Optically Variable Device for Security Documents
- Professor Hans I Bjelkhagen
- Centre for Modern Optics
- OpTIC Technium
- St Asaph Business Park
- ST ASAPH LL17 0JD, UK
- hansholo at aol dot com
The innovation is based on an old colour photographic technique, interferential photography or Lippmann photography, which was invented in 1891 by Gabriel Lippmann. He was awarded the Nobel Physics Prize for his invention in 1908. Lippmann photography was the first technique, which could record colour photographs directly in the camera. However, the technique had several limitations, e.g., a special type of isochromatic film was needed, the image could not be copied, the image switched between a negative and a positive colour image depending on viewing direction. These limitations are now real advantages for a photographic document security device.
A Lippmann photograph can be applied as a new type of Optically Variable Device (OVD) to be used on individually issued security documents, such as, e.g., identification cards, passports, credit cards, driving licences, and other documents where a high degree of security is needed. A Lippmann photograph is very similar to the embossed holograms currently used in this field; however, a unique recording of each document can be made to achieve a degree of security higher than that with mass-produced holograms. The recording of Lippmann photographs requires a special type of photosensitive recording material in contact with a reflecting layer. Modern panchromatic photopolymer materials or ultra-high-resolution silver halide emulsions can be used and, after being recorded and processed, laminated to security documents. A special type of recording equipment is required. Lippmann photographs are virtually impossible to copy and, certainly, cannot be copied by conventional photography or colour copying machines.
Brief description of the recording technique
A Lippmann photograph can be recorded on a photopolymer material in the following way. The photosensitive layer has to be rather thin, in the order of a few micrometers only. The light-sensitive layer must be coated on a flexible transparent base and a special type of reflecting foil has to be laminated on top of to the photosensitive polymer layer in perfect contact with it. Experimental photopolymer materials have been manufactured by DuPont, e.g., the HRF-700X071-3 film. This film was used to prove the concept of recording Lippmann photographs in modern photopolymer materials. The polymer film laminated to the reflecting foil must be exposed in a special camera. After being exposed to the image-forming information in the camera, the reflecting foil is detached from the photopolymer film and the photopolymer layer is exposed to strong white light or UV light for developing. The image brightness is increased by heat treatment of the recorded film. The whole processing technique of the photopolymer film is a completely dry process, which makes it suitable for manufacturing a recording/processing machine intended for Lippmann security labels: Lippmann OVDs. After being processed, the transparent photopolymer label is laminated to its corresponding security document. The polymer film contains no dyes or any fading chemicals, which means that the archival stability is expected to be very high. The photograph is simply a piece of plastic material with the information recorded in it as an optical phase structure (refractive index variations within the photopolymer layer).
In Fig. 1, a sample US passport is shown having a Lippmann OVD attached in the upper right corner. Figure 2a shows a close-up of the Lippmann OVD in colour, when observed perpendicular to the passport page. In Fig. 2b, the Lippmann OVD appears as a negative when viewed at an angle.
Fig. 1. Passport with a Lippmann OVD.
Fig. 2a. Lippmann OVD in colour.
Fig 2b. Lippmann OVD as a negative.
Note that these digital photographs cannot show the quality of the real Lippmann photograph with its extremely high image resolution.
Fig 3. A passport with a Lippmann OVD attached in the upper left corner. Light from a diffuser above the passport makes it easy to inspect the Lippmann OVD on the passport.
Advantages of the Lippmann OVD
As already mentioned holograms are common in the field of document security, where mass-produced embossed holograms are attached to many types of security documents and most commonly recognised are the holograms on VISA and MasterCard credit cards. In almost every case where holograms are used, exactly the same hologram image is attached to a large quantity of security documents of the same type, e.g., the embossed dove hologram on the VISA cards. Since holograms are difficult to manufacture and lasers are required for the actual recording of a hologram, the use of holograms has been a valuable security device over many years. Nowadays, however, it is possible to copy holograms and there are examples of illegally copied security holograms reported. Nevertheless, a hologram is a very valuable OVD for mass-produced security instruments such as bank notes, cheques, vehicle stickers, product labels, etc. Lippmann photography offers a new type of optical security device that is unique and can be individually produced for each security document issued. Some of the advantages of a Lippmann OVD as a security device are:
- Automatic recording and processing equipment for Lippmann OVDs can be manufactured to be used by security document producers and institutions issuing security documents.
- The recording is rather simple to perform, no specially equipped laboratory is required.
- The access to the recording photosensitive film, the special photopolymer material, can be strictly controlled by the manufacturer of the film. Only approved producers of security documents and institutions issuing such documents can order the material from the film manufacturer (e.g. like bank note paper).
- The Lippmann OVD has a very high archival stability.
- The Lippmann OVD is Bragg sensitive, which means it changes its colour depending on the angle of illumination and observation. It also switches between a positive and negative image. These features are extremely important as the effects are easily recognised when inspecting the Lippmann OVD. (Fig. 2 a and b)
- The Lippmann OVD cannot be copied by conventional colour photography nor can it be copied using colour copy machines or colour scanners.
- Since the resolution of the Lippmann OVD is extremely high, a reduced image of the security document can be laminated to the document, occupying only a limited area of it. In this case, magnifying techniques may be necessary to be able to read all the recorded information in the high-resolution Lippmann photograph.
One example of a Lippmann OVD used for document security and counterfeit-resistant purpose is on a passport. The Lippmann OVD can be recorded of the passport page including specific information about the individual, the signature, and the conventional colour photograph of each issued passport. Then, a reduced-size Lippmann image is laminated to the page at an appropriate place. The colour shift of the Lippmann OVD indicates that it is a genuine Lippmann photograph and not a conventional photograph. In addition, all the information recorded in this OVD can be compared with the corresponding information in the document itself. It is a very difficult process to go through if someone wants to tamper with Lippmann-protected documents. The most important advantage is that there is really no point in trying to copy a Lippmann OVD since it is unique to a particular passport and of no use applied to a different passport. The authenticity of a Lippmann OVD is easy to verify simply by looking at it. However, it is also possible to make automatic inspection equipment that can check the iridescence of the image or compare the information recorded on the document itself with the corresponding information stored in the Lippmann OVD.
The innovation (the application of Lippmann photography to security documents) is protected by a US patent and pending European patents. The Lippmann OVD was first described in a paper published in Optical Engineering. DuPont has expressed an interest in developing and manufacturing the special photopolymer film needed. Design and development of the recording and processing equipment are required. There is also more work needed to improve the quality of the recording material for Lippmann OVDs.
There is a worldwide interest in increasing the security of documents in general. In England, OVDs have been introduced to protect banknotes. In this case a kinegram is used, which is similar to embossed holograms. Personalised documents, e.g., the UK passport, also carries an OVD. However, identical holograms are used on all passports. Credit cards, although they are also personalised documents, all carry identical holograms. Recently, Germany introduced a new type of passport which contains a monochrome, individually-made 2D hologram, overlaminated to the passport page. There is no doubt about the fact that personalised documents need to be more difficult to fake, which means, adding personalised security features to them.
The worldwide market for document security is large and rapidly growing. The holographic security industry including the worldwide OVD market size in 2001 was $1.09 billion. There are many personalised documents which would benefit from a higher degree of security, for example, driving licences, traveling documents (Shengen Visa), corporate ID cards (air pilot's ID cards, people working in nuclear power stations), and military personnel, police officers, custom inspectors, etc., are all potential candidates for more secure ID documents.
The approach as regards Lippmann OVDs as security devices is to introduce a complete recording/processing system which is used in house for the manufacturing of the labels. Currently, mass-produced embossed holograms and other types of OVDs (e.g. the kinegram) are produced by different holographic companies and then delivered to the issuers of security documents to be attached/laminated to various security documents. Since the Lippmann OVD is unique to a particular document, there is no need to produce them in advance. Therefore special recording/processing machines have to be made which can be sold or leased to companies or authorities issuing security documents. The machine is similar to a photocopier. The security document issuer records the Lippmann OVD at the same time the document is prepared. In this way the issuer has full control of the process and the security around it.
- G. Lippmann, "La photographie des couleurs," C. R. Hebd. Séances Acad. Sci. 112, 274-275 (1891)
- H. I. Bjelkhagen, "Lippmann photographs recorded in DuPont color photopolymer material," Proc. SPIE 3011, 358-366 (1997)
- H. I. Bjelkhagen, "Secure photographic method and apparatus," US patent No. 5,972,546, Oct 26, 1999
- H. I. Bjelkhagen, "A new optical security device based on one-hundred-year-old photographic technique," Opt. Eng. 38, 55-61 (1999)