Glossary of Terms

Actuator

The element inside a piezo-electric printhead in which a pressure wave is generated to eject a stream or discrete drops of ink through the nozzle. The actuator will normally be found within the ink chamber, sometimes forming the actual chamber walls. See also Piezo Printhead and Thermal Printhead.

Adobe Illustrator

A vector based linework drawing program from Adobe Systems, available for Mac OS X and Windows operating systems. The current version is Illustrator CC 2014, part of the Adobe Creative Cloud suite of rental-only programs. Its main output file formats are the native AI, and the device-independent EPS and PDF.

Adobe Photoshop

The dominant program used in professional photography, design and print for processing, editing retouching and compositing of photographic and similar bitmap images. It was originally introduced in 1990 for the Apple Macintosh, and later developed as a parallel Windows version that now offers almost identical features.

AFP

Stands for Advanced Function Presentation. It's a printer architecture that's mainly used for financial, transactional and direct mail applications, which contain personalised and other variable content. It was originally developed by IBM (which called it Advanced Function Printing), but was handed over to the multi-developer AFP Consortium in 2004, which brought it up to date with respect to colour content.

Analogue

Infinitely variable, the stuff of everyday experience. A dimmer switch on a room light (or car instrument lights) is analogue, as the light increases or decreases smoothly and steplessly. In the printing sector, analogue is normally used as a distinction from digital, which splits things up into tiny chunks, all of the same size. Think of analogue as a ski-slope, and digital as the stairs you use to reach the top.

Aqueous ink

An ink using water as the main carrier. Also called water-based ink. This ink is considered non-toxic and safe for general domestic and office use. Aqueous ink is also often used for high quality fine art and photographic printing. It has little or no odour during or after printing. It's normally more expensive than solvent based inks.

Array

A general term that has more specific meanings in inkjet printing, as either an alternative name for a print head (in the nozzle sense), or the collective term for a number of such print heads arranged together. See print head.

Banding

Visible lines or stripes of differing density on a printed inkjet image, considered as a fault. It's always associated with scanning-carriage inkjets and appears across the print width. If the print is intended for viewing from a distance, such as a billboard, banding may not be a problem as it is hardly visible.

Bézier curves

These are mathematical descriptions of curves that are commonly used for vector drawing, with a graphical user interface that allows the user to create and modify them. On-screen the designer sees them as arcs linked by anchor points with extendable handles that are used to alter the shape to any extent.

Binary Inkjet

A drop-on-demand printhead which can either fire a drop of one particular size or no drop at all (binary being on or off with no in-between). This contrasts with greyscale heads, that can fire several different droplet sizes to give different ink densities. See Greyscale.

Bit

Binary Digit. Computers work with numbers built up from just two states: 0 or 1, equivalent to an electrical switch being off or on. They are normally grouped together in eights, called bytes. An 8-bit byte can contain any number between 0 and 255. See Byte.

Bit depth

Each pixel in a digital image can be assigned a particular shade, or grey level, between white and black. This is represented in a computer by a binary number, ie a string of 0 and 1 numbers.

Bitmap

Technical description for the way a computer builds up an image from building blocks of dots, or pixels. An image on a screen is a bitmap. A processed image that is output by a raster image processor (RIP) to a printer or imagesetter is a bitmap.

Byte

A block of bits, normally a group of eight. Using binary numerals, an 8-bit byte can be used to count from 0-255, giving 256 values. This figure appears frequently in graphic arts, as it is often used as the number of density levels per colour that a computer screen or halftone dot is capable of reproducing.

CAD

Stands for Computer-Aided Design. Generally vector-based software that is used in the printing industry for packaging structural design, but it can also drive laser forme cutters for cutting and creasing dies, or cutting tables for signage (though normally the cutting layout is incorporated as a separate layer within a graphics file).

Capping

Covering the printheads when the printer is not in use for an extended period. This helps to prevent solvent ink evaporating and drying in the heads, or in the case of UV inks, it protects them from stray UV light. Normally capping is performed automatically by the printer, either as part of the "sleep" process after a set time of disuse, or on shut-down.

CMYK

The abbreviation for the primary transparent ink colours needed to an acceptably full range of colours for something like a photograph. Stands for Cyan, Magenta, Yellow and Black. The letter K is used for black, partly because it avoids possible confusion with B for Blue. K actually stands for Key, dating back to when colours were printed in four separate passes (as is still often the case for screen printing). Black was printed first, then this image was used as a guide, or key, to register (align) all the other colours to. See: RGB, colour separation, process colour. See Spot Colour, Process Colour.

Colour

A sensation produced by different wavelengths of light falling on the retina of the eye. The eye perceives visible light in effect as mixtures of three components, red, green and blue. Equal mixes produce white, and no light produces black.

Colour gamut

The complete range of colours that can be printed possible from a particular combination of printer, inks and medium. Offset printing inks are quite limited when using CMYK colours only, but many screen process and inkjet ink sets have significantly wider CMYK gamuts. Some printers also offer orange, red, green or violet process colours to extend the gamut further.

Colour management system

A means of achieving standard colours from a variety of different devices by referring their results to a known colour space model.

Colour Management.

In current practise this usually means processing the colours in software using small files called profiles that compensate for the particular characteristics of the printer, ink and medium being used.

Colour Matching System

A way of defining and standardising colour for reproduction through printing and other visual technologies. Commercial systems such as Pantone supply reference books of colour patches, with instructions on how to match these colours by mixing standard colours of inks.

Colour separation

The result of splitting a coloured original image into its component parts for printing. A full-coloured photograph will normally be split into cyan, magenta, yellow and black separations, and these will be carried on individual films (for screen process), printing plates and cylinders (for lithography, flexography, gravure etc)or output channels (for inkjets and other digital printers).

Colour space

A description of the total range of colours achievable by a particular process. This includes the colours discernible by the average human eye; those for a bee or a dog would be quite different.

Computer-to-Screen (CtS)

A range of methods and technologies have been developed to print a screen process stencil directly onto the screen mesh, instead of using emulsion coated stencils exposed through a piece of film. The processes include inkjet (either a water-based ink or a wax phase-change ink), digital light process (essentially a digital projector system) or laser exposure.

Continuous Inkjet (CIJ)

A continuous Inkjet generates a stream of ink droplets all the time, with the stream directed towards or away from the media by deflectors of various types (typically electrical fields or air jets). Some early signage and photography inkjets such as Iris or Du Pont Digital Cromalin used continuous inkjets, but these have all been replaced by Drop On Demand Types. CIJ is mainly used today by Kodak in its Versamark and Prosper series of high speed commercial web inkjets. See: Drop On Demand.

Contone

Short for continuous tone. It originally applied to silver halide photographs, where intermediate tones can be infinitely variable between white and solid. Film is an analogue process. Some computer printers such as inkjets and dye sublimation devices can simulate contone, even though they use digital input information.

Corel Draw

A vector drawing program. The main competitor to Adobe Illustrator. It's seen as cheaper to buy and is supplied with a healthy set of fonts, clip art and other items (though the price differential has become less obvious since Adobe adopted its Creative Cloud rental policy). However, pre-press operators tend to prefer the EPS and PDF output from Illustrator. See: Adobe Illustrator, EPS, PDF

Creasing

The process of pressing or scoring a crease line into media, usually paper or card, so that it can easily be folded later. This is normally done after printing, for carton packaging, greetings cards and similar work that needs to be folded at a later stage of production, or perhaps supplied flat to the customer for later folding.

Cutting

A range of processes variously to reduce printable substrates to manageable sizes for handling, then to fit them to the size needed for the printing process, and then to trim them to the final size dictated by the job itself.

Database

A store of structured information, normally held on a computer storage system. This can be selectively searched, and information added or retrieved to order.

Degassing

Methods for removing dissolved air and other gasses from inks within a printer that might otherwise form bubbles and disrupt or completely stop the flow. Methods include passing the ink across a permeable membrane that has a reduced air pressure on the other side, before the ink reaches the printhead: the pressure differential induces air to come out of solution and form bubbles that can be safely removed before they reach the printhead and ink chamber.

DFE

Stands for Digital Front End. In graphic arts, the control software (and sometimes hardware) for a digital printer. It is often used as a alternate term to a RIP or Rip-Workflow.

Digital

All current commercial computers are digital, that is they base their operations on the rapid shuffling around of whole numbers, or digits, usually 0 and 1, representing off and on in an electrical switch. Digital is normally used as the opposite of analogue, where values are continually variable.

Dithering

One of the halftone patterning techniques commonly used by inkjet and other digital printers. It allows an extended range of tones and colours to be achieved in photographs and blends, by applying small dots with varying spacing. A variation called stochastic screening is used in offset printing and some other processes. See Halftone

DoD Inkjet

Stands for Drop on Demand. This describes a class of printheads that are precisely controlled to produce ink drops only when required. The term was coined to distinguish this type of head from Continuous Inkjet. All current large format and office/desktop inkjets use DoD printheads, whether they are based on thermal or piezo technologies. See Piezo, Thermal, Continuous Inkjet

Dot gain

A printed effect where ink droplets or halftone dots are larger than desired for a given tonal effect. Nearly all print processes are subject to dot gain of some degree, though the causes may differ.

DPD

The number, or maximum number, of sub drops in a printed drop. This is mostly of interest to inkjet scientists and developers, though it is important in understanding how greyscale heads work. See Greyscale Heads

DPI (aka Addressability)

Drops per Inch. A measure of the number of ink drops that appear on the final printed image from an inkjet printer. This can be different in the head pass and the media transport directions, so typically you'll see 720 x 360 dpi. Printed DPI is usually greater than the NPI (nozzles per inch) because of techniques like multiple passes and multiple heads.

Drop Interleaving

Also called Drop Interlacing or Interleave Printing. These are terms used for print patterning techniques that mitigate the problem of blocked nozzles, or reduce visual banding on low resolutions or low numbers of head passes. Typically the swathe of ink drops are controlled to create a scalloped (way) line at top and bottom edges. The scallop positions are varied for subsequent passes, breaking up the edges to reduce the visual impact.

Droplet size

Inkjet drop sizes are measured in picolitres. One picolitre is a millionth of a litre. Depending on the printhead configuration, the drop sizes normally range from 3 or 4 picolitres to more than 100 picolitres. Photo-quality printers such as Epson Stylus Pros generally produce the smallest drop sizes, and many of these use greyscale heads that vary the size of drop too. Printers used for signage and similar applications that are viewed from a distance can use larger drops, which cover a given area more quickly to allow greater print speeds. See Binary and Greyscale.

EPS

Stands for Encapsulated PostScript. Occasionally called EPSF, with an F for Format. A widely standard document format, often (but not always) used for vector files created by drawing programs such as Adobe Illustrator or Corel Draw, However, bitmap photographs can also be output as EPS by Photoshop, while layout programs can output EPS with all sorts of components.

Eyelets

Metal reinforcing rings that are used with banners and similar flexible signage material, or some curtainside vehicle media. These allow cords, ropes or straps to be passed through the holes in the eyelet rings and secured to scaffolding, frameworks, fences, vehicle structures or whatever else the material is being attached to.

Fast & Slow Scan Directions

In inkjets with moving carriages, the "fast scan" is the direction of the relative movement of the head and substrate. The "slow scan" direction is at 90 degrees to the fast one. These terms are useful when relating the printing motion to a particular image. Single-pass inkjets (such as digital label presses and web fed commercial inkjets) also have different characteristics for the print width and the length (ie the direction that the media travels in), as the width resolution is constant while the length varies with the speed that the media is fed under the heads.

File server

Also called Server. Central computing resource on a network of intelligent terminals. It's used for storage of things that many users might need access to, such as picture and page files. Also used for intensive background processing tasks, such as printing or Ripping. Acts as a controller and routing system for some types of network.

Film

Clear stuff that can carry photographic images. It has largely been replaced by digital cameras for original photography, but it's still used in pre-press as an exposure mask material for screen meshes, lithographic plates and some other analogue printing plates or cylinders.

Firing frequency

The number of drops ejected from each nozzle per second in a printhead. For example, a Xaar 1001 GS6 head has a firing frequency of 7 kHz, meaning 7,000 drops per second.

Flushing

Forcing ink through the printhead nozzles at high pressure, usually in an attempt to clear a blockage. It can waste a lot of ink and is not always successful. Sometimes a special flushing fluid is used. See Purging.

Foaming

The formation of a foam of air bubbles in ink, caused by dissolved gas. This only affects low viscosity inks. Depending on where the foaming occurs, it may be a problem or an intended effect. If bubbles or foaming occur in a printhead's ink chamber, it can cause misfiring or blockages.

Folders

A class of print finishing machines that takes a sheet or roll media and fold it over on itself. Sheet fed folders can fold in two directions to create multi-page sections, that typically are used in books or brochures after the spine has been glued or stitched and one or more of the edges is trimmed off to allow the pages to open.

Fonts

A font is a collection of characters and symbols all of the same style for a particular typeface. Thus Futura Light is a font, and so is Futura Extra Bold.

Four-colour

Referring to the four-colour printing process, where cyan, yellow, magenta and black inks are used to give full-colour halftones. See colour, separation.

Grey Levels

A measure of the number of tonal levels, or densities, in a digital image (which might be an original digital or scanned photograph, so it's relevant to screen printing). An image formed of a range of such tones is called a "Greyscale".The term "grey levels" is also used to refer to coloured images, in the specific sense of describing the tones within each colour channel or separation.

Greyscale heads

Greyscale printheads are able to vary the density of individually printed dots, contributing to the tonal variation of the final image. See Printhead.

GUI

Pronounced 'gooey'. Stands for Graphical User Interface, meaning the combination of menus, icons and mouse point-and-click method of controlling computer programs.

H&J

Stands for hyphenation and justification. It's common to arrange text in columns of constant width. However, words aren't constant in length, so some adjustment has to be made.

Halftone

This is the technique used to give an illusion of different tones (sometimes called greyscales) in printing processes that in reality only use inks of a single density (such as solid black).

HSS

Hybrid Side Shooter, a Xaar piezo printhead technology, so far only used in its 1001 and 1002 heads. See Printhead.

Illustrator

See Adobe Illustrator. A vector drawing program.

Imposition

The process of arranging pages for printing in sections, used for books, periodicals, newspapers and anything else that has a multi-page, book-like form. A four-page section has a simple pattern, but eight pages and above need ever more complex patterns to ensure that all the pages are the right way up and in the correct order.

Ink

In inkjet terms the fluid that is projected through an inkjet head. Normally this will dry or cure to form a visible image, although there are also clear inks and speciality fluids.

Inkjet

A digitally controlled printing technology that projects liquid ink through nozzles onto a substrate. There are many variations of printhead technologies for projecting the ink, and many different formulations of ink. Inkjets are used to produce graphical documents, photographs, signage etc, but also for industrial processes such as electronics, decorative laminates, textiles, and screen process mesh stencils.

JPEG

Stands for Joint Photographic Experts Group. A graphical file format that can be compressed to reduce its overall size. It is very widely used for photographic images for local storage, exchange, and placement in printed or web pages. Many graphics programs can compress and decompress JPEG files. It works with RGB or CMYK images, but cannot handle additional channels such as cut-out masks (for which TIFF or PNG can be used).

Laser

Light Amplification by Stimulated Emission of Radiation. Not the lead underpants sort of radiation, it's normally visible or near-visible wavelengths of light. Lasers in print are used as a source of intense, coherent light for exposing film, printing plates or screen mesh emulsions, or in laser printers and some digital presses as a way of discharging static charge to create image areas on photoelectric drums.

Laser printer

A non impact computer printer. It uses a laser to dissipate an electrostatic charge in specific areas of a light-sensitive drum, which then picks up toner by electrostatic attraction. The toner is then transferred to a sheet of paper to form an image which is fused into place by heat and pressure. Related but now obsolete analogue photocopiers use much the same process, but with reflected and focussed light bounced off the original that is being copied.

Latex ink

A water based inkjet ink suitable for outdoor signage, with similar applications and lifetime to eco solvent inks. Currently supplied by HP, Mimaki and Ricoh. It contains resins (called co-polymers) and pigments held in an emulsion in water. Despite the name, it is nothing to do with latex rubber. Latex is the US term for what in the UK is called an emulsion paint, and latex ink is a similar idea to the latter.

LED

Stands for light-emitting diode. A very efficient solid-state lighting technology that is increasingly replacing older technologies across a range of applications from domestic lighting through to UV ink curing. Red, green and blue LEDs are used to create variable colour lighting and sometimes exposure systems for film. Infra Red emitting LEDs are commonly used in remote controls for home entertainment systems.

LPI

Lines per inch. A term mainly used by offset printers to measure the fineness of a regular halftone screen. Most magazines are printed with 175 lpi screens. Inkjet screening works differently, especially with multiple passes, so the dot pitches don't exactly correlate with offset screens.

Media

The substrate or surface to be printed upon. For inkjets this might be paper, vinyl, wood, glass, metal, textile etc.

MEMS

Stands for "Microelectromechanical systems."Usually a way of making printheads using techniques similar to silicon chip fabrication. However Memjet is working on a mechanical printhead it calls "Pure MEMS."

Meniscus

In inkjet printing, this is curved surface of the ink at the outside of the nozzle when it is not being fired. The curve is due to a combination of the surface tension of the liquid and the difference in pressure between the liquid and the outside air. This tension stops ink dribbling out from a nozzle when it is not being fired. See Screen mesh, Surface Tension.

Mercury Vapour Lamps

The most commonly used technology for UV-cured inks, whether screen, offset or inkjet processes. There are various types, but they work on the principle of establishing an electrical arc current through a metal vapour in a glass tube. The resulting short circuit generates intense light with a high proportio0n of ultra violet wavelengths.

Moiré

A patterning effect, usually undesirable, that's produced by optical interference between two or more overlaid sets of closely spaced lines or grids. A common result is the appearance of large diamond patterns on the image.

Natural Resolution (or Natural DPI)

A measure of the number and pitch of nozzles on the printhead, for example 360 nozzles per inch (NPI). Because inkjet heads often perform multiple print passes, and/ or may have several printheads in line, the NPI is only loosely related to the final printed quality.

OEM

Original Equipment Manufacturer - A company that buys components in, such as printheads, to incorporate in machines it builds and sells under its own name.

Operating system

This is the fundamental program on a computer, the first thing it loads, which tells it to stop being an inert lump of plastic, silicon and metal and to start paying its way.

Outgassing

Although solvent and light solvent inks are touch-dry when they come off the printer, they continue to evaporate and 'outgas' for some hours. If you need to add a laminated surface it has to wait until outgassing finishes, or bubbles may form. Several types of ink have been developed that do not outgas, including aqueous, (though that has separate drying issues), UV-cured, latex and most recently, solvent-UV hybrids that contain very little solvent.

Pantone, PMS

The full name is Pantone Matching System, or PMS. It's commercial system for describing colours consistently, using reference patches in printed swatch books. These are usually encountered in printing as Pantone numbers specified for a certain colour, often on a company logo or similar house colours. Pantone is very wide ranging, with colour sets for plastics, paints, textiles as well as printable colours.

Paper

The original printing medium, still very widely used for books, newspapers, magazines, brochures, posters, fine art and many other applications. Any printing process will be able to print on paper, although different paper coatings may be needed for specific ink types.

PDF

Stands for Portable Document Formant. It's the dominant file format used for exchanging printable documents in the printing industry. It was developed by Adobe Systems in 1994 as was originally proprietary, though widely used. It is now an ISO standard, developed by a committee, which may explain why it has not changed much in recent years.

PDF/VT

A variant of PDF/X that can include variable data, which is particularly useful for digital printers where every copy can be different. PDF/VT-1 can contain lists of variable information internally, while PDF/VT-2 (so far not released) will be able to reference external databases, allowing the same file to print different content. As with other PDF variants, this is now an ISO standard (ISO 16612-2), with development by committee.

PDF/X

This is a subset of PDF that outputs documents in a tightly defined way, so there is less chance of failure due to unprintable elements if the file is opened and printed by a third party. This is called "blind transfer," because the receiving end doesn't need to have any knowledge of the creation settings, just that it is a PDF/X file (which self-identifies itself).

Phase change ink

A type of ink that is a wax-like solid at room temperature. It is heated in the printhead to become a liquid that is then fired at the medium. It is most commonly used by Xerox in its Phaser range of office printers, but the same company uses a variation of the process in its CiPress series of high end web-fed inkjet production printers.

Picolitre

One millionth of a litre. The usual measure for ink drop sizes generated by inkjet printheads. These typically range from 3to 100+ picolitres depending on the head and nozzle. The smallest sizes are usually confined to greyscale heads for high quality tonal work. See Greyscale Heads, Printhead.

Piezo printhead

One of the types of printhead that generate drop-on-demand printing. A piezo-electric material (which is a type of crystal) has the property of expanding or contracting when an electric current is passed though it. The effect is used within piezo inkjets to form an actuator, which is essentially a pump for the ink within the printhead chamber.

Pigment

A colorant within an ink. Pigments are insoluble, relatively large particles, generally making them more resistant to fading than smaller dye colorants that are fully dissolved.

Pinning

A curing method used with UV cured inkjets. A low intensity burst of UV light starts the curing and stops droplets spreading, but leaves the ink liquid enough to smooth over and give a glossy surface before it is fully cured by a second, higher intensity UV burst.

Pixel

Short for Pixel Element. This is the smallest element of a bitmapped image, visible on a computer screen if you enlarge the image, to show a mosaic of squares. The number of pixels in an image such as a photograph are often mistakenly called its resolution, but strictly speaking the resolution is a combination of the number of pixels and the enlargement factor, to give pixels per inch (PPI).

PNG

Stands for Portable Network Graphics. It's a bitmap file format that was originally developed for website graphics, as full-colour alternative to GIF (which is limited to 256 colours). It supports 24-bit RGB colours but not the CMYK print set. It can hold alpha channels, so objects can display on a website as cutouts. The compression is lossless.

Pop

An informal term for the appearance of an image that has bright, "punchy" colours, or other eye-catching characteristics. An older and entirely different usage to POP as an acronym for Point of Presence. See POS/POP.

POS/POP

Related terms standing for Point of Sale and Point of Purchase. In the printing sector it's often used as a generic description for small printed signs, special offer product boxes and other attention-grabbing items (such as wobblers), positioned on or near the checkout area or counters of a shop or similar retail environment.

PostScript

A device-independent page description language which was behind the desktop publishing revolution of the 1980s and 90s. A PostScript file generated by any program can be printed on any PostScript-compatible printer. Developed Adobe Systems in 1983, PostScript saw its first application in the Apple LaserWriter of 1985.

PPML

Stands for Personalised Print Mark-up Language. It's an XML-based printer language for variable data content. It has been developed by PODI, a multi-developer organisation.

Print Driver

Control software that supplies images to a printer for printing. The driver is specifically written to control a particular printer. It may be combined into a RIP.

Printhead

Also spelt as two words: print head. The core of an inkjet printer: a component containing an array of nozzles that project drops of ink towards the printing medium. See array, piezo printhead, thermal printhead.

Purging

See Flushing.

Recirculation

Systems that circulate ink continuously though either their storage cartridges or bottles, or through and past the printhead. This is usually to prevent heavy particles from settling under gravity.

Registration

In any printing process it's important to make sure that the printed image goes where you want it on the substrate, and can be repeated in the same position every time. In multi-colour printing this is particularly vital because the colours have to align on top of each other in the correct position, otherwise unwanted light and dark edges will show, and halftones will appear blurred.

Resolution

In print terms, it's a measure of the number of individual dots a printer or exposure system can produce within a unit of distance, typically given as dots per inch. In optics (where the term originally comes from), resolution describes the amount of detail a focused lens can project onto a surface and is usually described as line pairs per millimetre (or inch).

RGB

Red, Green, Blue, the main colours that the human colour vision system , perceives. Three types of cone cells in the human retina respond to different spreads of wavelengths in the visible spectrum. The brain perceives these responses as colour, with different proportions of red, green and blue giving all the colours that the human visual system can perceive.

RIP

Stands for Raster Image Processor, also called a Renderer.

Satellite drop

A smaller unwanted drop formed behind the main drop as it emerges from the nozzle. If it drifts away from the ink path it can cause blurred print.

Scanner

A device for converting physical images into electrical form. When most photography still used film, scanners were used from the 1960s onward to convert film images into electronic signals - analogue at first but later on to create digital files that could be stored on computers, edited in graphics programs such as Photoshop and placed into documents using layout programs.

Screen angle

In halftone printing with classic AM screens, the dot centres all align to an invisible cross-line grid, called the screen. The angle of the grid can be anything in a 360 degree rotation.

Screen mesh

In the early days of screen printing, from about 1,000 years ago to the early 20th Century, a screen printing mesh was made from silk.

Screen process

Also known as serigraphy or silk screen. A highly versatile analogue printing process, versatile enough to be used for a range of applications from fine art through to textiles, garments, signage and non-decorative industrial work such as printed electronics.

Screening

Screening is the process by which original continuous tone and multi- colour images are processed into halftones so they can be output realistically with the limited number of tones available in a printer. Nearly all printing processes use screens to reproduce variable tones.

Serigraphy

A fancier name for screen process. It's a more formally correct name than "screen process," which can become confused with halftone screens. It's also often used by the sort of art gallery that calls inkjet prints "giclée," and gravure prints "intaglio."

Solvent inks

The most common inks used for outdoor signage applications. They contain pigments suspended in volatile organic compounds (VOCs).

Spitting

Periodic automatic ejection of ink from the printheads when they are not being used, to keep the ink fresh and reduce the risk of drying in the nozzles.

Spot colour

Generally a special colour used in the printed job that cannot be achieved from standard CMYK process ink combinations. Many screen printing jobs, especially for fine art work or low value duplication, are entirely comprised of spot colours.

Squeegee

In screen printing, the squeegee is a rubber-tipped blade that is passed along the length of a screen mesh in a frame, forcing a measured amount of ink through the holes in the mesh and onto the substrate below.

Substrate

The medium that is being printed on. This can be paper, plastic, metal, wood, or virtually any other hard (and usually flat) surface. It's a generic term, used mainly because many printing processes, including inkjet and screen, can print on many different types of medium. In the signage world, the term "medium" is used more often than "substrate," but the meaning is the same. See Media.

Surface tension

The attraction between molecules at the surface of a fluid. The main significance in inkjets is the way it induces ink drops to form and contract to a roughly spherical shape as they are in flight from the head. In screen printing it is one of the factors that stops ink flowing through the holes in the mesh until forced through by the squeegee. See also Meniscus.

Swath

A band of print produced by one pass of a printhead. Larger heads can produce wider swathes so the media can be advanced more between passes if you don't need high quality.

Thermal printhead

A printhead that jets ink drops by forming a heated vapour bubble internally. This is the main alternative inkjet printhead technology to piezo and continuous inkjet.

TIFF

Tagged image file format. A widely used contone image file description format. It is able to handle 24-bit (RGB) or 32-bit (CMYK) colour as well as monochrome images, and extra channels including masks and spot colours, plus layers within Photoshop.

UV (ultra violet)

Short wavelengths of radiation beyond the visible range, in the 400 to 10 nanometre range. UV radiation in the 350 to 400 nm range is used to cure printing inks. See UV curing.

UV Curable Inks

Inks that are liquid until exposed to strong ultra-violet light, at which point they solidify almost immediately by polymerisation.

UV curing

The polymerisation process by which a UV-curable ink almost instantly changes from a liquid to a solid when exposed to UV light. The liquid ink contains long-chain molecules called monomers that can move freely. Exposure to UV light causes them to become entangled so they cannot move and so they become a solid.

Variable droplets

See Greyscale Heads.

Viscosity

The tendency for a liquid, such as ink, to resist flowing. Most solvent inks are relatively low viscosity, but UV curable inks are relatively high viscosity. In screen process, ink viscosity is a factor in determining the type of mesh to be used.

Wiping

A term for cleaning the inkjet printhead nozzle plate to remove excess ink or contamination. Often performed as part of the routine maintenance of before shutdown.

Xerography

A dry toner electrophotographic printing process. See laser printer.