1. Color Mixing
There are two ways to control how much red, green, and blue light reaches the eye:
“Additive Mixing” Starting with black, the right amount of red, green, and blue light are ‘added’ to an image.
“Subtractive Mixing” Starting with white, the right amount of red, green, and blue light are ‘subtracted’ from an image.
The most direct method of producing the sensation of colors is to add specific amounts of red, green, and blue light to a dark background. In this way, the colors are “added,” causing the sensation of color. This can be accomplished by projecting independent sources of red, green, and blue light onto a white screen in a darkened room.

2. Additive Color Mixing
By exciting the red, green, and blue sensitive cones, any color can be produced by adding together the three additive primaries (R,G,B).
Mixing the three color sources is known as “additive mixing” to distinguish it from mixing paints or dyes (“subtractive mixing”).
The most direct method of producing the sensation of colors is to add specific amounts of red, green, and blue light to a dark background. In this way, the colors are “added,” causing the sensation of color. This can be accomplished by projecting independent sources of red, green, and blue light onto a white screen in a darkened room.

3. Additive Color Mixing
For example, when blue and green lights overlap, the blue and green cones are illuminated, and we perceive cyan
Overlapping colored lights stimulate the three cone classes, leading to the perception of color. In this case, stimulating the blue and green sensitive cones in one area of the retina causes the perception of cyan. We describe this phenomenon with the “equation”
Blue + Green = Cyan

4. Additive Color Mixing green + blue = cyan red + blue = magenta
While the mixture of blue and green light to produce cyan and the mixture of blue and red lights to produce magenta seem intuitive, the mixture of red and green to create the sensation of yellow is surprising to many. Because we are more familiar with mixing pigments (like paints) we expect the mixture of red and green to produce an ugly brown color. It is important to understand that the colors we perceive are sensations created within the visual system by superimposing different colored lights on a dark background. If we examine the physical result of combining red and green wavelengths, we see only the superposition of the two wavelengths - not the isolated wavelength that we perceive as yellow.
Recall that we are talking about “ADDITIVE” mixing; we’re not mixing paints - we’re letting light fall on different kinds of photoreceptors in the back of the eye.
You can see the mixture of the additive primaries by examining display screens closely - color TVs and LCD displays have only three colors; red, green, and blue. The yellow you see is formed in your head, not on the screen.
red + green = yellow
red + green + blue = white

5. Additive Color Mixing red + green/2 = orange red + green = yellow
red/2 + green = lime
Colors other than RGB and CMY can be produced by mixing together variable amounts of the RGB components. E.g., orange is created by mixing equal amounts of red and green to create yellow, then adding more red to make orange.
Neutral values (black  gray  white) can be created by mixing equal parts of red, green, and blue light.
red + green + blue = gray
red + green + blue = gray
red + green + blue = white

6. Additive Color Reproduction
Color video projectors use additive color mixing
Projected red, green, and blue images contribute RGB components to create color images R G B
Examining a CRT or LCD display shows that a color image is actually produced by mixing together only the three additive primaries. Other colors, such as cyan, magenta, yellow and white are produced by exciting two or more cone classes.

7. Additive Color Mixing Methods
In addition to the superposition method described above, there are two other methods of mixing R, G, & B primaries.
- Spatial mixing (as in color TV)
- Temporal mixing (as in digital cinema)
Both rely on limitations of the visual system;
In all the previous examples of additive color mixing, it was assumed that the signal was produced by superimposing three light sources. A set of three slide projectors, or the three guns in a projection TV provides this kind of mixing. Such systems are often awkward for other color displays.

8. Spatial Mixing (Video Monitor)
Because the visual system has limited spatial resolution, small areas of different colors are mixed perceptually. x y
Spatial addressability of typical monitors goes from (640 x 480) to (1600 x 1280) pixels.
Color television monitors and color LCD displays rely on the limited spatial resolution of the human visual system to mix colors.
RGB sources close enough together, viewed from far away enough, blend in the visual system in the same way they would if they were superimposed on a screen. This spatial mixing is formally known as “partitive” additive mixing.
Examining a monitor or LCD screen with the loupe in your kit demonstrates the effect. The demonstration is especially effective when viewing a yellow object; up close only red and green are visible, but as you move away from the screen, yellow “appears.” It’s important to remember that this appearance is not the result of mixing of two physical phenomenon to create a third; rather it is the stimulation of two different cone classes in your retina, which induces the sensation of yellow in your visual system.

9. Temporal Mixing (Digital Cinema)
Because the visual system has limited temporal resolution, rapidly changing colors are mixed perceptually.
time
time
[The notes for this slide are almost identical to the previous slide. Differences are noted in italics to emphasize the minor differences between spatial and temporal mixing.]
Color digital “micro-mirror” displays (such as used in some digital video projectors and digital cinema systems) rely on the limited temporal resolution of the human visual system to mix colors.
RGB sources close enough together in time, viewed at repetition rates beyond the fusion frequency of the visual system (~30 Hz), blend in the visual system in the same way they would if they were superimposed on a screen.
Examining such a display with a slow-motion detector demonstrates the effect. The demonstration is especially effective when viewing a yellow object; at slow rates, red and green are visible, but as you speed up the presentation rate, yellow “appears.” It’s important to remember that this appearance is not the result of mixing of two physical phenomenon to create a third; rather it is the stimulation of two different cone classes in your retina, which induces the sensation of yellow in your visual system.
time

10. Color Monitors
A number of color monitors exist in most digital color document systems.
Different color monitors are likely to display the same digital file differently.
Note:
Monitors differ in sizes and phosphors. In addition, the same batch of monitors may differ in their hardware settings, I.e., brightness and contrast settings, and software-adjustable settings: white point and gamma.

11. Subtractive Color Mixing
Color hardcopy devices can’t use additive mixing because they aren’t sources of light; they can’t add Red, Green, or Blue components.
Instead, they use subtractive mixing. Starting with white light reflected by the substrate, they subtract the unwanted red, green, and blue components using cyan, magenta, and yellow colorants.
Additive systems are, by definition, systems that start with no light and add in the RGB components needed to induce the desired sensation.
While this works well for superimposition, spatial, and temporal additive systems, it does not work for ‘passive’ devices like toner, dyes, or ink on paper or transparent substrate. In this context, the term passive refers to systems that do not emit light, rather they control the incident light to produce the sensation of color.
Yet to produce the sensation of colors, we still have to control the amount of red, green, and blue power reaching the three cone classes in the retina. Passive color systems therefore rely on modifying the incident illumination to leave the RGB components necessary to produce the desired color sensation.

12. Subtractive Color Mixing
The goal is the same; to control the amount of Red, Green, and Blue light getting to the eyes’ three cone types
Each colorant absorbs 1/3 and transmits 2/3 of white light
g+b = c
White light
b+r = m
White light
White light
r+g = y
Because the goal of any color system is to manipulate the amount of red, green, and blue power reaching the retina, subtractive systems use colorants that can independently vary the amount of the red, green, and blue light incident on the substrate.
Cyan colorants absorb green light;
Magenta colorants absorb green light;
Yellow colorants absorb blue light;
By using varying amounts of the subtractive primaries (C, M, & Y), passive systems can produce the sensation of color by manipulating the amount of remaining R, G, and B components.
Note that the same effect cannot be produced by mixing red, green, and blue colorants, because each one would absorb 2/3 of the incident light (e.g., red colorant would subtract green and blue). As a result, mixing any two additive primary colorants would leave black, e.g., red plus green colorants:
Red colorant absorbs green and blue;
Green colorant absorbs red and blue;
Leaving no light.
white substrate
cyan colorant “minus red”
magenta colorant “minus green”
yellow colorant “minus blue”

13. Subtractive Color Mixing
Other colors are made by varying the amount of colorant in each layer.
White light
r+g/2 = orange
In the same way that mixing the additive primary (RGB) components can create a wide range of color sensations, using the subtractive primary (CMY) colorants can produce colors other than cyan, magenta,and yellow.
Placing magenta colorant over yellow produces the sensation of red because the magenta layer subtracts the green component of the incident light, and yellow subtracts the blue component, leaving only the red component.
Other colors are produced my using varying amounts of CMY colorants to selectively absorb varying amounts of the RGB components in the incident light. Note that all this discussion assumes that the incident light includes light covering a wide portion of the visible light spectrum. If this is not the case, the gamut of colors produced is reduced.
yellow & magenta = red
yellow
magenta
yellow
+ magenta/2
orange
+ cyan
black

14. Subtractive Color Reproduction
Color printing uses subtractive color mixing.
Adding black allows more accurate grays, and conserves the more expensive CMY colorants. C Y M K
While many photographic systems use only CMY colorants (i.e., dyes), most subtractive systems include a fourth colorant; black (‘K’ to distinguish it from Blue). Such systems are referred to as ‘CMYK’ systems. The black layer makes it possible to reproduce more accurate neutral grays and allows inks to be conserved by replacing equal amounts of C, M, & Y colorants with the equivalent amount of black ink. This “undercover removal” technique allows significant savings by conserving the more expensive CMY colorants.

15. Subtractive Color Imaging
Colors are rendered by different mixtures of cyan, magenta, and yellow inks printed.
Gradations in each channel can be achieved by halftone marking.
Contone
grayscale
Halftone

16. Subtractive Color Imaging
Process color printing is an example of subtractive color mixing
The spatial addressability of typical printers goes from 400 spots/in to 3,600 spots/in. C Y M K
Addressability—The spacing of marking spots a hard copy output device is capable of, and is expressed as spots in inch (spi); also known as printer resolution; Higher device addressability allows for more detail and subtle color transition in an image.

17. Subtractive Color Imaging
Assumptions:
White substrate (or paper) is used
It reflects all red, green, and blue light
Process inks are semi-transparent
Each ink absorbs ~1/3 of the visible spectrum cyan subtracts red, transmits green and blue magenta subtracts green, transmits red and blue yellow subtracts blue, transmits red and green
Note:
Process inks must behave like filters, and not as opaque colorants. Opaque colorants make good wall paint, just aren’t good for process color printing.
The problem with using red, green, and blue inks as subtractive primaries is that they absorb two-third of the light energies, and only reflect one-third. By doing so, all two-color overprints are black, and result in a very small color gamut.