

If the light source is enlarged or expanded in one direction or the other, the hologram actually sees the light source as an array of many spots at slightly different angles. Each of these points diffracts and is directed out of the hologram at a respective different angle and each of these source points forms an image. The images are typically superimposed, spaced by distance from the surface and by the sine of the scatter angle. A blurred rather than sharp image is produced from such a light source. The closer an image is to a hologram, the smaller the distance it is to the adjacent image. If the image is focused completely on the surface, the angle between the extended ends of the light sources will not cause the image to separate and the image will appear sharp. Color blurring produces a similar effect when the holographic label is illuminated with white or broadband light. White light consists of different colors. Each color is scattered at a slightly different angle when emerging from the hologram. Each color forms a separate image. Also, the further the image is from the film surface, the greater the separation between the focused images. Each image also has a slightly different color. They overlap each other and appear blurred in color. Volume holograms have some color filtering characteristics, but it is also difficult to emulate a laser. These two phenomena combine to further increase the blur.

In one example (see fig. 6), a physical model (603, 604) of all elements to be the final holographic label is made. Etched eagle coin-like model (603) and etched eagle coinThe elements of the oval mark (604) are located at desired positions relative to the surface of the final hologram. For aesthetic reasons, the support rods (601, 605) and the substrate (602) are intended to be invisible by minimizing the amount of laser light impinging thereon, by being painted black and/or made of glass. The substrate is typically glass. In addition, in order to be flat from an aesthetic point of viewA holographic marker is placed on the model with an element (607) not illustrated on the support rod (609).

Fig. 7 depicts a mold mounted on a vibration isolated optical table (701) and illuminated with a light source (706) that produces a diffused laser beam (702) from an angle of approximately 45 degrees from the normal to the engraved mold (603, 604). This is the H1 exposure step. In at least two color versions, selected mold elements are covered or removed during exposure. This prevents selected elements from being exposed to laser light during exposure to H1. This step can be repeated if additional colors are desired.

The diffused laser beam was then turned off and a 12.7 x 12.7cm glass plate coated with near clear dichromated gelatin was placed approximately 0.635cm above the center of the mold. Dichromated gelatin (DCG) (703) is an example of a common holographic recording material. The laser was turned on for a few seconds. Approximately 90% of the laser light passes through the coated plate, impinges on the object model, and is reflected back to the coated plate. The incoming light and the reflected object light meet to form an interference pattern and are recorded in the DCG coating. After chemical treatment, a change in refractive index corresponding to the interference pattern is produced within the coating. They are called interference fringes. The distance between these interference fringes is very small, which is smaller than the wavelength of the laser used. At this point, the coated plate still appeared very transparent. When these interference fringes encounter light of any color, the diffractive effect of the microfeatures filters the color and redirects the light to form a blurred image of the original object. However, when re-illuminated with the same laser as it was exposed and placed back in the same recording position, the image appears very sharp at all distances from the plate. At this point in the method, the detailed image is almost completely unrecognizable in scattered light because all image elements are more than 0.635cm from the hologram surface.

In embodiments having at least two colors, the first exposed elements will be covered and the first covered elements will be exposed. A second hologram is made using the same method as the first exposure with the elements covered or removed during the first exposure. This step may be repeated for each additional color until the desired number of colors is obtained.

In the next step (see fig. 8), the first exposed and processed DCG hologram (801) (commonly referred to as H1) is flipped over and placed back in its holder. The mold is removed. Turning on the laser, we see an undistorted three-dimensional image (802, 803, 806) floating above the DCG plate. This image is commonly referred to as a virtual image. The laser was turned off and the unexposed second DCG recording (804) plate was fixed approximately 0.635cm above the first hologram (H1). Now, the virtual image of the engraved eagle element is just on the surface of the second recording plate (804). The laser is then turned on again for a few seconds, and the plate is exposed and processed. This is a master hologram (H2) which can be put into a replication machine for copying.

In embodiments having at least two colors, the first H2 is treated so that it is only visible at a selected wavelength of the laser (e.g., 488 nm). The second H2 is treated so that it is only visible at a different laser color (e.g., 514 nm). The two H2 are clamped together and aligned so that the two elements are in the correct position. The sandwich is placed in a replicator and a photopolymer film is placed on the surface. In this case at least two laser wavelengths will be used for exposure and making the copy. The two wavelengths may be, for example, 488nm and 514 nm. These colors may cause each element to be rendered and recorded in its appropriate predetermined color. The photopolymer is then subjected to standard processing. By varying the method, the finished hologram can be adjusted or shifted so that the deeply hidden elements can only be recognized when illuminated with a suitable LED or the like.

When a photopolymer hologram is processed and turned over, it has a clear eagle image on the surface under the diffuse room light. However, the background image is blurred and unrecognizable until the image is illuminated with a light source (e.g., an LED or laser pointer) that is at least partially collimated and that is close to the point light source feature that at least partially overlaps the spectral bandwidth of the deep image hologram. Now, not only engravingThe hawk emblem is clear, moreoverOval background depth

The image can be recognized without being very blurred.

Background information is hidden by the natural property of holograms that blur under diffuse room lights, which are not point sources and are white (broadband). The desired background deep image will only be sharp if the light source has a center wavelength, spectral bandwidth, and an angle appropriate for the depth and detail of the image such that the deep image is recognized.

Examples

It should be noted that all spectral measurements found herein are radiometric, i.e., the electromagnetic energy flow is measured in a quantitative manner.

Example 1

To determine the depth of the deep image hologram that can be recognized under different light sources, a 42-piece 0.9525cm wide by 1.27cm high model was constructed. Etched to have a letter size of 0.0762-0.08128cm wide and 0.1778-0.22606cm highThe logo is adhesively secured to the stepped frame or bracket. The step depth increment was 0.15875 cm.

The mold was illuminated and exposed in the same manner as described above. First, H1DCG was prepared. The H1 hologram is next time back-illuminated to reproduce the true image of the identified step. The second hologram H2 was made in a similar manner to the original eagle hologram described above. The image plane is focused on the 8 th marking step down, which is about 1.27cm down. The resulting final DCG master hologram had images protruding 1.27cm forward of the surface, 8 images being surface images and 34 images deep in the hologram. The method was performed as described above for the eagle hologram. The data is compiled in table 1.

Example 2

A physical model is constructed, all elements of which are included in the final holographic label. The elements are placed relative to the surface of the final hologram. FIG. 6 depicts a physical model constructed. A model (603) of about 1.905cm in diameter and about 0.079375cm thick was designed to simulate the eagle emblem on a U.S. coin. It is made of polymer clay for engraving, painted in a certain grey scale and then associated with a metal ring support of about 0.15875cm for support, vibration isolation and thermal insulation. Photo-acid etching in magnesium of about 1.27cm by 0.47625cm by about 0.079375cmThe oval shaped logo (60) was painted, polished, and then attached to the same ring between the tips of the eagle emblem model wings. The structure was then attached to a black 5.3975cm x 0.318 cm screw. The assembly was then attached near the center of a 10.16cm by 12.7cm by 0.318 cm clear glass plate. Second smaller 0.9525cm by 0.635cm by 0.15875cm photo-acid etched in magnesium, painted and polishedThe marker model was attached to a 1.5875cm by 0.318 cm black screw. This smaller assembly was also attached to the same glass plate to which the eagle emblem assembly was attached, but moved each about 1.27cm to the right and down from the center of the base of the screw that secured the eagle emblem. This results in an eagle surface to small mark distance of about 2.7517 cm. Another magnesium block (607) having substantially the same rectangular dimensions and having random markings (608) on its top surface is placed on the glass sheetThe small marks are in approximately opposite positions. Rear endOne placement is to make the image visually balanced.

Illumination and exposure of H1

The assembly is then attached to a vibration isolated optical bench having a stable metal mount. On the model assembly, another stable support is attached to the table. The frame holds a dichromated gelatin (DCG) coated glass plate (H1) for the first exposure. Dichromated gelatin (DCG) is a commonly used holographic recording material, described in numerous documents, for example, in Rallison, "control of DCG and non-silver holographic material", the website is as follows:http://www xmission com/～ralcon/dcgprocess/p1.htm。

the spatially filtered slowly diverging laser beam was brought to 45 degrees from the normal to the model before the DCG plate was put in place. This beam is used to illuminate the mold element and the DCG recording material (when it is put in place). For aesthetic reasons, the bracket screws and base plate are rendered substantially invisible by minimizing the amount of laser light that impinges on them. This allows the laser light that does not illuminate the mold to be transmitted mostly to the black absorber below the fixed plate. The laser used was a Coherent Sabre argon ion laser (Coherent inc., Santa Clara, CA) set at 2 watts and 488 nm. Sufficient exposure typically requires about 50-70 mJ. The power on the surface of the future DCG was set to about 2500uw/cm using the Coherent inc.

Then, the diffused laser beam is turned off. During the construction of the model used in this example, the DCG plates were wetted in a room with a humidity of about 54% for 2 hours. The exposure and wetting times for each batch of DCG can be different. Therefore, trial and error successive approximation is required to find the correct exposure and processing conditions. Trial and error successive approximation is familiar to those skilled in the art. A 12.7 x 12.7cm glass plate coated with nearly clear DCG was fixed 0.635cm above the eagle emblem model surface, centered above the model. A 5 minute rest time was provided to dissipate heat and vibration in the mold, optics, optical table, and chamber prior to exposure. Then, the shutter in front of the laser was opened for 20 seconds to start exposure. Approximately 90% of the laser light passes through the coated plate, reflects off the mold, and then returns to the back side of the DCG plate. The incident light meets the reflected object light at the back side, forming an interference pattern and recorded in the DCG coating in the form of interference fringes (see fig. 7).

Chemical treatment

After exposure, the DCG plate was placed in a Kodak fixing bath for 45-60 seconds. It was then immediately placed in 80-120F water for about 4 seconds and then placed in another 80-120F water bath for about 4 seconds. Then, it was placed in a 20% water bath of 80% isopropanol at 130-. The exposed DCG plate was then immersed in the final 100% alcohol bath for several seconds, long enough for the plate to be slowly removed from the bath to promote uniform drainage and drying.

The plate was observable, but the film was swollen, so the image was reproduced too early for the next exposure step (green). The color was then brought to the original exposure wavelength of 488nm using the following dehydration method. The plate was placed back in an 80% ethanol 20% water bath for 4 seconds, then rinsed in a 100% alcohol bath and slowly removed. At this time, the image in H1 was almost completely unrecognizable under diffuse light because all image elements were more than 0.635cm from the hologram surface.

Exposure of H2

Prior to this exposure step, the exposed and processed H1 plate was flipped over and then placed back in the holder and the mold removed. The shutter of the laser at 488nm was opened and H1 was illuminated at 45 degrees but from the back (time reversal illumination) to visualize the observable image of the initial phantom on the DCG plate. The laser was turned off and a second unexposed DCG recording plate was fixed approximately 0.635cm above the first hologram H1. The observable image of the eagle model element was focused on the surface of the second recording sheet H2. The laser was turned on again and the exposure time, wavelength and power level were substantially the same as those used for H1 described above. The plate was chemically treated and conditioned in the same manner as H1 except that the target wavelength for conditioning was 476 nm. The time in the bath is slightly changed to allow adjustment to a new wavelength. This is now the master hologram. After a thin glass sheet is laminated to the film side and protected with a UV-curable adhesive, the master hologram is ready for replication.

Replication of H2

Placing the master hologram (H2) in a hologram copier (Holographics, Logan, UT. HRF734ng photopolymer transfer film (E.I. degree Pont de Nemours)&Co., inc., Wilmington, DE) is extruded/laminated onto its surface. A collimated coherent laser beam was directed at the film master laminate at 45 degrees, 476nm wavelength and a brightness of about 1.25mw for 40 seconds. The copy film was then picked up from H2 and moved in a roll (by web) to a UV 2.6+ Watt, 368nm immersion station (flood station) for 40 seconds. Lamination of the exposed UV-fixed film to the tinting film CTF 146(E.1.du Pont de Nemours)&Co., inc., Wilmington, DE) and passed through an oven at 101 ℃ at a speed of 3 meters/minute. This results in a finished hologram with a peak wavelength of about 530-550 nm. It was then passed through a 150 ℃ vortex oven (scroll oven) at a speed of 3 m/min. After the film was removed from the oven, its back side was laminated to a double-sided black adhesive. On the front side of which there is a clear protective overcoat on one side.

At this point, the hologram is complete. The eagle image element can now be easily seen under diffuse lighting conditions, but is hidden deepThe mark is blurred and is only recognizable with a monochromatic collimated or point source light. Useful types of light sources are lasers, LEDs and narrow band filtered white light. A suitable light source for this embodiment is an LED with a 518nm peak and a full width quarter peak (FWQM) of 40 nm. And a green laser indicator with peak of 532nm and FWQM of 8nm toAnd a narrow band filtered white light with a peak of 532nm and a FWQM of 14 nm. Light sources outside these ranges produce hidden images that are not discernable. See table 2 and fig. 9 showing the spectral curves of the illumination light source. FWQM is defined as the total spectrum from 25% intensity on one side of the peak to 25% intensity on the other side of the peak. In addition, FIG. 3 shows illumination angles where the light source is at least at an angle θ with respect to the normal to the surface of the holographic optical element.

TABLE 2

Sample (I)	FWQM(nm)	Peak wavelength (nm)	Wavelength range (nm)
				Blue color	42	470	450-492
Green colour	58	518	492-550
				Red colour	36	638	620-656
Orange colour	28	594	580-608
				Green #2	56	506	480-536
Green laser	4	532	530-534
				Red laser	8	652	648-656
Green filter	14	532	522-536

In order to work effectively, the spectrum FWQM of the light source should at least partially overlap the spectrum curve FWQM of the final adjusted hologram within an angle. Referring to table 3 and fig. 10, fig. 10 shows the spectral curves of holograms illuminated with white light at 3 different angles. See table 4 and the example of fig. 11 where the source curve is below the hologram curve.

TABLE 3

Sample (I)	FWQM(nm)	Peak wavelength (nm)	Wavelength range (nm)
				Hologram pattern35°Out at 0 ° (out)	88	590	534-622
Hologram pattern45°Go out at 0 DEG	92	580	522-614
				Hologram pattern55°Go out at 0 DEG	96	572	512-608

^*Hologram of eagle emblem and deep image

^**Irradiation angle

^***Normal viewing angle

TABLE 4

Combined light source and hologram data

Sample (I)	FWQM(nm)	Peak wavelength (nm)	Wavelength range (nm)
				Hologram 35 out at 0	88	590	534-622
Hologram 45 ° goes out at 0 °	92	580	522-614
				Hologram 55 out at 0	96	572	512-608
Green colour	58	518	492-550
				Green #2	56	506	480-536
Green laser	4	532	530-534
				Red laser	8	652	548-556
Green filter	14	532	522-536

Example 3 (prophetic example)

This example describes the fabrication of a deep image hologram with relief.

The first transmission hologram (H1) will be made in any recording material particularly useful for holography. Films include photopolymers, silver halides, photoresists, dichromated gelatin (DCG), thermochromic materials (thermal), photochromic materials, electrochromic materials, and the like. The model was constructed as in example 2, using volume reflection holography. The exposure time, exposure energy, laser, wavelength, optical geometry and post-exposure processing will be selected according to the requirements of the recording material, as will be within the ability of the skilled person. The recording apparatus is very similar to the apparatus specified in example 2 for the reflection method. For certain optical designs that employ photoresist or other surface relief producing recording materials and/or methods, the exposure process may be stopped at which point the H1 will be used directly for lithographic fabrication. See step 1 of fig. 5 for basic H1 geometry.

The processed H1 from step 1 is then illuminated in the reverse conjugate direction, producing a true image of the original object. A glass plate coated with a photoresist or other surface relief producing recording material and/or process is placed in the vicinity of the real image. The distance between the photoresist plate and the actual protrusions and the text size and/or image information determines the visibility of the deeply hidden image under various lighting conditions. The photoresist plate is then processed using standard photoresist processing techniques to produce a surface relief pattern with hologram information. In some cases, a plastic master is replicated directly from a photoresist master. In other cases, the photoresist or plastic master is converted to a metal (usually nickel) relief master by electroplating. Then, the relief original plate (shim) was attached to the drum of an embosser. The surface relief of the shim is then heated and pressed into the plastic sheet of the embossing machine. The plastic article may be coated with a relatively soft receiving layer. It may also be coated with a UV curable ink that can harden after pressing. The plastic article may also be pre-coated with a reflective metal on the receiving layer, which may also be applied after pressing. The plastic sheeting produces a deep image hologram that behaves in a manner similar to a volume reflection deep image hologram when illuminated with a particular light source, such as a near point source of light.

Example 4 (prophetic example)

The deep image hologram may operate without a surface holographic image (603). The hologram was made in much the same way as the deep image hologram in example 2. However, the original model of stage H1 includes only the part of the model that is deep and visible only with a special light source in its final form. In addition, the second stage hologram can now be eliminated by placing the deep element at the exact desired depth and slightly adjusting the model to change the angle of illumination.

Example 5 (prophetic example)

The deep image hologram may operate without a surface holographic image (603). The hologram was made in much the same way as the hidden image hologram in example 2. However, the original model of stage H1 includes only the part of the model that is deep and visible only with a special light source in its final form. See fig. 12. In addition, the second stage hologram can now be eliminated by placing the deep element at the exact desired depth and slightly adjusting the model to change the angle of illumination.

The picture or print may be printed on the hologram surface or on the over-laminate by means of typewriting, ink-printing or screen-printing. These prints are easily visible under normal conditions, but the use of special light sources is required to see deep or prominent hidden images.

Another variation is to build separate holograms with deep/raised hidden images or surface relief elements, respectively. In addition, they may be combined. The separately constructed hologram may also be laminated over the original hologram. The result is very similar to that of the original hidden image, which is based on the principle of having a surface image element that is easily visible.

Conventional adhesives such as Lintec 1 mil topcoat, CCG 2 mil Scratch resist, or Polatechno application documents-20 may be used. When these binders are in direct contact with the recording material, they all affect the photopolymer hologram. However, for someProducts such as "Value" in the variationsWithin a tolerable range. These variations can meet the needs of the user to some extent.

Claims (6)

1. A method of viewing a reconstructed image of a deep image hologram contained within a holographic optical element having a surface, the method comprising:

providing a deep image hologram, the hologram producing an image at least 0.318 centimeters from a surface of the holographic optical element, wherein the hologram producing an image is not recognizable to the human eye under a diffuse light source; and

illuminating the deep image hologram with an at least partially collimated incoherent light source having a spectral bandwidth at least partially overlapping the spectral bandwidth of the deep image hologram, wherein the image produced by the hologram is resolved.

2. The method of claim 1, wherein the deep image hologram is formed using a volume reflection method.

3. The method of claim 1, wherein the deep image hologram is formed using an embossing process.

4. A method of determining the authenticity of an article comprising a holographic optical element having a surface, the element comprising a deep image hologram, said method comprising the steps of:

(a) providing a holographic optical element on an article to be authenticated, said holographic optical element comprising a deep image hologram and having a surface, wherein an image produced by said hologram is at least 0.318 cm from the surface of the holographic optical element such that said image produced by said hologram is not recognizable to the human eye under a diffuse light source;

(b) illuminating the deep image hologram with an at least partially collimated incoherent light source having a center wavelength within the visible region of the electromagnetic spectrum and a spectral bandwidth, wherein the spectral bandwidth of the at least partially collimated light source at least partially overlaps the spectral bandwidth of the deep image hologram; and

(c) determining the article bearing the holographic optical element as authentic only if the hologram image is observable when the light source in step (b) is positioned at least one angle θ relative to the normal to the surface of the holographic optical element.

5. The method of claim 4, wherein the at least one angle θ relative to a normal to a surface of the holographic optical element is in a range of about +55 degrees to about-55 degrees, including all degrees.

6. A deep image hologram within a holographic optical element having a surface, said hologram producing a hologram image at a distance of at least 0.318 cm from the surface of the holographic optical element, wherein the hologram image is not recognizable by the human eye under a diffuse light source but is recognizable by the human eye under an incoherent light source that is at least partially collimated.