Chapter 2: Origin of Color What produces the color sensation?

1. Chapter 2: Origin of Color What produces the color sensation?

2. Light Stream of Photons (Energy: a measurable quantity) EM Waves Dispersion 700 nm 550 nm 400 nm

3. Direct Source (Illuminant) Indirect Object • Color sensation depends on: • Spectral composition of source / object • Intensity of light • Source type • Spectral sensitivity of the eye

4. Spectral Energy Distribution or Spectral Composition Relative amounts of light from different parts of the spectrum

5. Measurement of Light • Physical Units: • Joule (J): SI unit of energy. Energy required to lift a 1 kg object by 10.2 cm at sea level. • Watt (W): Rate at which energy is transformed (or work is done). One Watt is the same as one Joule/second. • Electron-Volt (eV): More useful unit of energy when dealing with atoms. 1 eV is equal to 1.6 x 10-19 J. Physical (Radiometric) Units Light as a form of energy Luminous (Photometric) Units Visual effect produced by light

6. Measurement of Light (Contd.) Luminous Units: Take into account the sensitivity of the eye at different wavelengths. So physical units must be scaled up or down! Units of Illumination:

7. Photometric Conversion • Formula to convert physical units to luminous units: • Luminous Units = Physical Units x RLE x 685 • Example: How many watts of power are required to produce 1 lux of illuminance by… • Red light (650 nm)? • Green light (550 nm)? • Useful Information: • Dark Night: 0.0001 lux • Star light: 0.001 lux • Moon: 0.1 lux • Office: 300 lux • Cloudy day: 1000 lux 0.0073 W 0.0015 W

8. Review Question • Both bulbs radiate the same amount of total energy. 100 Watts = 100 Joules per second. • 1900 Lumens appears brighter because it radiates more energy in the “useful” part of the spectrum. 100 Watts 1400 Lumens 100 Watts 1900 Lumens

9. Sources of Light • Depending on their spectra, light sources can be divided into two main categories. • Blackbody sources • Bright line sources

10. Blackbody Sources • “Hot” objects characterized by continuous spectra. • Examples: Sun, candle light, incandescent lamp… • Features: • Stephan’s Law: • 2. Wein’s displacement • law: ww2.unime.it/dipart/i_fismed/ wbt/ita/physlet/blackbody/ corponero.htm

11. Review Problems • Calculate the peak wavelength at which you radiate light (your body temperature is about 3100K). • How hot would a blackbody need to be in order to have its peak wavelength at 550 nm? • Color Temperature • Describes the kind of light produced by a blackbody source. • Higher color temperature  abundant in blue • Lower color temperature  abundant in red 9323 nm 5255 0K

12. Solar Spectrum (Blackbody Source)

13. Bright Line Sources • Generally single elements, • characterized by discontinuous • line spectra. • Examples: Sodium street light, • mercury lamp, neon sign, laser… http://mo-www.harvard.edu/Java/MiniSpectroscopy.html Hydrogen Helium Carbon How do atoms emit / absorb light?

14. Model of an Atom • Atoms = Nucleus (protons + neutrons) + Electrons. • Electrons in neutral atoms occupy definite energy levels (orbits) around the nucleus. • Electrons can jump between energy levels by absorbing or emitting energy.

15. Electronic Transitions • Example: Hydrogen Atom • Energy levels are given by: • Ground state: E1 = -13.6 eV • Higher states: E2 = -3.4 eV • E3 = -1.5 eV…. E2 E2 E1 E1 Jump to a higher level Energy equal to or greater than (E2-E1) must be supplied Jump to a lower level Excess energy (E2-E1) is released as a photon

16. The Hydrogen Spectrum n=5 -0.54 eV n=4 -0.85 eV n=3 -1.5 eV n=2 -3.4 eV n=1 -13.6 eV Visible lines in the hydrogen spectrum

17. Reflection, Transmission & Absorption • Incident Energy = Transmitted + Reflected + Absorbed • Colored objects can selectively reflect or transmit some part of the incident spectrum. • Absolute amount of reflected or transmitted light depends on: • Reflection / Transmission curve • Intensity of incident light at each wavelength (spectral composition). Object Incident Light Transmitted Light Reflected Light

18. Spectral Energy Curves & Reflectance Curves Percent of light reflected Rel. intensity White Bright 100 % Gray 50 % Black Dim 0 % 400 500 600 700 (nm) 400 500 600 700 (nm) Surfaces Lights

19. Reflection & Transmission • Important Rule: For each wavelength, Perceived Color Selective reflectivity or transmission of object Spectral content of source Rel. intensity % Reflectance Rel. intensity + = Red surface Dark appearance Blue light 700 nm 400 700 nm 400 700 nm 400 http://www.cs.brown.edu/exploratories/freeSoftware/repository/edu/brown/ cs/exploratories/applets/spectrum/reflection_guide.html

20. Review Problem Calculate the transmitted spectrum from the following data: Incident light intensity % Transmission of filter Rel. intensity % Transmission 10 100 5 50 0 0 400 400 500 600 700 (nm) 500 600 700 (nm) Rel. intensity 10 Transmitted Spectrum 5 0 400 500 600 700 (nm)

21. Absorption • Absorbed energy raises the temperature of the object. • Dark objects absorb more energy. • The Greenhouse Effect: • Absorbed light is converted • to heat (IR) which is • trapped by the greenhouse • because glass is opaque • to IR.

22. Rel. intensity 400 500 600 700 (nm) • Color Mixing • Where do colors like pink, brown, silver…come from? • Ideal white light source: • Produces equal energy in • all parts of the visible spectrum! • Additive primaries: Divide the ideal source into three equal parts. Rel. intensity Rel. intensity Rel. intensity Green Red Blue 400 500 700 (nm) 400 500 700 (nm) 600 600 400 500 700 (nm) 600

23. Rel. intensity Rel. intensity Rel. intensity + = Green Red Yellow 400 500 700 (nm) 400 500 700 (nm) 400 500 700 (nm) 600 600 600 • Additive Mixing • Additive primaries: Red, Green , and Blue. • Each primary is 1/3 of the spectrum. • Colors are produced by “adding” spectra. • Need three sources of light to produce colors. • Applications: Color TV, stage lighting…etc. • Example: http://www.cbu.edu/~jvarrian/applets/color1/colors_g.htm

24. Rel. intensity Rel. intensity Rel. intensity 400 500 700 (nm) 600 400 500 700 (nm) 600 400 500 700 (nm) 600 Magenta or - Green Yellow or - Blue Cyan or - Red • Subtractive Mixing • Subtractive primaries: Yellow, Cyan , and Magenta. • Each primary is 2/3 of the spectrum. • Colors are produced by “subtracting” part of the spectrum from white light source (i.e. by overlapping filters). • Need one white light source to produce colors. • Applications: Pigments, dyes, color printing…etc. http://lite.bu.edu/vision/applets/Color/Color/Color.html

25. Complementary Colors • Pair of colors that produce white when mixed additively. • Example: Yellow + Blue • Cyan + Red • Green + Magenta

26. Review • Explain how you would obtain the following colors by combining various intensities of the additive primaries: • a) Yellow b) Pink • c) White d) Orange • e) Purple f) Light cyan • 2. Explain how you would obtain the following colors by combining subtractive primary filters: • a) Red b) Green c) Blue • d) Black e) White f) Pink • g) Orange http://www.cs.brown.edu/courses/cs092/2000/py27/cmatchapp.html