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Additive and Subtractive Colors

HBB

Phoenix, US
8783 posts

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HBB Moderator Hal is an expert in several areas, including CLS Awarded for his excellent article contributions to the Resources. Donor Ribbon awarded for his very generous support to the Fundraising Campaign 2015 Charter Member
Mon 04-Jul-11 01:35 AM | edited Tue 19-Jul-11 02:52 AM by HBB

Colleagues:

I prepared these charts as an aid to a few people I am tutoring in basic color management concepts. This is not meant to be a rigorous treatment of color theory or management, simply an introduction for those interested.

Enjoy the brief tour. Comments/questions welcome.

Glossary

Primary (additive) Colors: The basic Red, Green, and Blue colors perceived by the human eye/brain system.

R = Red
G = Green
B = Blue
W = White


Subtractive Colors: Colors produced by selectively subtracting the three primary colors from white light.

C = Cyan
M = Magenta
Y = Yellow
K = Black (B already used for Blue)


Additive Colors/Filters

Click on image to view larger version


The human eye is sensitive to three primary colors: Red, Green, and Blue. If three projectors are equipped with filters, one with Red, one with Green, and one with Blue, and their images are converged (added) on a white screen, the image above appears. The black background simulates the portion of the screen not illuminated in the dark room.

The three primary colors are easily seen.

Example: Red equals White minus Green minus Blue.

In shorthand:

R = W - G - B
G = W - R - B
B = W - R - G

Where the three primary colors overlap in pairs, the three subtractive colors: Cyan, Magenta, and Yellow appear. See Subtractive Filters, below.

Example: Cyan equals Green Plus Blue.

In Shorthand:

C = G + B
M = R + B
Y = R + G

Where all three primary colors overlap in the center, white (W) appears.

W = R + G + B

The three primary colors are used in television sets, computer monitors and other devices that create the color perceived by the human eye/brain system by using projected light. By varying the three colors individually over a wide range, an enormous spectrum of colors can be created (Color space), ranging from black, the absence of all colors, to white, the presence of all three primary colors in equal intensity.

In eight-bit color depth, each of the three primary colors can have a value of 0 (zero) to 255, for a total of 256 shades. The total number of colors using red, green and blue equals 256 x 256 x 256 = 16,777,216 combinations. If all three primary colors are present in equal amounts less than 255 (pure white), the perceived color will be gray. All three colors at 127 (approximately one-half of 255) will produce a medium gray. All three at 0 (zero) produces black, the absence of color.


Subtractive Colors/Filters

Click on image to view larger version


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.

Example:

Cyan equals White minus Red.

In shorthand:

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 Ernesto points out below, 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.

Let me know if this is helpful ... or not.

Regards,

HBB in Phoenix, Arizona
Nikonian Team Member

Photography is a journey with no conceivable destination.

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