CoachA filter (colored glass, a gel, a layer of ink or paint) transmits part of the spectrum it receives and absorbs the rest. Producing colors by removing bands from white light is subtractive synthesis. Its primaries, complements of the additive ones, each subtract one band: cyan white R, magenta white G, yellow white B. Stacked filters subtract jointly: cyan yellow leaves green, cyan magenta blue, magenta yellow red, all three black. Printing and painting work this way, with C, M, Y inks.
示例
Example 11.9 (A filter chain)
White light crosses a yellow filter: the blue band is absorbed, leaving — yellow. A cyan filter then absorbs the red band, leaving : green. Swapping the filters changes nothing: each absorbs its own band regardless of order.