BAE2023 Physical Properties of Biological Materials Spectroscopy 11/15/2018 BAE2023 Physical Properties of Biological Materials 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 Frequency is a major factor in the primary characteristics Low frequency – “electrical” properties High frequency – “optical” properties 11/15/2018 2 2

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) 11/15/2018 3

The electromagnetic spectrum 11/15/2018 4

Relationship between frequency and wavelength 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 11/15/2018 5

Relationship between frequency and wavelength + - Antenna Plus Plus Minus Minus l KOSU = 3 x 108 / 97.1 x 106 l KOSU = 3 m l red = 6.40 x 10- 7 m = 640 nm Bohr’s Hydrogen = 5 x 10 - 11 m 11/15/2018 6

Plants light harvesting structure - model Jungas et. al. 1999 11/15/2018 7

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 11/15/2018 8

Frequency bands and photon energy 11/15/2018 9

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 11/15/2018 10

Measurement of reflected intensity – Typical Multi-Spectral Sensor Construction One Spectral Channel Photo-Diode detector / Amplifier CPU Analog to Digital Converter Optical Filter Illumination Collimator Computer Radiometer Target 11/15/2018 11

Measurement of reflected intensity - Fiber-Optic Spectrometer One Spectral Channel at a time Optical Glass Fiber Optical Grating CPU Analog to Digital Converter Element selection Computer Photo Diode Array 11/15/2018 12

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 400 nm 500 nm 700 nm 11/15/2018 13

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 11/15/2018 14

CIE XYZ model Y Attempts to describe perceived color with a three coordinate system model X Z= luminance 11/15/2018 15

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 11/15/2018 16

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 11/15/2018 17 15 14

Silicon Responsivity 11/15/2018 18 18 21

Primary and secondary absorbers in plants Chlorophyll-a Chlorophyll-b Secondary Carotenoids Phycobilins Anthocyanins 11/15/2018 19

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 11/15/2018 20

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, and r are the fractions taking each path Known as: fractional absorption coefficient, fractional transmittance, and reflectance respectively Il0 Il0 r Il0 a Iout = Il0 t 11/15/2018 21

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 11/15/2018 22

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 11/15/2018 23

Solar Irradiance UV NIR 11/15/2018 24

Soil and crop reflectance 11/15/2018 25

Soil Reflectances - Oklahoma 11/15/2018 26

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 11/15/2018 27

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 11/15/2018 28

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 11/15/2018 29

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!!!!) 11/15/2018 30

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 11/15/2018 31

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 11/15/2018 32

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) 11/15/2018 33

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) 11/15/2018 34

Electromagnetic properties Resistance, Capacitance and Dielectric Properties 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 11/15/2018 35

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 11/15/2018 36

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 11/15/2018 37

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 11/15/2018 38

Electromagnetic properties Resistance, Capacitance and Dielectric Properties Example 4.2 pg 176 of handout 11/15/2018 39

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) 11/15/2018 40

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 11/15/2018 41

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 11/15/2018 42