SCI 200 Physical Science Lecture 11 Sources of Color

SCI 200 Physical Science Lecture 11Sources of Color Rob Daniell August 4, 2011

Sources of Color • Part I: Self-luminous objects • Objects that emit their own light • Part II: Non-luminous objects • Object that a visible due to reflected light SCI200 - Lecture 11

Sources of Color • Part I: Self-luminous objects • Thermal radiators • Non-thermal sources • Sources: • Malacara, Daniel: Color Vision and Colorimetry: Theory and Applications,SPIE Press, Bellingham, 2001 • Especially Chapter 2. • Various Wikipedia articles, particularly “Liquid Crystal Display” and “Backlight” SCI200 - Lecture 11

Sources of Color: Self-luminous objects • Thermal radiation • Any object with a finite temperature radiates electromagnetic (EM) waves • “free” electrons vibrate (oscillate) due to thermal energy • Oscillating electrons emit EM waves • Light consists of EM waves between 400-700 nm • An ideal thermal radiator is termed a “black body” • If it radiates EM waves (light), why is it “black”? SCI200 - Lecture 11

Sources of Color: Self-luminous objects • Black body • A good emitter and a good absorber • Because of the “free” electrons • A black body not only emits EM waves, • It absorbs EM waves. • No reflection • At low temperatures • little radiation • good absorption • So a black body looks black SCI200 - Lecture 11

Sources of Color: Self-luminous objects • Black body radiation • Thermal equilibrium • Characterized by a definite temperature • Usually expressed in kelvins (K) • TK = 273 + TC • TK = 273 + (5/9)(TF –32) • Characterized by a specific spectrum • Intensity of radiation as a function of wavelength • First use of “quantum hypothesis” SCI200 - Lecture 11

Sources of Color: Self-luminous objects Visible wavelengths: 400-700 nm • Black body spectrum • Wavelength of peak emission intensity SCI200 - Lecture 11

Sources of Color: Self-luminous objects • Black body spectrum in the visible range SCI200 - Lecture 11

Sources of Color: Self-luminous objects • Real (as opposed to ideal) radiators • Sun • Surface temperature about 5500 K • Incandescent light bulbs • Filament (usually tungsten) heated by electric current • Bulb filled with inert gas (e.g., argon) to prevent oxidation • About 90% of the power is emitted as heat • Approximate black body spectrum SCI200 - Lecture 11

Sources of Color: Incandescent light bulbs • Incandescent light bulbs • Filament (usually tungsten) heated by electric current • Bulb filled with inert gas (e.g., argon) to prevent oxidation • Clear or frosted glass SCI200 - Lecture 11

Sources of Color: Self-luminous objects • Solar spectrum & equivalent black body spectrum SCI200 - Lecture 11

Sources of Color: Self-luminous objects • Color temperature • Temperature of a black body that would have the same color as the emitter • For daylight • Color temperature varies through the day • Color temperature varies with cloud cover etc. • For incandescent bulbs • Color temperature is very close to actual temperature SCI200 - Lecture 11

Sources of Color: Self-luminous objects • Color temperature & chromaticity SCI200 - Lecture 11

Incandescent light sources Incandescent light sources glow from their own heat and emit a “black body spectrum.” Incandescent light sources SCI200 - Lecture 11

Sources of Color: Self-luminous objects • Color temperature • Note that lower color temperatures correspond to redder colors • “Cooler” and “warmer” colors refer to psychological perception • To produce “warmer” (redder) colors, lower the color temperature • To produce “cooler” (bluer) colors, raise the color temperature SCI200 - Lecture 11

Sources of Color: Self-luminous objects • Non-thermal sources • Wide variety • Color temperature usually has little to do with the operating temperature • Color temperature is often a poor approximation to the actual color of the emitted light SCI200 - Lecture 11

Sources of Color: Self-luminous objects • Gas discharge lamps • Glass tube filled with • A low pressure gas such as hydrogen, argon, neon • A vaporized metal such as sodium or mercury • When an electric current is passed through the gas or vapor, the individual atoms are caused to emit light. • Atoms absorb discrete amounts of energy from the electrons • Atoms reemit the energy as photons, i.e., discrete amounts of energy • Discrete “lines” (wavelengths) rather than thermal continuum • Spectrum details depend on the pressure of the gas or vapor SCI200 - Lecture 11

Sources of Color: Self-luminous objects • Fluorescent lamps • A mercury vapor lamp with the interior of the glass tube coated with a fluorescent powder. • A fluorescent material • Absorbs short wavelength radiation • Emits longer wavelength radiation SCI200 - Lecture 11

Applications of self-luminous sources • Television, computer monitors, electronic displays • Cathode Ray Tube (CRT) • Largely obsolete • Liquid Crystal Display (LCD) • Many variations on the basic design SCI200 - Lecture 11

Applications of self-luminous sources • Cathode Ray Tubes • Date back to the 19th century • Vacuum tube • Electron “gun” • Electrons were first known as “cathode rays” • Screen coated with phosphors • Chemicals that glow when struck by electrons SCI200 - Lecture 11

Applications of self-luminous sources • Cathode Ray Tubes • Vacuum tube • Electron “gun” • Designed to produce a narrow beam • Electromagnet deflection coils to aim the beam • Screen coated with phosphors SCI200 - Lecture 11

Applications of self-luminous sources • Cathode Ray Tubes • Red, Green, & Blue signals • Each signal controls an electron beam • Screen devided into Red, Green, & Blue pixels • Phosphors continue to glow even after the electron beam has moved on SCI200 - Lecture 11

Applications of self-luminous sources • Liquid Crystal Displays • Based on the optical activity of Liquid Crystals • Liquids of long molecules, usually organic • Rotate the plane of polarization of light • Amount of rotation can be controlled through the applied voltage • Do not emit light themselves SCI200 - Lecture 11

Applications of self-luminous sources • Liquid Crystal Displays • 1-linear polarizer (vertical) • 2-glass substrate with electrodes • Shape of electrodes determines what can be displayed • 3-Liquid crystal • 4-glass substrate with common electrode • 5-linear polarizer (horizontal) • 6-reflecting screen or backlight SCI200 - Lecture 11

Applications of self-luminous sources • Liquid Crystal Displays • Backlight • White lights plus colored filters • Fluorescent light • “white”LEDs (Light Emitting Diodes) • Colored LEDs (RBG) SCI200 - Lecture 11

Sources of Color: Self-luminous objects • A note on units related to brightness • candela (cd) = the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540x1012 hertz (555.6 nm) and that has a radiant intensity in that direction of 1/683 watt per steradian • A common candle emits roughly 1 cd • A 100 W incandescent light bulb emits about 120 cd • Luminous flux • 1 lumen = 1 cd sr = flux from 1 cd into 1 sr • Illuminance (light received per unit surface area) • SI unit: 1 lux (lx) = 1 lumen m-2 • English unit: 1 foot-candle (fc) = 1 lumen ft-2 = 10.76 lx SCI200 - Lecture 11

Sources of Color: Self-luminous objects • Illuminance(light received per unit surface area) • SI unit: • 1 lux (lx) = 1 lumen m-2 • English unit: • 1 foot-candle (fc) • = 1 lumen ft-2 = 10.76 lx • Can be measured by commercially available lux meters: SCI200 - Lecture 11

Sources of Color: Self-luminous objects • Another definition of “foot-candle” SCI200 - Lecture 11

Sources of Color • Part II: Non-luminous objects • Various causes of color: • Pigments • Structural elements • Atomic, molecular, and crystalline properties • Sources: • Chapter 9 of textbook (Gilbert & Haeberli, Physics in the Arts) • Misc sources (mostly Wikipedia) SCI200 - Lecture 11

Sources of Color: Non-luminous sources • Pigments • Chemical compounds that absorb light of different wavelengths • Reflected (or scattered) light is colored • Appearance depends on • Source of illumination • Thickness of pigment coating • Mixtures of various pigments • Natural: “Animal, vegetable, and mineral” • Synthetic SCI200 - Lecture 11

Sources of Color: Non-luminous sources • Pigments • Animal pigments include • Melanin • Eumelanin: skin color and dark hair colors • Pheomelanin: red hair • Carotenoids - not strictly animal • Absorbed by animals from plants, algae, and photosynthetic bacteria • Responsible for pinks of flamingos and salmon • Blue eyes are due to lack of pigment • Rayleigh scattering (as in the atmosphere) causes more blue light to be scattered • Longer wavelengths are absorbed at the back of the iris SCI200 - Lecture 11

Sources of Color: Non-luminous sources • Pigments • “Vegetable” (i.e., plant) pigments include • Chlorophyll (green) • Participates in photosynthesis • Carotenoids (yellow, orange, pink, or red) • Tannin (black or brown) • Anthocyanins (red or bluish purple) SCI200 - Lecture 11

Sources of Color: Non-luminous sources • Pigments • Mineral pigments are extremely varied • Often oxides of metals • Each chemical compound has a unique absorption and emission spectrum • Discrete wavelengths • Bands of wavelengths • Used in paints, dyes, makeup, etc. • Usually sold as powders • Final colors depend on the liquid or paste base SCI200 - Lecture 11

Sources of Color: Non-luminous sources • Structural colors: Iridescence • If a material layer’s thickness is similar to the wavelength of visible light • Interference effects produce color SCI200 - Lecture 11

Sources of Color: Non-luminous sources • Structural colors: Iridescence • If a material layer’s thickness is similar to the wavelength of visible light • Interference effects produce color • Relative phase depends on angle of incidence • Colors vary with relative locations of source, layer, and observer SCI200 - Lecture 11

Sources of Color: Non-luminous sources • Structural colors: Iridescence • Examples: • Soap bubbles • Oil films • Bird feathers • Butterfly wings • Beetle shells • Pearls, mother of pearl • Opals • Fossils shells SCI200 - Lecture 11

Sources of Color: Non-luminous sources • Gemstones • Chemical composition • Sometimes impurities can affect colors • Ruby is corundum [Al2O3] with chromium impurities • Emerald is beryl [Be3Al2(SiO3)6] with chromium impurities • Beryl with other impurities takes on other colors: • Aquamarine is beryl with Fe2+ • Heliodor is beryl with Fe3+ • Marganite is beryl with Mn2+ • Etc., etc. SCI200 - Lecture 11

Sources of Color: Non-luminous sources • Gemstones • Active participation • Blue sapphire: corundum [Al2O3] with iron (Fe) and titanium (Ti) impurities. • When light hits the mineral it causes electrons to jump between the Fe and the Ti. Red and green light is absorbed in the process, leaving blue to be reflected back to the observer SCI200 - Lecture 11

Sources of Color: Non-luminous sources • Gemstones • Color centers • Defects in the crystal structure trap electrons which can absorb light • Often only certain wavelengths • Other wavelengths are scattered, giving color to the gem • Defects can be natural • Occurring when the crystal forms • Defects can be artificial • Produced by exposure of the crystal to various forms of radiation SCI200 - Lecture 11

Sources of Color: Non-luminous sources • Gemstones • Semiconductor band gaps • In order to conduct electricity, electrons must gain a minimum amount of energy • Recall that light of shorter wavelengths has higher energy • There is a maximum wavelength that can be absorbed • Threshold is different for different semiconductors SCI200 - Lecture 11

Sources of Color: Non-luminous sources • Gemstones • Semiconductor band gaps • Threshold longer than 700 nm: opaque • Threshold shorter than 400 nm: transparent • Threshold between 400 nm and 700 nm: colored • Red, orange, or yellow • Not blue SCI200 - Lecture 11

Sources of Color: Non-luminous sources • Gemstones • Impurities can cause multiple band gaps • Permits blue color SCI200 - Lecture 11

Summary • Self-luminous sources • Thermal • Non-thermal • Non-luminous sources • Pigments • Structural color (iridescence) • Mineral color SCI200 - Lecture 11

Summary • Self-luminous sources • Thermal • Approximately a black body spectrum • Wavelength of peak emission varies with temperature • Non-thermal • Atomic or molecular process • Excitation by electrons or other non-luminous processes SCI200 - Lecture 11

Summary • Non-luminous sources • Pigments • Chemicals whose composition determines which wavelengths are absorbed and which are reflected or transmitted • Structural color (iridescence) • Relies on destructive and constructive interference effects • Depends on relative location of source, object, and observer SCI200 - Lecture 11

Summary • Non-luminous sources • Mineral color • Wide variety of mechanisms • Color is frequently caused by • Impurities • Structural defects • Peculiarities of the electronic structure of the material (semi-conductors) SCI200 - Lecture 11

Homework Assignment • No Homework Packet • Lab 8: Color Subtraction • Thursday, August 4 • Make-up Lab • Thursday, August 11 • Test #3, Thursday, August 11 • Chapters 7, 8, & 9 • Lectures 9, 10, & 11 SCI200 - Lecture 11

SCI 200 Physical Science Lecture 11 Sources of Color