IDEAL SUBTRACTIVE PRIMARIES

Consider a set of three hypothetical filters that together let through the entire visible spectrum, but individually let through only the band of wavelengths that could serve as a red, green and blue additive primaries respectively. No such filters could be manufactured from physical colourants, but in the ideal subtractive mixing seen in the digital realm our colours work exactly like this. A moments consideration will show that these ideal red, green and blue filters will not be effective as subtractive primary colours. These filters by definition have no wavelengths in common, and so any pair will absorb all light. Changing the strength of one of the filters does not result in intermediate hues (Figure 5.3).

Figure 5.3. Subtractive mixing of ideal colourants corresponding to the additive primaries. Filters that let through only one additive primary at a time would not work for subtractive mixing. Varying the strength of the red (top slider) , green (middle slider) and blue (bottom slider) colourants does not produce intermediate hues. Rectangles show the phosphors glowing in each filter and (top left) in the overlap of all three. Copyright David Briggs and Ray Kristanto, 2007.

To be ideal primary colours for subtractive mixing, the filters would instead need to completely transmit a combination of two of the additive primaries at a time, and completely absorb the third. There are just three such possible combinations (R+G, R+B, G+B), and each pair of these combinations has one and only one of the additive primaries in common. As we saw in the last section, these combinations are seen as yellow, magenta and cyan respectively, and are each the additive complementary of the remaining additive primary (Figure 5.4). Yellow (R+G) and magenta (R+B), for example, having red in common, at full intensity mix subtractively to make red. Changing the intensity of one or other of the components produces a continuous range of intermediate hues between yellow and magenta (Figure 5.5). No colour is shared by all three primaries, and so subtractive mixture of the three at full intensity produces pure black.

Figure 5.4. Mixing of ideal subtractive primaries at full intensity.

Figure 5.5. Subtractive mixing of ideal subtractive primaries. Varying the strength of the cyan (top slider) , magenta (middle slider) and yellow (bottom slider) produces a full range of intermediate hues. Rectangles show the phosphors glowing in each filter and (top left) in the overlap of all three. Copyright David Briggs and Ray Kristanto, 2007.

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