The two most common types of hologram are surface relief and volume.
Surface relief holograms are mass-produced mechanically by embossing or casting the relief pattern or image into a thermoplastic film or a viscous coating on a film or paper. In embossing, the material is heated to accept the relief stamp, then rapidly cooled to fix it. In casting, the relief is rolled into a viscous coating which is then exposed to UV light to cure it and fix the image.
More rarely, the hologram might be injection moulded, or blazed directly onto the product using a high power laser or other method of scoring the surface relief pattern (click here to read about some of the latest alternative methods for reproducing holographic images by explosives, etching and moulding).
Surface Relief Holograms
Surface relief holograms are the most ubiquitous and the most familiar. Mainly they exhibit a characteristic rainbow-coloured pattern or image.
They can be made on a variety of carriers or substrates to suit the use they will be put to and the method of final application to the item to which they will be applied. Polyester (PET) is the most common substrate and is used in several configurations. Orientated polypropylene and paper are also used, as is PVC for labels (although this is increasingly falling out favour on environmental grounds). The most common of these configurations are as follow:
- Hot stamping foil - a process for applying the hologram as a very thin film, only a few nanometres thick so that it does not add appreciable thickness to the item it is on; almost always metallised which gives a silver or similar metallic appearance.
- Overlay/laminate - a transparent or semi-transparent film used to protect the personal data page of an identity card.
- Thread – for insertion into banknote or other security papers
- Laminating film –which differs from foils in that the carrier is retained during application. Most commonly used for either flexible packaging or for lamination to cartonboard for rigid packaging and for conversion to labels. In addition to polyester, orientated polypropylene is used, particularly for packaging applications involving food contact.
- Paper – used for packaging and for items such as bottle labels, especially shere a wet glue process is used.
- Label – normally self-adhesive, and die- or kiss-cut for application to the pack or product. For authentication, such labels are often frangible to demonstrate tamper-evidence
- Holomagnetic – the foil is laminated to a magnetic strip for application to a credit card or similar
One more facet of surface relief holograms to be aware of is the way in which they are coated. The diffractive effect depends on the definition of the lines of tiny ridges and grooves in the surface relief pattern. If the edges of the lines become worn, or the grooves fill with foreign matter, then the diffractive effect is diminished and the hologram becomes dull, eventually disappearing altogether. Therefore the surface relief pattern is always protected, either by being laid face down on the document or label, or by being coated with a protective lacquer.
In addition, most surface relief holograms require a reflective layer to bounce light back through the pattern to reveal the image. This reflective layer is usually aluminium, which appears silver, but other metals or metal oxides can also be used. Copper and gold are established alternatives to aluminium. Metal oxides, also known as high refractive index coatings (HRIC), refract light at specific wavelengths or angles, rather than reflecting all light, so that the film remains transparent at most viewing angles but the holographic image is revealed when viewed or illuminated at the correct angle.
Selectively removing the metal layer can serve the same purpose, by leaving metal dots, so the hologram is both reflective and translucent. A similar process, pattern demetallisation, allows removal of selected parts of the design.
Volume, or reflection, holograms have a very different appearance to their surface relief counterparts. They are mass-produced through optical copying of a master hologram, so they retain more of the optical properties of that master than the mechanical process of surface relief holograms. This allows them to be used for classical holograms which are fully 3D images, and recent developments also allow the use of colour in the image.
Volume holograms are generally used for authentication and the material is photopolymer, mainly in self-adhesive label format. The photopolymer is usually lined with a black backing, but it can be left transparent to be used, for example, as an overlay on a phone screen.