If I place the three subtractive filters on a light table, the image above appears. The white background simulates the white light shining up through the light table, or white paper placed under the subtractive filters or printer's ink.
The three subtractive filters work by subtracting (absorbing) specific colors from white light passing through them.
Cyan equals White minus Red.
C = W - R
M = W - G
Y = W - B
K = W - R - G - B
Where the subtractive filters overlap in pairs, the three primary colors (R, G, B) appear. Where all three overlap in the center, black appears, because all three colors have been absorbed (subtracted).
Example: Blue results where the cyan and magenta filters overlap, because the cyan filter subtracts red (-) but passes green (+) and blue (+), while the magenta filter passes red (+) subtracts green (-) and passes blue (+). Since the red and green are both subtracted, and blue is passed by both, the observer perceives blue.
The subtractive equations for each of the overlaps are shown on the chart above.
Early ink jet printers used four basic, subtractive colors of ink: Cyan, Magenta, Yellow, and Black. Black was an early addition to the CMY colors when they realized they could not get true, dense blacks with CMY inks alone. Newer models have expanded greatly on the ink set colors. My Epson Stylus Pro 7900 has eleven colors: four blacks, two cyans, two magentas, one yellow, one green, and one orange. By varying the density of the different inks, and enormous number of combinations are possible. This is called a color space.
Subtractive inks are used because they must work with reflected light, not transmitted light describe above with the additive colors. Subtractive inks work by absorbing their individual additive color from the white light reflected off the paper under them.
As fellow moderator Ernesto Santos (esantos) pointed out, we send RGB images to our printers, which then convert them to CMYK ... Why? Because the image on our monitor is generated using additive colors (RGB) using transmitted light. For the same image to appear on white paper, it must be converted to the subtractive CMYK colors which work by subtracting colors from the white light reflected off the paper. In other words, the white surface of the paper reflects the illuminating light source as shown above, roughly equivalent to the RGB transmitted light source used by the monitor.
White is produced by the absence of ink. The brightest white one can achieve is a function of the brightness of the paper. This can easily be seen by printing one copy of an image on a matte paper like Epson's Hot Press, Natural Bright, which has a definite off-white, tan cast, and a second copy on Epson's Exhibition Fiber paper, which has a very bright, smooth white surface.
Some ink jet papers are coated with Optical Brightening Agents (OBA's) that fluoresce under the ultraviolet component of the illumination shining on them, returning a blue/white color perceived to be brighter than the inherent brightness of the paper.
Over time, OBA's will fade with continued exposure to ultraviolet (UV) rays in sunlight and other illuminants, and the paper loses its artificial brightness. Placing the print under glass when framed, blocks the UV component and preserves the OBA's. It also prevents the UV component from reaching the OBA's, thereby defeating their purpose. Interesting.
Black is produced when all three subtractive colors are overlaid, thus absorbing all three of the additive colors.
Subtractive filters are used by photographers to balance the color temperature of different light sources, to create a uniform illumination of a scene. Amber filters (absorb/subtract blue) are used to bring daylight and speedlight color temperatures (5,000 K to 6,000 K) down to that of tungsten bulbs (2,600 K to 3,200 K). Similarly, blue filters (absorb/subtract red and green) are added to tungsten illumination sources to bring them up to daylight or speedlight color temperatures. Many other combinations are possible.
I hope this is helpful
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