PRINTING TECHNOLOGY

Colour resolution (or pixel depth) refers to the software’s ability to record colour differences. It has nothing to do with the resolution of area-related detail. The relationship between the number of grey levels or colours that a pixel can represent and the number of bits or bytes of computer memory needed to store it is listed given below. The minimum requirement is eight bits per colour (one byte), but often scanners are advertised as being 10-, 12- or even 16-bits per colour. Colour resolution: bits per pixel 1 bit per pixel = black or white 8 bits per pixel = 256 shades or colours 24 bits per pixel = 256 red 256 blue 256 green = 16.7 million colours each extra bit doubles the number of tones or shades that can be expressed the software convention is an 8-bit environment 10- or 12-bit scanners = redundancy of bits for tone transformations

Digital cameras use charge coupled devices (CCDs), instead of film, to record the light levels in the image being captured. CCDs are electronic chips that generate an electronic signal when light strikes one of its photosites. The brighter the light, the stronger the signal. The signals from each photosite are collected, digitised and stored. The stored value from each photosite is one sample point in the capture of the image. There are two ‘styles’ of CCD: ❑ the linear array ❑ the area array The smaller the photosites, and the closer they are together, the finer the detail that can be recorded. The resolution of the picture captured relates directly to the number of photosites. In the case of the linear array, the horizontal resolution is the number of photosites, but the vertical resolution is fixed by the way the image is moved across the sensor. The image must be ‘stepped’ across the sensor so that lines of picture information are generated. The number of lines (steps) form the ver...

Photographic materials are the general medium for image-data storage and transfer and still, in nearly all cases, they remain the most economic. But slowly the universal dominance of film as the only origination medium is being eroded by direct, electronic data storage. Scanning the artwork or photograph is the principle on which modern reproducti on is based – scanning the scene is becoming a realistic alternative. It is true that if the image is to be of prestige publication quality, then the scene being scanned must be controlled and stationary. However, the benefits of electronic, still photography are well documented and already being enjoyed. Digital photography has a great deal to recommend it. Some of the recognised advantages of recording the scene directly in a digital format are: ❑ image availability without chemicals; ❑ ease of transmission; ❑ direct entry to computer systems; ❑ ease of editing, retouching and enhancement; ❑ pre-separated data; ❑ convenience of archive stor...

An inherent characteristic of any photographic image is that its initial construction was of grains of silver. The faster the film, the larger the grains, and the smaller the degree of enlargement which can be tolerated. Once enlargement is such that the grain structure becomes obvious this will have a number of adverse effects on the reproduction. These are: ❑ areas of even tone become broken and granular; ❑ smooth transitions between tones can become harsh and, under very bad conditions, stepped – in other words, the tone changes occur in sharp steps like the contour lines on a map rather than a smooth, continuous transition; ❑ fine detail and textures can be lost due to the false image texture imposed on the subject by the grain structure of the film.

Loss of neutrality, in this context, can be defined as the state when any one colour is influenced adversely by another. It is easier to identify a colour shift from a neutral than it is to gauge the influence of one colour within another strong colour. In most instances the degree of shift will be slight and, therefore, have to be identified in areas where it will be most noticeable. These will be in shades close to grey. Normal colour vision is very accurate at gauging neutrals provided that the reference information of the surrounding conditions is correct. An unwanted, visible colour shift is known as a ‘cast’. There is no guarantee that a colour cast is going to be equal over the whole range of the image. Incorrect exposure, poor processing, poor film storage and some product characteristics can lead to tonal colour shifts. Shadows can be moved from the neutral when perhaps the midtones and highlights are correct. Similarly, the highlights might need special attention whilst othe...

In addition to controlling the conditions under which decisions are taken, it is also important to ensure that only people with ‘normal’ colour vision make colour decisions. It should be realised that approximately 8% of the male and 0.5% of the female population have defective colour vision. Conditions which satisfy the environment described abov e may be obtained by equipment that is designed and supplied in accordance with ISO 3664 or ANSI PH 2.30 1989. ISO 3664 is the international standard for viewing photographic materials and is currently under review. ANSI PH 2.30 1989 is the more recent of the two standards. It is an American standard which describes viewing conditions for colour prints, transparencies and photomechanical reproductions. A controlled working area, such as a viewing booth, requires regular maintenance to sustain the conditions demanded by the published standards. Aregular maintenance schedule should include the following tasks: ❑ All neutral surfaces, fluorescen...

Computer-generated graphics are widely used in publishing, from simple graphic elements for magazines and logos for corporate identity work to complex technical illustrations and exploded diagrams for instruction manuals. However, problems arise when publishers and/or printers do not understand the two basic categories of computer graphics and th e differences between them. There are two fundamentally different types of computer graphic files: ❑ raster, ❑ vector. Raster graphics Raster files describe the colour (and, therefore, tone) of every addressable position of a given image – the pixel. Complex pictures are built up by varying the attributes of the pixels within the image. The total image is stored as a reference grid, or map, of each pixel’s value. In an image which is just black and white, each pixel is either ‘on’ (black) or ‘off’ (white). Each pixel only needs a single bit to record its state, hence the term ‘bitmapped image’. In a tone or coloured image each pixel requires m...