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Filtering for Separations
When white light is directed through a prism the result will be a rainbow effect depicting the visible Colour Spectrum. Different wave lengths of light emerge from the prism at different angles ranging from the Ultraviolet region to the longer wave lengths of Infrared. In nature we often see this effect when light passing through water or ice particles produces a Rainbow. On the Intro to Colour start page, the first image shows how equal applications of the primary Additive colours, Red Green and Blue light, will produce White, or in effect create the reverse of the prism spectrum.

Visible light spectrum

Light reflecting off an object (even a photograph) gives us the sensation of colour because from the white light, some colours have been absorbed by the objects surface and we can not see them (subtracted). To record and reproduce the object on multiple surfaces (i.e. pictures on paper), the reflected light has to be separated into Subtractive Colour components that can then inturn be combined using pigments. The resulting images will also appear coloured to our eyes for same reasons i.e. absorbtion / reflection.

Light through R,G and B filters

Above is a graphic showing light travelling separately through three filters. Ideally only the Red, Green or Blue light will emerge depending on the colour of the filter. This and the following images help describe the process of creating separations.

Simple R,G & B separation

Here the original object is shown as a "transparency" (some people may only recognise them as "picture slides") where the light is projected from behind. (For "reflection originals", think of the light projected in front of the object and reflected back to the filters.)

The original object is a simple one with red, green, blue, cyan, magenta and yellow colours plus black and white. In each case the light travels from the source, through a filter, and exposes a film producing a film negative. What appears black in the film negative are areas of most exposure (most light transmitted through the filter from the original) and where the least light affects the film because colours have been blocked by the filter, the film negative will appear clear.

The Subtractive Colour name of the pigments that the final negatives (above) will be used for in each case are shown under the negatives.

Copy to filter, to negative, to plate

These images again show an original, followed by what the film would see through each of the red, green and blue filters (as samples above). Under the filters the resulting film negatives are shown followed by the individual Subtractive Colour plates.

Graphic of original reproduced by CMY inks

Studying this side view displaying the final result with the original, compare them and note where Cyan and Magenta inks create Blue, Magenta and Yellow inks create Red, and Yellow and Cyan inks create Green. Of course a real original may contain millions of different colours, so various percentages of each of the three Subtractive colours mixed together should reproduce them.
Once these steps are understood the Ideal Inks page (pigments) can describe a complex problem created because, indeed our pigments are far from perfect. Note that the best filters we can produce are also imperfect but not on the same scale as the pigments.

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