Glossary of Holograms
Below are definitions of some of the terms most commonly-used to describe holograms and related technologies, their properties and production processes.
2D – a holographic image style that places a two-dimensional plane of graphics at, in front of, or behind the image plane of the hologram.
2D/3D – a holographic image style created by a series of two-dimensional planes of artwork that are layered into the image volume of the hologram.
3D – a holographic image style that captures a three-dimensional object or scene.
Achromatic – an optical system free from dispersion or used to describe a colourless (grey tone monochrome) holographic image.
Augmented Reality (AR) – a technology that superimposes a computer-generated image onto a user’s actual view of the real world. AR devices often use the term ‘hologram’ to describe the heightened visual reality they provide.
Beamsplitter – an optical device that divides a beam of light into two beams.
Blazed grating – a diffraction grating which has a sawtooth profile structure.
Bragg diffraction – the diffractive principle which a layered stack of parallel reflecting surfaces of alternate higher and lower refractive index will reflect a beam of light, if and only when the reflected wavefronts are of an appropriate wavelength and orientation to produce constructive interference.
Bragg hologram – any hologram in which the Bragg condition is preferential than the grating condition in forming the image; a volume hologram.
Casting – a method of embossing in which a film of soft resin is applied to a nickel shim then exposed to ultraviolet light causing the resin to harden by cross linking. The resin film can now be removed and retains a faithful copy of the surface relief image that was present on the shim. This method of transferring an image from a nickel shim onto a plastic film, results in a bright image and causes very little wear to the shim.
Classic hologram – a classic hologram is usually made using a 3D object, often a model. A laser beam is split in two and part is diverted onto the object, reflected off it then allowed to combine (or interfere) with the other part of the beam known as the reference beam. The classic hologram is a recording of the interference pattern between the object and reference beams. A well-known example of a classic hologram is the dove used by Visa on their credit cards.
Coherence – the degree to which photons in a beam of light are in phase.
Computer generated hologram – a technique that uses computational methods to calculate the interference pattern that would have been created by a reference beam interfering with an object beam. The pattern is printed onto a mask or film to be viewed using illumination from a suitable light source.
Computer generated stereogram – as ‘stereogram’ but where the sequential images are generated from processing images on a computer.
Covert – a concealed or hidden feature within a hologram that is not apparent and can only be viewed with special methods or facilities, such as high magnification or lighting equipment (see overt).
Demetallisation – a two stage process in which metal, usually aluminium, is deposited onto the hologram and then selectively removed. The removal is achieved by either first printing a protective resin onto the metallic layer then dissolving away the unprotected metal or ablating the metal with a laser beam.
Dichromated Gelatin (DCG) – a chemical-gelatin mix that produces very bright images in a golden-yellow colour. The images have limited depth but can be viewed in extended light sources of poor quality. See Volume Hologram.
Die-cutting – the general process of using a die (a specialised tool to cut or shape material) to shear webs of low-strength materials into the final product, such as holographic labels.
Diffraction – the phenomenon whereby light waves spread out as a result of passing through small apertures whose dimensions are comparable to the wavelength of light. Of particular interest are gratings with line spacings close to the wavelength of light because these have the ability to bend light of different wavelengths by different amounts. Hence, they produce a prismatic effect of splitting white light into the various colours of the rainbow.
Diffractive Optically Variable Image Device (DOVID) – a generic term which comprises all security devices that are based on the diffraction of light by fine gratings. The different DOVIDs differ in their image resolution, brightness, and their animation capabilities. Examples of DOVIDS include holograms which by definition display three-dimensional images and other devices known by their tradenames, such as Kinegram®, DID®, Exelgram®, Alphagram™, to name a few.
Dot matrix – a surface relief hologram built up from an array of tiny diffractive gratings arranged at certain angles. The ‘dots’ are a point at which two microscopic beams of laser light meet at an angle and produce an interference pattern. In a dot matrix machine, a mechanical arrangement moves this point of light in a matrix pattern relative to a photosensitive plate. According to the final image required, the angle at which each dot is exposed into the matrix is determined by mathematical calculation. It is thus possible to construct fully synthetic images of objects that never existed in real life.
E-Beam (electron beam) – a method of creating a holographic image by ‘writing’ the individual interference fringes using a fine beam of electrons in a vacuum chamber. The challenge here is to calculate the exact position of each fringe according to where it would have resulted if a reference beam had interacted with an object beam.
Electroforming – the process of converting a holographic image from photoresist or plastic to metal (nickel). See Shim definition.
Eye-mark (or registration mark) – where some step in the overall manufacturing process requires a machine to process a single holographic image, an eye mark is invariably place next to each image to allow proper registration.
Frangibility – capable of being broken. Refers to holographic security labels which break up when removed or tampered with.
Grating – a grating is a series of parallel lines having a spacing comparable with the wavelength of light. Such an array of lines has the ability to split white light into a rainbow because the different wavelengths are bent, or diffracted, by different amounts. Such gratings were originally produced by using a diamond to rule closely spaced lines onto glass, but they are now more often produced by optical interference.
High Refractive Index (HRI) – in situations where a transparent hologram is required, for instance as a laminate to cover the variable data of a passport or other ID document, instead of coating the holographic image with a metallic film, a transparent coating is made using a material with a High Refractive Index. The HRI materials used in this process are typically oxides (e.g. titanium dioxide) or sulphides (e.g. zinc sulphide).
Hologram – the term ‘hologram’ is derived from two Greek words ‘holos’ meaning whole or complete and ‘graphos’ meaning an image. The term therefore describes a recorded image which is complete, in that it shows the whole volumetric space of the object or image, as opposed to a conventional picture, painting or photograph which displays an object from a single viewpoint.
Holographic Optical Element (HOE) – an optical device that uses diffraction to shape a wavefront, as opposed to a refractive optical element (using materials with differing refractive indexes) that depends on refraction.
Hot-stamping foil – a thin material which is applied to paper or other substrate through the combination of heat and pressure. Hot-stamping foil can be used to support holographic images which can be transferred, using heat and pressure, onto another substrate such as paper or plastic. The layers containing the holographic image are extremely thin, typically 5-6 microns. Therefore, they need to be supported on a thicker material such as 19-micron thick polyester. The action of stamping the foil with a heated die causes a ‘release’ layer between the hologram and the polyester support, to melt while simultaneously softening an adhesive ‘size’ coating. These combined effects allow the hologram to leave the polyester support and adhere to its new location.
Interference – a phenomenon in which two waves superpose to form a resultant wave of greater, lower, or the same amplitude. Interference usually refers to the interaction of waves that are coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency.
Kinetic – an adjective whose roots in the Greek word ‘kinesis’ indicate motion. A holographic image which displays movement of form or colour is said to be kinetic. Often, such images are patterns rather than objects. The patterns can be made up of fine lines or graphic elements and appear to scintillate when moved. The Swiss company Landis & Gyr found a particularly effective way of producing such designs and called them ‘Kinegrams’.
Kiss cuts – a tamper-resistant feature employing a die-cutting process where adhesive backed labels are cut through, but the laminated backing paper is not.
Laser – derived from ‘light amplification by stimulated emission of radiation’, a device that produces coherent light by stimulated emission of radiation.
Metallisation – the coating of films or surfaces with a very thin layer of metal using a metal deposition technique (generally vacuum deposition). Metallisation is what gives holograms their silver or similar metallic appearance.
Micron – a unit of measurement referring to the thickness or gauge of physical substrates and features. One micron is equal to one millionth (10-6) of a metre.
Microstructure – the very small scale structure of a prepared surface of a material as revealed by a microscope above 25× magnification. Diffractive microstructures comprise micro-protrusions or microgrooves or a combination thereof that are capable of diffracting light waves.
Microtext – text created within a holographic image that is so small that it cannot be reproduced by photocopying or scanning, and can only be read under magnification.
Multi-channel – a type of holographic image in which different, usually unrelated images, appear at different viewing angles.
Nanotechnology – the science and technology of precisely manipulating the structure of matter at the atomic and molecular level (one nanometre is one billionth of a metre). Nanotechology is being used to explore and develop unique optical phenomena for use in new material and security features, including holograms.
Object beam – the diffractive beam that is reflected from an object and incident on a sensitive material, usually a photosensitive material; also refers to the beam that is transmitted and diffracted by a holographic plate (H1) which is the “object” from which a copy (H2) is made. See also Reference Beam.
Overt – features such as holograms that are apparent and visible, and can be viewed without additional readers or instruments.
Parallax – the phenomenon in an image which allows depth to be judged from the movement of near image elements relative to more distant ones. Traditional embossed holograms only display parallax from side-to-side, but the lack of vertical parallax is rarely noticed.
PET – polyethylene terephthalate (polyester) – the base material used for most surface applied holograms.
Photopolymer holograms – holograms produced from photopolymer materials. The latter are materials that polymerise (the cross-linking of molecules) under the action of light energy and thus change their refractive index, and are used to record volume reflection holograms (see Volume Holograms) or Lippmann holograms.
Photoresist – a substance that becomes insoluble (negative) or soluble (positive) under the action of light. A surface grating image is produced which is the first stage in the making of an embossed hologram.
Pressure-sensitive – a label construction made up of three layers: a face material, a pressure-sensitive adhesive, and a backing sheet coated with a release agent.
Rainbow hologram – when illuminated with incandescent light (white light), rainbow holograms display images bathed in a changing rainbow spectrum of colours as the viewer moves their eye position up and down. Holographers have developed considerable control over the colours displayed in this type of hologram to produce images in a specific colour or in near natural colour.
Recombination – the ‘step and repeat’ process whereby a single, holographic image is laid out in rows and columns in preparation for shim production. It can either be carried out mechanically or optically. In the mechanical process, the single image is made into a stamper which is impressed at pre-determined intervals, into a plastic sheet. If done optically, the single image is exposed in a predetermined pattern onto a (glass) plate coated with a photoresist.
Reference beam – the unmodulated beam which, when directed at the photoresist forms a stationary interference pattern with the object beam. See also Object Beam.
Reflection hologram – this is viewed by the reflection of white light. The diffractive planes within the depth of the recording material have a spacing which corresponds only to a single wavelength of light. This wavelength, usually green or yellow, is reflected back and reconstructs the image, the other colours being transmitted and absorbed by a black backing placed behind the hologram.
Refraction – the bending of light as a beam passes from one medium (say air) to a different medium (say glass). For applications of refraction – see High Refractive Index (HRI).
Shim – a thin plate of metal, usually nickel, which is attached to a cylinder in preparation for the embossing process. The shim is produced by an electro-deposition process whereby the plate with the recombined images is immersed into a galvanic tank and metallic nickel is caused to accumulate on its surface. This metal plate is usually referred to as the ‘master’ or ‘mother’ shim. It is usually used to prepare ‘daughter’ shims which are used for the mechanical embossing process.
Shim line – at some point in the hologram replication process, one or more metal shims need to be mounted around the circumference of a metal cylinder (the embossing cylinder). The point at which the shims join often produces a visible line in the resulting embossed film. This line is most objectionable where the finished result is intended to be a continuous pattern. Those skilled in the art have developed procedures which successfully minimise or even eliminate such optical discontinuities in the final product.
Special effects – a whole range of optical special effects including Peppers Ghost, projection optics and image processing are combined to bring film footage of usually deceased celebrities (eg. Tupac Shakur, Michael Jackson, Robert Kardashian) back to life in an orchestrated theatrical display. The term ‘hologram’ is often used to describe the heightened sense of solidity and realism they provide.
Stereogram – a type of hologram prepared using the sequential images from a piece of movie footage. Each frame of the movie is converted into a vertical slit image and stacked against another slit image of the adjacent frame. The result is a hologram which, when the viewing angle changes side to side, the same image motion as the movie footage is seen as the frames are seen one after another.
Surface-relief hologram – a hologram which all the details of the image are recorded as an interference pattern on the surface, rather than through the volume, of a material. The value of such surface relief holograms is that they can be mass replicated by mechanical transfer from a master image to inexpensive plastic.
Transmission hologram – can be seen by light passing through the material containing the holographic image. Ironically, all metallised, embossed holograms are transmission holograms but are conveniently seen by reflected light because the incident light passes through the diffractive grating and is reflected back by the mirror coated on the reverse side.
Viewing angle – the angle at the surface of a hologram which defines the limits of the space relative to the hologram in which an image can be observed with acceptable visual performance.
Virtual Reality (VR) – a technology that immerses the viewer into a computer-generated simulation of a three-dimensional scene and allows them to interact with it using a digital interface. VR devices often use the term ‘hologram’ to describe the heightened sense of realism they provide.
Volume hologram – otherwise known as a Bragg diffraction hologram, is where the thickness of the recording material is much larger than the wavelength of the light used for recording. The advantage of volume over surface relief holograms is the gain in image quality.
Zero order device – often abbreviated to ZOD, these devices comprise gratings less than the wavelength of visible light (below 400 nanometres). These devices can be produced by embossing and display unique colour effects which are dependent on the plane of rotation of the device.