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

Light Stream of Photons (Energy: a measurable quantity) EM Waves Dispersion 700 nm550 nm400 nm

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

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

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 J. Physical (Radiometric) Units Light as a form of energy Luminous (Photometric) Units Visual effect produced by light

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: Description Physical Units Luminous Units Total energy (light) output Radiant flux (Watts) Luminous flux (Lumens) Light reaching a unit area Irradiance or Intensity (Watts/m 2 ) Illuminance (Lumens/m 2 = Lux)

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: lux Star light: lux Moon: 0.1 lux Office: 300 lux Cloudy day: 1000 lux W W

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

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

Blackbody Sources “Hot” objects characterized by continuous spectra. Examples: Sun, candle light, incandescent lamp… Features: 1.Stephan’s Law: 2. Wein’s displacement law: ww2.unime.it/dipart/i_fismed/ wbt/ita/physlet/blackbody/ corponero.htm

Review Problems 1.Calculate the peak wavelength at which you radiate light (your body temperature is about K). 2.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 K

Solar Spectrum (Blackbody Source)

Bright Line Sources Generally single elements, characterized by discontinuous line spectra. Examples: Sodium street light, mercury lamp, neon sign, laser… Hydrogen Helium Carbon How do atoms emit / absorb light?

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.

Electronic Transitions Example: Hydrogen Atom Energy levels are given by: Ground state: E 1 = eV Higher states: E 2 = -3.4 eV E 3 = -1.5 eV…. E2E2 E1E1 E2E2 E1E1 Jump to a higher level Energy equal to or greater than (E 2 -E 1 ) must be supplied Jump to a lower level Excess energy (E 2 -E 1 ) is released as a photon

The Hydrogen Spectrum eV -1.5 eV -3.4 eV eV eV Visible lines in the hydrogen spectrumTransition Photon Energy WavelengthColor E 3  E eV 653 nm Red E4  E2E4  E2E4  E2E4  E eV 486 nm Blue E5  E2E5  E2E5  E2E5  E eV 434 nm Violet 1 n=4 n=5 n=3 n=2 n=1

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). Transmitted Light Reflected Light Incident Light Object

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

Reflection & Transmission Important Rule: For each wavelength, Perceived Color Spectral content of source Selective reflectivity or transmission of object Rel. intensity % Reflectance + = nm Blue light Red surfaceDark appearance cs/exploratories/applets/spectrum/reflection_guide.html

Review Problem Calculate the transmitted spectrum from the following data: Rel. intensity (nm) Incident light intensity% Transmission of filter % Transmission (nm) Transmitted Spectrum Rel. intensity (nm)

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.

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 (nm) Rel. intensity (nm) Rel. intensity (nm) Rel. intensity (nm) Blue Red Green

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: Rel. intensity (nm) Rel. intensity (nm) Red Green Rel. intensity (nm) Yellow +=

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. Rel. intensity (nm) Magenta or - Green Rel. intensity (nm) Cyan or - Red Rel. intensity (nm) Yellow or - Blue

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

Review 1.Explain how you would obtain the following colors by combining various intensities of the additive primaries: a) Yellowb) Pink c) Whited) Orange e) Purplef) Light cyan 2. Explain how you would obtain the following colors by combining subtractive primary filters: a) Redb) Greenc) Blue d) Blacke) Whitef) Pink g) Orange