A commercial recording material for high-resolution colour holograms that is made from ultra-fine grain silver halide emulsions has been developed by a group of researchers working as part of the European SilverCross project. The resolution is comparable to modern digital photographs, which achieve 10,000 lines per millimetre. This could be used in advertisements, photo displays in galleries, or in packaging.
Silver halide materials have been employed in holograms for decades, however, their large grain sizes (200-300nm) mean that the recorded images are not of a high quality, and have difficulty displaying the full colour spectrum. The SilverCross researchers have restricted the growth of silver halide crystals to 5-10nm, producing images that have ultra-high resolution and low light scattering of the blue recording and replay wavelengths.
‘One technique we used to keep the crystals as small as possible was to mix them [with a Rousselot de-ionised, photo-type gelatin] at a lower temperature than normal [32-38ºC]. We then immediately froze them, and got rid of the water when we thawed them out,’ explains lead scientist Professor Hans Bjelkhagen of Technium OpTIC, an opto-electronics technology centre located in St Asaph, UK.
‘This freeze/thaw technique makes it possible to maintain small particles and get a high concentration [in order to achieve high sensitivity]. Otherwise you would get a low sensitivity because the particles are so tiny.’
The emulsion produced is blue sensitive, so green and red spectral sensitising cyanine dyes are added to broaden the colour spectrum to around 670nm. The emulsion is then manually or machine coated onto a 10.2 by 12.7cm clean glass plate (although Bjelkhagen says the size of the holograms could go up to two square metres).
Researchers are currently drawing up a proposal for the second phase of SilverCross to scale-up production. ‘Most photography companies know about holograms, but they don’t know what we are able to do with such emulsions. They are sitting on machinery and technology that could be adapted to holography,’ says Bjelkhagen.
His team is also working on the UK Department for Business, Enterprise and Regulatory Reform’s Holoendoscopy project, which began in 2006. The aim is to record holograms on the tip of an endoscope to create a holographic display of a person’s tissue that can be analysed, without the need for surgery.
They are also developing holograms that can be copied onto a polyester or tri-acetate film, which are lighter and more durable than glass.