Transmission Holograms
Transmission holograms are found on credit and bankcards are of this type. They are called transmission because ideally they are lit from behind, like a photographic transparency (slide), the light being transmitted through them.
Often it is not possible to light the hologram in this way so a mirror or aluminised foil backs them. This is why bankcard holograms have a silvery appearance. The light then bounces off the mirror backing and effectively lights the hologram from behind, as on a credit card.
A special type of transmission holograms are the so-called rainbow holograms as the light being used to illuminate them is split into a spectrum, the hologram representing the 'correct' colour(s) from one angle, while at other angles seen in different colours of the spectrum (from deep blue to red). Transmission rainbow holograms can be large (>1m2 ) or small (<10mm2 ), be one off's, or mass-reproduced using a technique known as embossing. This will be explained in the manufacturing section.
