Technical white paper

Overview of HP LaserJet Pro colour printing technology How HP Image Resolution technology delivers unsurpassed print quality

Table of contents Make an impression with HP Color LaserJet Pro printers............................................................................. 2

HP Resolution Enhancement technology ........................................................................................................ 2

Variable dot size and variable dot placement ............................................................................................ 2

Single vs. multi-level printing ......................................................................................................................... 3

38,400 x 600 dpi enhanced resolution ........................................................................................................ 4

Great colour out-of-the-box .......................................................................................................................... 4

Adaptive halftoning .......................................................................................................................................... 5

Trapping .............................................................................................................................................................. 5

Halo reduction ................................................................................................................................................... 5

Automatic closed loop colour calibration .................................................................................................... 5

Original HP Toner Cartridges with JetIntelligence ..................................................................................... 6

Conclusion: HP printers deliver superior print quality ................................................................................... 6

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Technical white paper | HP LaserJet Pro colour printing technology

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Make an impression with HP Color LaserJet Pro printers

Print quality has become increasingly important. Businesses produce documents to help them stand out and portray a professional appearance for customers and other external audiences. Of particular importance is the quality of colour documents, often developed to support sales and generate revenue. Consumers have also become more discriminating in their expectations. No longer is the specification of physical dots per inch (dpi), also known as optical, or spatial dpi resolution, adequate as the sole indicator of print quality. To deliver high-quality results, HP has developed a collection of innovative technologies collectively known as HP Resolution Enhancement technology (ImageREt).

HP Resolution Enhancement technology

HP ImageREt system sets itself apart from the competition by integrating proprietary technology advancements that optimise each element of the print process. This innovative system includes the ability to intelligently vary the size and position of the dots that make up text and images. Additional aspects of HP ImageREt and other innovative technologies include multi-level printing, adaptive halftoning, trapping, halo reduction, closed loop colour calibration, and Original HP Toner Cartridges with JetIntelligence.

This paper discusses these innovations and describes how HP Color LaserJet Pro printers achieve superior colour rendering and print quality.

Variable dot size and variable dot placement HP uses sophisticated laser scanning assemblies and industry-leading laser modulation control to vary the size and position of each laser exposure within each pixel on the addressable printer grid. This combination enables spectacular detail and reproduction fidelity. HP’s variable dot placement allows printed dots to be positioned in locations other than the centre of each pixel on the printer grid. Very small printer dots can be placed near pixel edges to achieve fine-line detail and images with crisp, smooth edges and improved print quality.

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Variable dot size and location on imaging drum

Pixel on photoconductor surface

Variable dot size and placement

Technical white paper | HP LaserJet Pro colour printing technology

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Single vs. multi-level printing

Single-level printing Single-level (or binary) printers use a binary printing process that cannot vary the amount of toner applied to a pixel. Each pixel receives either no toner (0) or a full pixel (1) level of toner for the four primary toner colours—yellow, cyan, magenta, and black. These toner colours can only be mixed to create red, green and blue (white is represented by the absence of colour). Therefore, only eight different colours can be applied to each pixel.

In binary printing, intermediate shades of colour within a single pixel are not possible, so in order to produce the many colours required for colour images, binary printers rely on a process known as dithering. Dithering creates the perception of other colours by combining the eight solid colours listed above in various groups of dots in an area called a halftone cell. This allows thousands of dithered colours to be produced, giving the appearance of colour variations or levels. The eye perceives these grouped patterns as additional colours. In the example below, magenta, cyan and black binary dots are combined to create the appearance of purple.

The downside of dithering is this grouping of individual dots effectively lowers the dpi resolution. Prints can appear grainy with rough colour gradations. Also, as larger halftone cells are used to create more colours, more area on the printed page is needed to represent a particular colour, resulting in a visible dot structure and reducing edge sharpness.

Multi-level printing HP multi-level technology yields millions of colours and delivers great print quality. It goes beyond single-level printing in two important ways:

• The printers can vary the amount of each toner colour, which is then blended to produce the exact shade desired.

• HP ImageREt uses complex algorithms to blend up to four toner colours together within the space of a single dot.

Multi-level technology enables HP printers to produce individual dots capable of millions of different colours. Combined with other ImageREt technologies, HP multi-level technology creates smooth colours, sharp images, and crisp text.

Desired appearance White

Very light cyan

Light cyan

Medium cyan

100% cyan

Binary halftone cells, 2 x 2 dots

Magnified binary print

Binary printers require grouping of multiple dots and use dithering to create the appearance of tone levels, with reduced effective resolution

HP multi-level single dot pixels

Magnified HP multi-level

print

HP multi-level printing can vary the amount of toner within a single dot, creating exact shades with high effective resolution

Binary dithering creates perception of colour

Technical white paper | HP LaserJet Pro colour printing technology

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Varying colour combinations In the simple example below, you can see how these millions of colours can be created. By varying the amounts of yellow and magenta toner, you can combine the two in different quantities to produce many shades in one dot area or pixel. This creates many more options for each dot than just “on (1)” or “off (0).” Each dot can have multiple shades of colour, requiring minimal dithering and clustering of multiple dots, and no large halftone cells.

38,400 x 600 dpi enhanced resolution Until recently, HP Color LaserJet Pro printer specifications focused on optical (or spatial) resolution, but optical resolution alone is an insufficient indicator of print quality. To address this, HP Color LaserJet Pro printers now include an enhanced resolution specification.

As previously discussed, binary printers rely on dithering to create the appearance of intermediate tone levels and to produce colours other than the printer primaries. The dithering process groups individual printer dots into halftone cells, which unfortunately reduces effective print resolution. Since multi-level printers produce intermediate tone levels, they rely less on dithering. As the number of tone levels a printer can produce increases, the need for dithering decreases. HP Colour LaserJet Pro printers offer 6-bit multi-level printing, meaning they can produce 64 tone levels per pixel for each primary colour. This 64 multi-level capability means that these printers require less dithering, resulting in an effective enhanced resolution of 38,400 x 600 dpi.

Great colour out-of-the-box HP printers implement several methods to produce the most satisfying colour rendering possible. Default colour tables are optimised to produce the best colour appearance match between a page viewed on a monitor and that same page printed and viewed in a typical office environment. To accommodate a variety of printing needs, optional colour renderings are available from the print driver, such as Vivid or Photo. There are also colour tables optimised for different media types which are automatically implemented based on the weight and type of media. The result is great colour out-of-the-box, taking full advantage of the increased colour gamut and image quality capabilities when using premium media types. In addition, colour renderings are optimised for a variety of printed documents, such as photographs or business graphics.

Various amounts of yellow

Various amounts of magenta

A variety of shades

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Technical white paper | HP LaserJet Pro colour printing technology

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Adaptive halftoning Adaptive halftoning is an HP-patented, image-enhancement feature that helps to smooth edges of colour text and graphics while maintaining the quality of area fills. One of the difficulties of digital colour printing is producing smooth, even area colour while at the same time producing clean and sharp edges. Adaptive halftoning provides a solution to this problem.

Smooth large area colour is optimised by selecting low-frequency digital halftones, which have larger spacing between halftone cells, and reproduce image areas of low detail with improved smoothness. The colour is built up by turning on and off dots while using multi-level printing as described earlier in this paper. At a normal viewing distance, your eye blends the dots together so the colour looks smooth. Using low-frequency halftones creates smoother, more accurate colour in large areas containing the same colour, such as in large text, bar charts, etc.

While the eye blends the dots in smooth areas, it is also very good at discriminating minor variations at edges. Edge sharpness is optimised by selecting high-frequency digital halftones, which have smaller spacing between halftone cells and better reproduce fine detail and eliminate jagged edges. Adaptive halftoning provides a solution to the requirement for smooth colour and sharp crisp edges by locating and increasing the digital halftone frequency at edges. This HP-patented technology is implemented in specialised electronics to provide fast printing performance.

12 point text without adaptive halftoning

12 point text with adaptive halftoning

Trapping Trapping uses HP-patented image enhancement that deliberately overlaps one colour with another to deliver high-quality edges between adjacent colours. HP trapping is different from traditional press implementation because it includes halo reduction (described below). While it used to take graphic designers or commercial press operators many hours to do this manually, the HP-enhanced version is offered as standard, performed automatically, and with no performance penalty.

Without trapping With trapping

Halo reduction Automatic halo reduction makes one colour intelligently recede from the edge of the other colour, reducing the overlap and making it less visible. For example, a process black line is composed of black, the dominant colour, and cyan and magenta, the subordinate colours. The cyan and magenta near the edge of the black line are reduced or removed. This prevents colour from being seen on the edge of the black line. Halo reduction can also be applied to other line colours, like green lines, where cyan is dominant and yellow is the subordinate colour.

Automatic closed loop colour calibration Printer calibration measures toner patterns and adjusts the amount of toner to a specified level to help ensure colours print reliably and consistently. The settings are stored in the printer’s memory and applied to every printed page. Under normal printing conditions, printed images are transferred directly to paper, but during the calibration sequence, the calibration image prints to the electrostatic transfer belt surface. The printer uses the surface of the belt as a reference point to make all calibration adjustments.

Technical white paper | HP LaserJet Pro colour printing technology

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© Copyright 2015 HP Development Company, L.P. The information contained herein is subject to change without notice. The only warranties for HP products and services are set forth in the express warranty statements accompanying such products and services. Nothing herein should be construed as constituting an additional warranty. HP shall not be liable for technical or editorial errors or omissions contained herein.

4AA6-1606EEW, September 2015

Original HP Toner Cartridges with JetIntelligence The latest HP LaserJet printers are built around a breakthrough in toner chemistry using the new Original HP Toner cartridges with JetIntelligence. The spherical shape of the precision black toner provides more control and precision in transferring toner to the page, which produces sharp text, bold blacks, and crisp graphics. HP ColorSphere 3 toner is specially engineered with a soft core and hard shell, which means greater energy efficiency and vibrant colour. The soft core facilitates quicker melting and a smooth flow onto the page, while the durable, hard shell reduces toner particle wear and maintains toner shape and size from the first printed page to the last.

Conclusion: HP printers deliver superior print quality

HP Color LaserJet Pro printers employ multiple innovative technologies integrated into a complete printing system. The result is maximised ease of use and high-quality output with exceptional image detail, crisp text, and brilliant colours with ultra-smooth tonal gradations. With these built-in image enhancement technologies, HP Color LaserJet Pro devices consistently provide the best print quality, automatically and right out of the box.

Polymer

Wax Hard shell

Soft core - Supports more pages - Enables faster printing - Uses less energy to print a page - Legendary colour print quality

- Enables higher yields

Durable shell