Chapter 9: Color

1. Chapter 9: Color • Color mixtures • Additive Mixing • Partitive Mixing • Subtractive Mixing • Colored Inks and Paint • Watercolors • CMYK Printing • Halftones

2. Additive Primary Colors 530-nm green yellow cyan 460-nm blue 650-nm red magenta

3. Additive Mixing: Yellow

4. Display Pixels: Partitive Mixing • What it instead, we make the different colors coming from separate, very small, very closely spaced points • You eye cannot see them as separate sources, so the colors mix and you see the same color. This is called partitive mixing

5. Concept Question: Partitive Mixing A computer pixel is composed of red, green and blue lights. If the red and green ones are illuminated, what color does it appear from a distance? • Green • Red • Yellow • Blue • Magenta Un-illuminated pixel Pixel with red and green lights illuminated

6. Partitive Mixing: Pixels

7. Subtractive Mixing • In additive mixing, we added the wavelengths that were hitting the eye: say red light and green light • What about things like filters and dye pigments? • These mix by subtractive mixing

8. Subtractive Mixing • Suppose we have a range of wavelengths hitting some object: • Three things can happen to each wavelength of light: • Reflection: that particular wavelength bounces off the object • Transmission: that wavelength passes through the object • Absorption: that wavelength is soaked up by the object • In general different things happen at different wavelengths

9. = Colored Filters • Recall that magenta is the additive mixture of blue and red • If a light looks magenta, it means that red and blue light is reaching our eyes, thus a magenta filter must transmit blue and red light, and subtract, by reflection or absorption, green light.

10. = Magenta filter subtracts green Cyan filter subtracts red Colored Filters • What happens if we layer colored filters? • Filters subtract light by absorption or reflection Incident white light Only blue gets through

11. Magenta filter subtracts green Yellow filter subtracts blue Colored Filters • Another example: = Incident white light Only red gets through

12. Concept Question: Colored Filters What color makes it through both the cyan and yellow filters? • Red • Blue • Green = Incident white light

13. Subtractive Mixing: Primaries • The subtractive primaries are • Cyan • Magenta • Yellow • In subtractive mixing, combining complementary colors produces black

14. Subtractive Mixing: Reflection • Colored filters subtract some colors and transmit others. • Subtractive mixing can also occur when a colored surface absorb some colors and reflect others White in Magenta out A magenta colored surface absorbs (subtracts) greenand reflects red and blue, thus appeared to be magenta

15. Subtractive Mixing: Reflection A green surface absorbs (subtracts) red and blue (magenta), and reflects green White in Green out

16. Subtractive Mixing: Reflection • The colors of light that are reflected from a surface mix additively when the hit your retina. • The subtractive part of the mixing is the light hitting the surface and part of it being absorbed or transmitted. • A surface that reflects both red and green light will appear yellow by additive mixing of the red and green light that reaches your eyes

17. Earth-orbiting satellites take pictures of Earth at different wavelengths to determine the health of crops and water

18. Subtractive Mixing: Real Filters • Real filters are non-ideal, they transmit a range of wavelengths, not just one. We can specify which wavelengths using an intensity distribution curve.

19. Magenta Filter Transmission

20. Cyan Filter Transmission

21. Subtractive Mixing: Colored Light • We have so far been assuming that we are illuminating our colored filters and surfaces with uniform white light. • But most light is not uniform white, or even white at all. • How can we figure out what objects will look like in non-ideal or non-uniform light?

22. Colored Light: Example Reflectance of a magenta shirt Fluorescent light emission

23. Combining Intensity Curves To combine two intensity curves, you multiply the curves at each point to get the combined curve When illuminated by this fluorescent light, this magenta shirt will appear gray (colorless)

24. Combining Intensity Curves: Example

25. Concept Question • The black line is the intensity distribution curve of a “daylight” CFL. • The purple line is the reflectance curve of a magenta colored surface. • What color does the surface appear to be when illuminated with this light? • Magenta • Red • Blue • Purple

26. Color Mixing: Ink and Paint • Color mixing with ink and pigment is in general, a complex mixture of additive and subtractive mixing. • Light rays hitting paint or ink on a piece of paper can interact with the pigments in several ways

27. Color Mixing: Ink and Paint • Some light is reflected from the surface • Some light passes through the ink and reflects from the paper underneath, while the ink absorbs some wavelengths • The rays from these two processes combine in the eye, mixing additively

28. Color Mixing: Ink and Paint • This process only occurs if the ink or paint is at least semi transparent, such as watercolors and some printing inks.

29. Watercolors • Watercolor is a challenging medium, because the transparent colors have to be layered carefully to avoid a muddy looking appearance • As light passes through each color layer, more wavelengths are subtracted, just like layering a set of filters • The light also reflects off the paper underneath, so the reflectance curve of the paper itself also subtracts some colors.

30. Process Ink Printing • The transparent inks used to print colored documents and packaging are called “process inks” • A printer wants to use the best primaries possible to create the largest number of colors from the smallest number of printing plates. • Each separate color requires a separate printing plate or roller

31. Process Ink Printing • The printing primaries are • Cyan • Magenta • Yellow • Black • Black is necessary because the printing inks are not ideal, and combining them together does not produce a true black, but a slightly color-tinged black. • This is also called the “CMYK” color system

32. Non-ideal Inks • The reason inks cannot be ideal is that they cannot be too saturated. • A narrow reflectance curve (more saturated) reflects less overall light, so saturated colors would be very dark

33. CMYK Printing

34. CMYK Printing

35. CMYK Printing • We have seen that we can generate a range of colors by subtractive mixing our CMYK primaries • But what it we want to print lighter, less saturated colors? • Diluting the inks would require a new printing plate for each color.

36. CMYK Printing: Halftones • To avoid having to add even more printing plates or rollers, printers use a variation of partitive mixing to generate lighter colors • They simply print less ink in a given area • This is called “half-tone“ printing, and is common in newspaper and magazine printing

37. Halftones When ink dots overlap: subtractive mixing Ink dots next to each other, partitive/additive mixing