A novel fluorescent security label has been produced that could replace numerous conventional fluorescent dyes in document security. This label utilizes rare earth ions doped in a borosilicate glass matrix to produce sharp spectral fluorescence peaks with characteristic long lifetimes due to the rare earth ions. These are subsequently detected by an online detection system based on fluorescence and the long lifetimes to avoid any interference from other fluorophores present in the background. Security is further enhanced by the interaction of the rare earth ions with each other and the effect of the host on the emission spectra and therefore the number of permutations that could be produced. This creates a very secure label with various applications for the security market.
Hologrammetry has many advantages over conventional imaging techniques for subsea visual inspection. Holograms recorded underwater can be replayed in the laboratory to provide an optical replica of the original subject. Real-image reconstruction allows planar 'optical sections' to be isolated and measured directly. However, these advantages can be removed by poor optimization of the reconstructed image. Furthermore, recording the hologram in water and replaying in air increases the magnitude of the optical aberrations which may be apparent. Such aberrations can be minimized using index compensation whereby the hologram is replayed in air with a wavelength which is equivalent to the effective wavelength of the beam in water. To monitor the influence of these effects and to establish the validity of the index compensation method, reconstruction takes place in a micrometer-controlled plate holder to allow precise positioning about all three rotational axes and the three translational axes. The image is viewed using a lensless TV camera or measuring microscope which is accurately moved through the image volume to provide dimensional information. Index compensation has been shown to work well for both back-lit and front-lit off-axis holograms and is effective over a wide range of field angles. Typically an on-axis resolution of around 1 1p/mm for a front-lit hologram replayed at the recording wavelength will increase to over 20 1p/mm when reconstruction takes place at the compensation wavelength. The corresponding astigmatic difference reduces from around 100 mm to less than 2 mm on employing compensation.
Natural full color holographic images can be perceived by the viewer by superimposing red, green, and blue images simultaneously. This is simplest with single stage reflection holograms and has been successfully accomplished by a number of workers using a variety of techniques on single emulsion or by `sandwiching' red and green/blue sensitive emulsions together. Alternatively using `rainbow' techniques has resulted in good natural color holograms and controlled swellings of a single emulsion has produced spectacular pseudo-color holograms. It would be advantageous to have a single emulsion, of broadband sensitivity, capable of recording color holograms. Such an emulsion, from Russia, was used initially at the University of Aberdeen to create color holograms with good results. Investigations were then carried out on emulsions commercially available in the UK to see if any were suitable for color holography, using single layers. It is shown that a near natural color image can be obtained from a single stage reflection hologram recorded on Agfa 8E75HD film, with no preparation, due to it being sensitive to green and blue wavelengths, as well as to red for which it is normally associated.
The advantages of holography for remote optical inspection and mensuration can be greatly reduced by loss of image quality caused by an unoptimized reconstruction geometry. For underwater holography it is essential that image quality is high to ensure mechanical defects, such as cracks, corrosion, or deformities, can be detected. In practical replay systems, a number of physical constraints limit image resolution to a value significantly lower than that theoretically possible in an ideal system. In most cases, even this limited resolution is unobtainable, due to mismatch of the reconstruction wavefronts caused by imprecise relocation of the hologram and inexact conjugation of the reference beam. It is shown how deviations from the optimum reconstruction geometry affect image resolution, both theoretically and experimentally, for a number of in-air and underwater recording geometries.