The missing ink

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.

Zink’s 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 colour.’

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 required.

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.’