In one part of the previous discussion, a dye was dispersed in a clear plastic and then ground up to make an ink. The advantage was the pos- sible increase in the stability of the colorant and the disadvantage was its scattering nature. Now let’s consider coating the particles with a partially cross-linked polymer and dispersing the whole combination in a material that makes the polymer semi-soluble. When the material is deposited and the solvent starts to evaporate, the polymer will coat the ink particles and create something close to a smooth layer, with close to only one index of refraction. The polymer will continue to cross-link and become durable. The result is that the scattering from the individual particles will be greatly reduced and the layer will act more like dye in a clear medium. This pro- duces a colorant system that has greater stability and more vibrant color capability.
The only disadvantage is a reduction in shelf life of the ink. Little bits of dye-impregnated glass dispersed in water will have a long shelf life. We might have to shake or stir the mixture before using, but the particles will disperse again as per the original state. Not so if there is a sticky polymer present. As the particles settle out, the polymer will begin to stick the particles together, and no amount of shaking or stirring will rever- se the process completely. These improved inks tend to have limited shelf life.
It is diffi cult to make this type of ink. The optics, chemistry, and rheol- ogy must all be just right, but progress is being made. In addition to better color stability and more vibrant colors, another special result is better reso- lution. In dye sublimation printers, the size of an individual pixel is con- trolled by the size of the heating element in the printer. The smaller it is, the sharper the image can be. With such a printer, the three colors can be superimposed and the size of the pixel on the print is the same as the size of the drop of dye that is deposited.
With ink printers, the story is much more complex. The ink can be laid down in very small drops, but it takes several drops to make a pixel. Since the ink is essentially opaque, one drop of ink cannot be placed on top of another. They have to be side by side. Each drop blocks some of the white of the underlying paper surface.
In the simplest model of a black-and-white laser printer, portions of the paper are reserved for pixels. To make white, no ink is deposited in the reserved pixel area. To make black, the whole area is covered with dots of black ink. To get a level of gray that is just above white, one dot is depos- ited. The next level of gray is accomplished by depositing two dots, and so on, until the whole area is covered with dots. The total number of dots that is required will be equal to the bit depth of the image (let’s say 256). If the array of dots is square then it is easy to see that the sides of the array will each be composed of 16 dots. The pixel will be composed of 16 rows and 16 columns of dots — 16 times 16 is 256. That is, the pixel will have a num- ber of dots on each side that is equal to the square root of the bit depth. This means that a printer rated at 1600 dots per inch, in the simplest case, will have an actual resolution of 1600 divided by the square root of 256 (16) and have a real resolution of 100 pixels per inch. Note that the dye sublimation printer may have a dot size that is 1/300 inch in size, which means it can lay down 300 dots per inch. But since each dot of a dye printer is actually a pixel, this printer has a resolution of 300 pixels per inch. In reality, in modern inkjet printers, there are ways to mathematically improve the resolution of the simple model shown here. But in any event, do not confuse dots per inch with pixels per inch. In dye sublimation printers, the number of dots per inch is the same as the number of pixels per inch. In ink and laser printers, the number of dots per inch is consider- ably higher than the number of pixels per inch. And since the viewer will be seeing pixels, the actual resolution will be lower than the dots-per-inch value.
As progress is made on new inks — ones that have more fi lm-forming properties that emulate dye layers — it will be possible to approach the one dot equals one pixel formula of the dye printers. At the same time, there is also progress in printer designs. They are using more than just the CMYK colorants, which can reduce the needed pixel depth for each color. These also have special patterns to maximize coverage, and they change the bit depth at sharp edges. The result will be greatly enhanced print resolution and better image brilliance.
Finally it is important to remember that modern digital printers perform signifi cant mathematical adjustments to the image sent to them by the com- puter. Resolution will be recomputed in order to fi t within the parameters of the physical printing mechanism. Bit depth might be altered in certain portions of the image to enhance apparent sharpness. Tone scales might be adjusted to compensate for the refl ectance properties of the materials used in the printer. All
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these adjustments are applied to all incoming image fi les and the adjustments are made to the entire image, not just preselected parts. They will not alter the image in any way that will lead to a false inclusion of a specifi c individual.