WO1990005944A2

WO1990005944A2 - Improvements in or relating to holograms

Info

Publication number: WO1990005944A2
Application number: PCT/GB1989/001365
Authority: WO
Inventors: Nicholas John Phillips
Original assignee: Scientific Applied Research (S.A.R.) Plc
Priority date: 1988-11-16
Filing date: 1989-11-16
Publication date: 1990-05-31
Also published as: AU4621189A; CA2003164A1; GB8826766D0; WO1990005944A3

Abstract

In a method of preparing a three-dimensional photograph of a subject, for example a three-dimensional portrait, the subject (10) is illuminated, through a diffusing screen by a source of laser light. A retro-reflective screen (14), comprising an array of cube-corner reflectors, is placed in front of the screen and a beam-splitting plate (12) is arranged, between the retro-reflective screen and the subject, the beam-splitting plate being so inclined relative to the subject as to reflect some of the light from the retro-reflective screen onto a photographic film or plate (18) disposed to one side of the subject, so that the laser light reflected from the subject, passes through the beam-splitting plate to the retro-reflective screen, is reflected back on itself to the beam splitting plate and is partially reflected thereby to form a real image (16) of the subject, in such laser light, on the opposite side of the beam-splitting plate from the subject. The photographic film or plate is located so as to receive the light rays passing through and forming said real image and is further illuminated by said source of laser light whereby there is formed on said film or plate, after conventional development, a holographic three-dimensional image of the subject, viewable in white light.

Description

Title: "Improvements in or relating to holograms"

THIS INVENTION relates to holograms and, more particularly, but not exclusively, to holograms which can be viewed in white light and which, when so viewed, present to the eye of the viewer a three-dimensional view of the original subject.

5

The invention is particularly applicable to three- dimensional portraiture-

Techniques are known for producing three-dimensional _Q holographic portraits of individuals, which are viewable directly in white light - i.e. without the requirement for a coherent light source to reconstruct the image of the original subject. However, these known methods have various disadvantages. 5

It is an object of the invention to provide an improved method and apparatus by which true three- dimensional representations of subjects may be produced holographically whilst avoiding the disadvantages inherent _Q in known methods.

Thus, one disadvantage of known techniques is that, to produce the final white-light viewable representation, it is necessary first to prepare a conventional hologram, not 5 viewable in white light directly and, in a subsequent stage, to combine this hologram with a further hologram, in a further exposure step, to provide the final white-light viewable three-dimensional representation.

0 According to one aspect of the invention there is provided a method of preparing a three-dimensional photo¬ graph of a subject, as herein defined, comprising so arranging the subject, a source of laser light, a beam- splitting plate, a retro-reflective screen as herein defined, and a photographic film or plate in relation to one another that the subject is illuminated by said source of laser light, that such, light, reflected from the subject, passes through the beam-splitting plate to the retro- reflective screen, is reflected back on itself to the beam splitting plate and is partially reflected thereby to form a real image of the subject,' in such laser light, on the opposite side of the beam-splitting plate, the photographic film or plate being located so as to receive the light rays passing through and forming said real image and being further illuminated by said source of laser light whereby there is formed on said film or plate, after conventional development, a holographic three-dimensional image of the subject, viewable in white light.

A further disadvantage is that, unless appropriate make-up techniques are applied to the subject before the holographic portrait exposure is made, in the resultant three-dimensional representation, as viewed, the subject tends to have an unattractive skin texture. It has been found that this results from the fact that the laser light conventionally used is of such a wavelength that it is not reflected totally at the skin surface, but penetrates the skin to a greater or lesser degree and is reflected differently by blood vessels, pockets of fat etc.

According to another aspect of the invention, there¬ fore, there is provided a method of preparing a hologram, or a holographic three-dimensional photograph of a human subject, using laser light, wherein the- laser light used is green light. The invention also extends to apparatus for use in preparing a three-dimensional photograph of a subject, as herein defined, comprising a laser, a beam-splitting plate, the retro-reflective screen as herein defined and means for supporting a photographic film or plate, the laser, beam- splitting plate, retro-reflective screen and film or plate supporting means being so disposed in relation to each other and to a subject area that when a subject, disposed in the subject area, is illuminated by laser light from the laser, laser light reflected from the subject can pass through the beam splitter plate to the retro-reflective screen to be reflected back on itself to the beam-splitter plate and be partly reflected by the latter to form a real image of the subject, in the region of said film or plate support, on the opposite side of the beam splitter plate from the subject, whereby the film or plate supported by said film or plate supporting means can be illuminated by light rays passing through and forming said real image, the arrangement being such^' that light from the laser source can also reach a film o plate supported by said film or plate-supporting means without first striking the subject.

The inventor has found that these defects may be avoided by using laser light of a different wavelength, lying in the green part of the spectrum. It has been found that such light is partially reflected at the skin surface, with the remainder being almost entirely absorbed by the underlying tissue, so that the light reflected from the subject is reflected entirely from the outer skin surface of the subject.

The term "retro-reflective" screen is used herein to denote a screen which has the property that a light beam striking any point thereon from any angle within a relatively wide range of such angles will be reflected back from the screen along substantially the same path as that along which the beam was incident on the screen. Such a screen may, for example, comprise a layer of minute glass- spheres which reflect light, after the manner of the well- known "cats' eyes", or an array of pyramidal reflectors, each affording a cavity, open towards the front of the reflector, defined by three mutually perpendicular reflective surfaces meeting at a common apex. In practice, in such screens, due to the finite size of the glass spheres

- or the pyramidal recesses, a ray striking the screen is, in general, reflected back from the screen, not exactly along the path of incidence of the beam onto the retro-reflective screen, but along a parallel path which is slightly displaced transversely relative to the incident path. This transverse displacement limits the resolution (if not more limited by some other factor) or, viewed in another way, the transverse displacement is negligible within the resolution of the system.

An embodiment of the invention is described below, by way of example, with reference to the accompanying drawing which is a schematic view, from above, illustrating a scheme for the production of three dimensional, white-light view¬ able holographic portraits.

Referring to the drawing, the head 10 of a subject is illuminated by light from a laser source (not shown) via a diffusive screen (not shown - which is used to avoid the risk of damage to the eyes of the subject). Some of the light reflected from the subject's head is reflected therefrom through an angled part-reflective/part-trans- missive plate 12, forming a "beam-splitter" and passes to a retro-reflective screen 14. The screen 14 has the property that a light-ray striking it from substantially any angle will be reflected back substantially along its path of approach. The screen 14 may, for example, comprise a layer of minute glass spheres which, after the manner of the well- known "cats' eyes" reflect light by internal reflection, or an array of pyramidal reflectors, each affording a cavity, open towards the front of the reflector, defined by three mutually perpendicular reflective surfaces meeting at a common apex. Such a reflective sheet may, for example, be formed by the technique disclosed in International (PCT) 5 Patent Application W089/06811. The light rays reflected from the retro-reflective screen 14 meet the beam-splitter 12 once more and part of the light so reflected is reflected, by the beam-splitter 12, laterally (since the beam-splitter 12 is inclined relative to the line connecting

10 the subject 10 and the reflector 14), to form a real image 16 of the subject 10 on the opposite side of the plane of the beam-splitter 12 from the subject 10. A photographic plate 18 or a portion of a photographic film 18 in a flat condition, is supported in the region of the image 16. The

15 plate or film 18 is likewise illuminated directly (in a manner not illustrated in the drawing, but which is conventional in the art) by light from the laser source, i.e. the plate or film 18 is also illuminated by light which has not first struck the subject 10 and a holographic image

20_. is consequently formed upon the plate or film 18.

Upon development of the plate or film 18, a white- light viewable three-dimensional holographic photograph of the subject is obtained, i.e. the resulting, developed film 25 can be viewed directly in white light and the viewer will see, within the confines of the edges of the developed film or plate, a three-dimensional image of the original subject.

The laser light used to make the exposure is prefer- 0 ably green light and may, for example, be light, of a wavelength of 532 x 10 -9 ^J m produced by applying a frequency- doubling technique to the light produced by a laser, using, as the active material, a rod of glass doped with neodymium. Thus, the beam from such a laser is passed through a

35 frequency doubling crystal from which emerges a beam of half

_q the wavelength, namely 532 x 10 m, which lies in the green part of the spectrum. The preferred photographic material is Ilford blue/green sensitive emulsion - Type SP672T which has a peak sensitivity very close to the wavelength of the green light produced as above. The photographic material is so constituted as to be insensitive to light of the fundamental wavelength of the laser.

It is contemplated that the elements of the apparatus illustrated in the drawing may be mounted within a booth having a seat therein upon which the subject may sit and which is so located as to place the head of the subject in the appropriate region (herein referred to as the subject area) relative to the other parts of the apparatus. The person desiring to have his portrait made will, it is contemplated, insert an appropriate coin or token into an appropriate mechanism within the booth and, after the fashion of known photographic booths, a warning will be given that an exposure is about to be made and the exposure will subsequently be made. In the present instance, to make the exposure, the laser will be energised only very briefly,^' so that the exposure will, effectively, be a "flash" exposure. After the plate or film 18 has been exposed, the apparatus will automatically, in much the same way as in a conventional automatic photography booth, transfer the plate 18, or the appropriate section of film 18, to a processing facility, which, being entirely conventional, is not described here, and the finished product will be delivered conventionally to an outlet receptacle where it may be collected by the sitter.

The three-dimensional photograph is preferably made somewhat smaller in scale than the subject, by incorporat¬ ing, in known manner, a suitable image reduction lens system (not shown) in the apparatus.

Whereas, in the embodiment so far contemplated, the resultant photograph will be essentially monocromatic, it may, of course, be made to present to the eye of the beholder an attractively "tinted" appearance, for example having a bronze-tinted appearance, for example by utilising, in processing of the photographic material, a finishing bath, known per se adapted to produce slight thickening or swelling of the emulsion layer.

Besides affording a more attractive mode of por¬ traiture than hitherto, providing a more realistic and comprehensive record of the appearance of the sitter, the technique referred to may have useful security applications, for example in affording more accurate and reliably identi¬ fiable pictures in passports or the like identification documents or in providing more comprehensive records of subjects or scenes. Thus, for example, it might become standard procedure during surgery to take three-dimensional photographs using the technique described so that the progress of operations may be more effectively examined in retrospect, allowing errors or oversights such as the leaving of instruments or swabs in cavities, to be detected more easily.

The technique described, using green laser light, avoids the need for special make-up to be applied to subjects before the holographic portrait is made, for the green laser light used is reflected from the skin surface, but is substantially completely absorbed, without reflection, by the underlying muscle, fat, blood vessels etc.

Furthermore, the use of the retro-reflective screen, in combination with the beam-splitting plate, to form a real image of the subject in the region of photographic film or ■plate, eliminates the need for a second holographic exposure, necessary in known techniques for producing white- light viewable holograms. Consequently, the described technique embodying the-" ⁱnvention is well suited for an automatic _boot_h-_base_d operation requiring no preparation of the subject and with minimal delay between the sitting phase and the delivery of the finished portrait.

Claims

1. A method of preparing a three-dimensional photograph of a subject, as herein defined, comprising so arranging the subject, a source of laser light, a beam-splitting plate, a retro-reflective screen as herein defined, and a photo¬ graphic film or plate in relation to one another that the subject is illuminated by said source of laser light, that such light, reflected from the subject, passes through the beam-splitting plate to the retro-reflective screen, is reflected back on itself to the beam splitting plate and is partially reflected thereby to form a real image of the subject, in such laser light, on the opposite side of the beam-splitting plate, the photographic film or plate being located so as to receive the light rays passing through and forming said real image and being further illuminated by said source of laser light whereby there is formed on said film or plate, after conventional development, a holographic three-dimensional image of the subject, viewable in white light.

2. A method of preparing a hologram, or a holographic three-dimensional photograph of a human subject, using laser light, wherein the laser light used is green light.

3. Apparatus for use in preparing a three dimensional photograph of a subject, as herein defined, comprising a laser, a beam-splitting plate, the retro-reflective screen as herein defined and means for supporting a photographic film or plate, the laser, beam-splitting plate, retro- reflective screen and film or plate supporting means being so disposed in relation to each other and to a subject area that when a subject, disposed in the subject area, is illuminated by laser light from the laser, laser light reflected from the subject can pass through the beam splitter plate to the retro-reflective screen to be reflected back on itself to the beam-splitter plate and be. partly reflected by the latter to form a real image of the subject, in the region of said film or plate support, on the opposite side of the beam splitter plate from the subject, whereby the film or plate supported by said film or plate supporting means can be illuminated by light rays passing through and forming said real image, the arrangement being such that light from the laser source can also reach a film or plate supported by said film or plate-supporting means without first striking the subject.

4. Apparatus according to claim 3 including means oper¬ able, in response to a command operation, to illuminate the subject area and the film or plate-supporting means briefly with laser light from the laser source, to expose the film or plate supported by said film or plate supporting means and for subsequently, automatically, removing the film or plate from the film or plate supporting means, developing and processing the same and delivering the finished holo- graphic photograph to an outlet for collection.