Hans I. Bjelkhagen
De Montfort University
Network
Latest external collaboration on country level. Dive into details by clicking on the dots.
Publication
Featured researches published by Hans I. Bjelkhagen.
Applied Optics | 2001
Jong Man Kim; Byung So Choi; Yoon Sun Choi; Jong Min Kim; Hans I. Bjelkhagen; Nicholas J. Phillips
Silver halide sensitized gelatin (SHSG) holograms are similar to holograms recorded in dichromated gelatin (DCG), the main recording material for holographic optical elements (HOEs). The drawback of DCG is its low sensitivity and limited spectral response. Silver halide materials can be processed in such a way that the final hologram will have properties like a DCG hologram. Recently this technique has become more interesting since the introduction of new ultra-high-resolution silver halide emulsions. An optimized processing technique for transmission HOEs recorded in these materials is introduced. Diffraction efficiencies over 90% can be obtained for transmissive diffraction gratings. Understanding the importance of the selective hardening process has made it possible to obtain results similar to conventional DCG processing. The main advantage of the SHSG process is that high-sensitivity recording can be performed with laser wavelengths anywhere within the visible spectrum. This simplifies the manufacturing of high-quality, large-format HOEs.Silver halide sensitized gelatin (SHSG) holograms are similar to holograms recorded in dichromated gelatin (DCG), the main recording material for holographic optical elements (HOEs). The drawback of DCG is its low energetic sensitivity and limited spectral response. Silver halide materials can be processed in such away that the final hologram will have properties like a DCG hologram. Recently this technique has become more interesting since the introduction of new ultra-fine-grain silver halide (AgHal) emulsions. In particular, high spatial-frequency fringes associated with HOEs of the reflection type are difficult to construct when SHSG processing methods are employed. Therefore an optimized processing technique for reflection HOEs recorded in the new AgHal materials is introduced. Diffraction efficiencies over 90% can be obtained repeatably for reflection diffraction gratings. Understanding the importance of a selective hardening process has made it possible to obtain results similar to conventional DCG processing. The main advantage of the SHSG process is that high-sensitivity recording can be performed with laser wavelengths anywhere within the visible spectrum. This simplifies the manufacturing of high-quality, large-format HOEs, also including high-quality display holograms of the reflection type in both monochrome and full color.
electronic imaging | 2002
Hans I. Bjelkhagen; Dalibor Vukicevic
Until recently, display holography was usually associate with 3D imaging. After the appearance of color holography it has become possible, however, to record holographic images of 2D objects, such as, for example, oil paintings. The realistic-looking virtual image recorded in a Denisyuk reflection hologram is the most suitable for such reproductions. A holographic contact recording of a painting reproduces the painting with all its texture details preserved, such as brush strokes, the painters signature, etc. This means that an exact copy of the painting can be made, which can then be displayed at art exhibitions, museums, etc., when the original is not available. If an expensive painting is concerned, possessing an exact copy of the painting may also be important for insurance purposes, in case the painting is stolen or damaged. The advantage of a color contact hologram is that the hologram reconstruction process can be relaxed, as there is no need of spatial coherence of the white light source used to illuminate the hologram. In addition, no depth distortions are introduced as a function of the light sources distance from the plate. Only the angel of illumination is of primary importance if good color reproduction is to be obtained. The paper discusses the rendition of color in hologram, which is extremely important in this case. The holographic reproduction process of an oil painting is also described, and the major advantages of holographic reproduction are discussed together with its limitations.
Optical Engineering | 1999
Hans I. Bjelkhagen
A new optical security application of an old color photographic technique (Lippmann photography, invented in 1891) is presented. To- day, this type of photography can be applied as a unique security device on security documents, such as, e.g., identification cards, passports, credit cards, and other documents where a high degree of security is needed. A Lippmann photograph is very similar to the holograms cur- rently 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 medium in contact with a reflecting layer. Panchromatic photopolymer materials can be used and, after being re- corded 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 pho- tography or color copying machines.
Holography: A Tribute to Yuri Denisyuk and Emmett Leith | 2002
Hans I. Bjelkhagen
Two imaging techniques are presented which can create remarkable images. The first technique is color holography which provides full parallax 3D color images with a large field of view. The virtual color image recorded in a holographic plate represents the most realistic-looking image of an object that can be obtained today. The extensive field of view adds to the illusion of beholding a real object rather than an image of it. The other technique is interferential color photography or Lippmann photography. This, almost forgotten, one-hundred-year-old photographic technique, is also remarkable. It is the only color recording imaging technique, which can be record the entire visible color spectrum. It is not based on Maxwells three- color principle, the dominating principle behind most current color imaging techniques. The natural color rendition, make this 2D photographic technique very interesting. The reproduction of human skin and metallic reflections, for example, are very natural looking, which is not possible to record in ordinary photography.
Archive | 2005
Hans I. Bjelkhagen
Optical variable devices (OVDs), such as holograms, are now common in the field of document security. Up until now mass-produced embossed holograms or other types of mass-produced OVDs are used not only for banknotes but also for personalized documents, such as passports, identification cards, travel documents, driving licenses, credit cards, etc. This means that identical OVDs are used on documents issued to individuals. Today, there is a need for a higher degree of security on such documents and this chapter covers new techniques to make personalized OVDs.
Proceedings of SPIE | 2001
Jong Man Kim; Byung So Choi; Yoon Sun Choi; Sun Il Kim; Jong-Min Kim; Hans I. Bjelkhagen; Nicholas J. Phillips
Silver halide sensitized gelatin (SHSG) holograms are similar to holograms recorded in dichromated gelatin (DCG), the main recording material for holographic optical elements (HOEs). The drawback of DCG is its low energetic sensitivity and limited spectral response. Silver halide materials can be processed in such a way that the final hologram will have properties like a DCG hologram. Recently this technique has become more interesting since the introduction of new ultra- fine grain silver halide (AgHal) emulsions. In particular, high spatial frequency fringes associated with HOEs of the reflection type are difficult to convert employing SHSG processing methods. Therefore, an optimized processing techniques for reflection HOEs recorded in the new AgHal- materials is introduced. Diffraction efficiencies over 90 percent can be obtained for both transmission and reflection diffraction gratings. Understanding the importance of the selective hardening process has made it possible to obtain results similar to conventional DCG processing. The main advantage of the SHSG process is that high-sensitivity recording can be performed with laser wavelengths anywhere within the visible spectrum. This simplifies the manufacturing of high-quality, large-format HOEs, including also high-quality display holograms of the reflection type, both monochrome and full color.
electronic imaging | 2000
Jong Man Kim; Hans I. Bjelkhagen; Nicholas J. Phillips
Silver halide sensitized gelatin (SHSG) holograms are similar to holograms recorded in dichromated gelatin (DCG), the main recording material for holographic optical elements (HOEs). The drawbacks of DCG is its low sensitivity and limited spectral response. Silver halide materials can be processed in such a way that the final hologram will have properties like a DCG hologram. Recently, this technique has become more interesting after the introduction of the new ultra-high-resolution silver halide emulsions. An optimized processing technique for transmission HOEs recorded in these materials is reported. Diffraction efficiencies over 90% can be obtained for transmissive diffraction gratings. Understanding the importance of the selective hardening process has made it possible to obtain results similar to conventional DCG processing. The main advantage of the SHSG process is that high sensitivity recording can be performed employing laser wavelengths anywhere within the visible spectrum. This simplifies the manufacturing of high-quality, large-format HOEs.
Proceedings of SPIE | 2013
Hans I. Bjelkhagen
A review of recent improvements and applications in color holography is provided. Color holography recording techniques in silver-halide emulsions and photopolymer materials are discussed. Both analogue Denisyuk color holograms and digitally-printed color holograms are described. The light sources used to illuminate the recorded holograms are very important to obtain ultra-realistic 3D images. In particular the new light sources based on RGB LEDs are significant improvements in displaying color holograms with improved image quality over today’s commonly used halogen lights. Color holograms of museum artifacts have been recorded with new mobile holographic equipment.
electronic imaging | 2004
Marc Jager; Hans I. Bjelkhagen; Martin J. Turner
Lippmann photographs can, in principle, reproduce the entire incident spectrum at every point in the recording. This paper presents a comprehensive model of the Lippmann process, including exposure, chemical processing and subsequent reproduction. The main emphasis is on the optical properties of emulsions, where the theory of radiative transfer is used to obtain a detailed description of how interference patterns are formed in the presence of scattering particles and absorption. The results presented illustrate the reproduction fidelity of Lippmann photographs and highlight the most significant factors influencing their quality. Whereas the reproduction of monochrome sources is excellent, locally broadband signals are more problematic. Several practical measures to improve upon broadband performance are discussed.
electronic imaging | 2002
Jong Man Kim; Yoon Sun Choi; Hans I. Bjelkhagen; Nicholas J. Phillips
The recording and processing technique for color HOEs in ultrafine-grain panchromatic silver halide emulsions is presented. It is possible to obtain high diffraction efficiency employing the silver halide sensitized gelatin (SHSG) process. SHSG holograms are similar to holograms recorded in dichromated gelatin (DCG). The drawback of DCG is its low sensitivity and limited spectral response. Panchromatic silver halide materials from Slavich can be processed in such a way that the final holograms have properties like a DCG hologram. The processing method or microvoid technique has been optimized for three laser- wavelength recordings in Slavich PFG-03C emulsion. For example, applying this new processing technique high- efficiency white holographic reflectors can be manufactured. The technique is also suitable for producing efficiency color display holograms. In particular, masters for mass production of color holograms or color HOEs can be performed by contact-copying into photopolymer materials because the reconstruction wavelengths are identical to the recording wavelengths.