I confess that I did not perform a DEEP read of this article, but it did not strike me as a particularly new thing at all?
People have been shining colored lights onto silver halide paper to make digital prints for a long time. I don't quite get how Lumejet is any
different from, say, a Durst Theta printer, which has been around for quite a while.
What am I missing?
The essential statement here is correct – and this goes for many other comments made (especially re gamut). We are not doing anything new in terms of the basic principles of shining RGB lights on to silver halide paper, and we are using the same silver halide paper as everyone else. What is different is our focus on the precision with which we do that, both in terms of the physical direction of the light onto the paper, and the control of those light pulses, as well as the way we interact with the paper itself.
There is some (intentionally relatively superficial) description of this at
https://www.lumejet.co.uk/technology/. Our technology has been under development since approximately 2000 and many millions of $ (close to $25m) have been spent to date on developing the machine.
Fundamentally, silver halide paper has one basic limitation - the gamut of the paper and the emulsions. This is something we can do relatively little about; we use the best Fuji Crystal Archive Professional DPII papers – their exhibition quality papers – that are available to everyone else (although not all pro labs use these papers, and consumer-oriented labs do not). What we have done ourselves is to re-characterize each media (paper type) using our RGB print head and created bespoke target files for each paper, and then build the best colour profiles we possibly can for our machines.
For the purpose of colour management, a printer must be calibrated to the centre line of the Lab colour space. This calibration produces a neutral Lab grey line from the media white point through to the media black point (DMAX). The calibration provides a repeatable reference point that can be calibrated to before a shift. The reference point is used as the base line foundation upon which the machine’s media colour profile (.icc) is characterised and built.
A media's neutral grey line calibration is specified in the media target (.tgt) file by specifying a number of input R,G,B values and corresponding output C,M,Y colour densities. The target file has a number of incremental steps of input RGB values from white (0xFFFFFF) through grey to black (0x000000). For each step, there is a corresponding output C, M, and Y media colour density that when measured in D50 Lab colour space, produces a Lab neutral colour. When all steps are put together, a Lab neutral grey line is produced by the media. The target file is used by the printer to calibrate the media's DMAX (black). After black calibration, the target file is used to balance the grey to produce the neutral grey line for colour management.
Our target files yield highly accurate neutral LAB gray lines and high DMAX. This is a skilled and ‘black art’ process that few know how to do: Fuji themselves do not do this (or do not share the results if they do), so we had to develop a technique and process in-house to do it ourselves. We believe our target file generation process is very accurate along the whole tone curve and hence we follow the neutral grey line in the 3D gamut. This then gives us the foundation to build our colour profile more precisely through the colour space to the outer edges of our gamut and to determine how we handle out-of-gamut colours.
The key advantage of silver halide is that it is continuous tone. So a single image pixel of any colour is represented on the page by a single pixel of that colour – as opposed to the structured combination of dots of different coloured inks required to create a pixel of a single colour with an inkjet. This does not always matter, but complex effects like subtle split-toning and gentle tonal variation over a page can be very difficult to achieve correctly with inkjet half-tone processes. Halftone is a trick of the eye (side by side dot combinations) that requires a K layer to make up for CMY inks not producing a dense black. Contone is real Newtonian light combination of just CMY dyes as tiny colour filter plates in a vertical stack (dot on dot). And our CMY unit cells are much better aligned, to microns, than the laser and other LED printers, as those were not designed to handle fine text and graphics (requiring 400dpi, the Nyquist minimum for hand held prints). We estimate that inkjet systems would require 12 colours and 10x the pixel density to produce something approaching contone quality .
What really makes the difference with silver halide is how the paper receives light. This is where the majority of our improvement lies. The emulsion layers of the paper contain silver crystals and colour couplers that together, when activated by light, form colour clouds. These clouds are approximately 5 microns in diameter. So the paper is itself capable of resolving extremely small (invisible to the human eye) levels of detail. But this depends on how accurately the paper is imaged. In a traditional darkroom enlarger set-up, light from a small source was beamed through a negative onto paper of the relevant size. The enlargement process caused softening of the image because of the enlargement itself and edge effects as light towards the edge of the paper was not hitting the paper vertically. All silver halide print machines attempt, in various ways, to address this issue – more or less successfully. Some use lasers, some use LEDs, but each of them has imperfections in the accuracy of this imaging process.
We developed the Lumejet printer from scratch over the past 15 years, starting from research projects in Warwick Manufacturing Group’s laboratory in Warwick, UK. The research came about to address the fact that it was then impossible to print sharp text accurately on silver halide. Our printer uses a proprietary print head that incorporates a patented optical fibre taper to reduce an array of 576 individually-addressable RGB LEDs down to a very precise spot. Each LED is 300 microns in diameter, and these are reduced by the optical fibres to 60 microns each. The fibre taper is essentially a pixel projector that takes an array of larger LEDs and produces smaller pixels on the paper to produce ultra-sharp images, text and graphics. The print head is positioned just above the paper and scanned, to micron accuracy, across the paper (like an inkjet with light). As the head passes over the paper, successive RGB LEDs are triggered so that each pixel area on the paper is imaged successively by one LED of each colour, all placed exactly on top of each other. The finished pixel is therefore formed extremely accurately and receives only the light signal from the three R,G,B LEDs forming that pixel. There is minimal cross-talk between pixels. Because the print head is always directly above the paper, light beams always enter the paper exactly vertically, eliminating edge effects. The motion control and signal processing software that allows each and every pixel to be placed precisely where it should be is highly complex and at the core of our proprietary process. The printer prints 12mm swathes at approximately 1.5m/s across the paper (Y axis) to a pixel placement of c. 1 micron. The paper is then indexed forward by the swathe width (X axis), to a precision of c. 5-7 microns, and the next swathe is printed, with a small overlap (which is blended) to eliminate any motion errors.
So the difference between our technology and other silver halide printers boils down to:
(i) our pixel is smaller at 63.5 microns (400dpi) - which is the smallest the human eye can resolve for prints held at 14”.
(ii) our pixels are placed more accurately next door to each other so there is no overlap;
(iii) the paper is always imaged directly from above so cross-talk between pixels is minimized;
(iv) we have made a huge investment in calibrating the printer to the latest papers and profiling our printer to ensure that it gets the most out of those papers without compromise. For instance, some other printers will operate at a higher DMax. This produces deeper blacks, but it also causes yellow flaring on whites due to cross-talk and over-exposure. We currently dial back our DMax slightly to eliminate this problem – we believe it is better to have pure greyscales than to push blacks to the max, although we accept that this will not always be optimal. We continue to work on this area and expect to increase DMax progressively over time.
(v) Our printers are manufactured today by us, and constantly updated – unlike most of the high-end competition that has been discontinued.
(vi) We are the only photo lab in the world that builds its own printers from scratch, so far as we are aware.
In terms of the more detailed technical differences that make our printer stand out:
Principally it is in the design of the print head, which came about from first principles of physics laid out at the initial Warwick R&D stage more than 15 years ago e.g.
1) How the 5:1 fibre taper bundle is made and drawn (so there are roughly 300 fibres in front of each LED) and structured, with interstitial black rods between the fibres used to reduce light scatter;
Attached is an image of the top of a tapered fibre bundle:
2) How the fibres are made (by multiple draws of glass with different indices) and the Numerical Aperture of the fibres themselves, to couple the LED light into the fibre internal core, rather than scatter sideways. The NA of our fibres is c. 0.80, which couples light efficiently down the fibre core with little sideways cross-talk, producing a “tapered light funnel” effect in front of each LED with very sharp edges;
3) How the individual RGB LEDs are robotically placed (to c. 20 microns), wirebonded and aligned on the print head array and exposed through a very accurate and hard edged black mask. This is Gerber plotted at 2400dpi, with precise pitch and interlacing between odd and even rows, to provide c. 10% pixel overlap and avoid micro-banding. We are in fact imaging the mask edges, rather than the LEDs (which vary in size and position);
4) How the fibres are drawn from 20 microns (top) to 4 microns (bottom), so although we print with a 63.5um (400ppi) spot, the edge of the spot is constrained by the 4 micron fibres at the exit end and gives very little halation (flare, light scatter);
5) How 4-6 microns is also the grain size of the AgX emulsions – so we are effectively dropping photons on grains (all part of the design); and
6) How the RGB light at the fibre exit is transferred 1:1 to the paper using a telecentric relay lens (9 stack lens) that has been designed to minimize chromatic aberration (different paths of different wavelengths) and produce parallel light rays into the emulsion.
Sample scans of text and images are shown at
https://www.lumejet.co.uk/technology/ Our unit pixel cells are clean with little cross-talk. All of the above design elements were initially addressed and produce really clean, sharp text and graphics: they make for a beautifully sharp photographic reproduction – while older photographic print techniques are inherently less sharp and produce beautiful photos partly through that inherent softness!
To pick up on a few well-known machines:
The Durst Lambda was an RGB laser spinner system, which imaged at 200dpi natively (switchable to 400dpi) and suffered from RGB pixel alignment from middle to edge due to the F-Theta flat field lens (it is also 50” wide). It was aimed at poster making, not small format prints.
The Durst Epsilon and Theta (30”) used multiLED prints heads, imaging at 256dpi, but their LEDs were placed to one side and the light was piped by fibre cables and a large lens onto the paper. Again they were built mainly for images, not text and graphics, and the final beam shaping/delivery was not done as LumeJet.
Other machines are also very good, but none was designed to print text and therefore all suffer from some compromises in sharpness and accuracy relative to our machine.
