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 user 2008-04-04 at 3:06:42 pm Views: 62
  • #19806

    The missing ink
    2008  The instant photo is dead, long live the instant photo. After 45
    years, Polaroid says it will no longer make the iconic film for the
    press-click-develop cameras, blaming the rise of digital photography
    for a fall-off in sales. But its replacement represents a startling
    jump in printing technology: a pocket-sized printer that produces
    full-colour photos without using ink.Developed by Zink, formerly part
    of Polaroid’s research and development operation but spun off as an
    independent company in 2005, the technology has two components: a
    thermal printer, and a new type of photographic paper.Within this paper
    is the key to the photos — three layers of a dye material which is
    transparent and colourless in a solid crystalline form but which
    becomes highly coloured when melted.’The image-forming technology is
    embedded in the paper,’ said Steve Herchen, Zink’s chief technology
    officer. ‘There’s never been anything like this before. It’s the first
    entirely new colour printing system for over 20 years.’

    At the
    heart of the Zink system is the crystalline dye, which the company
    refers to as an ‘amorphochromic’ material. Development of the material
    started around 2000, said Herchen, when Zink was part of Polaroid. ‘Our
    chemists came up with the molecular design for them, designed
    syntheses, and worked with other companies to develop the processes to
    make them at a larger scale,’ he said.

    Like all organic dyes,
    the compounds are coloured because their molecular structure contains a
    chain of carbon atoms that are joined together in such a way that
    electrons can zip up and down the chain freely. This requires energy,
    and because electrons obey the arcane laws of quantum mechanics, they
    can only absorb specific amounts of energy, corresponding to specific
    wavelengths of electromagnetic radiation.

    The quirk of this type
    of structure is that the amount of energy the electrons absorb
    corresponds to the wavelengths of visible light. Different-length
    chains absorb different wavelengths and the colour you see is known as
    the complementary colour to the wavelengths that have been absorbed.
    For example, if a substance absorbs indigo light, it appears yellow; if
    it absorbs green light, it will appear red.

    amorphochromic dyes add a literal extra twist to this molecular
    racetrack. Organic molecules are flexible and, when they crystallise,
    they flex into a shape — known as a conformation — that allows them to
    pack together into a geometrical lattice structure.In the case of the
    Zink dyes, they can only form a lattice structure if they twist into a
    shape that does not allow the electrons to zip up and down the carbon
    chain. That means they cannot absorb visible light, and therefore
    appear colourless.’When they melt, they’re released from the rigid
    conformation that they’re held in by the crystal lattice,’ explained
    Herchen. ‘They’re then free to be in many conformations, and that
    enables the resonance within the chemical structure that leads to the

    Having developed this class of molecules, which Herchen
    said is unique to the company and covered by patents, the Zink chemists
    had to tune them in two ways.
    First, they had to make three types of
    compound, each giving a specific tone when melted: one magenta, one
    yellow and one cyan. Second, they had to ensure that the crystalline
    form of each dye melts — and therefore became coloured — at distinct
    and widely- separated temperatures. The yellow dye melts at 200°C, the
    magenta at 150°C, and the cyan at 100°C.

    To make the photo
    printing paper, the crystals are processed to a size around 2µm and
    dispersed in water, along with some additives that help to control the
    way the colour develops and stabilise it once the photo is printed.A
    multi-layer thin-film coating process sprays several layers
    simultaneously onto the backing, a white plastic film. Between each dye
    layer is a thin layer that helps regulate the way heat flows through
    the structure, and over the top is a UV-protective layer to prevent the
    colours fading in light, then a clear polymer which protects the
    crystals from the hot print head and also makes the photos glossy and
    waterproof.The whole structure is then dried to leave the multi-layer
    structure with the cyan-forming crystals nearest the backing, the
    magenta layer in the middle and the yellow on the top.This arrangement
    is designed to work with the type of thermal print head the Zink system
    uses. ‘Tens of millions of thermal printers are sold every year, it’s a
    well-established technology,’ Herchen said. ‘But the way the head is
    driven is unique to Zink.’

    Thermal print heads contain a line of
    tiny resistors that heat up when a current is passed through them. The
    Zink print head is 2ins long and has 600 resistors, each corresponding
    to a colour pixel on the printed photo. ‘What’s crucial to Zink is that
    we had to be able to control them independently, both in terms of how
    hot they get and how long a thermal pulse lasts,’ said Herchen.To
    obtain a yellow pigment on a single pixel, the resistor element
    delivers a hot but short pulse, hot enough to take the crystals below
    it to 200°C but short enough that the heat does not have time to
    percolate through the structure and melt the layers underneath. A
    cooler, but longer, pulse will have time to reach the middle layer and
    be hot enough to melt the crystals there, but will not melt the top
    layer or penetrate to the bottom, resulting in a magenta pixel.Cooler
    and longer still, and the heat reaches the bottom to produce a cyan dot
    but leaves the upper layers transparent. Precise control of the exact
    temperature and duration dictate the amount of crystals melted, and
    therefore the richness of the tone.For each pixel, the resistor must
    pulse up to three times, to develop the exact proportions of yellow,
    magenta and cyan which, when mixed together, produce the colour

    The dyes are transparent, so looking through the
    layers against the pure white background gives clean, pure colours.
    ‘There are a number of printing technologies which use a separate black
    along with the cyan, magenta and yellow, but we’ve found that we can
    get very acceptable neutral blacks and greys with good density by
    turning on all three colour formers,’ said Herchen.Once the colours are
    melted, the other additives come into play to keep the colours stable.
    ‘The melted, coloured material is a highly viscous, amorphous glass,’
    Herchen said. ‘If the softening temperature of the glass is kept high
    enough, the molecules can’t move around much, so they can’t reorganise
    into a crystal lattice.’

    The printing process takes about 30
    seconds to print a 2in x 3in picture, during which time there are some
    200 million heat pulses.
    The small size of the prints has attracted
    some scepticism but it is designed to capture a specific market,
    Herchen explained. ‘There isn’t any real limit to the size of print but
    we wanted to capitalise on the compactness of the technology — when you
    do away with ink cartridges and ribbons and so forth, you can eliminate
    a lot of volume. We thought there would be a great benefit for a mobile
    or pocket-size printer. We picked this size because it’s the size of a
    credit card or a photo that will fit in your wallet.’

    Also, each
    picture has a peelable sheet on the back revealing a sticky layer,
    allowing the pictures to be pasted into reports, for school and college
    reports, as well as for users such as estate agents, insurance
    assessors and police, who may need to produce instant pictures for
    documentation.The trend for camera phones was also an important factor.
    ‘In 2008, the projections for multi-megapixel camera phones say that
    the number of those that will be sold, just in this year, is more than
    the total number of cameras of any type that have ever existed,’
    Herchen said.’It’s staggering. And these camera phones will capture
    billions of images, and it’s awkward, inconvenient and sometimes nearly
    impossible to print them. We didn’t want to enter the market with a
    me-too product, like a home photo printer — people already have options
    to do that. We wanted to fill a need which had no way of being
    fulfilled. So we thought of making a printer that was very small,
    portable, battery powered, and can print wirelessly via Bluetooth or
    via a USB cable. There’s no other product or technology that’s capable
    of doing that.’

    Zink’s old parent, Polaroid, will be the first
    to release products using the technology, with both a mobile printer
    and a digital camera incorporating a printer in the shops for next
    Christmas.The company has several other licensees who plan to
    incorporate the printer into their own equipment. ‘We’re very proud of
    the technology,’ Herchen said. ‘And all image printing systems, from
    silver halide on to ink jets and lasers, have got better over their
    lifetime. This is just our first year.’