1. BAE2023 Physical Properties of Biological Materials
Spectroscopy
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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
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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)
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4. The electromagnetic spectrum
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5. 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
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6. 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 m = 640 nm
Bohr’s Hydrogen = 5 x m
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7. Plants light harvesting structure - model
Jungas et. al. 1999
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8. Light emission / absorption governed by quantum effects
Planck
Einstein
One “photon”
DE is light energy flux
n is an integer (quantum)
h is Planck’s constant
n is frequency
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9. Frequency bands and photon energy
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10. Changes in energy states of matter are quantified
Bohr
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
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11. 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
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12. 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
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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
400 nm
500 nm
700 nm
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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
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15. CIE XYZ model Y
Attempts to describe perceived color with a three coordinate system model X
Z= luminance
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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
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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
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18. Silicon Responsivity
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19. Primary and secondary absorbers in plants
Chlorophyll-a
Chlorophyll-b
Secondary
Carotenoids
Phycobilins
Anthocyanins
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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
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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, 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
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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
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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
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24. Solar Irradiance UV
NIR
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25. Soil and crop reflectance
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26. Soil Reflectances - Oklahoma
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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
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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
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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
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30. Electromagnetic properties
Xrays and gamma rays have shortest wavelengths m and highest frequencies 1020 hz
60 cycle AC: 60 hz and 5 x 106 m (coast to coast distance for 1 wavelength!!!!)
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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
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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
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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)
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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)
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35. 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
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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
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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
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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
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39. Electromagnetic properties Resistance, Capacitance and Dielectric Properties
Example 4.2 pg 176 of handout
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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)
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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
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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 in Stroshine book
Problem in Stroshine book
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