Visual Imaging and the Electronic Age Color Science

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1 Visual Imaging and the Electronic Age Color Science Grassman s Experiments & Trichromacy Lecture #5 September 8, 2015 Prof. Donald P. Greenberg

2 What is Color Science? Quantifying the physical energy which reaches the eye (physical) Determining the perceptual responses of the human visual system (perceptual) How does the brain interpret what is seen? (cognitive)

3 Color Rainbow over London.

4 Color

5 Visible Light Spectrum

6 Frequency Spectrum

7 Rods & Cones Comparison of a rod cell (right) and cone cell (left). This shows how each cell acquired its name from its shape.

Visible Light Spectrum 400 500 600 700 S M L

8 Visible Light Spectrum S M L Dominant wavelengths of human receptor system

9 Question? How can we map a Physical light description to a Perceptual color sensation? P? l Physical Perceptual

10 Question? Why can a TV display reproduce (almost) every color sensation that we can experience using only 3 color phosphors? R,G,B?

11 Question? Why can a printer reproduce (almost) every color sensation that we can experience using only 3 color inks? CMY?

12 Question? How can we reproduce such a vast range of color using two completely different sets of three primary colors? R G B vs C M Y

13 Additive Color

14 Subtractive Color

15 Printer Inks Black Cyan Magenta Yellow

16 Subtractive Reflection Processes

17 Additive & Subtractive Color Spaces RGB CMY

18 What is Color Science? Quantifying the physical energy which reaches the eye (physical) Determining the perceptual responses of the human visual system (perceptual) How does the brain interpret what is seen? (cognitive)

19 Power Power Spectral Distributions of Emissive Light Sources Test lamp CIE standard 400 Wavelength (nm) Wavelength (nm) 700 D 55 : typical sunlight D 65 : typical average daylight D 75 : typical north-sky light

20 Power Spectral Distributions of Emissive Light Sources Test lamp P E l l E400 E410 E420 E 430 E 460 E 550 E Wavelength (nm)

21 Emitted Light P l E l 700 P = Eλ dλ 390 P energy reaching the eye at all wavelengths E l emitted light energy at each wavelength

22 Spectral Distributions of Reflective Colors

23 Reflected Light P P P l E l E l l l (Emitted Light) P = E λ ρλ dλ energy reaching the eye at all wavelengths E l l emitted light energy at each wavelength reflected light energy at each wavelength

24 Reflected Light E l l E l l Roy S. Berns. Billmeyer and Saltzman s Principles of Color Technology, 3 rd Ed. 2000, John Wiley & Sons, Inc. p. 15.

25 Transmitted Light P l E l (Emitted Light) P E (ReflectedLight) l l l P T l E T l l l (Transmitted Light) transmitted light energy at each wavelength

26 Transparency

27 Transparency

28 What is Color Science? Quantifying the physical energy which reaches the eye (physical) Determining the perceptual responses of the human visual system (perceptual) How does the brain interpret what is seen? (cognitive)

29 Cross Section of Eye & Retina

30 Rods and Cones

31 Rods and Cones

32 Receptor Distribution fovea Adapted from Levine, Vision in Man and Machine McGraw-Hill, 1985.

33 Receptor Distribution

34 Receptor Distribution fovea parafovea periphery far periphery Cone Rod Adapted from Levine, Vision in Man and Machine McGraw-Hill, 1985.

35 Cone Responses S,M,L cones have broadband spectral sensitivity S,M,L neural response is integrated with respect to l results in a trichromatic visual system

36 Relative Spectral Sensitivity (normalized) The relative spectral sensitivity of the L, M, and S cones (Stockman 1993). These spectral sensitivities are based on measurements in front of the eye rather than of isolated photoreceptors. Strictly speaking, these are called cone fundamentals. Roy S. Berns. Billmeyer and Saltzman s Principles of Color Technology, 3 rd Ed. 2000, John Wiley & Sons, Inc. p. 14.

37 Grassmann s Color Matching Experiments (1853) Roy. S. Berns. Billmeyer and Saltzman s PRINCIPLES OF COLOR TECHNOLOGY, 2000 John Wiley & Sons, Inc.

38 Matching a Test Lamp with 3 Primary Lights We can match a color sensation from any spectrum using only 3 primary colors (R,G,B) spectral test lamp 700 nm 546 nm 436 nm observer

39 Matching a Test Lamp with 3 Primary Colors

40 Matching a Test Lamp with 3 Primary Lights Need to allow negative light Can t match a bright yellow (Y) light with R,G,B. But can match Y + B with R + G. spectral test lamp 436 nm 700 nm 546 nm observer

41 Matching a Test Color (Lamp) with 3 Primary Colors

43 Trichromatic Generalization Many colors can be matched by additive mixtures of suitable amounts of three fixed primary colors. Others have to be mixed with a suitable amount of one before it can be matched by the other two. All the colors can be matched in one of these two ways: The restriction is that none of the primary colors can be matched by an additive mixture of the other two.

44 Trichromatic Generalization Proportionality and additivity are valid over a large range of observing conditions. Proportionality - Additivity - If A=B, then ka=kb If A=B, and C=D, the A+C=B+D

45 Experiment to Determine the Response Matching Functions of the Average Human Observer 400 nm 700 nm 700 nm 546 nm 436 nm observer Individually match the RGB primary lights to the unit values of each of the spectral lamps.

46 Response Matching Functions of the Average Human Observer These are the response matching functions of the average human observer for these three primary lights.

47 Observer Response x = x = Response Matching Functions Roy S. Berns. Billmeyer and Saltzman s Principles of Color Technology, 3 rd Ed. 2000, John Wiley & Sons, Inc. p. 46.

48 Computing Tristimulus Values with the Response Matching Functions For each test lamp we can compute the equivalent RGB tristimulus values using the color matching functions l l l l l l l l l d b P B d g P G d r P R ) ( ) ( ) ( ) ( ) ( ) (

49 Observer Response The integrated L, M, and S responses that result from the light entering the eye from an illuminated object. This can be calculated as the product of the spectral properties of the light source, the object, and the observer s sensitivities, followed by integration over wavelength, essentially, calculating the areas under the last row of curves. Roy S. Berns. Billmeyer and Saltzman s Principles of Color Technology, 3 rd Ed. 2000, John Wiley & Sons, Inc. p. 15.

50 Observer Response Roy S. Berns. Billmeyer and Saltzman s Principles of Color Technology, 3 rd Ed. 2000, John Wiley & Sons, Inc. p. 46.

51 Trichromacy P S P l s l dl X l M P l m l dl L P l l l dl S,M,L

52 End...

Visual Imaging and the Electronic Age Color Science

Visual Imaging and the Electronic Age Color Science Grassman s Experiments & Trichromacy Lecture #5 September 6, 2016 Prof. Donald P. Greenberg Light as Rays Light as Waves Light as Photons What is Color