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Spectroscopy

Spectroscopy. 1. Electrical and magnetic properties. Electromagnetic fields are propagated through and reflected by materials Characterized as: Current flow at low frequencies Magnetism in metals Optical absorbance / reflectance in light

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Spectroscopy

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  1. Spectroscopy BAE2023 Physical Properties of Biological Materials 1

  2. Electrical and magnetic properties Electromagnetic fields are propagated through and reflected by materials Characterized as: Current flow at low frequencies Magnetism in metals Optical absorbance / reflectance in light Frequency is a major factor in the primary characteristics Low frequency – “electrical” properties High frequency – “optical” properties 4/18/2011 2

  3. Fundamentals of high frequency electromagnetic waves (Light) • Light = Energy (radiant energy) • Readily converted to heat • Light shining on a surface heats the surface • Heat = energy • Light = Electro-magnetic phenomena • Has the characteristics of electromagnetic waves (eg. radio waves) • Also behaves like particles (e.g.. photons) 3

  4. The electromagnetic spectrum 4

  5. Relationship between frequency and wavelength l Plus Plus Wavelength = speed of light divided by frequency (miles between bumps = miles per hour / bumps per hour) l= Wavelength [m] n= Frequency [Hz] c = 3x108 m/s in a vacuum Minus Minus 5

  6. Relationship between frequency and wavelength l + - Antenna Plus Plus Minus Minus lKOSU= 3 x 108 / 97.1 x 106 lKOSU= 3 m lred= 6.40 x 10- 7 m = 640 nm Bohr’s Hydrogen = 5 x 10 - 11 m 6

  7. Plants light harvesting structure - model Jungas et. al. 1999 7

  8. Light emission / absorption governed by quantum effects Planck - 1900 Einstein - 1905 One “photon” DE is light energy flux n is an integer (quantum) h is Planck’s constant n is frequency 8

  9. Frequency bands and photon energy 9

  10. Changes in energy states of matter are quantified Bohr - 1913 • Where Ek, Ej are energy states (electron shell states etc.) and frequency, n , is proportional to a change of state • and hence color of light. Bohr explained the emission spectrum of hydrogen. Hydrogen Emission Spectra (partial representation) Wavelength 10

  11. Measurement of reflected intensity –Typical Multi-Spectral Sensor Construction One Spectral Channel Photo-Diode detector / Amplifier Analog to Digital Converter CPU Optical Filter Illumination Collimator Radiometer Computer Target 11

  12. Measurement of reflected intensity - Fiber-Optic Spectrometer One Spectral Channel at a time Optical Glass Fiber Optical Grating Analog to Digital Converter CPU Element selection Computer Photo Diode Array 12

  13. Visual reception of color • Receptors in our eyes are tuned to particular photon energies (hn) • Discrimination of color depends on a mix of different receptors • Visual sensitivity is typically from wavelengths of ~350nm (violet) to ~760nm (red) Wavelength 700 nm 400 nm 500 nm 13

  14. Quantification of color • Spectral measurements can be used to quantify reflected light in energy and spectral content, but not very useful description of what we see. • Tri-stimulus models – represent color as perceived by humans • Tri-stimulus models • RGB - most digital work • CYM - print • HSI, HSB, or HSV - artists • CIE L*a*b* • YUV and YIQ - television broadcasts 14

  15. CIE XYZ model Y • Attempts to describe perceived color with a three coordinate system model X Z= luminance 15

  16. CIE Lab model • An improvement of the CIE XYZ color model. • Three dimensional model where color differences correspond to distances measured colorimetrically • Hue and saturation (a, b) • a axis extends from green (-a) to red (+a) • b axis from blue (-b) to yellow (+b) • Luminance (L) increases from the bottom to the top of the three-dimensional model • Colors are represented by numerical values • Hue can be changed without changing the image or its luminance. • Can be converted to or from RGB or other tri-stimulus models 16

  17. Photo-Chemistry • Light may be absorbed and precipitate (drive) a chemical reaction. Example: Photosynthesis in plants • The wavelength must be correct to be absorbed by some participant(s) in the reaction • Some structure must be present to allow the reaction to occur • Chlorophyll • Plant physical and chemical structure 17

  18. Silicon Responsivity 18

  19. Primary and secondary absorbers in plants • Primary • Chlorophyll-a • Chlorophyll-b • Secondary • Carotenoids • Phycobilins • Anthocyanins 19

  20. Chlorophyll absorbance Chla: black Chlb: red BChla: magenta BChlb: orange BChlc: cyan BChld: bue BChle: green Source: Frigaard et al. (1996), FEMS Microbiol. Ecol. 20: 69-77 20

  21. Radiation Energy Balance • Incoming radiation interacts with an object • and may follow three exit paths: • Reflection • Absorption • Transmission • a + t + r= 1.0 • a, t, andrare the • fractions taking each path • Known as: • fractional absorption coefficient, • fractional transmittance, and • reflectance respectively Il0 Il0r Il0 a Iout = Il0 t 21

  22. Internal Absorbance (Ai) • Lambert's Law - The amount of light absorbed is directly proportional to the logarithm of the length of the light path or the thickness of the absorbing medium. Thus: l = length of light path k = extinction coefficient of medium • Normally in absorbance measurements the measurement is structured so that reflectance is zero 22

  23. Reflectance • Ratio of incoming to reflected irradiance • Incoming can be measured using a “white” reflectance target • Reflectance is not a function of incoming irradiance level or spectral content, but of target characteristics 23

  24. Solar Irradiance NIR UV 24

  25. Soil and crop reflectance 25

  26. Soil Reflectances - Oklahoma 26

  27. Electromagnetic properties Review: Electromagnetic radiation is energy Interaction with materials is affected by the properties of the material Can give indication of physical damage, mold presence, foreign material, contaminating chemicals or ID of materials 27

  28. Electromagnetic properties Applications Near-infrared: measuring moisture, % oils and proteins Xrays: internal defects Microwaves: heating/cooking Magnetic properties: moisture content and composition Gamma Rays: sterilization of food products during processing 28

  29. Electromagnetic properties Electromagnetic radiation (ER) is transmitted in the form of waves Wavelength λ (lambda) Frequency ν (nu) λ ν = c, speed of light in a vacuum 3.0 x 108 29

  30. Electromagnetic properties Xrays and gamma rays have shortest wavelengths 10-12 m and highest frequencies 1020 hz 60 cycle AC: 60 hz and 5 x 106 m (coast to coast distance for 1 wavelength!!!!) 30

  31. Electromagnetic properties Interactions with visible light, Infrared and UV radiation Used for sorting and quality evaluation Iref = reflected I1 = energy entering the object I2 = energy striking the opposite face after rectilinear transmission Iout = leaving the opposite face 31

  32. Electromagnetic properties Transmittance: T=Iout/I0 Absorbance: Ai=-log (I1/I2) Reflectance: R=Iref/I0 Optical Density: log10(I0/Iout) Amount of energy transmitted through the material 32

  33. Electromagnetic properties Flourescence: excited by energy at a particular wavelength and then emits energy at a different wavelength (aflatoxin test for aspergillus...fungi) Delayed-light emission: radiation is emitted for a time after the exciting radiation is removed (chlorophyll) 33

  34. Electromagnetic properties Resistance, Capacitance and Dielectric Properties Biological materials act as a combination of resistors and capacitors Varies with moisture content and internal structure Used to evaluate quality and composition Dielectric loss factor is important in heating (microwave) 34

  35. Electromagnetic properties Resistance, Capacitance and Dielectric Properties Resistance: measure by placing material between two metal plates and incorporating into an electric circuit Value of R is inversely correlated with moisture content Pressure of plates and temperature also affect R 35

  36. Electromagnetic properties Resistance, Capacitance and Dielectric Properties Resistivity: ρr (rho) R = (ρr L)/A , Ω-1m-1 or Siemen/m, S/m Resistance and resistivity are variable So…we use capacitance instead. In an AC circuit, capacitor causes a phase shift between voltage and current. (perfect vacuum = 90°) With biomaterials in place < 90° See Figure 11.5 36

  37. Electromagnetic properties Resistance, Capacitance and Dielectric Properties Dielectric Properties: dielectric constant ε' and dielectric loss factor ε”. ε‘ = ability of material to store energy ε” = ability of mateials to dissipate energy Loss tangent = ε” / ε‘ Rate of heat generation per unit volume (Q) at a location inside material: Q = 2πf ε0 ε”E2, where f = frequency, ε0 = free space dc (8.854E-12 F/m), E = electric field 37

  38. Electromagnetic properties Resistance, Capacitance and Dielectric Properties Distance waves will penetrate material before being reduced to 36.8% of original value….power penetration depth (δp) δp = λ0((1+ (ε”/ ε‘)2)1/2)-1/2) / (2π(2 ε' )1/2 λ 0 = wavelengh in free space 38

  39. Electromagnetic properties Resistance, Capacitance and Dielectric Properties Example 4.2 pg 176 of handout 39

  40. Electromagnetic properties Resistance, Capacitance and Dielectric Properties Moisture content effects on dielectric properties Pg 177 handout figure 4.18 Free water : found in capillaries (I) Bound water: physically adsorbed to the surface of dry materials (II) 40

  41. Electromagnetic properties Resistance, Capacitance and Dielectric Properties Example of dielectric properties: Page 183 handout Table 4.2 Measuring dielectric properties pg 187 handout figure 4.23 41

  42. Electromagnetic properties Problem 1. Estimate the penetration depth of raw beef during cooking in a home microwave oven. Assume that dielectric properties are constant throughout heating. Problem 2. Determine the angle of signal lag for wheat, corn and rice. Problem 3. 11.2 in Stroshine book Problem 4. 11.4 in Stroshine book 42

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