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Multiple Spot-Colour Bitmap Images

duotoneThese screen captures are from Adobe's Photoshop, but several other programs have similar options. Under the Mode Menu, an excellent option available to graphic designers and prepress technicians is the DUOTONE option.

Firstly, to use it a bitmap must be set to GreyScale Mode, and only then will the Duotone mode menu item become available.

Duotone Options

Once selected, a window appears enabling the choice of the Spot Colours needed to print the image as a Duotone picture (two spot colour inks) and adjustment of the tone curves of the chosen colours. Photoshop offers further spot colour plates also. i.e. Tri-tone, Quad-tone.

• Clicking on the colour square will open a familiar colour picker window. From here you pick:
1. a Spot-Colour colour system i.e. Pantone Spot Colours
2. one of the required spot colour Names. i.e. Pantone ????.

Repeat for the other spot colour(s). (in a spot colour job realise black would also be a spot colour)

• Clicking on the Gamma square will allow adjustment to the colours via 'tonal curves' enabling one of the colours to become dominant - most common choice for duotones. Changes can be made to both colours to get the desired effect.

tonecrv1.gif (7407 bytes)The crossed cursor placed on the default curve line will produce a dot and a corresponding value in the percentage boxes on the right. Manually change the value or drag the dot up or down to get the desired value.

Here the quarter tones have been lightened so that the other colour used will dominate in those areas, while the 90%+ tones of this colour will dominate slightly over the other colour. This sample assumes that the other colour used has not been adjusted from a straight line curve.

Selecting an .EPS Header imageWhen saving the file, the .EPS format must be used because that is the only way the extra spot colour references can be transferred to another program or PDL – some programs that can import the duotone require a bitmap header so that the image can be recognised on the page.
Pagemaker Separations optionsHere we see  the Adobe PageMaker 'Colour' Print dialogue window and the Colour Palette. When the .EPS (Duotone) file is placed onto a page, the Spot Colours chosen in Photoshop are automatically included in the Colour Palette list.

Later, when the publication is ready to print, the relevant Colour Plates can be chosen (Separations option selected) from the 'Print-Ink' list where the duotone Spot Colour inks will be listed. The resulting separations will include the separated duotone plates as well as any other colour plates included in the job. i.e. CMYK..

Notes: a few imagination starters.
  1. Duotone, Tritone etc. images can be converted back to RGB or CMYK.if it is just the tool and colour effects that you want to use.
  2. Usually one of the colours will be made to dominate the other although of course an even mix of each can be used.
  3. A classic example used and seen by me over many years is one where a blue and black might be the two spot colours used to print a publication. Any picture subjects like stainless steel, aluminium etc. (i.e. an oil refinery) can be made to look excellent with the blue reduced considerably in the highlight and quarter-tones while the blue dominates slightly in the mid-tones and three-quarter tones. Using a selection, the black could also be reduced in any sky areas.
  4. Practised use of black and about a 1/3 grey colour can enhance black and white pictures if the client wants to pay for the extra print run for a classy result.
  5. Not relevant to duotones but, in high quality publications included black and white pictures can look a little special if produced as CMYK images although the stock and press need to be of good quality too to avoid colour shifts.

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